Optical and Structural Characterization of Amorphous Carbon Films

Transcription

1 Optical and Structural Characterization of Amorphous Carbon Films By Pratish Mahtani A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Department of Electrical and Computer Engineering University of Toronto Copyright by Pratish Mahtani 2010

2 Optical and Structural Characterization of Amorphous Carbon Films Abstract Pratish Mahtani Master of Applied Science Department of Electrical and Computer Engineering University of Toronto 2010 A fundamental study of the correlations between ion energy, substrate temperature, and plasma density with hydrogen content, percent sp 2 bonding, optical gap, and refractive index of hydrogenated amorphous carbon (a-c) films is presented. A strong dependency between the ion energy used during deposition and the film s microstructure is shown. Moreover, it is revealed that the optical properties of the a-c films are controlled by the concentration and size of sp 2 clusters in the film. Through N 2 mixing in the source gas, room-temperature nitrogen doped polymeric-like a-c films were demonstrated for the first time. X-ray Photoelectron Spectroscopy revealed an increase in the Fermi level of these films with increased nitrogen content. A proof-of-concept a-c based transparent heat mirror (THM) was demonstrated. It was shown that a-c acts as an oxygen-free protective barrier and anti-reflective coating for Ag films in the THM, increasing the transmission in the visible region by 10-20%. ii

3 Acknowledgments First and foremost, I would like to thank my supervisors, Professor Nazir Kherani and Professor Stefan Zukotynski. The fundamental scientific approach and forward thinking that they provided was a great base from which to conduct research. I am indebted to Professor Kherani for his support, patience, guidance, and endless hours of analysis and discussion of my research. I am grateful to Professor Zukotynksi for his careful and well thought out critique of my work. This thesis would not have been possible without them. A special thank you to Dr. Davit Yeghikyan and Dr. Tome Kosteski for mentoring me through the early stages of my experiments and providing a structured, safe, and enjoyable work environment in the lab. I would also like to thank them for supervising and assisting me in the building of the RF PECVD system. Their experience in the field was a resource that I drew upon countless times during my research. Keith Leong has been a great colleague, tutor, and friend. I would like to express my deepest gratitude for his extensive assistance in my research including providing training on the spectral ellipsometer and FTIR spectrometer. I can only hope that his steadfast adherence to the scientific approach has in-part rubbed off on me. I would like to thank Paul O Brien for his assistance with SEM, transmission/reflection, and conductivity measurements. I would also like to thank Paul for his friendship and for being an outlet for my hockey obsession. Thank you to Dr. Adel Gougam for assisting me with four-point probe measurements and to Dr. Honggang Liu for performing lifetime measurements. Also, I would like to express my gratitude to Raymond Tsai for providing training on the DC Saddle Field system and introducing me to the field of amorphous carbon. Thank you to Avikshit Mathur and Manish Goyal for providing training on the E-beam, thermal evaporation, and sputtering systems. Thank you to Anton Fischer for assisting me with the optical microscope. I would also like to thank Dr. Rana Sodhi for performing XPS measurements. Thank you to the NSERC Solar Buildings Research Network for their support of this research. A special thank you to Professor Andreas Athienitis and Professor Stephen Harrison for their iii

4 collaboration and development of this research project. I would also like to express my gratitude to Professor Michael Collins for his assistance in transmission/reflection and emissivity measurements. Thank you to Jarrett Carriere for assisting in developing the initial research plan for this project. Lastly, I would like to express my deepest appreciation to my friends and family for providing support, encouragement, and when I needed it, a distraction from my research. Nothing is possible without you. iv

5 Table of Contents Acknowledgments... iii Table of Contents... v List of Tables... viii List of Figures... ix List of Symbols and Acronyms... xvii 1 Introduction Properties of carbon Current state and trends in amorphous carbon research Amorphous carbon as an optical coating Research overview Amorphous Carbon: An Overview Deposition methods Plasma kinetics RF PECVD Process variables in RF PECVD Film growth processes Experimental Apparatus & Characterization Techniques Deposition system RF PECVD system Sample preparation Sample set space UV-VIS-NIR Spectral Ellipsometry X-ray photoelectron and X-ray excited Auger electron spectroscopy Overview v

6 3.6.2 Elemental composition Film Density Fermi level shifts Carbon sp 2 / sp 3 bonding ratio Fourier Transform Infrared Spectroscopy Profilometry Experimental Results Overview Growth rate Nitrogen content Hydrogen content Percent of sp 3 Bonding Film density Impurity doping Optical properties Analysis Overview Effect of deposition conditions on growth, electronic- and micro-structure of films C-RT and A-RT sample sets A-20 sample set C-N and A-N Sample Sets Relationship between film microstructure and optical properties Applications: Transparent Heat Mirror Overview Background Experimental vi

7 6.4 Results Conclusions and Future Work Conclusions Future Work vii

8 List of Tables Table 1.1: Categories of amorphous carbon (a-c) films... 3 Table 3.1: C1s and N1s core-level binding corrected for charging effect (CN sample set) Table 3.2: Vibrational modes of molecular groups commonly found in a-c films Table 4.1: Summary of sample sets Table 4.2: N1s shifts for C-N sample set Table 4.3: N1s shifts for A-N sample set Table 6.1: Experimental details for deposition of three-layer THM structure viii

9 List of Figures Figure 1.1: sp 3 and sp 2 bonding hybridizations of carbon. Black dots represent atomic positions. Taken from [1] Figure 1.2: Ternary phase diagram of the various forms of amorphous carbon films. Abbreviations are defined in Table Figure 2.1: Typical RF-PECVD System Figure 2.2: Generation of plasma in RF-PECVD. Neutrals are represented by N, ions are represented by +, and electrons are represented by Figure 2.3: Formation of sheath regions near anode and cathode in RF PECVD. Neutrals are represented by N, ions are represented by +, and electrons are represented by Figure 2.4: Typical potential profile for RF PECVD chamber with cathode of smaller area than anode Figure 2.5: (a) Example of PECVD chamber where asymmetric gas flow leads to asymmetric ion and radical distribution and consequently non-uniform film growth; (b) Example of PECVD chamber which employs showerhead gas inlet which leads to symmetric gas flow profile and uniform film growth Figure 2.6: Film growth processes in a-c (taken from [28]) Figure 3.1: Schematic of RF PECVD chamber used for research discussed in this thesis Figure 3.2: Schematic of optical system used for fitting ellipsometry measurements Figure 3.3: (a) XPS: measurement of electrons emitted from core-level due to x-ray absorption (b) XAES: measurement of secondary electrons emitted from valence-level carrying excess energy created from core-level hole, created in process shown in (a), being filled Figure 3.4: Band diagram depicting relationship between binding energy (BE), work function of the sample (Φ sample ), work function of the electron analyzer (Φ analyzer ), Fermi energy of the sample (E f ), measured kinetic energy of the electron (E k ), and photon energy (hν) for XPS. Note ix

10 that since the sample and analyzer are both grounded, their Fermi energies are aligned. E k represents the kinetic energy of the electron when it is emitted from the sample, and E k represents the kinetic energy of the electron measured on the electron analyzer Figure 3.5: Example XPS measurement used to determine elemental composition. The measurement was taken on a sample in the A-20 sample set and shows two distinct peaks at two binding energies: (i) 526eV corresponding to the binding energy of 1s electrons in oxygen and labeled O1s in the figure, (ii) 284eV corresponding to the binding energy of 1s electrons in carbon and labeled C1s in the figure Figure 3.6: Dependence of penetration depth, d, of x-ray incident at angle of θ on a film with x- ray absorption length of L Figure 3.7: Example of angle-resolved XPS measurements providing qualitative information of film density. Note the curves are just a guide to the eye Figure 3.8: (a) sandwich method of measuring conductivity transversely through a thin-film (b) shunting and unknown length issues that can occur with soft PLC:H film with pores, pinholes and scratches Figure 3.9: Auger emission in sp 3 -hybridized carbon atom Figure 3.10: Two potential Auger processes in sp 2 -hybridized carbon atom that produce emission of electrons with unique kinetic energies Figure 3.11: (a) XAES measurement (b) derivative spectra of XAES measurements with D- parameter indicated on figure Figure 3.12: Normalized transmission spectrum from FTIR measurement Figure 3.13: Transmission spectrum with background removed Figure 3.14: Absorption coefficient spectrum with vibrational modes indicated Figure 4.1: Change in growth rate with temperature for A-20 sample set Figure 4.2: Change in growth rate with RF power for C-RT and A-RT sample sets x

11 Figure 4.3: Change in growth rate with N 2 partial pressure for C-N and A-N sample sets. Note the C-N sample set and A-N sample set had identical deposition parameters other than the placement of the substrate and RF power used. For the C-N sample set, substrates were placed on the cathode and an RF power of 5W was used while for the C-N sample set, substrates were placed on the anode and an RF power of 20W was used Figure 4.4: Change in nitrogen content (at. %) with N 2 partial pressure for films in A-N and C-N sample sets. The curves serve as guides to the eye Figure 4.5: Absorption of C-H modes for a-c films in the C-RT sample set Figure 4.6: Change in hydrogen concentration in a-c films in the C-RT sample set deposited at different RF powers. The curve is a guide for the eye Figure 4.7: Percent of CH x bonding in the CH 2 sp 2 mode for C-RT sample set Figure 4.8: Absorption of C-H modes for a-c films in the A-RT sample set Figure 4.9: Change in hydrogen concentration in a-c films in the A-RT sample set deposited at different RF powers Figure 4.10: Percent of CH x bonding in the CH 2 sp 2 mode for A-RT sample set Figure 4.11: Absorption of C-H modes for a-c films in the A-20 sample set Figure 4.12: Change in hydrogen concentration in a-c films in the A-20 sample set deposited at different substrate temperatures Figure 4.13: Percent of CH x bonding in the CH 2 sp 2 mode for A-20 sample set Figure 4.14: Change in the percent of sp 3 -bonded carbon atoms for a-c films deposited at different RF Powers in the C-RT and A-RT sample sets. The curves are guides to the eye Figure 4.15: Change in the percent of sp 3 -bonded carbon atoms for a-c films deposited at different substrate temperatures in the A-20 sample set xi

12 Figure 4.16: Difference in the oxygen concentration (O.C.) in the bulk and in the near-surface (O.C. surface -O.C. bulk ) for samples in the A-20 sample set. Measurements made by AR-XPS with measurement at 70 o representing the near-surface and measurement at 30 o representing the bulk Figure 4.17: Difference in the oxygen concentration (O.C.) in the bulk and in the near-surface (O.C. surface -O.C. bulk ) for samples in the C-RT sample set. Measurements made by AR-XPS with measurement at 70 o representing the near-surface and measurement at 30 o representing the bulk Figure 4.18: Difference in the oxygen concentration (O.C.) in the bulk and in the near-surface (O.C. surface -O.C. bulk ) for samples in the A-RT sample set. Measurements made by AR-XPS with measurement at 70 o representing the near-surface and measurement at 30 o representing the bulk Figure 4.19: Refractive index (n) of a-c films in the A-20 sample set. These are intrinsic films that were deposited on the anode at an RF power of 20W at several different substrate temperatures. For clarity, an error bar is only shown for the first data point on the sample deposited at 200 o C Figure 4.20: Absorption coefficient (α) of a-c films in the A-20 sample set. These are intrinsic films that were deposited on the anode at an RF power of 20W at several different substrate temperatures. For clarity, an error bar is only shown for the first data point on the sample deposited at 200 o C Figure 4.21: Refractive index (n) of a-c films in the C-RT sample set. These are intrinsic films that were deposited on the cathode at several different RF powers. For this sample set there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample deposited at 60W Figure 4.22: Absorption coefficient (α) of a-c films in the C-RT sample set. These are intrinsic films that were deposited on the cathode at several different RF powers. For this sample set there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample deposited at 60W xii

13 Figure 4.23: Refractive index (n) of a-c films in the A-RT sample set. These are intrinsic films that were deposited on the anode at several different RF powers. For this sample set there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample deposited at 80W Figure 4.24: Absorption coefficient(α) of a-c films in the A-RT sample set. These are intrinsic films that were deposited on the anode at several different RF powers, For this sample set there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample deposited at 80W Figure 4.25: Refractive index (n) of a-c films in the C-N sample set. Each film has a different level of nitrogen content (at. %) due to the difference in the N 2 partial pressure that was used in the source gas. For this sample set the substrate was held on the cathode, the RF power was set to 5W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown for the last data point on the sample with a nitrogen content of 8.97% Figure 4.26: Absorption coefficient (α) of a-c films in the C-N sample set. Each film has a different level of nitrogen content (at. %) based on the N 2 partial pressure that was used in the source gas. For this sample set the substrate was held on the cathode, the RF power was set to 5W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample with a nitrogen content of 8.97% Figure 4.27: Refractive index (n) of a-c films in the A-N sample set. Each film has a different level of nitrogen content (at. %) based on the N 2 partial pressure that was used in the source gas. For this sample set the substrate was held on the anode, the RF power was set to 20W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample with a nitrogen content of 17.04% Figure 4.28: Absorption coefficient (α) of a-c films in the A-N sample set. Each film has a different level of nitrogen content (at. %) based on the N 2 partial pressure that was used in the source gas. For this sample set the substrate was held on the anode, the RF power was set to 20W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample with a nitrogen content of 17.04% Figure 4.29: Change in the E 04 gap with substrate temperature for A-20 sample set xiii

14 Figure 4.30: Change in the E 04 gap with RF power for C-RT and A-RT sample sets. The curves are a guide to the eye Figure 4.31: Change in the E 04 gap with nitrogen content for C-N sample set. The curve is a guide to the eye Figure 4.32: Change in the E 04 gap with nitrogen content for A-N sample set. The curve is a guide to the eye Figure 5.1: Change in plasma potential profile with increasing RF power for RF-PECVD chamber with area of cathode smaller than area of anode Figure 5.2: Difference in relationship between growth rate and RF power for A-RT and C-RT sample sets. The curves serve as guides to the eye Figure 5.3: Relationship between hydrogen concentration and RF power for C-RT and A-RT sample sets. The curve is a guide to the eye for the data points in the C-RT sample set Figure 5.4: Relationship between %sp 3 bonding and RF power for C-RT and A-RT sample sets. The curves are a guide to the eye. Note that for the C-RT sample set, there appears to be two distinct regions in the relationship; one at low power (<20W) in which only hydrogen displacement is occurring and one at higher powers (>20W) in which both hydrogen displacement and film penetration is occurring. For the A-RT sample set, due to the weaker relationship between RF power and ion/radical energy, only the hydrogen displacement region is apparent Figure 5.5: Temperature dependent etching processes creating a net negative effect of substrate temperature on growth rate of a-c films Figure 5.6: Relationship between growth rate and N 2 partial pressure for A-N and C-N sample sets. The curves are intended only as a guide to the eye Figure 5.7: Increasing N content (at. %) with increasing N 2 Partial Pressure (%) for A-N and C- N sample sets xiv

15 Figure 5.8: Relationship between charge-corrected N1s shifts and nitrogen content for C-N and A-N sample sets. The curves are a guide to the eye. Note that charge-corrected N1s shifts can be taken as a qualitative measure of shifts in the Fermi level. An increase in the charge-corrected N1s peak would represent an upward shift of the Fermi level toward the conduction band Figure 5.9: Potential bonding configurations between nitrogen and carbon. Doping is only possible in the configurations shown in (b), (e), and (h). Figure taken from [64] Figure 5.10: Simplified diagram of the density of states in a-c films Figure 5.11: Photoemission spectra of a-c measured through Ultraviolet Photoelectron Spectroscopy (UPS). The vertical axis represents the photoemission counts measured through UPS and provides a qualitative assessment of the density of states in the valence band with the Fermi level lying at 0eV. Note that the peak of the π-band lies closer to the Fermi level than the peak of the σ-band. Taken from [28] Figure 5.12: Relationship between %sp 2 bonding with E 04 gap (black) and refractive index at 350nm (grey) for C-RT sample set. The curves are a guide for the eye Figure 5.14: Relationship between %sp 2 bonding with E 04 gap (black) and refractive index at 350nm (grey) for A-20 sample set. The curves are a guide for the eye Figure 5.15: Relationship between E 04 gap and nitrogen content for C-N and A-N sample sets. Note for both sample sets the refractive index remained constant at 1.6. For visual clarity the refractive index curves were not included in the figure. The curves are a guide for the eye Figure 6.1: Normalized spectral emissive power of a blackbody 5780K and blackbody 300K. Note that the solar spectrum can be approximated by a blackbody 5780K Figure 6.2: Overview of multi-layer transparent heat mirror design Figure 6.3: Infrared reflectance of state-of-the-art SnO 2 (38nm)/NiCr (1nm)/Ag (9nm)/NiCr (3nm)/Sn0 2 (38nm) structure. Taken from [67] xv

16 Figure 6.4: Transmittance of state-of-the-art SnO 2 (38nm)/NiCr (1nm)/Ag (9nm)/NiCr (3nm)/Sn0 2 (38nm) structure. Taken from [67] Figure 6.5: Infrared reflection of a-c/ag/a-c/glass THM optical system. Note that the spectra for Ag/glass and glass are shown as references Figure 6.6: Visible transmission of a-c/ag/a-c/glass THM optical system. Note that the spectra for Ag/glass and glass are shown as references Figure 6.7: Visible transmission of a-c/ag/a-c/glass THM using different Ag layer thicknesses. The Experimental curve represents experimental measurements done on the a-c/ag(20nm)/a- C/glass THM that was fabricated. The Model: 20nm Ag curve represents the simulated transmission for an a-c/ag(20nm)/a-c/glass THM. The Model: 10nm Ag curve represents the simulated transmission for an a-c/ag(10nm)/a-c/glass THM xvi

17 List of Symbols and Acronyms A-20 Anode 20W sample set. In this sample set, substrates were placed on the anode and the RF power was fixed at 20W. The varied parameter was the substrate temperature. a-c Ag A-N A-RT at. % Au C1s CH 4 CH x C-N Amorphous Carbon (either hydrogenated or non-hydrogenated) Silver Anode Nitrogen-incorporation. In this sample set, substrates were placed on the anode, no intentional heating was performed, and the RF power was fixed at 20W. The varied parameter was the partial pressure of n 2 in the source gas. Anode Room Temperature sample set. In this sample set, substrates were placed on the anode and no intentional heating was performed. The varied parameter was RF power. Atomic Percent Gold Carbon core electron energy level Methane gas Hydrocarbon Molecule Cathode Nitrogen-incorporation. In this sample set, substrates were placed on the cathode, no intentional heating was performed, and the RF power was fixed at 5W. The varied parameter was the partial pressure of n 2 in the source gas. C-RT Cathode Room Temperature sample set. In this sample set, substrates were placed on the cathode and no intentional heating was xvii

18 performed. The varied parameter was RF power. Cu DC DCSF DLC:H Copper Direct-Current Dc Saddle Field Hydrogenated Diamond-Like a-c E 04 gap Photon energy at which the absorption coefficient reaches 10 4 cm -1 E-beam evaporation ECR Electron Beam Evaporation Electron Cyclotron Resonance E f Fermi Energy FTIR Ge GLC GLC:H H Fourier Transform Infrared Spectroscopy Germanium Graphitic-Like a-c Hydrogenated Graphitic-Like a-c Hydrogen H 2 Hydrogen gas ICP k MW n N1s Inductively-Coupled Plasma Extinction Coefficient Microwave Refractive Index Nitrogen core electron energy level xviii

19 N 2 Nitrogen gas NiCr O.C. O1s Nickel Chromium Oxygen Concentration Oxygen core electron energy level O 2 Oxygen gas PECVD PL PLC:H PV RF RF-PECVD sccm SnO 2 TAC TAC:H THM ULSI UV-VIS-NIR VLSI XAES Plasma Enhanced Chemical Vapor Deposition Photoluminescent Hydrogenated Polymeric-Like a-c Photovoltaic Radio Frequency Radio Frequency Plasma Enhanced Chemical Vapor Deposition Standard Cubic Centimetres Per Minute Tin Oxide Tetrahedral Amorphous Carbon Hydrogenated Tetrahedral a-c Transparent Heat Mirror Ultra Large Scale Integrated Circuits Ultraviolet Visible Near-Infrared Very Large Scale Integrated Circuits X-Ray Excited Auger Electron Spectroscopy xix

20 XPS ZnS ZnSe α λ %sp 3 bonding X-Ray Photoelectron Spectroscopy Zinc Sulphide Zinc Selenide Absorption Coefficient Wavelength Percent of sp 3 -hydridized carbon atoms xx

21 1 1 Introduction 1.1 Properties of carbon Carbon is one of the most widely studied elements. It is the basis of all life forms on our planet. An entire branch of chemistry, organic chemistry, is dedicated to the study of hydrocarbon-based compounds. Part of the fascination of carbon is the widely varying properties of its allotropes. There are three main allotropes of carbon: diamond, graphite, and fullerene. Diamond is highly transparent, an excellent electrical insulator, and the hardest and most thermally conductive of all naturally occurring materials. In contrast, graphite is opaque, an excellent electrical conductor, a thermal insulator, and one of the softest naturally occurring materials. Fullerenes represent a relatively newly discovered set of carbon allotropes. As opposed to the semi-infinite network of carbon atoms found in diamond and graphite, fullerenes exist in finite molecular forms such as hollow spheres. Fullerenes are the focus for much of the work in nanotechnology and are the basis for carbon nanotubes. The versatility of carbon-based materials is a function of the different bonding hybridizations that carbon atoms can form. The ground state electron configuration of carbon, 1s 2 2s 2 2p 2, has four valence (L-shell) electrons. Under appropriate conditions these valence electrons can form hybridized orbitals such as sp 3 or sp 2 orbitals. This is illustrated in Figure 1.1. The formation of sp 1 orbitals are also possible, however, they do not play a significant role in the properties of a-c films and thus will not be discussed further [1]. Figure 1.1: sp 3 and sp 2 bonding hybridizations of carbon. Black dots represent atomic positions. Taken from [1].

22 2 In sp 3 hybridization, four identical tetrahedrally oriented orbitals are formed with each orbital being separated by an angle of o. Each of the four sp 3 orbitals can form a single covalent bond along the axis joining the nuclei of the two atoms involved in the bond. This type of bond is known as a σ-bond and is the strongest of all the covalent bonds. Carbon bonded in the sp 3 configuration forms an interlocking network of strong covalent bonds with each carbon atom forming a σ-bond with its four nearest neighbours. Crystalline carbon bonded in this type of configuration is known as diamond. It is this highly dense interlocking microstructure that leads to the extraordinary hardness, transparency, and high electrical resistivity of diamond. In sp 2 hybridization, three identical co-planar trigonally directed sp 2 orbitals are formed with each orbital being separated by an angle of 120 o. Each of the sp 2 orbitals contains a single valence electron with the remaining electron being held in the 2p orbital whose axis is perpendicular to the plane containing the three sp 2 orbitals. Each of the three sp 2 orbitals can form a single σ-bond with neighbouring atoms while the remaining 2p orbital can form a bond with another 2p orbital of a neighbouring atom. The bond between the 2p orbitals is perpendicular to the axis joining the nuclei of the two atoms involved in the bond. This type of bond is known as a π-bond and is illustrated in Figure 1.1. The π-bond is weaker than the σ- bond since there is a greater overlap of orbitals in a σ-bond than in a π-bond. As sp 2 hybridized carbon atoms come together, each carbon atom uses its sp 2 orbitals to form single σ-bonds with its three nearest neighbours. The remaining 2p orbital of each atom overlaps with the 2p orbitals of each of its three nearest neighbours. This leads to a distributed π-bond which lies across the entire plane of carbon atoms. Since all four valence electrons are used to bond with coplanar atoms, adjacent planes are only held together by weak Van Der Waals forces. Crystalline carbon bonded in this type of configuration is known as graphite. It is this microstructure of delocalized π-bonding and weak inter-planar forces that lead to the high conductivity, opaqueness, and softness of graphite. Amorphous carbon (a-c) is a non-crystalline form of carbon that only has short range structural order. The term a-c is also used to include hydrogen-containing (hydrogenated) forms of noncrystalline carbon. While the two primary crystalline forms of carbon exhibit either 100% sp 3 bonding (diamond) or 100% sp 2 bonding (graphite), a-c films contain a mixture of sp 3 and sp 2 bonding. It is the ability to control the ratio of sp 3 to sp 2 bonds through growth conditions that allows the development of a-c films that exhibit astonishingly different mechanical, electrical,

23 3 and optical properties. In addition to bonding hybridization, a second parameter, hydrogen content, plays an important role in determining the film properties. Based on the percentage of sp 3 bonds and the percentage of hydrogen in the film, a-c films are separated into different categories as shown in Table 1.1 and qualitatively illustrated in Figure 1.2 [1]. The naming of these categories is based on historical publications and can be confusing at times. For instance, some types of hydrogenated diamond-like carbon (DLC:H) are dark in appearance and thus the optical properties are not at all like those of diamond. Moreover, it could be argued that the mechanical properties of tetrahedral amorphous carbon (TAC) are more diamond-like than DLC:H. Thus special care must be taken to distinguish between the properties associated with the name of the category and the actual properties of a-c films in that category. Table 1.1: Categories of amorphous carbon (a-c) films Category Abbreviation sp 3 (%) H (at.%) Hardness (GPa) Optical Gap (ev) Density (g/cm 3 ) tetrahedral a-c TAC >65 < hydrogenated tetrahedral a- C hydrogenated diamond-like a-c hydrogenated polymeric-like a-c hydrogenated graphitic-like a-c TAC:H > DLC:H PLC:H soft GLC:H < soft graphitic-like a-c GLC <30 <10

24 4 Figure 1.2: Ternary phase diagram of the various forms of amorphous carbon films. Abbreviations are defined in Table 1.1.

25 5 1.2 Current state and trends in amorphous carbon research In 1971, two researchers in the space science division of the Whittaker Corporation, Aisenberg and Chabot, reported the room-temperature deposition of a-c films that exhibited many diamond-like properties [2]. Ever since this time, researchers from across the world have been working to understand this material and to find applications for it. Right from the beginning, the most prevalent application explored for a-c was as a hard, scratch-resistant coating. Research into this application reached a fever-pitch in 1989 when Liu and Cohen predicted the existence of a β-c 3 N 4 phase in nitrogen-incorporated a-c that was harder than diamond [3]. Although the existence of the β-c 3 N 4 phase has yet to be clearly achieved experimentally [4], the prevalent research in the field of hard a-c coatings (DLC:H, TAC:H, TAC) has led to a maturity in the understanding and fabrication of these films [5]. Today, the market for hard a-c coatings is rapidly growing and it currently represents close to a billion dollar industry [6]. The most popular application of a-c is as a wear and corrosion resistant coating for magnetic storage media (eg. computer hard drives) [5; 6]. Hard a-c coatings (DLC:H, TAC:H, TAC) have also found applications as a low-friction, scratch-resistant coating for polycarbonate sunglasses, razor blades, machining tools, and automotive components [5; 6; 7]. In addition to their high hardness and low friction, these forms of a-c have also been shown to be impermeable to liquids and to be chemically inert. This has led to exploration of a- C coatings to protect biological technologies, such as artificial heart valves and joint implants, against corrosion and diffusion [8; 9; 10; 11]. With the breadth of applications for hard forms of a-c, much of the research in the mechanical properties of a-c has shifted from academia to industry [6]. Today, industrial institutions are researching methods of optimizing hard a-c coatings to meet the needs of their specific applications.

26 6 1.3 Amorphous carbon as an optical coating The superior mechanical properties of a-c have motivated most of the research in the field over the past thirty years. While a-c films have excellent mechanical properties, the infrared transparency, large optical gap, and the tunable refractive index ( ) of these films also present intriguing possibilities for optical applications. Also, while research into the mechanical properties of a-c has been in-depth and has reached a level of maturity, studies into the optical properties of a-c have been sparse and application specific. Several researchers have explored the potential of using a-c as a replacement to SiO 2 for the interconnect dielectric used for Very Large Scale Integrated Circuits (VLSI) and Ultra Large Scale Integrated Circuits (ULSI) [12; 13; 14; 15]. The high infrared transparency in a-c has also led to some work in utilizing a-c as an anti-reflection coating for Ge and ZnSe infrared detectors [16; 17; 18; 19; 20]. In addition, the large optical gap and tunable refractive index of a-c has led researchers to explore the potential of a-c as an anti-reflection coating for Si solar cells [21; 22; 23]. There has also been some work in studying the photoluminescent (PL) properties of a-c and looking into the possibility of using a-c as a replacement to ZnS in electroluminescent devices [24; 25; 26]. Although these studies have examined the potential of a-c as an optical coating, they are scattered and application-specific. There has been a lack of fundamental research in these publications examining the relationship between the optical properties and the structural properties and their correlation with the growth conditions of a-c. This type of fundamental study is necessary in order to achieve the full potential of a-c as an optical coating.

27 7 1.4 Research overview Amorphous carbon films possess a combination of chemical inertness, high infrared transmissivity, and tunable mechanical and optical properties. It is this unique combination of properties coupled with the fact that these films can be deposited at room-temperature using simple deposition techniques that make a-c an appealing alternative to other thin-film materials. This is especially true in the solar industry where the exponential growth in demand and competition in the industry has forced researchers to develop advanced cost-effective alternatives to the standard technologies currently being used. The unique properties of a-c make it appealing for a number of applications in the solar industry including: anti-reflection coatings for Si photovoltaic (PV) cells, window layers for thin-film and heterojunction PV cells, surface passivation layers, and low-emissive transparent heat mirrors for high efficiency windows. Before a-c films can be utilized in any of these applications it is necessary to complete fundamental research relating the optical properties of a-c to the structural and growth conditions of these films. In this thesis, experiments exploring the effect of growth conditions on the microstructural properties and optical properties of a-c films are presented. The structure of the thesis is as follows. In the following chapters (Chapters 2 and 3), a background on a-c film growth and details regarding the experimental apparatus and characterization techniques used are discussed. The results of these experiments are presented in Chapter 4, while the analysis of these results and the defining relationships between the growth conditions, microstructural properties, and optical properties of a-c films are discussed in Chapter 5. In Chapter 6, several applications for a-c as an optical coating are suggested and one of these applications, a-c as a transparent heat mirror coating, is experimentally demonstrated. Conclusions of this research are presented in the final chapter.

28 8 2 Amorphous Carbon: An Overview 2.1 Deposition methods A variety of methods have been used to deposit a-c thin films. Typically, GLC and GLC:H are deposited from sputtering a graphite target, while TLC and TLC:H are deposited using vacuum arc deposition or pulsed-laser deposition, and PLC:H and DLC:H are deposited using either sputtering or plasma-enhanced chemical vapor deposition (PECVD) [27; 28; 29; 30]. Since this thesis investigates PLC:H and DLC:H films deposited using PECVD, only the PECVD method is discussed further. In this section an overview of the important concepts relating to a-c films deposited by PECVD is given. A complete review of plasmas and the PECVD technique is beyond the scope of this thesis; interested readers are directed to the following sources for more information [28; 31; 32]. One of the key features of PECVD is that a variety of substrates can be used because the substrate temperature is kept low, typically below 250 o C. Deposition can occur at a low temperature because as indicated by the name, a plasma or glow discharge is used to generate the reactive species necessary for film growth. A plasma is a volume of gas consisting of a high density of charge carriers (ions and electrons) [27]. The high density of charge carriers in plasmas, give it a number of distinguishing properties from non-ionized gases, and thus plasmas are often referred to as the fourth state of matter. In PECVD, a plasma is generated by an external electric field being applied to a volume of lowpressure gas, typically less than 1 Torr. Before the electric field is applied, the volume of gas has a very low density of charge carriers, created by cosmic radiation. Once the field is applied, the charge carriers that are present accelerate and collide with neutral molecules (neutrals) in the gas. Through these collusions, energy is transferred from the energetic charge carriers to the neutrals causing ionization of the neutrals thus generating more charge carriers. Energetic carriers also collide with electrodes and walls of the chamber causing ejection of new carriers into the gas. As charge density builds in the chamber, the rate of recombination of charge increases. The recombination of charge carriers occurs as electrons and ions in the plasma recombine with each other or are lost due to collisions with the walls of the chamber. In time, a steady-state is reached between the generation of new charge carriers and the recombination of existing charge carriers.

29 9 2.2 Plasma kinetics The plasma produced in typical PECVD systems consists of a mixture of ions, electrons, and neutrals, with neutrals representing the majority of the species. The energy of the applied field is transferred primarily to the electrons since their lower mass allows them to rapidly respond to the applied field. Thus, the species within the plasma consist of high energy electrons and low energy ions and neutrals; this is known as a low-temperature plasma [27]. The energetic electrons in the plasma play an important role as their high energy and low mass allow them to collide inelastically with stable neutral molecules in the plasma resulting in one of the following processes: (i) Ionization: The energy transferred in an inelastic collision is used to eject an electron from a neutral molecule, producing a positively charged ion and an additional electron. Example: CH 4 + e - CH H + 2e - (ii) Dissociation: The energy transferred from the inelastic collision is used to breakdown a stable neutral molecule into neutral highly-reactive molecular fragment(s) known as neutral radical(s). Note that in the context of this thesis, the term neutral radical(s) will be shortened simply to radical(s). Thus in this thesis, the term radical is used to refer to neutral radicals. Example: CH 4 + e - CH 3 + H + e - (ii) Excitation: The energy transferred from the inelastic collision is used to promote electron(s) in the neutral molecule to a higher energy state. When these electron(s) relax to their ground state, a photon is emitted. This relaxation process is responsible for the visible glow commonly seen in plasmas. The ions and radicals produced in the plasma are predominantly responsible for film growth in the PECVD process. Various types of ions and radicals can be produced depending on the type of source gas used to produce the plasma. In the work reported in this thesis, methane (CH 4 ) was used as the source gas. The following lists the main ionization and dissociation processes found in methane plasmas (note radicals are indicated by: `*`) [33; 34]:

30 10 CH 4 + e - CH * 3 + H * + e - CH 4 + e - CH H * + 2e - CH 4 + e - CH * 2 + H 2 + e - CH 4 + e - CH * + H 2 + H * + e - CH 4 + e - CH e - H 2 + e - 2H * + e - H 2 + e - 2H + + 3e - While there are a number of ionic and radical species in methane plasmas, the three predominant species are CH + 3, CH * 3, and H * [35]. The specific role of these species in film growth is discussed in Section 2.5.

31 RF PECVD PECVD systems are classified based on the source used to generate the electric field. The primary types of PECVD sources are direct-current (DC), radio-frequency (RF), and microwave (MW). There also exist a number of advanced configurations such as DC saddle-field (DCSF), electron cyclotron resonance (ECR), and inductively-coupled plasma (ICP). DC configurations (standard DC or DC saddle-field) are typically not used for a-c deposition since in these configurations the source is coupled to the plasma through an external DC current. During the deposition, highly resistive a-c film builds up on the DC electrodes and it becomes increasingly difficult to maintain a stable plasma [27]. The work reported in this thesis uses the RF PECVD configuration, which is the most commonly used configuration for a-c film deposition [28]. Rather than uses an external current, the RF PECVD configuration uses an alternating RF field to couple power from the source to the plasma [27]. The typical configuration for an RF PECVD chamber is shown in Figure 2.1. The main features are the power source, the impedance matching network, the two circular electrodes, pumping outlet, and source gas inlet. The RF source, which operates at a frequency of 13.56MHz based on regulatory requirements, is capacitively coupled to the chamber in order to suppress any DC current. An impedance matching network is connected in series with the RF source in order to prevent reflection of the RF power from the chamber. The two electrodes are typically of different radii, with the smaller electrode typically being the cathode (powered) and the larger electrode typically being the anode (grounded). The substrate can be placed on either the anode or cathode but for hard a-c films, the substrate is typically placed on the cathode. The reasons for this asymmetry in electrode area and the choice of substrate placement are explained later in this section.

32 12 Figure 2.1: Typical RF-PECVD System. The generation of a plasma in an RF PECVD chamber is depicted in Figure 2.2. The RF source creates a field oscillating at a frequency of 13.56MHz. While the electrons in the gas volume are able to oscillate at this frequency, the ions and neutrals 1 with their lower mobility, a factor of 10 2 lower than electrons, are unable to react to this oscillating field [27]. Thus the ions and neutrals in the gas remain relatively stationary while the electrons oscillate between the electrodes. As the electrons oscillate between the electrodes they collide inelastically with the neutrals and induce ionization generating new ions and electrons. The generated ions remain stationary while the generated electrons add to the oscillating flux of electrons and induce more ionization of neutrals. This process continues until a steady-state is reached between the generation and recombination of charge carriers. 1 Note the term neutral(s) is referring to both the stable neutral molecules introduced by the source gas and neutral radicals.

33 13 Figure 2.2: Generation of plasma in RF-PECVD. Neutrals are represented by N, ions are represented by +, and electrons are represented by -. As shown in Figure 2.3, in addition to the central plasma (glow) region, there also exist dark space-charge regions near the electrodes known as the sheath regions. Due to the higher mobility of electrons compared to ions, during the positive phase of electric field, the electron current towards a given electrode is significantly higher than the ion current to the same electrode during the negative phase of the electric field. This leaves a positive space charge region near the electrodes known as the sheath regions. Since the system is capacitivelycoupled, the electron and ion currents must be balanced, thus a static electric field forms in the sheath regions such that the electrodes develop a negative DC potential with respect to the plasma. This negative potential is known as the self-bias potential. Figure 2.3: Formation of sheath regions near anode and cathode in RF PECVD. Neutrals are represented by N, ions are represented by +, and electrons are represented by -.

34 14 Due to the capacitor-like nature of the sheath regions, the applied RF voltage is divided between the sheaths according to their inverse capacitance and can be described by the following equation [28]: VV 1 = AA 2 2 VV 2 AA 1 (1) where V 1 is the self-bias potential of the electrode with area A 1 and V 2 is the self-bias potential of the electrode with area A 2. Thus if the electrodes are asymmetric in area, the electrode with the smaller area will acquire the dominant self-bias potential. This is important when considering the energy of ions and neutrals impinging on the electrode. Although, the sheath region forces the net electron and ion fluxes towards the electrodes to become equal, the energy that they have when they impinge on the electrodes is not. Due to formation of the field in the sheath region, ions that enter the sheath regions are accelerated by the field and impinge on the electrode with a high energy, as opposed to electrons which are retarded by the field. Ions that accelerate through the sheath region collide with neutrals and thus both energetic ions and neutrals impinge on the electrodes. As discussed in Section 2.5, for hard forms of a-c, high energy ions and neutrals are desired. Thus the RF PECVD chamber is typically designed such that the substrate lies on the smaller electrode. According to Equation 1, the smaller electrode will acquire the dominant self-bias and thus the energy of the ions and neutrals impinging on the smaller electrode will be greater than that of the larger electrode. An example of a typical potential profile found in RF PECVD systems is shown in Figure 2.4.

35 15 Figure 2.4: Typical potential profile for RF PECVD chamber with cathode of smaller area than anode.

36 Process variables in RF PECVD There are several process variables in RF PECVD that affect film growth. Understanding the role of each of these process variables is essential in obtaining stable plasma conditions that will deposit reproducible, high-quality films. The following provides a qualitative description for the key process variables in a standard RF PECVD chamber: (i) RF power: As described in the Section 2.3, the peak RF potential is divided between the two sheath regions with the sheath region surrounding the electrode with the smaller area developing the larger potential drop. Thus the greater the applied RF power, the larger the potential drop in the sheath region. Since ions are accelerated through the sheath region towards the substrate, the energy of ions impinging on the substrate increases with increasing RF power. In addition to affecting the energy of ions, the applied RF power also affects the density and types of species present in the plasma. As described in Section 2.2, gaseous state neutral molecules introduced into the PECVD chamber can be ionized or dissociated into several different types of ions and radicals. In each case, the energy produced through inelastic collisions with electrons acts as the catalyst for the production of these species. Since the energy of electrons in the plasma is coupled with the intensity of the applied field, the likelihood of production of a particular species is dependent on the applied RF power. (ii) Area of electrodes: As described in Section 2.3 and Equation 1, the self-bias developed by each electrode is dependent on their respective areas, with the electrode with the smaller area developing the larger self bias. Therefore, the potential profile of the plasma is dependent on the ratio of areas of the two electrodes. If both electrodes are chosen with equal areas, then a symmetric potential profile will develop. Typically, RF PECVD chambers are designed such that one electrode is smaller than the other so that an asymmetric profile, like that shown in Figure 2.4, develops. With an asymmetric profile, high energy ions will impinge on the smaller electrode even at low RF powers. The ratio between the areas of the electrodes should be chosen to create the potential profile that is suited for the type of film being deposited. (iii) Pressure: Since many of the processes involved in the PECVD process are based on collisions such as collisions of electrons with neutrals, the source gas pressure that is maintained in the PECVD chamber plays an important role in determining the characteristics of the plasma. Generally, lower pressures lead to higher energy ions and electrons due to a lower collision

37 17 frequency at lower pressures. If the pressure is driven below a threshold, the collision frequency between electrons and neutrals reaches a point where there is not enough generation of charge carriers to maintain a stable plasma. If the pressure is driven too high, due to the high frequency of collisions, electrons are not be able acquire enough kinetic energy to ionize the neutrals in the chamber and thus the generation of charge carriers will again be too low to maintain a stable plasma. Thus for a given source gas, power, and flow rate, a range of pressures exist for which a stable plasma can be generated. In addition to the requirements of striking a stable RF PECVD plasma, pressure has a strong effect in determining the value and distribution of ion energies impinging on the substrate. With lower pressures, the initial flux of electrons towards the electrodes is higher. This necessitates a larger developed DC self-bias on the electrodes to equalize the ion and electron fluxes. With a higher potential drop in the sheath region, the energy of ions impinging onto the substrate will be higher. Moreover, with a lower pressure, the number of collisions that these ions undergo in their path through the sheath region to the electrode will be lower which will lead to a larger mean energy and a sharper linewidth in the distribution of ion energies [27]. (iv) Gas flow rate: The rate and profile of how the source gas flows in and out of the PECVD chamber also affects the film growth. There are two key events in the deposition process: (1) breakdown of the source gas into ions and radicals, and (2) transport of the ions and radicals towards the substrate. If the gas flow rate is too high, not enough time is given for this deposition process to take place and the film growth rate will be low. In the extreme case, growth will not occur at all and a plasma may not strike. If the gas flow rate is too low, the plasma will reach an exhausted state where there will be a diminished concentration of neutral molecules in the plasma leading to a reduced deposition rate. The optimal gas flow rate is dependent on a number of factors including the geometry of the chamber, the other deposition parameters being used, and the importance of growth rate in the deposition process. The profile of how gas flows in and out of the PECVD chamber also affects film growth. If profile of gas flow in the chamber is asymmetric then the production of ions and radicals in the plasma will also be asymmetric which will result in non-uniform film growth. This is illustrated in Figure 2.5. The source gas inlet ports and the pumping outlet ports in the PECVD chamber should be designed to provide a symmetric gas flow profile to ensure uniform film growth.

38 18 Figure 2.5: (a) Example of PECVD chamber where asymmetric gas flow leads to asymmetric ion and radical distribution and consequently non-uniform film growth; (b) Example of PECVD chamber which employs showerhead gas inlet which leads to symmetric gas flow profile and uniform film growth.

39 19 (v) Substrate Temperature: The temperature of the substrate plays an important role as it provides thermal energy to solid-phase molecules in the growing film. Several temperature dependent processes can occur during film growth and need to be chosen based on type of film being grown and the type of substrate being used. Some of the processes include crystallization of the film, surface migration, thermal desorption of solid-phase molecules in the film, thermal expansion, increase/decrease of film growth rate, and increase/decrease in film stress.

40 Film growth processes The deposition process in RF PECVD can be broken down into three steps: (1) the production of a plasma through inelastic collisions between energetic electrons and neutral molecules from the source gas, (2) the formation of sheath regions near the electrode surfaces, and (3) the interaction of energetic ions and radicals with the growing film. While the first two steps have already been discussed in some detail, the third step is the focus of this section. Much of the work in modeling the growth and microstructure in a-c films has been done by Robertson [28; 36; 37; 38; 39]. Robertson describes the microstructure of a-c as follows. The skeleton of the film is a continuous network of sp 3 -bonded carbon atoms. The sp 2 -bonded carbon atoms form small localized clusters that lie within this network. The quantity and size of these clusters can vary depending on the energy of ions/radicals impinging on the growing film, the substrate temperature, and the relative hydrogen concentration. The surface of an a-c film is fully passivated with hydrogen, leaving it chemically inert [28]. Since the surface of the film is chemically inert, film growth can occur only through abstraction of hydrogen from the surface or subsurface bonding (subplantation) by energetic ions/radicals. The following is a list of the primary film growth processes in a-c; an illustration of these processes is given in Figure 2.6. (i) ion/radical subplantation: In order to penetrate through an interstitial site on the surface of a growing a-c film, an ion or radical must have enough energy to overcome the repulsive interatomic potential when passing through the films surface. This threshold energy is known as the penetration threshold, E p, and is approximately 32eV for the surface of a typical a-c film [28]. High energy ions/radicals that can overcome this threshold can penetrate several nanometers into the film and bond with subsurface carbon atoms [38]. This subplantation process increases the density and hardness of the a-c film. It is for this reason that high energy ions/radicals are useful for the deposition of hard a-c films such as DLC:H, TAC:H, and TAC. The energy of ions/radicals also affects the percentage of sp 3 hybridized bonding in the film. For each type of a-c film, there exists an optimal ion/radical energy for which maximum sp 3 bonding can be achieved. If the ion/radical energy is increased beyond this maximum, the excess energy not used during the surface penetration is released as thermal energy. This thermal energy is used to relax sp 3 bonded carbon atoms to the lower energy sp 2 configuration, and thus the proportion of sp 3 bonding in the film decreases.

41 21 (ii) hydrogen abstraction from surface: The surface of a growing a-c film is almost completely passivated by hydrogen. However, surface hydrogen atoms can be abstracted leaving highly reactive dangling bonds at the surface. There are two ways hydrogen can be abstracted from the surface: (1) the surface hydrogen dissociates from its current bond to form a bond with a passing hydrogen or hydrocarbon radical, (2) a high energy ion or radical collides with a surface hydrogen atom and removes it from its current bond. (iii) radical bonding to surface dangling bond: If a surface dangling bond is present, an incoming hydrogen or hydrocarbon radical can form a bond with the dangling bond and contribute to the film growth. (iv) subsurface hydrogen abstraction: With their small size, hydrogen ions and radicals can penetrate deep into the film s bulk and abstract a hydrogen atom that is bonded in the bulk of the film. The formed H 2 molecule diffuses to the surface and desorbs from the film [28]. (v) ion or radical bonding to subsurface dangling bond: An energetic ion or radical can form a bond with a subsurface dangling bond and contribute to the film growth. If the dangling bond is deep within the film s bulk, hydrogen radicals are the most likely to be able to penetrate to the required depth to re-passivate the dangling bond. Figure 2.6: Film growth processes in a-c (taken from [28]).

42 22 3 Experimental Apparatus & Characterization Techniques 3.1 Deposition system The primary part of the research plan for the work discussed in this thesis, was to relate the growth conditions (temperature, plasma density, ion/radical energy, and nitrogen content) of a-c films with their structural properties (film density, sp 3 /sp 2 ratio, and hydrogen content) and optical properties (refractive index and optical gap). In order to obtain meaningful results, precise control over the growth conditions was necessary and thus the choice of the deposition system used was vital. In the initial stages of the research, a DC Saddle Field (DCSF) PECVD system was utilized for deposition of the a-c samples. The DCSF system was designed and patented at the University of Toronto and was the primary deposition tool available at the commencement of this research. The primary advantage that the DCSF system has over the RF PECVD system is that it allows independent control of ion energy and plasma density. During the initial stages of the research, it became apparent that while the DCSF offers advantages over the traditional RF PECVD system, it was not suitable for deposition of highlyresistive a-c films. It was found that after the growth of more than 100nm of a-c film, the DCSF system would become unstable. This loss of stability in the plasma was attributed to the fact that the DCSF system uses an external DC current to couple the power source to the plasma. As resistive a-c film built up on the electrodes of the DCSF system, it became increasingly difficult to draw the external DC current and the plasma would become unstable. As stability and reproducibility in film growth was particularly important for this research, it was determined that it would be necessary to design and build an RF PECVD system to deposit the a-c films needed for this research.

43 RF PECVD system The RF PECVD system that was designed and built for the research discussed in this thesis is shown in Figure 3.1. As shown in the figure, a 13.56MHz power supply is connected to an impedance matching network to prevent reflection of power from the chamber. The impedance matching network consists of variable series and shunt capacitors that are connected to a controller that automatically adjusts their impedance to obtain zero reflected power. The electrodes for the system consist of a 12.5cm diameter stainless steel cathode and a 20cm diameter stainless steel anode separated by 2.5cm. The asymmetry in electrode area was chosen in order to create a potential profile that would lead to high energy ions impinging on the cathode and low energy ions impinging on the anode. Since both the anode and cathode can act as the substrate holder, this provides a wide range of ion energies that can be used for a-c film deposition. The chamber shell, which is held at ground potential, is made of stainless steel and has two optical viewports that allow the plasma and sheath regions to be viewed. The viewports have stainless steel shutters that allow the chamber shell to maintain a constant potential profile. It should be noted that even though the pumping arrangement of the system is asymmetric the showerhead configuration of the cathode provided uniform deposition across the substrates. This was confirmed through ellipsometry measurements which showed less than 1% variance in film thickness across a 2cm x 4cm substrate.

44 Figure 3.1: Schematic of RF PECVD chamber used for research discussed in this thesis. 24

45 Sample preparation The substrates used for the a-c film depositions were double-side polished crystalline silicon (c- Si) wafers with resistivity of Ω-cm and thickness of 500µm. Prior to deposition, contaminants such as dust and grease were removed from the surface of the substrates using the following cleaning procedure: 15 minutes ultrasonic bath in acetone 5 minutes ultrasonic bath in de-ionized water 15 mutes ultrasonic bath in isopropyl alcohol 5 minutes ultrasonic bath in de-ionized water blow dried with desiccated air 1 minute dip in buffered hydrofluoric (HF) acid to remove native oxide rinse in de-ionized water blow dried with desiccated air Following the cleaning, substrates were loaded into the PECVD chamber. Then the chamber was sealed and pumped overnight with a mechanical and turbo-molecular pump to achieve a base pressure of 10-6 mbar. The following day, the a-c film deposition was performed. The first step in the deposition process was the introduction of the source gas. Once the source gas was introduced into the chamber and the pressure stabilized, the substrate heater (if needed) was set to the desired temperature. After the temperature had stabilized, the RF power was enabled, the plasma ignited, and the a-c film deposition commenced. Once the film deposition was complete, the remaining source gas was pumped out, then the chamber was brought back up to atmospheric pressure, and the a-c samples were unloaded and stored in an N 2 atmosphere to prevent contamination of the film. During the deposition, the electrodes and walls of the chamber are covered with a-c film. In order to maintain consistent plasma conditions and prevent contamination for future depositions, this a-c film was removed through reactive ion etching using an oxygen plasma struck in the RF PECVD chamber. The reactive ion etching removed the a-c film by using oxygen ions and radicals to dissociate the

46 26 carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds in the film to form stable CO 2 and H 2 gas phase molecules that could then be pumped out of the chamber. The reactive ion etch was performed using the following parameters: source gas: O 2 flow rate: 20sccm chamber pressure: 40mTorr RF power: 25W duration: 45 minutes

47 Sample set space In order to examine the effect of growth conditions on the optical and structural properties of a- C, five different sample sets were studied. For each sample set either RF power, temperature, or the source gas was used as the independent varied parameter. The choice of RF power and temperature as variable parameters was based on studies which showed that these parameters had the strongest effect on the microstructure of a-c [40; 41; 42; 43]. The composition of the source gas was also varied in order to allow for doping studies to be carried out. The details of each sample set are explained below: I. Cathode Room Temperature (C-RT): In this sample set, substrates were placed on the cathode and no intentional heating was performed. The varied parameter was RF power. Seven different RF powers were used in this sample set: 3W, 5W, 10W, 15W, 20W, 40W, and 60W. Due to the strong linkage between RF power and the energy of ions/radicals impinging on the cathode, a wide range in ion/radical energies was produced by varying the RF power. The aim of this sample set was to determine the effect of ion/radical energy on the microstructural and optical properties of a-c films. Specifically, the a-c film properties that were analyzed for this sample set were: growth rate, film density, hydrogen content, percent of sp 3 -bonded carbon atoms, refractive index, and absorption coefficient. II. III. Anode Room Temperature (A-RT): In this sample set, substrates were placed on the anode and no intentional heating was performed. The varied parameter was RF power. Five different RF power were used in this sample set: 5W, 10W, 20W, 40W, and 80W. Due to the relatively weak relationship between RF power and the energy of ions/radicals impinging on the anode, the ion/radical energy will increase only slightly while the plasma density will show a strong increase with increasing RF power. The aim of this sample set was to determine the effect of the density and types of species in the plasma on the properties of a-c films. Specifically, the a-c film properties that were analyzed for this sample set were: growth rate, film density, hydrogen content, percent of sp 3 - bonded carbon atoms, refractive index, and absorption coefficient. Anode 20W (A-20): In this sample set, substrates were placed on the anode and the RF power was fixed at 20W. The varied parameter was the substrate temperature. Four

48 28 different substrate temperatures were used in this sample set: 35 o C (no intentional heating), 100 o C, 150 o C, and 200 o C. The aim of this sample set was to determine the effect of substrate temperature on the properties of a-c films. Specifically, the a-c film properties that were analyzed for this sample set were: growth rate, film density, hydrogen content, percent of sp 3 -bonded carbon atoms, refractive index, and absorption coefficient. IV. Cathode Nitrogen-incorporation (C-N): In this sample set, substrates were placed on the cathode, no intentional heating was performed, and the RF power was fixed at 5W. The varied parameter was the partial pressure of N 2 in the source gas. Five different partial pressures were explored: 0%, 5%, 10%, 25%, and 50%. The aim of this sample set was to determine the potential of room-temperature doping in a-c films and the effect of nitrogen content on the optical properties of the films. Specifically, the a-c film properties that were analyzed for this sample set were: growth rate, nitrogen content, Fermi level shifting, refractive index, and absorption coefficient. V. Anode Nitrogen-incorporation (A-N): In this sample set, substrates were placed on the anode, no intentional heating was performed, and the RF power was fixed at 20W. The varied parameter was the partial pressure of N 2 in the source gas. Five different partial pressures were explored: 0%, 5%, 10%, 25%, and 50%. An RF power of 20W was selected so that the ion/radical energies impinging on the substrate were similar to those for the C-N sample set. This was done so that the only significant difference between this sample set and the C-N sample set was the plasma density. The aim of this sample set was to determine the effect of plasma density on the room-temperature doping of a-c films. Specifically, the a-c film properties that were analyzed for this sample set were: growth rate, nitrogen content, Fermi level shifting, refractive index, and absorption coefficient. The deposition parameters that remained fixed for all depositions were the flow rate and pressure. The flow rate was set to 20sccm and the pressure was set to 60mTorr for all depositions. These values were chosen based on an initial parameter scan that showed that the use of these parameters led to a uniform and stable plasma profile.

49 29 In order to gauge the error associated with the deposition process (i.e. reproducibility of the samples), one sample from each sample set was reproduced and the thickness and optical properties of these samples were measured. It was found that the samples were highly reproducible with variance in growth rate being less than 2% and variance in the optical properties being less than 1%. The method of accounting for these errors is discussed in Section 3.5. Moreover, it should be noted that errors in the deposition parameters were accounted for in the results. These errors, RF power (+/-3%), temperature (+/- 1 o C), and partial pressure (+/- 0.7%), were assigned based on the specified tolerances of the respective controllers.

50 UV-VIS-NIR Spectral Ellipsometry UV-VIS-NIR spectral ellipsometry was used to determine the thickness and the complex index of refraction of the a-c samples. The complex index of refraction, nn (EE), is defined as: nn (EE) = nn(ee) + iiii(ee) (2) where n(e) is the refractive index which describes the relative phase velocity of electromagnetic radiation propagating through the material and k(e) is the extinction coefficient which describes the absorption of electromagnetic radiation as it propagates through the material. The extinction coefficient, k(e), is related to the absorption coefficient of the material, α(e), by: αα(ee) = 4ππππ(EE) λλ (3) The method of ellipsometry is based on measuring the change in polarization state of light reflected from an optical system (eg. air-film-substrate). The thickness and/or complex index of refraction of the film can be found by performing a regression fitting of the measured data against the theoretical change in polarization state of the optical system. This requires an initial approximation of the thickness, n(e), and k(e) for each of the components of the optical system. For the a-c samples, the optical system consists of three layers: air a-c film c-si substrate. The only unknowns lie in the a-c film. A 1 st -order approximation for the thickness of the a-c film was made by using values collected from profilometry measurements. Since the optical properties of a-c vary greatly, a 1 st -order approximation for n(e) and k(e) could not be made. Thus in order to fit the measured data and obtain continuous values for n(e) and k(e), a theoretical dispersion model for a-c was needed. There are several common dispersion models that are used to describe semi-transparent films such as the Cauchy, Sellmeier, Lorentz, Tauc-Lorentz, and Forouhi-Bloomer models. These dispersion models can be differentiated on how they describe absorption in the material. For amorphous semiconductors such as a-c, the description of absorption is complex due to the existence of a band gap, tail states, and defect states. The Tauc-Lorentz and Forouhi-Bloomer models were developed to describe the absorption characteristics of amorphous semiconductors. Both models have been widely used, however, it has been shown that the parameters used in the

51 31 Forouhi-Bloomer model provide greater consistency with the actual electronic structure of a-c films [44]. The Forouhi-Bloomer model describes the extinction coefficient for amorphous materials using the following equation: kk(ee) = AA(EE EE gg ) 2 EE 2 BBBB + CC (4) where E g, A, B, and C are positive non-zero constants characteristic of the medium. Using Kramers-Kronig analysis, the real part of the refractive index can be found and is described by the following equation: nn(ee) = nn( ) + BB oo EE + CC oo EE 2 BBBB + CC (5) where n( ) is a positive non-zero constant representing the real part of the refractive index at large photon energies, and B o and C o are constants that depend on A, B, C, and E g. Thus the Forouhi-Bloomer dispersion model consists of five independent constants: A, B, C, E g, and n( ). For more information on the Forouhi-Bloomer model and the method of spectral ellipsometry, interested readers are directed to the following sources [44; 45; 46]. Ellipsometric measurements were performed using the Sopra UV-VIS-NIR spectral ellipsometer. The measurements were taken in the wavelength range between 275nm and 825nm at an angle of incidence of For fitting the measured data, a four layer optical system as shown in Figure 3.2 was used. The void+a-c diffusion layer was used to effectively describe the surface of the a- C film. This layer provided greater accuracy to the physical representation of the optical system as angle-resolved XPS measurements showed that the surface of the a-c layer was highly porous.

52 32 Figure 3.2: Schematic of optical system used for fitting ellipsometry measurements. Fitting was done using the Levenberg-Marquard method with a maximum of 50 iterations. The parameters that were allowed to vary were the five constants of the Forouhi-Bloomer dispersion model, the thickness of the a-c layer, the thickness of the void+a-c diffusion layer, and the concentration of a-c in the void+a-c diffusion layer. All samples were fitted with an R-squared convergence of approximately Based on reproducibility experiments, an error of +/-1.8% was estimated for thickness, while errors +/-0.2% and +/- 1.0% were estimated for the refractive index and extinction coefficient, respectively.

53 X-ray photoelectron and X-ray excited Auger electron spectroscopy Overview X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) were used to measure the composition, relative density, sp 2 /sp 3 bonding ratio, and relative position of the Fermi level for the a-c samples. A detailed review of XPS and XAES is beyond the scope of this thesis. Interested readers are directed to the following sources for details on XPS and XAES [47; 48; 49]. Both XPS and XAES are analytical methods that use an x-ray source to irradiate a sample under ultra-high vacuum (UHV) in order to induce photoelectron emission in the sample. The emitted electrons are collected by an electron detector and analyzer to measure the quantity and kinetic energy of electrons emitted from the sample. As shown in Figure 3.3, XPS and XAES are differentiated based on the photoelectron emission process being analyzed. In XPS, the energy and quantity of core-level electrons emitted from the sample are measured, while in XAES the energy and quantity of electrons emitted through the Auger process are analyzed. Core-level and Auger electrons can be differentiated since the kinetic energy of electrons emitted from these two processes lie in different energy ranges. XPS was used to provide quantitative measurements of elemental composition, qualitative measurements of film density, and qualitative measurements of Fermi level shifts for the a-c samples doped with Nitrogen. XAES was used to provide quantitative measurements of the sp 2 /sp 3 bonding ratio in the a-c samples.

54 34 Figure 3.3: (a) XPS: measurement of electrons emitted from core-level due to x-ray absorption (b) XAES: measurement of secondary electrons emitted from valence-level carrying excess energy created from core-level hole, created in process shown in (a), being filled. XPS and XAES measurements were performed on a Thermo Scientific K-Alpha spectrometer. Measurements were taken using a monochromatic source and the pass energy was set to 30eV. In order to compensate for charging effects, a flood gun was used for the XAES measurements. To determine the sp 2 /sp 3 bonding ratio in the a-c samples, XAES measurements were also taken on graphite and chemical vapour deposited (CVD) diamond references Elemental composition Elemental composition can be found through XPS measurements by relating the kinetic energy measured of an emitted core-level electron, E k, to the core-level binding energy, BE, of a particular element through the following equation: EE kk = hν BE Φ analyzer (6) where hν is the x-ray photon energy and Φ analyzer is the work function of the electron analyzer. This relationship is graphically illustrated in the band diagram shown in Figure 3.4. As can be seen in the figure, the binding energy is defined as the energy difference between the core-energy level and the Fermi level. Since this value is unique for each element, the elemental composition of the sample can be found by comparing the counts of electrons occurring at different binding energies. An example of elemental composition measurement is shown in Figure 3.5. The energy scale was converted to binding energy using Equation 6 and each peak

55 35 was assigned to a specific element based on the best match between the peak s energy and known core-level binding energies. The counts were then corrected based on known photoemission cross-sections of the selected elements and then the elemental composition was found through the ratio between the areas under each peak. An error of 5% of the measured value was accounted for these measurements based on the known tolerance of the system. It should be noted that due to their low binding energies, electrons emitted from hydrogen and helium cannot be detected in XPS systems, and thus composition found excludes contribution from hydrogen. As discussed in the following section, the hydrogen content of the samples was measured using Fourier Transform Infrared Spectroscopy. Figure 3.4: Band diagram depicting relationship between binding energy (BE), work function of the sample (Φ sample ), work function of the electron analyzer (Φ analyzer ), Fermi energy of the sample (E f ), measured kinetic energy of the electron (E k ), and photon energy (hν) for XPS. Note that since the sample and analyzer are both grounded, their Fermi energies are aligned. E k represents the kinetic energy of the electron when it is emitted from the sample, and E k represents the kinetic energy of the electron measured on the electron analyzer.

56 36 Figure 3.5: Example XPS measurement used to determine elemental composition. The measurement was taken on a sample in the A-20 sample set and shows two distinct peaks at two binding energies: (i) 526eV corresponding to the binding energy of 1s electrons in oxygen and labeled O1s in the figure, (ii) 284eV corresponding to the binding energy of 1s electrons in carbon and labeled C1s in the figure Film Density Qualitative measurements on film density were obtained through XPS by taking measurements at three different angles of incidence: 30 o, 50 o, and 70 o. By varying the angle of incidence, the depth that the x-ray beam penetrates the sample also varies with larger angles of incidence leading to lower penetration depths. For example, if the angle of incidence of the x-ray is denoted by θ, and the absorption length of x-ray photons in the film is denoted by L, then the relationship between the penetration depth, d, with the angle of incidence can be approximated by: dd = LL cosθ (7)

57 37 This relationship is illustrated in Figure 3.6. Figure 3.6: Dependence of penetration depth, d, of x-ray incident at angle of θ on a film with x-ray absorption length of L. By analyzing the change in film composition measured at different angles (depths), qualitative information on the film s density can be found. For example, a film with an oxygen concentration that remains fairly constant with increasing depth can be concluded to be more porous and less dense than a film that shows a strong decrease in oxygen concentration with increasing depth. An example of this can be seen in the measurements shown in Figure 3.7. In the figure, sample A shows a fairly constant oxygen profile while sample B shows a clear decrease in oxygen concentration with decreasing angle (i.e. increasing depth).

58 38 Oxygen Concentration (at. %) Sample A Sample B Angle of Incidence ( o ) Figure 3.7: Example of angle-resolved XPS measurements providing qualitative information of film density. Note the curves are just a guide to the eye Fermi level shifts In order to investigate the potential of nitrogen as a dopant in a-c, the dependence of the Fermi level position with nitrogen content needed to be measured. For other semiconductors, this is typically performed by measuring changes in the activation energy indicated in conductivity versus temperature measurements. However, this method presents several challenges when applied to the a-c samples studied in this thesis. Perhaps the greatest difficulty is presented by the fact that PLC:H films, which were the type of a-c films investigated for doping in this thesis, are highly resistive ~ Ω-cm [50]. With the equipment available, the only method of reliably measuring the resistivity of this type of film is by performing the measurement transversely through the sandwich method, illustrated in Figure 3.8. However as shown in the figure, due to the soft, porous nature of PLC:H films, the sandwich method also poses several difficulties

59 39 as one needs to ensure that the metal contacts are not deposited over any pores, pinholes, or scratches. Figure 3.8: (a) sandwich method of measuring conductivity transversely through a thinfilm (b) shunting and unknown length issues that can occur with soft PLC:H film with pores, pinholes and scratches. In addition to issues associated with the high resistivity of the film, the fact that the PLC:H samples were deposited at room-temperature also presented challenges. Since the samples were deposited at room-temperature, any measurements on the film needed to be performed at or below room temperature in order to prevent any annealing of the film. However, in order to find the activation energy, conductivity versus temperature measurements are typically performed up to at least 200 o C to 300 o C [51; 52]. XPS presents an optical alternative for measuring Fermi level shifts. As discussed in Section 3.6.2, binding energy is defined as the energy difference between a core-level electron and the Fermi level. Thus shifts in the Fermi level can be found by measuring changes in the binding energy of a material. Several authors have shown XPS to be a reliable method for detecting Fermi level shifts in a-c films [53; 54; 55]. In these studies, Fermi level shifts through XPS for DLC:H, TAC:H, and TAC films were detected. To the best of our knowledge there is no study published on detecting Fermi level shifts through XPS for PLC:H films. When measuring Fermi level shifts through XPS, care must be taken not to confuse a shift in the Fermi level with other effects that can cause shifts in the binding energy such as increased resistivity or heteropolar bonding. Increased resistivity in the material can cause charging effects that lead to a built-in retarding potential in the material which will decrease the kinetic energy of

60 40 electrons. This decrease in kinetic energy due to charging effects should not be falsely associated with an increase in the binding energy of core-level electrons. In order to compensate for charging effects, the C1s and N1s core-level binding energies were corrected using the method outlined in [56]. In this method, the O1s binding energy on the surface of the a-c samples is measured. Any shifts in the O1s peak in the samples is attributed to the charging effect, and the C1s and N1s peaks are corrected by this shift. This method is argued to be valid because the bonding environment of oxygen on the surface of a-c films does not change significantly under different deposition conditions [56]. An example of C1s and N1s corrected core-level energies is shown in Table 3.1. Table 3.1: C1s and N1s core-level binding corrected for charging effect (CN sample set) Nitrogen C1s N1s O1s ΔO1s C1s corrected N1s corrected (at. %) +/ / / / /-0.03 (ev) +/-0.03 (ev) (ev) (ev) (ev) (ev) Heteropolar bonding, which is covalent bonding where the bonded electron(s) are not equally shared by the atoms due to differences in electronegativity, can also cause a shift in binding energy. Since nitrogen is more electronegative than carbon, the C1s binding energy can be expected to increase due to an increase in nitrogen content [53; 55]. Therefore both the C1s and N1s peaks need to be considered to determine if a shift in the Fermi level has taken place. Only if both the charge-corrected C1s and N1s peaks have increased could one attribute the cause to a shift in the Fermi level [55].

61 41 It should be noted that a quantitative value of the Fermi level shift cannot be obtained from XPS measurements since the shift in the charge-corrected C1s peak is caused by both a shift in the Fermi level and electronegativity effects. Thus as a means of comparing samples, the shift in the charge-corrected N1s peak can be used as a qualitative measure of the shift in the Fermi level [53]. Note an error of +/-0.01eV was accounted for in the measured C1s, N1s, and O1s peaks based on the resolution of the measurement. This uncertainty also was accounted for when assigning errors to the ΔO1s, C1s corrected, and N1s corrected values Carbon sp 2 / sp 3 bonding ratio Using the Lascovich method [57; 58; 59; 60], XAES measurements were used to estimate the ratio between sp 2 -hybridized and the sp 3 -hybridized carbon atoms in the film. The Lascovich method is an empirical method which uses XAES measurements on carbon allotropes: diamond (100% sp 3 hybridized) and graphite (100% sp 2 hybridized) to determine the percentage of sp 2 hybridization in a-c films, which contain a mixture of sp 3 and sp 2 hybridization. The method relies on the fact that the kinetic energy of an electron emitted from the Auger process is sensitive to the energy levels present in the atom. For example, a sp 3 -hybridized carbon atom has one core-level (1s) and one valence level (2sp 3 ), and thus only one type of Auger emission can take place; this is depicted in Figure 3.9. A sp 2 -hybridized carbon atom has one core-level (1s) and two valence levels (2sp 2 and 2p) and thus multiple Auger emission processes can take place; this is depicted in Figure Since sp 2 -hybridized carbon atoms have multiple valence levels that can potentially be involved in Auger emission, one can expect the kinetic energy of Auger electrons to have a broader profile in sp 2 -hybridized carbon atoms than in sp 3 -hybridized carbon atoms. This can be extended from isolated atoms to condensed matter, as sp 2 - hybridization leads to the presence of π-bands and thus a broadening of the valence band.

62 42 Figure 3.9: Auger emission in sp 3 -hybridized carbon atom. Figure 3.10: Two potential Auger processes in sp 2 -hybridized carbon atom that produce emission of electrons with unique kinetic energies. A sample Auger measurement is illustrated in Figure Note that the width of the dominant peak, D, is more easily found by the derivative spectra, where the D-parameter in the derivative spectra is represented by the distance between the maximum of the positive-going excursion and the minimum of the negative-going excursion.

63 43 Figure 3.11: (a) XAES measurement (b) derivative spectra of XAES measurements with D- parameter indicated on figure.

64 44 Lascovich proposed that the percentage of sp 2 -hybridized carbon in a-c films can be found by a linear extrapolation of the D-parameter measured in the a-c film with the D-parameters of diamond and graphite [57]. Thus the percentage of sp 2 -hybridized carbon atoms can be found through the following: %ssss 2 = DD ssssssssssss DD dddddddddddddd DD gggggggg hiiiiii DD dddddddddddddd 100% (8) An error of +/-1% was accounted for the calculated value based on the step size used for this measurement.

65 Fourier Transform Infrared Spectroscopy Fourier Transform Infra-Red (FTIR) spectroscopy was used to measure the hydrogen concentration in the a-c samples. A complete review of FTIR is beyond the scope of this thesis. Interested readers are directed to the following sources for details on the method of FTIR [61; 62; 63]. FTIR is an infrared (IR) spectroscopy method that uses a broadband source coupled with a Michelson interferometer to perform IR transmission and reflection measurements. Using postacquisition processing with the Michelson interferometer allows the Fourier-transform of the measured transmission or reflection intensity as a function of wavenumber to be found. Therefore, even though a broadband source is used, measurements at particular wavenumbers can be made. FTIR measurements were made using a Perkin Elmer 2000 spectrometer. The spectral range used was 400cm -1 to 5200cm -1 with a resolution of 1cm -1. Before a measurement was taken, samples were left in the sample compartment for 10 minutes in order to allow desiccated air to purge the compartment. This purge was found to be necessary as it prevented absorption from water vapor and CO 2 to effect measurement data. Moreover, in order to reduce background noise in the acquired data, the average of 20 measurements was taken for each sample. All measurements were taken on samples deposited on c-si substrates. These substrates were chosen for FTIR measurements due to the near-flat transmittance of c-si in the spectral range of interest. FTIR spectroscopy is commonly employed in measuring the molecular bonding in solids and liquids. These measurements are based on observing the absorption arising from the vibrational modes of different molecular groups. Since every molecule has its own natural vibrational mode with the frequency generally lying in the IR region, by measuring the absorption in the IR, the existence and concentration of particular molecular groups can be found. For example, Table 3.2 shows the vibrational modes of different molecular species commonly found in intrinsic and nitrogen incorporated a-c films [39; 64]. By examining the IR spectra of an a-c sample, information on the types of bonds and the quantity of particular molecular groups present in the sample can be found.

66 46 Table 3.2: Vibrational modes of molecular groups commonly found in a-c films Wavenumber (cm -1 ) Vibrational Mode Assignment sp 2 -bonded CH 2 & CH 3 bending sp 2 C-C, C-N, and C=N 1375 sp 3 -bonded CH 2 & CH 3 bending C=N 1450 sp 3 -bonded CH 2 and CH 3 bending mode C-N, C=N, and C=C C=N 1600 C=N, and C=C C=C (higher), and C=N(lower) CN triple bond stretch (lower) ; CC triple bond stretch (higher) 2855 sp 3 CH CH 2 and CH sp CH 2 sp NH mode 3300 sp 1 CH mode 3400 OH mode

67 47 Figure 3.12 shows an example of an FTIR transmission measurement on one of the a-c samples normalized against the transmission of a bare c-si substrate. Figure 3.12: Normalized transmission spectrum from FTIR measurement. In order to better indentify the absorption modes present in the a-c film, the background of the spectra was removed using polynomial curve-fit algorithm provided in the TableCurve 2D Automated Curve Fitting and Equation Discovery 5.0 software. As shown in Figure 3.13, once the background was removed the location and size of the absorption peaks could easily be identified.

68 48 Figure 3.13: Transmission spectrum with background removed. The absorption spectrum of the a-c film was found using the following relation [65]: αα = 1 dd ln TT (9) where α is the absorption coefficient, d is the thickness of the film, and T is the normalized transmission of the a-c sample with the background removed. The resulting absorption spectrum of the a-c sample is shown Figure Each absorption peak was assigned to the nearest known vibrational mode for intrinsic a-c. As can be seen, the dominant absorption peak lies between 2750cm -1 and 3050cm -1 and can be attributed to hydrogen-carbon stretching modes.

69 49 Figure 3.14: Absorption coefficient spectrum with vibrational modes indicated. A commonly used method of determining the hydrogen concentration in a-c films is to integrate the area under the dominant CH x peak (2750cm -1 to 3050cm -1 ) in the absorption coefficient spectrum and multiply this value by an empirically found oscillator strength [39; 66; 67; 68]: HH. CCCCCCCC. = AA αα(ωω) ww dddd (10) where A is the constant 5X10 20 cm -2 which represents the empirically found absorption strength correction factor. An error of 1% was estimated for the calculated hydrogen concentration. This error was based on comparing the calculated hydrogen concentration for different fits of the raw FTIR background spectra.

70 Profilometry Profilometry was used to obtain a 1 st -order estimate of the thicknesses of the a-c films deposited. This estimate was needed in order to perform spectral ellipsometry modelling. Profilometry is a mechanical method of measuring the thickness of a thin-film material. Designed similar to a record player, profilometers use a stylus connected to a tracking arm which is free to move along two axes (vertical and horizontal) and is held down by a counter-balance load. The thickness of a thin-film is measured by moving the stylus across a step between the bare substrate and the film. The deflection of the tracking arm connected to the stylus is converted to an electrical signal and displayed on screen. As opposed to other methods of measuring thickness of thin-films such as scanning electron microscopy (SEM) or transmissive electron microscopy (TEM), measurements made using profilometry are simple, fast, and independent of the electrical and optical properties of the film and substrate used. However, the advantage of simplicity and speed is countered by a loss in accuracy. Since profilometry measures thickness by measuring deflection, a mask is required during film growth to provide a step between the bare substrate and the film. Due to edge effects, the growth rate of film near the mask will differ from the growth rate near the center of the substrate and thus the thickness measured by profilometry near the mask will differ from the thickness in the rest of the substrate. Moreover, since profilometry is a mechanical method, measurements are dependent on the mechanical properties of the film and substrate (eg. hardness, friction, surface roughness, etc.). Since profilometry was used only for a 1 st -order estimate of thickness, the loss in accuracy was more than justified by the simplicity and speed of the method. Profilometry measurements were made on a KLA-Tencor P16+ with the counter balance load set to 0.5mg.

71 51 4 Experimental Results 4.1 Overview In this chapter, the results from measurements of growth rate, nitrogen content, hydrogen content, percent of sp 3 -bonded carbon atoms, film density, and optical properties for the a-c films studied in this thesis are presented. For convenience, a summary of the five sample sets explored is provided in Table 4.1. Table 4.1: Summary of sample sets Sample Set Description A-20 Anode 20W: These were samples deposited on the anode at an RF power of 20W. The variable deposition parameter for this sample set was temperature which was varied from ambient temperature (35 o C) to 200 o C. C-RT A-RT C-N A-N Cathode Room Temperature: These were samples deposited on the cathode with no applied heating. The variable deposition parameter for this sample set was RF power which was varied from 3W to 60W. Anode Room Temperature: These were samples deposited on the anode with no applied heating. The variable deposition parameter for this sample set was RF power which was varied from 5W to 80W. Cathode Nitrogen incorporated: These were samples deposited on the cathode with no applied heating at an RF power of 5W. The variable deposition parameter for this sample set was the partial pressure of N 2 in the source gas which was varied from 0% to 50%. Anode Nitrogen incorporated: These were sample deposited on the anode with no applied heating at RF power of 20W. The variable deposition parameter for this sample set was the partial pressure of N 2 in the source gas which was varied from 0% to 50%.

72 Growth rate As explained in Section 3.5, the thickness of the a-c films were measured using spectral ellipsometry with profilometry measurements acting as an initial estimation for the regression fitting. The thickness of the a-c films ranged from 150nm to 1µm with most films having a thickness between nm. Growth rates were measured on all five sample sets. Figures 4.1 to 4.3 show the dependence of growth rate on the varied deposition parameter for sample sets A-20, C-RT, A-RT, C-N, and A- N, respectively. As can be seen in Figure 4.1, the growth rate of a-c for the A-20 sample set shows a strong dependence on the substrate temperature used for film deposition; with higher substrate temperatures leading to lower growth rates. Figure 4.2 shows the dependence of growth rate on the applied RF power for sample sets C-RT and A-RT. Both sample sets show a significant increase in growth rate with increasing RF power. By comparing the figures, it is evident that the growth rate in the C-RT sample set increases more rapidly with increasing RF power than the A-RT sample set. This is the first indication that the placement of the substrate (at the anode or cathode) plays a role in the film growth process. 6 5 Growth Rate (nm/min) Temperature ( o C) Figure 4.1: Change in growth rate with temperature for A-20 sample set.

73 Growth Rate (nm/min) A-RT C-RT RF Power (W) Figure 4.2: Change in growth rate with RF power for C-RT and A-RT sample sets. Figure 4.3 shows the dependence on growth rate on the percentage of N 2 introduced in the source gas. For the C-N sample set, the growth rate appears to be independent of the amount of N 2 introduced when the nitrogen content is in the 0-25% range but as the nitrogen introduction reaches 50% there is a small decrease in the film growth rate. The dependency of growth rate with the percentage of N 2 in the source gas appears to be quite different for the A-N sample set as the growth rate increases monotonically as the nitrogen content is increased.

74 54 7 Growth Rate (nm/min) A-N C-N N 2 in Mixing Bottle (% pressure) Figure 4.3: Change in growth rate with N 2 partial pressure for C-N and A-N sample sets. Note the C-N sample set and A-N sample set had identical deposition parameters other than the placement of the substrate and RF power used. For the C-N sample set, substrates were placed on the cathode and an RF power of 5W was used while for the C-N sample set, substrates were placed on the anode and an RF power of 20W was used.

75 Nitrogen content As described in Section 3.6, the nitrogen content in the a-c samples was measured using XPS. Figure 4.4 shows the change in the nitrogen content (at. %) in a-c films deposited at different N 2 partial pressures in the source gas. Only samples included in the A-N and C-N sample sets are shown since only these sample sets included N 2 in the source gas. The deposition parameters in the A-N and C-N sample sets were identical other than the RF power and the placement of the substrate. In the A-N sample set, an RF power of 20W was used and the substrate was held on the anode, where as in the C-N sample set, an RF power of 5W was used as the substrate was held on the cathode. As can be seen in Figure 4.4, for both sample sets, the nitrogen content in the film increased as the N 2 partial pressure was increased. One clear difference between the sample sets is the rate at which the nitrogen content increased. As can be seen in Figure 4.4, films in the A-N sample set showed a more rapid increase in the nitrogen content with increasing N 2 partial pressure than films in the C-N sample set N Content (at. %) % 10% 20% 30% 40% 50% 60% N 2 Partial Pressure (%) A-N Sample Set C-N Sample Set Figure 4.4: Change in nitrogen content (at. %) with N 2 partial pressure for films in A-N and C-N sample sets. The curves serve as guides to the eye.

76 Hydrogen content Hydrogen content measurements were made on the three intrinsic a-c sample sets: C-RT, A-RT, and A-20. As described in Section 3.7, the concentration of hydrogen in the amorphous carbon samples was estimated by integrating the area under the overlapping CH x peaks (2750cm -1 to 3050cm -1 ) in the infrared absorption spectra. The absorption spectra in the 2750cm -1 to 3050cm -1 region and the corresponding hydrogen concentration for a-c films in the C-RT sample set are shown in Figure 4.5 and Figure 4.6, respectively CH 2 sp CH &CH 2 sp W Absorption (cm -1 ) 2000 CH 3 sp 3 5W 10W 15W 20W W 60W Wavenumber (cm -1 ) Figure 4.5: Absorption of C-H modes for a-c films in the C-RT sample set.

77 Hydrogen Concentration (10 22 cm -3 ) RF Power (W) Figure 4.6: Change in hydrogen concentration in a-c films in the C-RT sample set deposited at different RF powers. The curve is a guide for the eye. As can be seen in Figure 4.6, the hydrogen concentration monotonically decreases with increasing RF power. In addition, by observing Figure 4.5, one can see that there are clear trends in the ratios of the different CH x modes with increasing RF power. For example, as the RF power is increased, the relative intensity of the CH 2 sp 2 mode decreases in comparison to the CH x sp 3 modes. This trend is quantified in Figure 4.7 which shows the percent of CH x bonding in the CH 2 sp 2 mode for films deposited at different RF powers. As can be seen in the figure, there is a rapid decline in the relative concentration of CH 2 sp 2 bonding in the a-c films as the RF power is increased.

78 CH 2 sp 2 mode (%) RF Power (W) Figure 4.7: Percent of CH x bonding in the CH 2 sp 2 mode for C-RT sample set. The absorption spectra in the 2750cm -1 to 3050cm -1 region and the corresponding hydrogen concentration for a-c films in the A-RT sample set are shown in Figure 4.8 and Figure 4.9, respectively. As can be seen in Figure 4.9, there does not appear to be any clear trend between the hydrogen concentration and the RF power used during deposition. The hydrogen concentration changes only slightly as the RF power is decreased from 80W to 10W, however, once the RF power is dropped to 5W there is a significant drop in the hydrogen concentration in the film. While there does not appear to be any clear trend in hydrogen concentration with RF power for this sample set, there is a trend in the ratio of the CH 2 sp 2 peak to the CH x sp 3 peaks with increasing RF power. Similar to the C-RT sample set, the relative intensity of the CH 2 sp 2 mode decreases as RF power is increased. This trend is depicted in Figure However, by comparing Figure 4.10 with Figure 4.7, it is clear that this trend is more prominent in the C-RT sample set than in the A-RT sample set.

79 CH &CH 2 sp 3 CH 2 sp CH 3 sp Absorption (cm -1 ) W 10W 20W 40W 80W Wavenumber (cm -1 ) Figure 4.8: Absorption of C-H modes for a-c films in the A-RT sample set.

80 60 6 Hydrogen Concentration (10 22 cm -3 ) RF Power (W) Figure 4.9: Change in hydrogen concentration in a-c films in the A-RT sample set deposited at different RF powers CH 2 sp 2 Mode (%) RF Power (W) Figure 4.10: Percent of CH x bonding in the CH 2 sp 2 mode for A-RT sample set.

81 61 The absorption spectra in the 2750cm -1 to 3050cm -1 region and the corresponding hydrogen concentration for a-c films in the A-20 sample set are shown in Figure 4.11 and Figure 4.12, respectively CH &CH 2 sp 3 CH 2 sp CH 3 sp 3 Abdorption (cm -1 ) C 100C 150C 200C Wavenumber (cm -1 ) Figure 4.11: Absorption of C-H modes for a-c films in the A-20 sample set.

82 62 7 Hydrogen Concentration (10 22 cm -3) Temperature ( o C) Figure 4.12: Change in hydrogen concentration in a-c films in the A-20 sample set deposited at different substrate temperatures. As can be seen in Figure 4.12, there appears to be a local maxima in the relationship between the hydrogen concentration in the a-c films and the temperature of the substrate during deposition. As the temperature is increased from ambient temperature, 35 o C, where no intentional heating was applied to the substrate, to 100 o C, there is a significant increase in the hydrogen concentration in the film. As the temperature is increased further to 150 o C, the hydrogen concentration remains relatively constant. However, once the temperature is increased to 200 o C, there is a significant decrease in the hydrogen concentration in the film. The ratio of the CH 2 sp 2 mode to the CH x sp 3 modes shows a different trend with increasing substrate temperature. As can be seen in Figure 4.13, the relative concentration of CH 2 sp 2 bonding decreases as the temperature is increased from 35 o C to 100 o C and again as the temperature is increased from 150 o C to 200 o C, however, between 100 o C to 150 o C, there does not appear to be any change in the relative concentration of the CH x modes.

83 CH 2 sp 2 Mode (%) Temperature ( o C) Figure 4.13: Percent of CH x bonding in the CH 2 sp 2 mode for A-20 sample set.

84 Percent of sp 3 Bonding As discussed in Section 3.6, the percent of sp 3 -bonded carbon atoms in the a-c samples was estimated using XAES. Measurements were performed on the three intrinsic a-c sample sets: C- RT, A-RT, and A-20. Due to overlapping modes introduced by nitrogen, the percent of sp 3 - bonded carbon atoms could not be extracted from XAES measurements for the nitrogenincorporated sample sets: C-N and A-N. Figure 4.14 shows the change in the percent of sp 3 -bonded carbon atoms (%sp 3 bonding) for a-c films in the C-RT and A-RT sample sets. For both these sample sets, the variable deposition parameter was the applied RF power with all other parameters being held constant. These samples sets only differ in the placement of the substrates, with substrates being held on the cathode for the C-RT sample set and substrates held on the anode for the A-RT sample set. As can be seen in the figure, both sample sets show decreasing trends in %sp 3 bonding with increasing RF power with the C-RT sample set showing a more rapid decrease than the A-RT sample set. It is interesting to note that at low RF powers (below 20W), the C-RT and A-RT sample sets show similar relationships between %sp 3 bonding and RF power. However, as the RF power is increased further, the %sp 3 bonding in the C-RT sample set begins to decreases rapidly while the A-RT sample set only shows a slight decrease. Figure 4.15 shows the change in the percent of sp 3 -bonded carbon atoms (%sp 3 bonding) for a-c films in the A-20 sample set. For this sample set, all films were deposited at an RF power of 20W with the substrates held on the anode, however, each sample was deposited at a different substrate temperature. As can be seen from the figure, the substrate temperature has a similar effect as the RF power had in the C-RT and A-RT samples sets, as the %sp 3 bonding shows a generally decreasing trend with increasing substrate temperature. However, it is clear that for the range in substrate temperatures explored, the %sp 3 bonding is more weakly related to substrate temperature than it is to RF power.

85 %sp 3 bonding C-RT A-RT RF Power (W) Figure 4.14: Change in the percent of sp 3 -bonded carbon atoms for a-c films deposited at different RF Powers in the C-RT and A-RT sample sets. The curves are guides to the eye. %sp 3 bonding Temperature ( o C) Figure 4.15: Change in the percent of sp 3 -bonded carbon atoms for a-c films deposited at different substrate temperatures in the A-20 sample set.

86 Film density In order to augment other structural characterization measurements and assist in the analysis, qualitative film density measurements were performed on the three intrinsic sample sets: A-20, C-RT, and A-RT. As explained in Section 3.6, the a-c film density can be qualitatively measured by examining the change in oxygen concentration at increasing depth from the surface. Samples that have a significantly lower oxygen concentration in the bulk of the sample as compared to the surface can be deemed more dense than samples that have near constant oxygen concentration from the surface to the bulk. Thus by measuring the change in oxygen concentration in the film, a qualitative assessment of the a-c film density can be made. For the A-20 sample set, the change in film density for different substrate temperatures can be seen in Figure As can be seen in the figure, the samples deposited at higher substrate temperatures show a larger gradient in oxygen concentration and thus can be concluded to have a relatively greater film density. 200 Temperature ( o C) Change in Oxygen Concentration (at. %) Figure 4.16: Difference in the oxygen concentration (O.C.) in the bulk and in the nearsurface (O.C. surface -O.C. bulk ) for samples in the A-20 sample set. Measurements made by AR-XPS with measurement at 70 o representing the near-surface and measurement at 30 o representing the bulk.

87 67 The change in film density for samples deposited at different RF powers in the C-RT sample set can be seen in Figure As can be seen in the figure, there is a significant change in the oxygen gradient and thus film density as the RF power is increased from 3W to 10W. As the RF power is increased from 10W to 20W the oxygen gradient appears to remain near constant (i.e. changes are within the range of measurement error). As the RF power is increased beyond 20W, the oxygen gradient and thus the film density once again increases significantly RF Power (W) Change in Oxygen Concentration (at. %) Figure 4.17: Difference in the oxygen concentration (O.C.) in the bulk and in the nearsurface (O.C. surface -O.C. bulk ) for samples in the C-RT sample set. Measurements made by AR-XPS with measurement at 70 o representing the near-surface and measurement at 30 o representing the bulk. The change in film density for samples deposited at different RF powers in the A-RT sample set can be seen in Figure As can be seen in the figure, as the RF power is increased there is a significant increase in the oxygen gradient and thus density of the samples. It is also interesting to observe that for this sample set, the samples deposited at 5W, 10W, and 20W, all show a negative oxygen gradient meaning that there is more oxygen in the bulk of the film than near the surface. This is an indication of the highly porous structure of these films.

88 68 80W 40W RF Power (W) 20W 10W 5W Change in Oxygen Concentration (at. %) Figure 4.18: Difference in the oxygen concentration (O.C.) in the bulk and in the nearsurface (O.C. surface -O.C. bulk ) for samples in the A-RT sample set. Measurements made by AR-XPS with measurement at 70 o representing the near-surface and measurement at 30 o representing the bulk.

89 Impurity doping In order to determine the effect of nitrogen content in a-c films, the relative position of the Fermi level for films in the C-N and A-N sample sets were estimated through binding energy shifts measured using XPS. This method was described in Section 3.6. For both the C-N and the A-N sample sets, the percent of N 2 in the source gas was varied from 0% to 50%. Both the C-N and A-N sample sets produced PLC:H type a-c films with the deposition conditions of the sample sets being identical other than the RF power and the placement of the substrate. In the A-N sample set, an RF power of 20W was used and the substrate was held on the anode, where as in the C-N sample set, an RF power of 5W was used as the substrate was held on the cathode. Due to these differences, the nitrogen content (at. %) for these two sample sets differed. As explained in Section 3.6, the change in the charge-corrected N1s binding energy provides a qualitative measure of shifts in the Fermi level position with an increase in the N1s binding energy representing a shift of the Fermi level toward the conduction band. The change in the charge-corrected N1s binding energy for films with different levels of nitrogen content (at. %) in the C-N and A-N sample sets are shown in Table 4.2 and Table 4.3, respectively. For both sample sets it is clear that the Fermi level shifts towards the conduction band with increasing nitrogen content (at. %). This indicates that nitrogen is acting as an n-type dopant for the a-c films produced in these sample sets. Table 4.2: N1s shifts for C-N sample set N 2 in source gas (%) Nitrogen (at. %) N1s corrected +/-0.03 (ev) ΔN1s +/-0.03 (ev)

90 70 Table 4.3: N1s shifts for A-N sample set N 2 in source gas (%) Nitrogen (at. %) N1s corrected +/-0.03 (ev) ΔN1s +/-0.03 (ev)

91 Optical properties In this section, the optical properties of the a-c samples, measured through spectral ellipsometry, are presented. The results included in this section are the refractive index and absorption coefficient from 300nm to 825nm as well as the calculated optical energy gap of the a-c samples for all five sample sets: A-20, C-RT, A-RT, C-N, and A-N. The refractive index and absorption coefficient spectra for the a-c films included in sample set A-20 are shown in Figure 4.19 and Figure 4.20, respectively. For this sample set, all films were deposited at an RF power of 20W with the substrates held on the anode, however, each sample was deposited at a different substrate temperature. As can be seen from the figures, the films deposited at 35 o C, 100 o C, and 150 o C show similar refractive index and absorption coefficient spectra (i.e. differences are within error bars), however, as the substrate temperature is increased to 200 o C, there is a noticeable shift in both the refractive index and absorption coefficient spectra n C 0 C 100C 0 C 150C 0 C 200C 0 C λ (nm) Figure 4.19: Refractive index (n) of a-c films in the A-20 sample set. These are intrinsic films that were deposited on the anode at an RF power of 20W at several different substrate temperatures. For clarity, an error bar is only shown for the first data point on the sample deposited at 200 o C.

92 α (10 4 cm -1 ) C 0 C 100C 0 C 150C 0 C 200C 0 C λ (nm) Figure 4.20: Absorption coefficient (α) of a-c films in the A-20 sample set. These are intrinsic films that were deposited on the anode at an RF power of 20W at several different substrate temperatures. For clarity, an error bar is only shown for the first data point on the sample deposited at 200 o C. The refractive index and absorption coefficient spectra for the C-RT and A-RT sample sets are presented in Figures 4.21 to For both these sample sets, the variable deposition parameter was the applied RF power with all other parameters being held constant. These samples sets only differ in the placement of the substrates, with substrates being held on the cathode for the C- RT sample set and substrates held on the anode for the A-RT sample set. As can be seen in Figures 4.21 and 4.22, the applied RF power has a significant effect on the refractive index and absorption coefficient for samples deposited on the cathode (C-RT sample set). As the RF power is increased from 5W to 40W, both the refractive index and absorption coefficient spectra show significant increases. However, as the RF power is increased from 40W to 60W, the increase is

93 73 less significant with the absorption coefficient spectra of these two samples being within the range of the error bars n W 40W 20W 15W 10W 5W 3W λ (nm) Figure 4.21: Refractive index (n) of a-c films in the C-RT sample set. These are intrinsic films that were deposited on the cathode at several different RF powers. For this sample set there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample deposited at 60W.

94 α (10 4 cm -1 ) W 40W 20W 15W 10W 5W 3W λ (nm) Figure 4.22: Absorption coefficient (α) of a-c films in the C-RT sample set. These are intrinsic films that were deposited on the cathode at several different RF powers. For this sample set there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample deposited at 60W. The trends for samples deposited on the anode (A-RT sample set) are not as apparent. As shown in Figure 4.23, the only noticeable shift in refractive index occurs as the RF power is increased from 40W to 80W, however, the magnitude of this shift is relatively small. The trend in the absorption coefficient spectra, shown in Figure 4.24, is more apparent. Similar to the C-RT sample set, the absorption coefficient increases with increasing RF power, however, once again the magnitude of the shift is relatively small in comparison to the C-RT sample set. It is clear from these results, that the refractive index and absorption coefficient of the samples deposited on the anode (A-RT sample set) show a significantly weaker dependency on the RF power than the samples deposited on the cathode (C-RT sample set). It should also be pointed out that the refractive index and absorption coefficient spectra of the samples deposited on the anode appear to be similar to that of the samples deposited at low RF powers (3W-5W) on the cathode.

95 n W 40W 20W 10W 5W λ (nm) Figure 4.23: Refractive index (n) of a-c films in the A-RT sample set. These are intrinsic films that were deposited on the anode at several different RF powers. For this sample set there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample deposited at 80W.

96 α (10 4 cm -1 ) W 40W 20W 10W 5W λ (nm) Figure 4.24: Absorption coefficient(α) of a-c films in the A-RT sample set. These are intrinsic films that were deposited on the anode at several different RF powers; for this sample set there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample deposited at 80W. The refractive index and absorption coefficient spectra for the C-N and A-N sample sets are shown in Figure 4.25 to 4.28, below. For both these sample sets, the variable deposition parameter was the percent of N 2 in the source gas, which was varied from 0% to 50%. The fixed deposition parameters of these samples sets differed only in the RF power used and the placement of the substrate. In the A-N sample set, an RF power of 20W was used and the substrate was held on the anode, where as in the C-N sample set, an RF power of 5W was used and the substrate was held on the cathode. Due to these differences, the nitrogen content (at. %) for these two sample sets differed.

97 77 As can be seen in the figures, for both sample sets, the refractive index and absorption coefficient spectra show a weak dependency with nitrogen content. For the C-N sample set, both the refractive index and absorption coefficient spectra show only minor increases (i.e. shifts are close to or within range of error bars) as the nitrogen content is increased from 0% to 4.41%; however, when the nitrogen content is increased to 8.97%, a noticeable downward shift in the refractive index and absorption coefficient spectra can be seen. The A-N sample set shows a similar trend to the C-N sample set as the refractive index and absorption coefficient spectra show only minor increases (i.e. shifts are close to or within range of error bars) as the nitrogen content is increased from 0% to 4.6%. However, unlike the C-N sample set, as the nitrogen content is increased beyond 8.97%, both the refractive index and absorption coefficient spectra shift upwards rather than downwards % 4.41% 2.96% 5% 0% n λ (nm) Figure 4.25: Refractive index (n) of a-c films in the C-N sample set. Each film has a different level of nitrogen content (at. %) due to the difference in the N 2 partial pressure that was used in the source gas. For this sample set the substrate was held on the cathode, the RF power was set to 5W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown for the last data point on the sample with a nitrogen content of 8.97%.

98 α (10 4 cm -1 ) % 4.41% 2.96% 5% 0% λ (nm) Figure 4.26: Absorption coefficient (α) of a-c films in the C-N sample set. Each film has a different level of nitrogen content (at. %) based on the N 2 partial pressure that was used in the source gas. For this sample set the substrate was held on the cathode, the RF power was set to 5W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample with a nitrogen content of 8.97%.

99 % 9.73% % n % 0% λ (nm) Figure 4.27: Refractive index (n) of a-c films in the A-N sample set. Each film has a different level of nitrogen content (at. %) based on the N 2 partial pressure that was used in the source gas. For this sample set the substrate was held on the anode, the RF power was set to 20W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample with a nitrogen content of 17.04%.

100 α (10 4 cm -1 ) % 9.73% 4.60% 2.58% 0% λ (nm) Figure 4.28: Absorption coefficient (α) of a-c films in the A-N sample set. Each film has a different level of nitrogen content (at. %) based on the N 2 partial pressure that was used in the source gas. For this sample set the substrate was held on the anode, the RF power was set to 20W, and there was no intentional heating of the substrate. For clarity, an error bar is only shown for the first data point on the sample with a nitrogen content of 17.04%. Typically for a-c films the optical energy gap is defined as the E 04 energy, which is the photon energy at which the absorption coefficient of the film equals 10 4 cm -1. The E 04 energies for the a- C films in the A-20, C-RT, A-RT, C-N, and A-N sample sets are shown below in Figure 4.29 to Figure Based on the 1.0% error attributed to the absorption coefficient, an uncertainty of +/-0.2% was calculated for the E 04 values. Since the E 04 gap is linked directly with the absorption coefficient of the film, the trends in the E 04 gap for each sample set are the same as the trends in absorption coefficient already discussed.

101 81 E 04 gap (ev) Substrate Temperature ( o C) Figure 4.29: Change in the E 04 gap with substrate temperature for A-20 sample set E 04 Gap (ev) A-RT C-RT RF Power (W) Figure 4.30: Change in the E 04 gap with RF power for C-RT and A-RT sample sets. The curves are a guide to the eye.

102 E 04 Gap (ev) Figure 4.31: Change in the E 04 gap with nitrogen content for C-N sample set. The curve is a guide to the eye. Nitrogen Incorporation (at. %) Optical Gap (ev) Nitrogen Incorporation (at. %) Figure 4.32: Change in the E 04 gap with nitrogen content for A-N sample set. The curve is a guide to the eye.

103 83 5 Analysis 5.1 Overview In this chapter, the results presented in the previous chapter are analyzed. This analysis is broken up into two sections. In Section 5.2, the effect of ion/radical energy, plasma density, substrate temperature, and nitrogen incorporation on the growth rate, microstructural properties, and Fermi level position are discussed. In Section 5.3, the relationship between the microstructure and optical properties of a-c films is explored

104 Effect of deposition conditions on growth, electronic- and micro-structure of films When reviewing the results presented in Chapter 4, it is apparent that many of the film properties show similar trends as the primary deposition parameter is varied. Therefore, in order to analyze the results and determine the relationships between different film properties, it is best to look at all of the results within a sample set together and analyze how the deposition conditions affect these film properties. Moreover, since some of the sample sets share the same variable deposition parameter: (i) C-RT and A-RT, (ii) C-N and A-N, it will also be helpful to group these sample sets together in the analysis C-RT and A-RT sample sets The C-RT and A-RT sample sets shared the same variable deposition parameter, RF power, and other than the placement of the substrate, they also shared identical fixed deposition parameters. As explained in Section 2.3, one of the effects of increasing the RF power in an RF PECVD system is that the types and density of ions and radicals generated in the plasma region will change. This is intuitive since by increasing the RF power more energy is coupled to the free electrons in plasma region and as these energetic electrons collide with neutral particles in plasma region, new species and an overall greater density of ions and radicals are generated. This increase in plasma density with increasing RF power will occur in both the C-RT and A-RT sample sets since the placement of the substrate will not have an effect on this process. However, by reviewing the results in the previous sections it is apparent that the placement of the substrate has a significant effect on the film growth rate and microstructure. To appreciate how the placement of the substrate (anode or cathode) can affect the growth rate and microstructure of a-c films, one needs to review the potential profile of an RF PECVD chamber. As explained in Section 2.3, the electrodes on a capacitively-coupled RF-PECVD system develop negative DC biases with respect to the plasma potential. If the areas of the electrodes are asymmetric, then the electrode with the smaller area develops the dominant self-bias potential with the ratio of the self-biases being described by Equation 11: V cathode V anode = A anode A cathode 2 = 2.6 (11)

105 85 where V cathode and V anode are the magnitude of the self-biases on the cathode and anode with respect to the plasma potential and A cathode and A anode are the areas of the cathode and anode for the RF PECVD system used for these experiments. Since the area of the cathode was smaller than the area of the anode, the dominant self-bias fell on the cathode and thus an asymmetric potential profile like that shown in Figure 5.1 would develop in the chamber. As shown in the figure, as the RF power applied to the plasma is increased, the magnitude of the self-biases on both electrodes would increase; however, in order to maintain the ratio of the biases governed by Equation 11, the self-bias on the cathode would have to increase by a factor of 2.6 greater than the increase in self-bias on the anode. As explained in Section 2.3, the mean energy of ions and radicals impinging on an electrode is primarily determined by the self-bias potential of the electrode. Since in the RF PECVD system used here the dominant self-bias falls on the cathode, the energy of ions and radicals impinging on the cathode is greater than on the anode. Moreover, as the RF power is increased the change in self-bias and consequently ion/radical energy is significantly more pronounced on the cathode than on the anode.

106 86 Figure 5.1: Change in plasma potential profile with increasing RF power for RF-PECVD chamber with area of cathode smaller than area of anode. Growth Rate: As presented in Section 4.2, the growth rate for both the A-RT and C-RT sample sets increased with increasing RF power. Figure 5.2 shows the relationship between growth rate and RF power was similar for both sample sets; however, the growth rate in the C-RT sample set was consistently 20-30% more than the A-RT sample set. The fact that both sample sets showed very similar growth rates even at high RF powers suggests that plasma density is playing a primary role in controlling the growth rate. This is intuitive since an increase in plasma density and the introduction of new reactive species in the plasma

107 87 would cause an increase in surface reactions on the growing film leading to an increase in growth rate Growth Rate (nm/min) A-RT C-RT RF Power (W) Figure 5.2: Difference in relationship between growth rate and RF power for A-RT and C- RT sample sets. The curves serve as guides to the eye. The fact that the growth rate in the C-RT sample set was consistently 20-30% more than the A- RT sample set can be explained through the higher ion/radical energies impinging on the cathode. As explained in Section 2.5, low energy ions & radicals do not have enough energy to penetrate the surface of a growing a-c film and thus can only bond to the film if there is an unpassivated dangling bond on the surface of the film. In contrast, higher energy ions and radicals do not need to rely on surface dangling bonds since they can penetrate the surface and bond to a carbon cluster within the bulk of the film. This increase in bonding possibilities for high energy ions and radicals can explain the enhancement in growth rate seen in the C-RT sample set in comparison to the A-RT sample set. Microstructure: As presented in Section 4.4, for the C-RT sample set there was a strong correlation between the hydrogen concentration in the film and the RF power used during deposition while for the A-RT

108 88 sample set the hydrogen concentration seemed invariant with the RF power used. This comparison is depicted in Figure 5.3. These results indicate that the ion/radical energy rather than the plasma species/density is primarily responsible for determining the hydrogen concentration in the film. By reviewing the film growth processes presented in Section 2.5, this relationship between high energy ions/radicals and hydrogen concentration can be understood. Due to the relatively low displacement energy of hydrogen, 2.5eV, surface hydrogen atoms can be displaced from their bond through collisions with high energy ions/radicals. Moreover, high energy hydrogen ions & radicals impinging on the film can penetrate into the bulk of the film, bond with another hydrogen atom, and then desorb from the film. Thus there is a tendency for the hydrogen concentration in the film to decrease as the mean energy of ions/radicals in film deposition is increased. This explains why the hydrogen concentration has a strong dependency on RF power for the C-RT sample set but not for the A-RT sample set. 7 Hydrogen Concentration (10 22 cm -3 ) C-RT A-RT RF Power (W) Figure 5.3: Relationship between hydrogen concentration and RF power for C-RT and A- RT sample sets. The curve is a guide to the eye for the data points in the C-RT sample set. To understand why there is a significant drop off in hydrogen concentration for the sample deposited at 5W in the A-RT sample set, density measurements need to be taken into account. Without taking into account film density, hydrogen concentration measurements can be

109 89 misleading at times since a film with a relatively low hydrogen concentration can actually have a relatively high percentage of hydrogen if the film has a low enough density. Qualitative film density measurements presented in Section 4.6, showed that the film density increased with increasing RF power for both the C-RT and A-RT sample sets. Since for the C-RT sample set, the hydrogen concentration and film density showed opposite trends with increasing RF power (i.e. hydrogen concentration decreased while film density increased), it is evident that the percent of hydrogen (at. %) in the films also decreases with increasing RF power. This is not the case in the A-RT sample set where other than the sample deposited at 5W, the hydrogen concentration remained near-constant. Thus it is not directly apparent what the trend in hydrogen (at. %) is for the A-RT sample set. However, insight into this trend can be extracted by examining the ratio of the C-H peaks in the infrared absorption spectra presented in Section 4.4. The relative area of the CH 2 sp 2 peak can be used as a qualitative measure of the percent of hydrogen (at. %) in the films, with a larger relative CH 2 sp 2 peak being attributed to a higher percentage of hydrogen. The rationale for this stems from the fact that hydrogen prefers to bond to sp 3 hybridized carbon atoms because it represents a lower energy state than a bond with a sp 2 hybridized carbon atom. Thus C-H sp 2 bonding would be unlikely to occur unless the available sp 3 carbon bonds are near or at saturation due to a high percent of hydrogen (at. %) in the film. It should be pointed out that it is also possible for a high relative intensity of the CH 2 sp 2 peak to occur if a high percentage of the carbon atoms in the film are sp 2 hybridized; however, as indicated by the results shown in Section 4.4 and Section 4.5, for both the C-RT and A-RT sample sets the percent of sp 2 bonded carbon atoms and the relative intensity of the CH 2 sp 2 peak show opposite trends with increasing RF power. As shown in Section 4.4, the relative area of the CH 2 sp 2 peak decreases with increasing RF power with the rate of decline being significantly larger in the C-RT sample set. This is consistent with the ion/radical energy argument made above. For both the C-RT and A-RT sample sets, the ion/radical energy will increase as the RF power is increased. However, this increase is significantly larger for ions/radicals impinging on the cathode. Thus it would be expected that the percent of hydrogen (at. %) in the film would decrease for both sample sets with increasing RF power, with the rate of decrease being significantly larger for the C-RT sample set.

110 90 This relationship between ion/radical energy and hydrogen (at. %) in the film can help explain the trends in percent of sp 3 -bonded carbon atoms (%sp 3 bonding) presented in Section 4.5. For both the A-RT and C-RT sample sets, the trends in %sp 3 bonding followed the same trends as the hydrogen content (at. %). As the RF power increased, the %sp 3 bonding in both sample sets decreased, with the rate of decline in %sp 3 bonding being significantly greater for the C-RT sample set than the A-RT sample set. The coupling between the hydrogen content (at. %) and the %sp 3 bonding in the film occurs due to the fact that the H-C sp 3 bond is a lower energy bond for hydrogen in a-c than the H-C sp 2 bond. Thus a high hydrogen content (at. %) will promote sp 3 bonding in a-c. This type of a-c film is classified as polymeric-like hydrogenated amorphous carbon (PLC:H) because the high hydrogen content (at.%) causes most of the available carbon sp 3 bonds to be filled with hydrogen, leading to the microstructure of the film being a low density network of H-C sp 3 clusters. As the mean ion/radical energy rises, the hydrogen content (at.%) begins to decline due to more ions/radicals impinging on the film with energies greater than the displacement energy of hydrogen. Due to the relationship between hydrogen content (at. %) and %sp 3 bonding in the film, this decline in hydrogen content (at. %) will lead to a decrease in %sp 3 bonding in the film. In addition to reducing the hydrogen content (at. %), as the mean ion/radical energy rises further, some ions/radicals will have enough energy to overcome the penetration threshold of the film (32eV). Any excess energy that these ions/radicals possess above the penetration threshold is transferred to thermal energy to the film. This thermal energy can be used to relax C-C sp 3 bonds to the more stable C-C sp 2 configuration [28], leading to a further decrease in %sp 3 bonding in the film. This relationship between %sp 3 bonding and ion/radical energy is demonstrated in the results shown in Figure 5.4. As can be seen from the figure, for the C-RT sample set, the relationship between %sp 3 bonding and RF power appears to have two distinct regions. For low RF powers (<20W), the C-RT sample set appears to be following a similar relationship to the A-RT sample set, but at higher RF power (>20W), the decline in %sp 3 bonding in the C-RT sample set begins to decline more rapidly. These two regions can be attributed to the two processes explained above. At low RF powers (i.e. low ion/radical energies), ions/radicals impinging on the film

111 91 have only have enough energy to displace hydrogen from the film and thus indirectly decrease the %sp 3 bonding in the film. At higher RF powers (i.e. high ion/radical energies), ions/radicals impinging on the film continue to displace hydrogen from the film but in addition ions/radicals are now able to overcome the surface penetration threshold of a-c whereby their excess energy transforms C-C sp 3 bonds to more stable C-C sp 2 bonds The reason why the A-RT sample set never appears to enter this second region is due to the fact that the ion/radical energies rise more rapidly on the cathode than on the anode with increasing RF power. Therefore, even at high RF power (>20W), the ions/radicals impinging on samples held on the anode do not have enough energy to penetrate the film and transform C-C sp 3 bonds to C-C sp 2 bonds %sp 3 bonding hydrogen film penetration & hydrogen displacement C-RT A-RT RF Power (W) Figure 5.4: Relationship between %sp 3 bonding and RF power for C-RT and A-RT sample sets. The curves are a guide to the eye. Note that for the C-RT sample set, there appears to be two distinct regions in the relationship; one at low power (<20W) in which only hydrogen displacement is occurring and one at higher powers (>20W) in which both hydrogen displacement and film penetration is occurring. For the A-RT sample set, due to the weaker relationship between RF power and ion/radical energy, only the hydrogen displacement region is apparent.

112 92 It is important to emphasize here that the relationship between hydrogen content (at. %) and %sp 3 bonding in a-c is concurrent and dynamic in nature as both of these film properties are directly and indirectly related to film growth conditions such as ion/radical energy A-20 sample set Growth Rate: As presented in Section 4.2, there was a strong dependence between the growth rate and the substrate temperature for the A-20 sample set. It was observed that as the substrate temperature was increased, the growth rate of the film dropped significantly. This phenomenon has been observed by other researchers [69; 70; 71], and has been attributed to a temperature dependent etching of the growing a-c film [28]. Hydrogen radicals (i.e. atomic hydrogen) are highly reactive and can act as an etchant for a-c when it impinges on the film. It has been reported, that this etch rate increases with increasing temperature [28]. Therefore, even though the processes involved in the growth of a-c film are independent of temperature, the temperature dependent etching of the film causes increasing substrate temperature to have a net-negative effect on the growth rate of the film. This is illustrated in Figure 5.5 [28].

113 93 Figure 5.5: Temperature dependent etching processes creating a net negative effect of substrate temperature on growth rate of a-c films. Micostructure: As presented in Section 4.4, no trend can be seen between the hydrogen concentration and the substrate temperature for the A-20 sample set. Thus it is necessary to look beyond hydrogen concentration and density measurements in order to gain insight into the relationship between substrate temperature and the percent of hydrogen (at. %) in the film. Following the rationale provided in Section for the A-RT sample set, the percent of hydrogen (at. %) in the film can be extracted by examining the ratio of the C-H peaks in the infrared absorption spectra. In Section 4.4, it was shown that the relative area of the CH 2 sp 2 peak declined with increasing substrate temperature. This indicates that the percent of hydrogen (at. %) in the film also declined with increasing temperature.

114 94 This relationship between hydrogen content (at. %) with substrate temperature can be understood through the relatively low displacement energy of hydrogen in a-c, 2.5eV. Due to the low displacement energy of hydrogen, even at low temperatures, some of the hydrogen atoms in the a-c film will acquire enough thermal energy to dissociate from their carbon host, diffuse, and desorb from the film. As the substrate temperature is increased, the proportion of hydrogen atoms that acquire this threshold energy will increase and the consequently the hydrogen content (at. %) in the film will decrease. This relationship between hydrogen content (at.%) and substrate temperature is partially responsible for the decreasing trend in the percent of sp 3 -bonded carbon atoms (%sp 3 bonding) in the film with increasing substrate temperature presented in Section 4.5. Due to the promotion of sp 3 bonding by hydrogen, as the hydrogen content (at. %) in the film decreases, there is a tendency for the %sp 3 bonding in the film to also decrease. However, in addition to this process, as the substrate temperature is increased, more thermal energy is provided to the growing film which can be used to relax sp 3 bonded carbon atoms to the more stable sp 2 bond. Thus it is likely that a combination of these two processes is responsible for the decrease in %sp 3 bonding in the film with increasing substrate temperature C-N and A-N Sample Sets The C-N and A-N sample sets represent the two sample sets with nitrogen incorporation. The only differences in the deposition conditions between these two sample sets were the placement of the substrate and the RF power used. For the C-N sample set, the substrate was placed on the cathode and an RF power of 5W was used, while for the A-N sample set the substrate was placed on the anode and an RF power of 20W was used. These RF powers were chosen such that intrinsic films deposited under the respective conditions had similar optical and structural properties. Due to the strong dependence of ion energy on the structural properties of the film (eg. hydrogen content, %sp 3 bonding, etc.), it can be assumed that the only significant difference in the deposition conditions between these two sample sets lies in the plasma species/density. Therefore, by analyzing the differences in the trends between these two sample sets, the dependence of the type and density of species in the plasma on the nitrogen content and doping efficiency can be found.

115 95 Growth Rate: As presented in Section 4.2, the trend in growth rate with increasing N 2 partial pressure was quite different for the C-N and A-N sample sets. As can be seen in Figure 5.6, even with no N 2 in the source gas, the growth rate in the A-N sample set was significantly higher than the C-N sample set. This is to be expected due to the higher RF power and thus higher plasma density used in the A-N sample set. As the N 2 partial pressure was increased, the growth rate in the A-N sample set increased while the growth rate in the C-N sample set stayed near-constant. This suggests that the higher RF power used in the A-N sample set is playing a role in determining the effect of N 2 partial pressure on the growth rate of the film. This can be explained by the fact that at a higher RF power, the types and density of nitrogen ions and radicals will increase. The reactive species produced at this higher RF power can provide additional bonding possibilities for the growing a- C film and thus increase the growth rate. Therefore, increasing the N 2 partial pressure can increase the growth rate of the film, however, a threshold RF power is needed in order to generate the necessary reactive species that will allow this effect to be seen. 7 Growth Rate (nm/min) A-N C-N N 2 in Mixing Bottle (% pressure) Figure 5.6: Relationship between growth rate and N 2 partial pressure for A-N and C-N sample sets. The curves are intended only as a guide to the eye.

116 96 The small drop-off in growth rate seen in the C-N sample set as the N 2 partial pressure is increased to 50% can be attributed to a change in the microstructure that was necessitated by the increase in nitrogen content (at. %) in the film. This change in microstructure was confirmed from AR-XPS measurements which showed a significant decrease in film density as the N 2 partial pressure was increased to 50% in this sample set. As discussed in Section 5.3, this change in microstructure is consistent with the trends in optical properties seen in this sample set. Microstructure & Electronic Structure: As presented in Section 4.3, the nitrogen content for both the C-N and A-N sample sets increased with increasing levels of N 2 partial pressure in the source gas. The results are repeated in Figure 5.7 for convenience. As can be seen from the figure, the rate of increase for the A-N sample set was significantly higher than the C-N sample set. This difference can be explained through the plasma species/density. The A-N sample set used a higher RF power than the C-N sample. This higher RF power would lead to an increase in the plasma density and the introduction of new reactive molecular nitrogen species in the plasma. With an increase in the density and types of nitrogen reactive species in the plasma, the likelihood of nitrogen species impinging and bonding on the growing film will increase and thus the nitrogen content in the film will also increase N Content (at. %) % 10% 20% 30% 40% 50% 60% N 2 Partial Pressure (%) A-N Sample Set C-N Sample Set Figure 5.7: Increasing N content (at. %) with increasing N 2 Partial Pressure (%) for A-N and C-N sample sets.

117 97 The change in electronic structure has showed a dependency on the plasma density/species. From the results presented in Section 4.7, it is evident that for the same percent of N 2 in the source gas, the charge-corrected N1s shifts and thus the Fermi level shifts were larger in the A-N sample set than the C-N sample set however; this is at least partially due to the different nitrogen content levels in these films. In order to more effectively compare these two sample sets, the relationship between charge-corrected N1s shifts with nitrogen content needs to be compared. Figure 5.8 illustrates this relationship for the two sample sets. As can be seen from the figure, the C-N sample set shows a near-linear relationship between nitrogen content and the chargecorrected N1s shifts, while the A-N sample set shows a near-logarithmic relationship Charge-Corrected N1s Shift (ev) C-N Sample Set A-N Sample Set Nitrogen Content (at. %) Figure 5.8: Relationship between charge-corrected N1s shifts and nitrogen content for C-N and A-N sample sets. The curves are a guide to the eye. Note that charge-corrected N1s shifts can be taken as a qualitative measure of shifts in the Fermi level. An increase in the charge-corrected N1s peak would represent an upward shift of the Fermi level toward the conduction band.

118 98 As explained in Section 3.6.4, charge-corrected N1s shifts can be taken as a qualitative measure of shifts in the Fermi level with an increase in the charge-corrected N1s peak representing a shift in the Fermi level toward the conduction band. Thus the differences shown in Figure 5.8 represent differences in doping efficiencies between the two sample sets. This difference in doping efficiency can be explained through the different nitrogen species present in the plasma of the two sample sets. As shown in Figure 5.9, only particular carbon-nitrogen bonding configurations are potential doping configurations. One of the factors which determines the carbon-nitrogen bonding configuration in the film is the type of molecular species in the plasma. Of course, the types and density of reactive molecular species in the plasma are dependent on the RF power used during deposition. The higher doping efficiency in the A-N sample set indicates that the higher RF power used in this sample set increased the density of molecular species necessary for carbon-nitrogen bonding in potential doping configurations. Figure 5.9: Potential bonding configurations between nitrogen and carbon. Doping is only possible in the configurations shown in (b), (e), and (h). Figure taken from [72]. It should also be noted that that although the microstructure of the intrinsic films formed in the A-N and C-N sample sets are similar they are not identical. Thus it is also possible that microstructure of films in the A-N sample set are more susceptible to doping than films in the C-

119 99 N sample set. Without having identical microstructures, it is not possible to know for certain how much of a role the plasma species/density is having on the doping efficiency of the film.

120 Relationship between film microstructure and optical properties In the previous section, trends in growth rate, microstructure, and electronic properties were analyzed in order to define relationships between these properties and the deposition conditions of the film. In this section, the relationship between the microstructure and optical properties of a-c films is explored. This relationship can be best explained through Robertson s model of a-c films [28; 38]. Robertson describes the microstructure of a-c as a continuous network of sp 3 bonded carbon atoms with the sp 2 bonded carbon atoms forming small localized clusters that lie within this network. As discussed in the previous section, the proportion of the film composed by these sp 2 clusters can vary depending on film growth conditions. The quantity and size of the sp 2 clusters play an important role in determining the film s optical properties. Carbon bonded in the sp 3 configuration makes four strong σ-bonds with its bonding partners, while carbon bonded in the sp 2 configuration makes three strong σ-bonds and one weak π-bond with its bonding partners. The σ-bonds form occupied σ-states in the valence band and empty σ * -states in the conduction band, separated by a large σ- σ * gap. Similarly, the π-bonds from the sp 2 bonding form occupied π-states in the valence band and empty π * -states in the conduction band, but separated by a smaller π - π * gap [1; 28]. This is illustrated in the simplified density of states shown in Figure For comparison, an experimentally measured density of states of the valence band of an a-c sample is shown in Figure 5.11 [28]. Figure 5.10: Simplified diagram of the density of states in a-c films.

121 101 Figure 5.11: Photoemission spectra of a-c measured through Ultraviolet Photoelectron Spectroscopy (UPS). The vertical axis represents the photoemission counts measured through UPS and provides a qualitative assessment of the density of states in the valence band with the Fermi level lying at 0eV. Note that the peak of the π-band lies closer to the Fermi level than the peak of the σ-band. Taken from [28]. Due to their smaller gap, the π - π * states create the dominant band gap in a-c. Therefore, the quantity and size of the sp 2 clusters in the a-c microstructure will determine the optical properties of the film. This relationship between the sp 2 content and the optical properties of the film is illustrated in Figure 5.12 for the C-RT sample set. As can be seen, both the E 04 gap and refractive index show a strong dependency with the sp 2 content as the E 04 optical gap monotonically decreases and refractive index monotonically increases with increasing sp 2 bonding in the film. It is also interesting to note that the E 04 gap and refractive index show an S-shape dependency with the sp 2 content with the peak dependency occurring at around 50% sp 2 bonding.

122 E 04 Gap (ev) nm %sp 2 content Figure 5.12: Relationship between %sp 2 bonding with E 04 gap (black) and refractive index at 350nm (grey) for C-RT sample set. The curves are a guide for the eye. The dependency between sp 2 content and the optical properties of a-c films is also seen in the A- RT and A-20 sample sets. Figure 5.13 and Figure 5.14 show the dependence of the E 04 gap and refractive index on the sp 2 content for the A-RT and A-20 sample sets, respectively. As seen from the figures, for both sample sets, the E 04 gap decreases and the refractive index increases with increasing sp 2 content. It should be noted that the refractive indices displayed in Figures 5.12 to 5.14 are for a wavelength of 350nm. The choice of this wavelength was based on the fact that for the films with E 04 gaps above 3eV, the refractive index only showed appreciable changes at wavelengths below 400nm. Therefore in order to show the dependence between sp 2 content and the refractive index for all the films, it was necessary to show the refractive index at a wavelength below 400nm.

123 E 04 Gap (ev) nm %sp 2 bonding 1.6 Figure 5.13: Relationship between %sp 2 bonding with E 04 gap (black) and refractive index at 350nm (grey) for A-RT sample set. The curves are a guide for the eye E 04 Gap (ev) nm %sp 2 bonding 1.55 Figure 5.14: Relationship between %sp 2 bonding with E 04 gap (black) and refractive index at 350nm (grey) for A-20 sample set. The curves are a guide for the eye.

124 104 Due to difficulties associated with overlapping nitrogen and carbon modes in the XAES spectra, %sp 2 bonding measurements were not taken on the C-N and A-N sample sets. Thus the relationship between %sp 2 bonding with the optical properties of the film cannot be directly verified for these sample sets. However, as illustrated in Figure 5.15, it is apparent that there is a strong relationship between the E 04 optical gap and the nitrogen content (at. %) of the films. For both sample sets, the E 04 optical gap decreases with increasing nitrogen content. This relationship is expected since increasing the nitrogen content will lead to an increase in the density of donor states that lie within the band gap and thus lower the E 04 optical gap in the film. It should also be noted that for the C-N sample set, when the nitrogen content is increased to 9% the trend in declining E 04 gap is broken and the E 04 gap shows a slight increase. This change suggests a change in microstructure of the film related to the increase in nitrogen content. This is consistent with the growth rate measurements analyzed in Section 5.2.3, where the C-N sample set showed a sudden decrease in growth rate when the nitrogen content was increased beyond 9%. The fact that no discontinuity in the trends in growth rate or E 04 gap are seen for the A-N sample set suggests the microstructure of intrinsic films in this sample set have a greater capacity for nitrogen incorporation and thus do not require a change in microstructure as the nitrogen content is increased.

125 105 E 04 Gap (ev) C-N A-N Nitrogen Incorporation (at. %) Figure 5.15: Relationship between E 04 gap and nitrogen content for C-N and A-N sample sets. Note for both sample sets the refractive index remained constant at 1.6. For visual clarity the refractive index curves were not included in the figure. The curves are a guide for the eye.

126 106 6 Applications: Transparent Heat Mirror 6.1 Overview In the previous sections, it was shown that the a-c films show a wide range in optical properties which can be precisely tuned through film growth conditions. In addition, it was shown that these films can be moderately doped even at room-temperature deposition. These properties coupled with the superior mechanical properties of a-c make these films appealing for a number of optical applications including: anti-reflection coating for Si photovoltaic (PV) cells, window layer for thin-film heterojunction PV cells, and low-absorption surface passivation layer for c-si wafers. One of the more interesting applications of a-c which has yet to be explored in depth is the use of a-c in a multi-layer transparent heat mirror (THM) coating. In the following section, an overview of THM coatings is provided and the application of a-c films in these coatings is discussed. In addition, results for initial THM designs are provided and an outlook for future work is given.

127 Background A transparent heat mirror (THM) coating is an optically selective coating which is transparent to solar radiation but reflective for mid-infrared radiation. These coatings are used in a number of applications with the most common being high-efficiency, low-emissive windows for residential and commercial buildings. These low-emissive windows have been shown to significantly reduce heating loads in homes and buildings by suppressing radiative thermal losses [73; 74]. The key in understanding THM coatings is to note that the wavelength spectrum emitted by the sun, which can be approximated by a blackbody radiator at T~5780K, is different from the spectrum emitted by an object at room temperature, which can be approximated by a blackbody radiator at T~300K. This is illustrated in Figure 6.1. Notice that the peak of the solar spectrum is between 400nm-900nm, where as the peak of the spectrum of a blackbody radiator at 300K is between 5μm-20μm. Therefore, an effective THM coating can be made by designing it such that it has a high transmittance from 400nm-900nm and a high reflectance from 5μm-20μm. Figure 6.1: Normalized spectral emissive power of a blackbody 5780K and blackbody 300K. Note that the solar spectrum can be approximated by a blackbody 5780K.

128 108 One method of designing a THM is to sandwich a thin-layer of a transition metal (eg. Au, Ag, Cu, etc.) between films with low absorption coefficients (low-k) and appropriate refractive indices. This is depicted in Figure 6.2. The thin-metallic layer provides the required reflection in the mid-infrared while the low-k layers act as anti-reflective layers thereby enhancing the transmission in the solar region. Additional requirements of the low-k film are that it be nonabsorbing in the mid-infrared to allow reflection from the thin-metallic layer and also have a high hardness so that it can prevent scratches and oxidation to the metallic film. Figure 6.2: Overview of multi-layer transparent heat mirror design. Since most wide band gap materials cannot satisfy all the specified requirements, typically multiple layers of different materials are used. For example, the state-of-the-art THM coating developed by Martin-Palma et al. uses a five-layer SnO 2 (38nm)/NiCr (1nm)/Ag (9nm)/NiCr (3nm)/Sn0 2 (38nm) structure [75]. In this structure, the Ag layer provides the reflection in the mid-infrared while the NiCr layers act as an oxygen-free protective layer for the Ag and the Sn0 2 layers act as anti-reflective layers to maximize transmissivity in the solar region. The NiCr and SnO 2 layers also provide a small boost to mid-infrared reflection of this structure. The transmission and reflection spectra for this structure are shown in Figure 6.3 and Figure 6.4.