Electrical characterization of impurity-defect complexes in silicon

Transcription

1 Electrical characterization of impurity-defect complexes in silicon Naveen Goud Ganagona Department of Physics University of Oslo A thesis submitted for the degree of PhilosophiæDoctor (PhD) 2014 November

2 Naveen Goud Ganagona, 2015 Series of dissertations submitted to the Faculty of Mathematics and Natural Sciences, University of Oslo No ISSN All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission. Cover: Hanne Baadsgaard Utigard. Printed in Norway: AIT Oslo AS. Produced in co-operation with Akademika Publishing. The thesis is produced by Akademika Publishing merely in connection with the thesis defence. Kindly direct all inquiries regarding the thesis to the copyright holder or the unit which grants the doctorate.

3 Abstract Thermal evolution of impurity-defect complexes in proton-irradiated mono-crystalline silicon material has been investigated by deep level transient spectroscopy (DLTS). Especially, the interaction between common impurities such as oxygen, carbon and hydrogen, and intrinsic defects is addressed. Oxygen and carbon are introduced during the materials growth while hydrogen occurs during device processing of Si wafers, like fabrication of cells. The defect dynamics was investigated by post-irradiation annealing of the samples, isochronally or isothermally, at increasingly higher temperatures up to 475 C while measuring the concentration of the various electrically active defects. Firstly, the kinetics of the transition from divacancy (V 2 ) centers to divacancyoxygen (V 2 O) pairs was studied. A simultaneous transition of the donor and acceptor states of V 2 to those of V 2 O was established by applying optical DLTS together with ordinary DLTS measurements. An experimental value for the diffusivity of V 2 in the neutral charge state has been deduced from the isothermal annealing data and the results seem to favor partial dissociation of V 2 as the predominant migration mechanism and may challenging an one-stage mechanism proposed in the literature. Further, firm evidence for the identification of trivacancy (V 3 ) and trivacancy-oxygen (V 3 O) deep levels in the bandgap has been established, enabling data on the formation and annealing kinetics of these complexes in both n- and p-type samples. In particular, the formation kinetics of V 3 O has been studied in detail and experimental values of the migration energy and diffusivity pre-factor of V 3 have been determined. Secondly, the annealing of the interstitial carbon-interstitial oxygen (C i O i )pairs has been studied, and it is concluded that a dissociation is the prevailing mechanism. The binding energy between C i and O i is estimated to be 1.7 ev. A correlated growth of a defect level at 0.39 ev above the valence band edge (E v ) was observed with the iii

4 disappearance of C i O i. Correlation with data from Photoluminescence spectroscopy (PL) measurements on similar samples suggested that the defect may be a interstitial carbon-interstitial oxygen dimer (C i O 2i ) complex. Finally, an attempt to study vacancy-hydrogen related defects was made using hydrogen implanted p-type samples and tentative evidence for a hydrogen-related deep level center with an acceptor state located 0.45 ev below the conduction band edge was formed. iv

5 Dedicated to my parents...

6 Acknowledgements I would like to take the opportunity to thank everyone who supported me to accomplish this work. First of all, I would like to express my sincere gratitude to my main supervisor Prof. Edouard Monakhov for giving me this opportunity and for guiding me through many research and scientific problems. Thank you for your patience and effort in proof-reading the articles and your feedback has been helped me to improve my writing skills. Secondly, a large thank you to my co-supervisor Prof. Bengt Svensson for his extraordinary supervision, interesting discussions and shaping our articles into high quality ones. Thirdly, thanks to my co-supervisor Dr. Lasse Vines for his hands on guidance, countless day to day discussions and initial corrections to articles. I would like to thank all the colleagues of the MiNa-Lab for their cooperation and discussions. Thanks to E-lab colleagues, Chi-kwong Tang, Vincent Quemener and Helge Malmbekk, for their assistance, discussions on DLTS and creating good working environment. Special thanks to Bahman Raeissi and Augustinas for PL measurements. My office mates Bhoodoo and Per Lindberg deserve a special mention for creating a friendly environment and outings. I would like to thank our lab engineers, Viktor Bobal and Mikeal Sjodin, for their assistance especially Viktor for performing all ion-implantations in this work. I would also like to thank Vishnukanthan and Raja for their wonderful company and making life comfortable in Oslo. I also must thank my friends Satish, Vikas, Varsha and Amul Anganti for sharing personal stuff and joyful trips.

7 Finally, I would like to thank all my family members who supported me during this period, especially my mom (Vijaya) and brothers (GLN goud and Praveen). This work would not have completed without the constant support and companionship of my beloved wife (Navya) and I would never ever enough thank her, love you forever!

8 iv

9 List of included papers I. Formation of donor and acceptor states of the divacancy-oxygen centre in p-type Cz-silicon N. Ganagona, B. Raeissi, L. Vines, E.V. Monakhov and B.G. Svensson Journal of Physics: Condens. Matter 24, (2012) II. Transformation of divacancies to divacancy-oxygen pairs in p-type Czochralski-silicon; mechanism of divacancy diffusion N. Ganagona, L. Vines, E.V. Monakhov and B.G. Svensson Journal of applied Physics 115, (2014) III. Formation of single and double donor states of trivacancy-oxygen complexes in p-type silicon N. Ganagona, L. Vines, E.V. Monakhov and B.G. Svensson Solid State Phenom , pp (2013) IV. Formation kinetics of trivacancy-oxygen pairs in Silicon N. Ganagona, L. Vines, E.V. Monakhov and B.G. Svensson Journal of applied Physics 116, (2014) V. Identification of the carbon-dioxygen complex in silicon N. Ganagona, L. Vines, E.V. Monakhov and B.G. Svensson In manuscript (2014) v

10 VI. Defects in p-type Cz-silicon irradiated at elevated temperatures N. Ganagona, B. Raeissi, L. Vines, E.V. Monakhov and B.G. Svensson Physica Status Solidi C 9, No. 1011, (2012) VII. PL and DLTS analysis of carbon-related centers in irradiated ptype Cz-Si B. Raeissi, N. Ganagona, A. Galeckas, E.V. Monakhov and B.G. Svensson Solid State Phenomena Vols (2014) pp vi

11 Contents 1 Introduction Introduction Basic concepts in semiconductors Properties of semiconductors Point defects Electrical properties of defects Defect Annealing P-N junction Experimental methods Capacitance-voltage measurements Deep Level Transient Spectroscopy (DLTS) DLTS spectra Optical-DLTS and MCTS Photoluminescence (PL) Ion Implantation Sample preparation Results and Discussion The divacancy and divacancy-oxygen complexes The divacancy (V 2 ) The divacancy-oxygen complex (V 2 O) Diffusion mechanism of V The trivacancy and trivacancy-oxygen complexes Carbon-oxygen complexes vii

12 CONTENTS 4.4 Hydrogen related complexes Conclusions and suggestions for future work 47 References 49 viii

13 1 CONTENTS

14 CONTENTS 2

15 Chapter 1 Introduction 1.1 Introduction The discovery of the transistor by Bardeen, Brattain and Schockley in 1947, triggered a revolution in semiconductor electronics. During the last 60 years there has been an exponential expansion both in development and use of semiconductor devices. Semiconductors are omnipresent in applications such as computers, satellites, solar cells and indeed in general electronics applications. Among the different semiconductors, Silicon (Si) has been the dominant material for the semiconductor industry due to its abundance, cheaper to produce silicon based devices than other materials and superior processing properties. However, several new materials replaced Si in some of the areas, due to its limitations in certain applications. Especially, it is not well suited in (i) optical applications as it has and indirect bandgap, (ii) high power and (iii) in high frequency applications. The recent growth in the photo-voltaic (PV) industry together with the need for specialized applications like radiation hard detectors, e.g. in the Large Hadron Collider, has lead to renewed efforts in basic research in Si. Interestingly, while electronic industry has been thriving reducing the size of devices, the PV industry has focused on reducing the material cost and increasing the efficiency. Since the production of high quality Si is expensive, a low cost processes have been developed to produce low quality Si often called as solar grade Si. This creates a situation where a precise control of impurities and material quality is decisive. The material properties are very sensitive to the atomic imperfections in the crystal, so called point defects which are present in the material. The main 3

16 1. INTRODUCTION sources for the point defects are: non-ideal material growth, high temperature processing steps during device fabrication, doping techniques e.g. implantation, spin-on-diffusion and laser doping, and/or irradiation. These defects are a serious concern in most of the applications as they act as a recombination centers deteriorating the device performance, e.g reducing the carrier lifetime in photovoltaic applications. Defects are much more pronounced when the material is exposed to radiation. For example, radiation damage is a major concern in certain area of semiconductor applications; including device operation in a space and for particle detectors in high energy experiments. On the other hand, irradiation is a perfect tool in defect studies, since they can be created and the concentration can be tuned in accordance with the limitations of the characterization technique. Impurities present in the material can interact with intrinsic defects which result in higher order complexes, crucial for device operation. As such, high quality material is generally a good starting point to investigate point defect reactions, given that the defects can be introduced in a controllable and reproducible way. Carbon (C) and oxygen (O) are the most common and dominant impurities in Si, and are introduce during the growth. C and O are normally found on substitutional and interstitial site, respectively, and natively do not influence the electronic properties of the material since they are electrically inactive. However, O i and C s can form a wide variety of electrically active complexes with other intrinsic defects such as vacancies and interstitials or together with other impurities, e.g. thermal double donors. Hydrogen (H) is another common impurity introduced during processing of Si wafers. H is known to passivate different types of electrically active centers in Si, making it an important impurity in multi and single crystalline silicon. However, theoretical and experimental studies show that H can also form electrically active complexes from the interaction with vacancy related defects. In this thesis, thermal evolution of multivacancy-oxygen complexes and impurity related defects (O,C and H) has been investigated using, mainly, the Deep level transient spectroscopy (DLTS) technique. In order to get more information on some of the defects also Photoluminescence (PL) spectroscopy has been employed. The thesis is organized as follows: Chapter 2 introduces the basics for understanding point defects and semiconductors relevant to the present study. 4

17 1.1 Introduction Chapter 3 describes the experimental techniques used in the work. Chapter 4 summarizes the work and the main experimental results are disseminated in Papers I - VII. In particular, the results reported in Papers I and II focus on the divacancy related defects. Papers III and IV point out the annealing results for trivacancy and related complexes. The results reported in Papers V, VI and VII focus on the investigation of annealing behavior of carbon-oxygen complexes and formation of carbon-dioxygen complexes. Interaction of hydrogen with irradiation induced defects will be discussed in section

18 1. INTRODUCTION 6

19 Chapter 2 Basic concepts in semiconductors This chapter introduces a brief overview of the fundamentals of semiconductor physics to better understand the experimental methods and the results described in subsequent chapters and attached papers. 2.1 Properties of semiconductors Electrons in an isolated single atom can occupy discrete energy levels. In solids, a large number atoms are brought close together, discrete energy levels merge into so called energy bands which contains positioned packed energy levels.[1] The occupancy of energy states by electrons is given by the Fermi-Dirac distribution 1 F (E) = where E 1+exp[(E E f )/k B T ] f is the Fermi level, k B Boltzmann constant and T is the absolute temperature. [2] The energy bands are filled by charge carriers or empty at sufficiently low temperatures. The uppermost energy band occupied by electrons is called valence band (E v ) while the lowermost unoccupied band is referred to as conduction band (E c ). Electrons in the conduction band are free to move in response to an applied electric field and, thus, transport current. Solids can be classified into three types depending on their ability to conduct an electric current and they are: insulators, metals and semiconductors. In semiconductors, the valence band and conduction band are separated by a band gap (E g ) which is free of energy levels in an ideal materials. Electron can be thermally excited to the conduction band and the empty state, hole, left in the valence band can also move, and lead to electrical conduction in semiconductors. In insulators, valence and conduction bands are separated by a large band gap and no availability of electrons even with an applied electric field. In metals, valence bands is overlap 7

20 2. BASIC CONCEPTS IN SEMICONDUCTORS with conduction bands and free electrons are readily available. Conductivity of semiconductors can be greatly enhanced by so-called doping in which specific impurities are introduced in order to control the type of doping. By doping the semiconductor with an impurity that donates an electron to the conduction band, the material will have excess of electrons, and is called as n-type; while material with excess of holes is referred to as p-type semiconductor. 2.2 Point defects In an ideal crystal structure atoms are ordered periodically in a lattice. However, defects always occur in the crystal. A point defect is an imperfection in a crystal, basically characterized by one unoccupied lattice position or one interstitial atom, molecule, or ion. Point defects exist in thermal equilibrium, but can in addition be introduced during growth and processing. Point defects can be accumulated under irradiation and/or implantation. The following section mainly focuses on irradiation induced point defects. Point defects by irradiation When a semiconductor is subjected to irradiation with sufficient energy the elastic collisions between the energetic particle and a native lattice atom can result in a displacement of the atom out of its lattice site. As shown in Fig 2.1a, an extra atom placed between the atoms in the lattice (interstitial (I)) and a vacant lattice site atom (a vacancy (V)) together known as a Frenkel pair. If the transferred energy between the particle and the atom is above a threshold value, it results in a generation of a Frenkel pair. The threshold energy for defect production is defined as the minimum energy to produce a Frenkel pair. For silicon, it is around 15 ev.[3] The nature of the radiation damage mechanism depends on the energy and type of particle. In Si, electrons, due to their small mass, need above 250keV in order to transfer an energy equal to the threshold energy. The collision cascades are more pronounced for heavy ions compared to light ions and electron irradiation. An energy of several MeV is needed to create secondary collisions in electron irradiation, while a few hundred kev is needed in the case of proton irradiation which is mainly used in the present work. In secondary collision cascades, nearby 8

21 2.3 Electrical properties of defects Figure 2.1: Frenkel pair formation a), annihilation b), and impurity interstitial c) vacancies can pair together to form divacancy (V 2 )[4], trivacancy (V 3 )[5] etc. In Si, only a few percent of the irradiation-induced vacancies and interstitials survive as they annihilate themselves, as shown in Fig 2.1b.[6] The survived V s and I s are highly mobile at room temperature and migrate through the lattice, and can react with other impurities (X) that are present in the material, for example X + I X i, as illustrated in Fig 2.1c. The resulting defects are crucial since electrical properties of the material strongly depend on the defects. The electrical properties of the defects will be discussed in the following section. 2.3 Electrical properties of defects The electrical and optical properties of a semiconductor can be greatly influenced by the presence of defects. In a perfect semiconductor, no energy levels exist in the band gap. However, the presence of defects and dopants can introduce unwanted energy levels. Defects can be categorized as either electrically active or inactive. Electrically active defects form states in the forbidden band gap and 9

22 2. BASIC CONCEPTS IN SEMICONDUCTORS can emit/trap electrons to/from the conduction band or valence band. If the trap is negatively charged after it captures an electron then it is referred to as an acceptor. If the trap is positively charged after emitting an electron then the state is called as a donor state. Electrically active defects have different characteristics depending on their level position in the band gap and they can be either shallow or deep levels. Shallow states are located close to the valence band or conduction band, which are generally used for doping, while deep states are usually further away from the band edges and act as generation/recombination centers. Emission and capture Consider a point defect with an energy level (E t ) in the band gap and uniformly distributed with a concentration,n t, throughout the semiconductor. Charge carriers, i.e, electrons and holes in the conduction and valence band respectively, can interact with the defect state resulting in a filling and emptying of the defect state. According to Shockley-Read-Hall theory, this is a statistical process.[7, 8] The possible transitions are shown in Fig 2.2. The rate of capture of electrons and holes is c n n and c p p (c n /c p are capture coefficient for electrons/holes), respectively, while the rate of emission for electrons and holes from a defect state is e n and e p, respectively, where n and p are the concentration of electrons in the conduction band and holes in the valence band, respectively. Taking all the reactions into account, the change in the fraction of the state occupied by carriers, n t, as function of time, t, is given by dn t dt =(c nn + e p )(N t n t ) (c p p + e n )n t (2.1) The capture rate can be defined as c n,p = σ n,p ν th(n,p) (2.2) where σ n,p is the capture cross section of an electron or a hole and ν th(n,p) is the thermal velocity. The thermal electron (hole) velocity is defined as ν th(n,p) = 3kB T,wherem m n,p n(p) is the effective electron (hole) mass and T is the absolute temperature. Further, the emission rate for electrons ans holes from a level can expressed by the following equation [9]: e n,p(t )=ν th(n,p) g 0 g 1 σ n,p N c,v e ΔG k B T (2.3) 10

23 2.4 Defect Annealing Figure 2.2: Emission and capture transitions mechanism of a defect level where N c,v is the effective density of states in the conduction/valence band g 1 is the degeneracy of an occupied state, g 0 is the degeneracy factor of an nonoccupied state, ΔG= ΔH -TΔS is Gibbs free energy of the defect state, ΔH is the enthalpy, and ΔS is the entropy. Depending on the values of the emission and capture coefficients, different transitions will dominate, as shown in Fig 2.2. It is also useful to define another parameter, apparent capture cross section (σ app ): σ app = σ n e ΔS k B (2.4) By measuring the emission rate as a function of temperature T, we can extract the activation enthalpy and capture cross section from the Arrhenius plot of ln(e n /T 2 ) versus 1/T. ΔH is commonly interpreted as ΔG ignoring the entropy factor in DLTS measurements (which is mainly used in this thesis). For deep states in the band gap T ΔS is usually assumed to be small, however, this is not necessarily valid in all cases. 2.4 Defect Annealing Defects are generally stable up to a certain temperature range and can undergo several reactions. Information about the defect can be revealed by a careful examination of its annealing behavior, for example using different characterization techniques (such as DLTS, Infrared (IR) absorption, PL etc.). Defect annealing can often be divided into two processes: migration and dissociation. For mi- 11

24 2. BASIC CONCEPTS IN SEMICONDUCTORS gration, the defects become mobile at a certain temperature and migrate until captured at other impurities and/or defects that are present in the material. In this process defects can form new complexes (such as V 2 +O i V 2 O [10]) with higher thermal stability or they can annihilate, for example a vacancy can be eliminated when it meets an interstitial. Dissociation of a defect occurs if the annealing temperature is high enough to overcome the binding energy of the defect, for example C i O i can dissociates into its components C i and O i, when the temperature is above 350 C(C i O i C i +O i ).[11] A first-order annealing kinetics process can be described by [12]: dn = cn (2.5) dt where c is the annealing rate constant and N is the defect concentration. A characteristic of this behavior is that it involves only one process with a rate proportional to the defect concentration. The solution to the above equation is N(t)=N 0 e ct, where N 0 is the initial defect concentration. For a typically thermally activated process the rate constant depends on temperature T and is given by: c(t )=c 0 e Ea k B T (2.6) where E a is the activation energy and c 0 is the frequency factor. From the frequency factor, an indication about the observed process can be obtained, i.e, migration or dissociation. Dissociation typically gives a frequency factor of c 0 k BT while migration usually would result in c h 0 k BT,where k BT h h is the frequency of the phonons in the lattice. (h is Planck constant) [13, 14] Further, a diffusion limited reaction between two defects A and B with a concentration of N A and N B, respectively, can be described by the following differential equation dn A = 4πR(D A + D B )N A N B (2.7) dt where R is the capture radius for the reaction and D A and D B are the diffusion coefficients for defects A and B, respectively. If only one of the two defects are mobile at the temperature of reaction eg., D A D B,thenD=D A +D B D A and D A = DA 0 e Ea k B T,whereDA 0 is the exponential pre-factor. In this case, the time 12

25 2.5 P-N junction evaluation of N A and N B is generally not exponential and so-called second order kinetics occur. 2.5 P-N junction A general overview of pn-junctions will be given in this section which are building blocks for many electronic applications and in understanding of other semiconductor devices. A pn-junction is a two terminal device. We can obtain an asymmetric pn junction when the concentration of the donors side(n D )ismuch more pronounced than the acceptor concentration (N A ) side or vice-versa. In these junctions, charge neutrality implies that the depletion region extends into the low doped region. In this thesis we have used asymmetric junctions in most of the experiments. When two semiconductors, n-type and p-type, are joined together, a large concentration gradient in charge carriers occurs at the junction, which will lead to diffusion. Under thermal equilibrium conditions, highly mobile electrons and holes from the n and p-type regions, respectively, diffuse across the junction. Electrons leave behind positively charged donor atoms, and holes leave behind negatively charged acceptor-atoms, thereby creating a spacechargeregion (W) at the junction (see Fig 2.5). An electric field, built up around the p- and n-region, prevents the further diffusion of charge carriers entering the junction, and W is often referred to as a depletion region. A potential difference built-up around the junction is called built-in potential (V bi ). The potential barrier can be modified by applying an external bias (V) to the junction. The net current will be zero, the drift current is exactly balanced by the diffusion current. Assuming an abrupt junction and by applying the Poissons equation, one can derive the following equation for the depletion width W [2]: [ ] 1/2 2ɛ0 ɛ r (V bi V ) (N a + N d ) W = (2.8) q N a N d where ɛ 0 is the permittivity of free space, ɛ r is the relative permittivity of the semiconductor material, V is the applied voltage and is positive/negative for forward/reverse bias, q is the elementary charge and N a /N d is the doping concentration of acceptors/donors. 13

26 2. BASIC CONCEPTS IN SEMICONDUCTORS Figure 2.3: a) Schematics of p- and n-type semiconductors with energy bands and b) pn-junction with energy bands, depletion region, contact potential and electric field - The capacitance of the depletion region is given by C = ɛ 0ɛ r A W [ ] 1/2 = ɛ q N a N d 0ɛ r A (2.9) 2ɛ 0 ɛ r (V bi V ) (N a + N d ) where A is the area of the junction cross section. For an abrupt n + p junction (N d N a ), the capacitance given in Eq. 2.9 can be reduced to [ ] 1/2 q C = ɛ 0 ɛ r A 2ɛ 0 ɛ r (V bi V ) N a (2.10) 14

27 Chapter 3 Experimental methods The different experimental techniques, Capacitance-voltage measurements (C-V), Deep level transient spectroscopy (DLTS) and Photo-luminescence (PL) spectroscopy, which have been used in this thesis will be discussed in this chapter. 3.1 Capacitance-voltage measurements Capacitance-voltage (C-V) measurements are a technique to characterize a p-n junction or Schottky barrier junction. During the C-V measurements, a small AC voltage typically in the range of mv superimposed on a DC voltage is applied to the structure at typical frequency range of 1 khz - 1 MHz. The resulting charge (Q) variation gives rise to a differential capacitance, which is a measured quantity. The capacitance is defined by: C = dq dv By rewriting the Eq. 2.10, we get (3.1) 1 C = 2 2V bi V +. (3.2) 2 qɛ 0 ɛ r A 2 N A qɛ 0 ɛ r A 2 N A From the above equations, by plotting 1/C 2 versus V, the concentration (in this case N A ) can be extracted from the slope and the extrapolation of 1/C 2 =0 gives V bi. Moreover, from C-V measurements, information about the effective charge carrier concentration as function of depth can be extracted by the following 15

28 3. EXPERIMENTAL METHODS 3 x 1013 Hole concentration [cm 3 ] Depth [μm] Figure 3.1: Charge carrier concentration profile in a p-type Si sample with a Schottky barrier contact of Al. The profile reveals passivation of boron acceptors close to surface. relation. N(W )= 2 ( ) ΔC 2 1. (3.3) qɛ 0 ɛ r A 2 ΔV a Fig 3.1 shows an example from CV measurements, hole concentration versus depth of a Schottky contact on p-type-si. Hydrogen can unintentionally accumulate at the surface of the sample during the Schottky contact formation. Hydrogen can passivate boron acceptors by forming neutral B-H complexes [15], and thus a reduction in the net carrier concentration occurs near the surface, as shown in Fig Deep Level Transient Spectroscopy (DLTS) DLTS is a powerful technique that is widely used to study electrically active defects in semiconductor materials. DLTS was introduced by D.V. Lang in 1974.[16] The principle of DLTS is based on the measurement of capacitance transients due to the emission of charge carriers trapped by a defect in the depletion region of the material. From DLTS data, one can extract the activation enthalpy, apparent capture cross section and concentration of the levels in the bandgap of materials. 16

29 3.2 Deep Level Transient Spectroscopy (DLTS) A general DLTS principle of operation can be explained by using a Schottky or pn-junction. An asymmetrical pn junction using a n-type substrate is shown in Fig 3.2. Consider the junction when a deep donor level with an energy position E t is present. When E t is below the Fermi level, the level is filled with electrons. The junction initially starts with a reverse bias V r, as shown in Fig 3.2. The resulting depletion region has a width of W r and all the traps above the Fermi level are empty while the traps below the Fermi level are filled with electrons. 2ɛ The width λ, which is given by 0ɛ r q 2 N d (E F E t ), is the region between the reverse bias width and crossing of the defect level and the Fermi level. A filling pulse V f, forward bias or rather by decreasing the reverse bias, is then applied for a short amount of time usually a few milliseconds in order to fill the traps. As shown in Fig 3.2, the band bending decreases and traps are moved below the Fermi level and being filled. During this time, the depletion region width reduces to W f and the corresponding capacitance is C 0. When the pulse is removed after a time t p, the reverse bias is returned to its original value and opposite to filling occurs: the traps, which were below the Fermi level, again move above the Fermi level, and will emit their trapped carriers. In the first case immediately after removing the pulse, the depletion width is more than W r and the corresponding capacitance is C r ΔC, ΔC is proportional to the amount of traps. All the filled traps start to emit their carriers and with the time the capacitance approaches its initial reverse bias condition. This gives a capacitance transient which is then recorded. The procedure is repeated several times at each temperature interval in a temperature scan. The capacitance transient in response to the voltage pulsing sequence is measured at each temperature throughout. The temperature interval used in this thesis is K. By assuming that W r is greater than W f and the amount of traps are small ΔC C, the capacitance transient can be expressed as if uniform depth distributions apply for N t and N d. ΔC(t) = C rn t 2N d e ent, (3.4) 17

30 3. EXPERIMENTAL METHODS 1) 2) 3) V f 2 V r C r Figure 3.2: Illustration of DLTS principle of operation with three steps: 1) Reverse bias, 2) filling pulse and 3) charge carrier emission 18

31 3.2 Deep Level Transient Spectroscopy (DLTS) Figure 3.3: DLTS spectrum resulting from the transient capacitance difference measured from two xed times t 1 and t 2 at different temperatures. (box car weighting function) DLTS spectra A DLTS spectrum is a representation of the collection of capacitance transients recorded after each filling pulse. Transients are measured consecutively as a function of time at different temperatures, as shown in Fig 3.3. The DLTS signal can be deduced by the following equation S = 1 t d +t i n i t d ΔC(t)ω(t) (3.5) where n i is the number of measurement points of the capacitance transient for the so-called i th window, t i is the n i τ t d is the delay time and ω(t) isthe specific weighting function, several types of weighing functions are possible where the so-called Box car is illustrated in Fig 3.3. A deep level in the band gap will result in a peak in the DLTS spectrum. Since the emission rate is temperature dependent, at certain T, the DLTS signal reaches a maximum value. By varying the length of time to measure the capacitance transient, the so-called time window, the peak shifts in the temperature scale 19

32 3. EXPERIMENTAL METHODS C i O i ΔC/C rb Temperature [K] 10 2 E t = 0.36 ev σ na = cm 2 e n /T 2 (s 1 K 2 ) /T [1/K] x 10 3 Figure 3.4: (a) DLTS spectra for six different rate windows in the range of ( ms) 1 and (b) Arrhenius plot for the defect level 20

33 3.2 Deep Level Transient Spectroscopy (DLTS) and results in a set of DLTS spectra with different time windows. An illustration of a DLTS spectrum with different time windows is shown in Fig 3.4a. A set of values [e n, T] are available for the peak maximum at the different windows. By using the Arrhenius relation: ln(e n /T 2 )=ln(βσ na ) ΔH/k B T (3.6) where β is a constant factor, the slope of the line in Fig 3.4b is proportional to the activation energy of the defect level and the extrapolated intercept at 1/T 0 gives the apparent capture cross section. The concentration of the deep level can be extracted from Eq Weighting functions As mentioned earlier, several types of weighting functions are possible depending on the required signal to noise ratio and the energy resolution. The main ones used in this thesis were the lock-in and the GaverStehfest (GS4) functions. The lock-in weighting function is a simple mathematical conversion and has a good signal-to-noise ratio. The lock-in weighting function is given by: 1 t d t t d +2 i 1 τ ω(t) = 1 t d +2 i 1 τ t t d +2 i τ The GS4 weighting function was first suggested by Istratov in 1997 and derived by Stehfest-Gaver algorithm.[17, 18] It gives better energy resolution with higher capability of separating the peaks but has a poorer signal-to-noise ratio. The GS4 weighting function is given as 1 t d t t d +2 i 2 τ 25 t d +2 i 2 τ t 2 i 1 ω(t) = 48 t d +2 i 1 τ t t d +3/2 2 i 1 τ 24 t d +3/2 2 i 1 τ t t d +2 i τ 21

34 3. EXPERIMENTAL METHODS DLTS signal arb. unit Lock in GS Temperature [K] Figure 3.5: DLTS spectra from a lock-in and GS4 weighting functions. A difference between lock-in and GS4 is shown in Fig 3.5. The peaks are broader for lock-in compared to GS4. Hence, GS4 can more easily resolve overlapping peaks, but at expense of an increased noise level. DLTS is a very sensitive technique in terms of trap concentration as it can detect defects with low concentrations less than 0.01% of doping concentration. Hence, it can go down to 10 8 cm 3 range in pure and high resistivity samples, and it is a non-destructive technique. Another important feature of DLTS is the possibility to obtain depth profiling of the defects. On the other hand, the defect concentration should not be exceed 10-20% of the doping concentration for the above mentioned assumptions to be valid. DLTS does not give structural or chemical identity of the defects, and makes direct assignment of the defect levels difficult. However, one can make comparison studies with other complementary techniques such as infrared absorption (IR), electron paramagnetic resonance (EPR), and photoluminescence (PL) to identify the defects. 22

35 3.2 Deep Level Transient Spectroscopy (DLTS) Optical-DLTS and MCTS DLTS is commonly used to study majority carrier defects and, thus, limited to probe only parts of the band gap. Ideally, one would like to probe the entire band gap which also includes information about the minority carrier defects. In normal DLTS, only majority carriers are injected into the probing region where as, in so called minority carrier transient spectroscopy (MCTS) both majority and minority carriers are injected in to the probing region. This can be done by using pn-junctions where minority carriers are injected electrically by applying a forward bias pulse. A selective injection of minority carriers is done optically during the filling sequence, sometimes called optical DLTS (ODLTS).[9] In the technique based on forward biasing a pn-junction during the filling pulse, both majority and minority carriers are simultaneously present in the investigated region, the probability of minority carrier occupation of levels is low for the defects with higher majority carrier capture rates. In ODLTS, optical excitation can be done either from back side or front side through the diode (Schottky or pn junction). Back side excitation generate carriers in the bulk if the absorption length is shorter than the sample thickness. Further, only minority carriers will diffuse to the depletion region if the diffusion length is sufficiently large, while the majority carriers diffusion is negligible due to the electrical field of the junction. In front side excitation, both types of charge carriers will be generated close to or within the junction and signicant contributions from majority carriers must be taken into account. In the present work, back side optical excitation is used. Fig 3.6 shows the comparison of DLTS, MCTS and ODLTS spectra from some selected p-type samples. The DLTS spectrum reveals only majority carrier (in this case holes) traps, H1 and H2. The MCTS spectrum is similar to the DLTS one in addition to one minority carrier trap E1. On the other hand, ODLTS analysis reveals two other minority carrier traps E2 and E3 which were not observed by DLTS/MCTS in addition to E1. It shows the applicability of ODLTS for identifying certain traps, and one can obtain a more firm identification of the defect levels by combining both DLTS and ODLTS measurements. One of the major advantages of ODLTS over forward biased-mcts is that it can be applied to Schottky diodes. Fig 3.6 shows the comparison of DLTS, MCTS and ODLTS spectra from some 23

36 3. EXPERIMENTAL METHODS H1 H2 DLTS MCTS ODLTS ΔC/C rb E1 E E Temperature [K] Figure 3.6: Comparison between DLTS, MCTS and ODLTS spectra of proton irradiated p-type CZ-Si selected p-type samples. The DLTS spectrum reveals only majority carrier (in this case holes) traps, H1 and H2. The MCTS spectrum is similar to the DLTS one in addition to one minority carrier trap E1. On the other hand, ODLTS analysis reveals two other minority carrier traps E2 and E3 which were not observed in DLTS/MCTS in addition to E1. It shows the applicability of ODLTS for identification certain traps. One can obtain a clear identification of the defects by combining both DLTS and ODLTS measurements. One of the major advantages of ODLTS over forward biased-mcts is that it can be applied to Schottky diodes. 3.3 Photoluminescence (PL) Photoluminescence (PL) spectroscopy is a contact-less and non-destructive method to probe the electronic structure of materials and defects/impurities. The basic principle of PL is that the excitation light is absorbed by creating electronhole pairs, which then may recombine radiatively, giving away the excess energy through the emission of light (photons). The sample exhibits luminescence via photo-excitation, hence the term photoluminescence. 24

37 3.3 Photoluminescence (PL) Electron-hole pairs are formed and subsequently recombine by one of the several mechanisms as shown in Fig 3.7. The excitation causes electrons within the material to move into permissible excited states. These electrons return to their equilibrium states, the excess energy is released and may include the emission of light referred to as a radiative process, or may not emit light through a nonradiative process. The energy of emitted light (Photoluminescence) relates to the difference in energy levels between the two electron states involved in the transition between the excited state and equilibrium ground state. Figure 3.7: mechanism of absorption and luminescence between valence and conduction band (direct excitation) - The advantages of PL are band gap determination, which is useful in particular when working with new compound semiconductors, and impurity levels, recombination mechanisms, and material quality can be determined. Radiative transitions in semiconductors involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence intensity can be used for the indication of their concentration. Direct band gap materials give stronger PL than indirect band gap materials such as Si. As an example, from paper VII, Fig 3.8 shows the PL spectra of proton irradiated p-type Si and after the heat treatment at 385 C. The so called C-line ascribed to the C i O i complex, dominates in the as-irradiated sample while the intensity of the so-called P-line increases after heat-treatment at 385 C concurrent 25

38 3. EXPERIMENTAL METHODS Figure 3.8: PL spectra of p-type Si as-irradiated and after the heat treatment at 385 C for 180 min with a decrease in the intensity of the C-line. 3.4 Ion Implantation Ion implantation has been widely used for introducing impurity profiles with precise control of the position and dose for many decades. Although the technique did not enter mass production before the mid 1970s, Shockley issued a patent already in 1954 with a detailed description of the relevant processes involved.[19] During ion implantation, ionized atoms are accelerated by an electrical field and bombard a target, for example a semiconductor wafer. When an energetic ion enters a solid target, it starts to lose energy. The distance that the ion travels in the semiconductor is called as ion range which can be projected into a certain direction, typically perpendicular to the surface (so-called projected range). The projected range of the implanted ions can be manipulated by varying the ion energy typically from about 1 kev up to several MeV. The energy loss in a target material is a result of two mechanisms, electronic stopping and nuclear stopping. Electronic stopping involves collisions between the incoming ion and 26

39 3.4 Ion Implantation the target electrons. The interaction between the incident ion and a target atom can be treated as a Coulomb scattering event. Nuclear stopping is due to the elastic collision between the two constituents i.e, incoming ion and the target atoms. Nuclear stopping causes atomic displacement (damage) in the target material and post implantation annealing is normally required to restore the crystal structure. Implantation as Irradiation The implantation damage, caused by the ions penetrating the material, can also be advantageous for studies of defects in semiconductors, since the defect generation can be controlled by the energy, dose and type of implanted ions. When light elements such as hydrogen are implanted, a measurable amount of point defects can be formed. Here, we regard ion implantation as irradiation when the implantation peak is far away from the probed region. In the present work, irradiation done with 1.8 MeV protons gives rise to a projected range of 40 μm, while the DLTS measurements probed depths of 1-2um. During the annealing out-diffusion of H from the region around the projected range may be anticipated but no indications of any H-related levels in the DLTS spectra were found. In the studies of H-related defects, the implantation peak of hydrogen into Si will be in the probing region. Implantation generates primary defects which in turn will interact with each other as well as with hydrogen, generating multiple secondary defect complexes. The damage generated from 1 MeV proton implantation extends in principle all the way from the projected range ( 17 μm) to the surface. However, in this case we have used an Al-foil in front of the sample to reduce the penetration depth of the protons, more closely corresponding to the DLTS probe depth. High temperature proton irradiations were also used in some of the experiments performed in this thesis. High temperature irradiations are useful in understanding the defect formation with higher thermal stability, in particular for lifetime control of power devices.[20] This is done by, first, heating the sample to the required temperature. Irradiation is then done at the elevated temperature and the temperature fluctuations are not more than 10 C. After the irradiation, 27

40 3. EXPERIMENTAL METHODS Figure 3.9: Schematic structure of n + p-junction which is used in this thesis the samples were cooled down to RT in order to minimize the annealing of defects. 3.5 Sample preparation The samples used in the present work were mainly n + p structures and a short description of the fabrication will be given here. A sample structure is shown in Fig.3.9. CZ-Si wafers were cleaned with a standard RCA (RCA1-3) cleaning procedure prior to oxidation. After this cleaning process, the wafers were dry oxidized at 1100 C for 3 hours to grow a 250 nm thick SiO 2 layer. The oxide formed on the back of the wafer is used as a protective layer during the diffusion, to ensure that no dopants enter the backside. The protection of back side oxide can be done by spin coating of photoresist. Standard positive photolithography and wet etching using buffered oxide etch (BOE) were then applied to open holes with a diameter of μm in selected areas. When the photoresist is completely removed under the exposed regions, a buffered oxide etch (BOE) is used to open the holes in the oxide. After the oxide etch is complete, most of the photoresist can be removed by rinsing in acetone for a few minutes. 28

41 3.5 Sample preparation The n + layer was formed by indiffusion of phosphorous (P) from gas phase in a commercial tube furnace. One can also use ion implantation and subsequent activation in a tube furnace or by rapid thermal processing. In order to improve the electrical contact between the diode and a probe needle, an Ohmic contact is formed by evaporation of Aluminum (Al) over the oxide opening, thereby capping the hole with a metal as well. The thin oxide layer that has been formed over the diode areas during the diffusion process is removed by HF cleaning. A BOE etch is applied to remove the backside oxide. Al Ohmic contacts are then prepared by thermal evaporation on the front side (n + side), having a thickness of about 200 nm. 29

42 3. EXPERIMENTAL METHODS 30

43 Chapter 4 Results and Discussion This chapter aims to give a brief background related to the studies that have been done in the thesis. A summary of the obtained main results will be given with suggestions for the future work. The present chapter deals with three different point defect complexes. In the first part, the most prominent intrinsic point defect stable at room temperature (RT) is discussed, namely, the divacancy (V 2 ), its derivative the divacancy-oxygen (V 2 O) complex, and the diffusion mechanism of V 2. Second part reveals the recently identified trivacancy (V 3 ) and trivacancy-oxygen (V 3 O) complexes, and their detailed transformation kinetics. The third part is about the annealing of carbon-oxygen complexes and formation of new defects with higher stability in the range of C. 4.1 The divacancy and divacancy-oxygen complexes The divacancy (V 2 ) Irradiation-induced vacancies and interstitials in silicon are highly mobile at room temperature and can easily form higher order complexes or complexes with impurities present in the material, which are stable at RT. Among the higher-order vacancy complexes, V 2 is the most prominent intrinsic point defect stable at RT and it is strongly enhanced in concentration after particle irradiation and ion implantation. The V 2 can form in two ways: direct displacement of two neighboring Si atoms during the irradiation and secondly, by agglomeration of two migrating single vacancies (V) which are produced in two different collisions. The diva- 31

44 4. RESULTS AND DISCUSSION Figure 4.1: Visualization of the basic atomic structure of V 2 with its six neighboring Si atoms. cancy structure can be regarded as two missing Si atoms next to each other and is schematically shown in Fig 4.1. Initially, there are six broken Si-Si bonds around the divacancy, however, four of these bonds (e.g., atoms 1-2 and 4-5, as shown in Fig 4.1) form into two bent pair bonds.[21] The remaining two bonds are broken, so called dangling bonds. Extensive studies of V 2 have been done by Watkins and Corbett in the 60 s using primarily Electron Paramagnetic Resonance (EPR) measurements. It was shown that V 2 can appear in three different charge states: positive, neutral and negative. Later, DLTS studies established a doubly negative charge state. Energy levels of V 2 observedbydltsataboute v ev, E c ev, and E c ev are attributed to positive (+/0), negative (-/0) and doubly negative (=/-) transitions of V 2, respectively.[21, 22, 23, 24, 25] In p-type silicon, only V 2 (+/0) can be detected by majority carrier DLTS, however, it has been reported that V 2 (-/0) can be observed in p-type samples by optical-dlts (see section for detailed explanation).[22] In turn, in n-type material the acceptor states of V 2, single and 32

45 4.1 The divacancy and divacancy-oxygen complexes double negative, have been well studied by DLTS while V 2 (+/0) is not observed by MCTS and ODLTS due to its large electron capture cross section.[23] The details of the energy levels of V 2 are summarized in Table 4.1. Charge state Energy Comments level (ev) V 2 (+/0) E v DLTS in p-type [22, 25] V 2 (-/0) E c DLTS in n-type[21, 23] and ODLTSinp-type[22] V 2 (=/-) E c DLTS in n-type [21, 23] Table 4.1: Details of V 2 levels The early EPR studies by Watkins and Corbett investigated the annealing behavior of V 2 in both CZ- and FZ-Si materials irradiated with high doses of MeV electrons (10 18 cm 2 ). It was found that V 2 is stable up to 200 C and starts to migrate beyond this temperature with an activation energy, E a,of 1.3 ev.[21] The study suggested that V 2 is more stable in FZ-Si, as compared to CZ-Si, due to lower impurity concentration. A dissociation process was suggested as the main annealing mechanism of V 2 in FZ-Si. The dissociation energy was estimated to 1.9 ev. In CZ-Si, V 2 s anneal by migration and trapping at other impurities that are present in the material. One of the most abundant impurities in CZ-Si is interstitial oxygen (O i ) which is present at concentrations of cm 3. V 2 s are trapped by O i, and the divacancy-oxygen interaction is normally the main mechanism of the V 2 annealing in irradiated CZ-Si, as other early studies have suggested.[26, 27] The annealing behavior of V 2 in Si irradiated with low doses is also dependent on other, less abundant, impurities. Previous DLTS studies showed that hydrogen can influence the annealing of V 2.[28, 29] However, the present section deals with oxygen as the main impurity trap during the migration of V 2 while the section 4.4 presents the effect of Hydrogen on V 2. 33