A thesis presented to. the faculty of. the College of Arts and Sciences of Ohio University. In partial fulfillment. of the requirements for the degree

Transcription

1 The Study of Coupling in InGaAs Quantum Rings Grown by Droplet Epitaxy A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Master of Science Samar M. Alsolamy May Samar M. Alsolamy. All Rights Reserved.

2 2 This thesis titled The Study of Coupling in InGaAs Quantum Rings Grown by Droplet Epitaxy by SAMAR M. ALSOLAMY has been approved for the Department of Physics and Astronomy and the College of Arts and Sciences by Eric A. Stinaff Associate Professor of Physics and Astronomy Robert Frank Dean, College of Arts and Sciences

3 3 ABSTRACT ALSOLAMY, SAMAR, M., M.S., May 213, Physics and Astronomy The Study of Coupling in InGaAs Quantum Rings Grown by Droplet Epitaxy Director ofthesis: Eric A. Stinaff The use of metal droplet epitaxy may provide a novel method of growing laterally coupled nanostructures. We will present optical studies of InAs/GaAs nanostructures which result in twin quantum dots (QD) formed on a single quantum ring (QR). Previous studies have investigated the coupling between vertically grown quantum dot pairs. In this thesis, we have used photoluminescence (PL) and photoluminescence excitation (PLE) to examine the possibility of energy transfer and coupling between quantum dot pairs in a single InGaAs quantum ring grown by droplet epitaxy. Power dependent photoluminescence spectra reveal a few peaks at low power, which are identified with emission from the ground state of the individual dots. As the power is increased we observe multi-exciton and excited state emission. We then perform PLE, tuning the excitation laser energy continuously from the high energy ring emission down to the individual dot states. We have observed resonant PLE emission in the QD/QR structures both at high laser energy and when resonant with the identified states of one of the QDs which may indicate energy transfer and/or coupling between the dots.

4 4 ACKNOWLEDGMENTS First, I would like to give my thanks and acknowledge to my adviser, Professor Eric Stinaff, not only for his guidance and mentoring through my research from working in the lab to writing this thesis, but also for his patience and care during this year. I would like also to thank my lab partners, Ramana Thota, for showing me the basic equipments in the lab and being my second teacher in the lab. I thank the collaborators in University of Arkansas for preparing the samples and discussing the research experiments. I thank Dr. Candace Stewart for giving me feedback to revise my thesis. I would like to thank Saudi Arabian Cultural Mission in the USA and Umm al-qura University for their financial support and giving me the opportunity to have this scholarship. Finally, I would to thank my beloved husband for his care, support, and patience during the three years of my study.

5 5 TABLE OF CONTENTS Page Abstract... 3 Acknowledgments... 4 List of Tables... 7 List of Figures... 8 Chapter 1: Introduction Outline: Semiconductor Physics Chapter 2: physics and Growth of Heterostructures Heterostructures Quantum Well Quantum Wire Quantum Dot Growth Techniques Molecular Beam Epitaxy (MBE) Stranski-Krastanow Technique Droplet Epitaxy... 3 Chapter 3: Quantum Rings Fabrication Approach of Quantum Rings Energy Transfer in a Quantum Ring Chapter 4: Experimental Measurement and Setup Experimental Measurement Photoluminescence Spectra Photoluminescence Excitation Spectra Experimental Setup Sample Lab Description Photoluminescence Setup Photoluminescence Excitation Setup... 49

6 6 Chapter 5: Data and Result Photoluminescence Data Result Result Result Result Photoluminescence Excitation Data Result Result Chapter 6: Discussion and Conclusions References Appendix A: Subtractive Mode in TriVista System... 92

7 7 LIST OF TABLES Page Table 2.1: The number of the confinement D c and of degrees of freedom D f in electrons motions for the four basic systems...19 Table 5.1: Different QRs in ARK_QRings_SC695 sample with laser wavelength 78 nm at room lights off and monitors on...52

8 8 LIST OF FIGURES Page Figure 1.1: Crystal structure of diamond and zinc blende...15 Figure 1.2: a) Bonding and antibonding orbitals in a single atom and a crystal. b) Bands structure of metals, semiconductors and insulators...16 Figure 2.1: Densities of states of bulk, quantum well, quantum wire, and quantum dot...19 Figure 2.2: InAs quantum well structure in GaAs substrate; the black circle represents an electron and the empty circle represents a hole...2 Figure 2.3: Two types of band alignments between two semiconductors a) Type I. b) Type II staggered. C) Type II misaligned...21 Figure 2.4: Different heteroepitaxial growth modes for strained layer growth...25 Figure 2.5: Schematic diagram of the growth chamber in molecular beam epitaxy process...28 Figure 2.6: Diagram of Stranski-Krastanow growth mode...3 Figure 2.7: Diagram of droplet epitaxy growth for GaAs quantum nanostructure...31 Figure 3.1: a) 2 x 2 µm two- dimensional AFM image of QRs in the sample. b) Three-dimensional AFM image of a QR magnify. c) Details of one QRs dimensions along [1-1] and [11] directions from part b...32 Figure 3.2: Schematic of the formation process of the InGaAs quantum rings...34 Figure 3.3: Three types of coupling between a QD pair. a) Tunneling. b) Emission and absorption. c) Förster resonant energy transfer...37

9 9 Figure 3.4: a) The conduction band schematic of the two quantum dots of quantum ring. b) micro-pl spectrum of two QDs in QR. c) varying the excitation intensities of micro-pl spectrum of two QDs in a QR...4 Figure 4.1: Photoluminescence process in a single QD...42 Figure 4.2: a) Photoluminescence spectra from a single QD shown emission from X, X -1, X -2, X -3, X -4 and X -5 excitons. b) Power dependent Photoluminescence spectra from a single QD isolated shown exciton and biexciton in s-shell and p-shell...43 Figure 4.3: Photoluminescence excitation process in a single QD...44 Figure 4.4: Position (Horizontal, Vertical, Focus) of an isolated QR...45 Figure 4.5: Lab and equipments setup...48 Figure 4.6: Three Labview programs; on the right side is the main system controlling the spectrometer, laser, CCD and show PL spectra. In the middle is PLE measurement of the Labview program. On the left is a QR position program...48 Figure 4.7: Schematic of photoluminescence setup in oblique or normal incidences by adjusting the mirror to right position...49 Figure 4.8: Schematic of photoluminescence excitation setup in oblique or normal incidences by adjusting the mirror to right position for one and three stages of TriVista system...5

10 1 Figure 5.1: Different QRs from table 5.1 in ARK_QRings_SC695 sample with laser wavelength 78 nm shown three main groups of emissions: wetting layer, wetting ring, and a quantum dot pair...53 Figure 5.2: The emission from the same two QRs at different conditions: monitors ON or OFF...54 Figure 5.3: Two groups of peaks emit from QD pairs in QRs at lower power a) 2 nw (4.8 nw/cm 2 ). b) 2 nw (48 nw/cm 2 )...57 Figure 5.4: Moving the sample small horizontal steps from QR in Normal incidence setup to show the peaks that emit from witting ring and a QD pair...6 Figure 5.5: PL depending on the varying power wavelength and fixing the laser wavelength and other conditions for the same QR...63 Figure 5.6: PLE spectra of three QRs shown the changing of QDs peaks between narrow and broad...67 Figure 5.7: PLE data where the second QD monitored and laser scanned to it. a) and b) No sign of coupling in QR. c) and d) possibility of coupling in QRs...73 Figure 5.8: a) PLE for the same QR in figure 5.7 c focusing in part of where a photon was observed when laser wavelength at nm in Bristol reading. b) PL spectrum for the same QR shown where two wavelength of the laser and photon emission...76 Figure 5.9: a) Two PL spectra for QR in figure 5.7 d shown where two wavelength of the laser and photon emission. b) The same QR with different powers at

11 11 laser wavelength 82 nm, in addition, two wavelengths of the laser and photon emission were shown...78 Figure 5.1: a) PLE for the same QR in figure 5.7 e focusing in part of where a photon was observed when laser wavelengths at nm, 95.8 nm and nm when room light and monitors ON. b), c) and d) Two PL spectra for the same QR shown where three wavelengths of the laser and photon emission. e) The same QR with different powers at laser wavelength 82 nm, in addition, three laser wavelengths lines and a photon emission line were shown...84 Figure A.1: a) Schematic of what inside the three stages of TriVista system. b) Subtractive mode process...92

12 12 CHAPTER 1: INTRODUCTION Ever since the concept of the band structure for an ideal crystalline solid was introduced by Bloch in 1928, scientists have been reducing the size of semiconductor structures. Reaching the nanoscale and creating structures such as quantum wires and dots is now relatively routine. This has resulted in the ability to create nanoscale structures in a crystalline material with discrete energy levels similar to atoms 1. In the early 193s, a dedicated study of semiconductor physics was started at the Physico- Technical Institute. After that, theoretical researches and experimental discoveries have continued in the semiconductors field 2. In 1947, the first transistor at Bell Labs was invented 3. Since then, more innovations and devices have been appearing in the field. For example, microprocessors have continued to progress with a 5 % improved performance, a 15 % smaller size and a 3 % decrease in cost each year, as predicted by Moore s Law. This has resulted in the size of devices reaching to nanometer scales 4. In addition to several applications of a single semiconductor nanostructure (such as a single quantum dot) new types of nanotechnology have been created, for example, the coupling between two or more semiconductor nanostructures, such as the coupling between a quantum dot pair in quantum molecules 5. Another occurrence of this type of coupling may be in quantum rings, which is investigated in this thesis Outline: In this thesis, a background of semiconductors will be discussed in chapter one. In this discussion, the band theory will be described as arising from bonding and antibonding bands resulting in the valence, and conduction bands; as a result, the

13 13 difference among conductors, semiconductors, and insulators will be inferred. Furthermore, the behavior of the charge carriers in semiconductors as a function of temperature and doping will be discussed. Finally, the density of states and wave function of the charge carriers in the bulk semiconductors at three dimensions will be introduced. The general concept of semiconductor heterostructures and their properties will be illustrated in the first section of chapter two. The density of states from two dimensions to zero dimensions will be attributed to the physical dimensionality of the three nanostructures (quantum wells, weirs, and dots), in addition to their shapes, properties and applications. In the second section of chapter two, the nanostructures growth technique will be discussed. One of the most important crystal epitaxial growth techniques, molecular beam epitaxy (MBE), will be included. In addition, two growth modes, Stranski-Krastanow (SK) and droplet epitaxy (DE), will be explained. In chapter three, the focus will turn to a special type of nanostructures, the quantum ring (QR). The growth by droplet epitaxy of InGaAs quantum rings on a GaAs substrate using molecular beam epitaxy technique will be illustrated in the first section. The concept of coupling a quantum dot pair, in general, and in a single quantum ring, in particular, will be discussed in the second section. Chapter four highlights the experimental equipment. In the first section, the concepts behind the measurement of the photoluminescence (PL) and photoluminescence excitation (PLE) spectra will be explained. The experimental samples and setup of the lab and photoluminescence and photoluminescence excitation will be described in the second section of chapter four.

14 14 The fifth chapter will display the results of the photoluminescence and photoluminescence excitation spectra to investigate the coupling in a ring-shaped nanostructure. The final chapter will discuss explicitly the results of chapter five and conclude the thesis Semiconductor Physics: Semiconductors are materials with intermediate electrical conductivity, in the range of 1-2 to 1 9 Ω cm at room temperature, between that of conductors (1-6 Ω cm) and insulators (1 14 to 1 22 Ω cm) 6, 7. Typical elements in a semiconductor are the fourth group elements in the periodic table such as silicon and germanium. They crystallize in the diamond structure (figure 1.1) and have four valence electrons, which participate in the chemical bonding between the atoms. In addition to elemental semiconductors as described above, compound semiconductors are the binary compounds of a trivalent element and a pentavalent element such as GaAs and InP or a divalent element and a hexavalent element such as ZnS. Indeed, IV-IV compounds are also compound semiconductors such as SiC. The III- V or II-VI compounds, which have the crystal structure of zinc blende, (figure 1.1) form sp 3 hybrid orbitals similar to the group four elements but here there ends up being some ionicity to the bonding 6, 8.

15 15 Figure 1.1. Crystal structure of diamond (right) and zinc blende (left) (after Leese) 9. In atomic physics, electrons are distributed following the Pauli Exclusion Principle, which states that no two electrons can occupy the same quantum state. An atom can be split into its core (nucleus and occupied electrons states) and outer shell (valence electrons). When two atoms combine, the valence electrons wave functions overlap and form bonding and antibonding orbitals. The bonding orbital is the lower energy of sp 3 bond, while the antibonding orbital is the higher energy one as shown in figure 1.2.a. Hence, if a large number of atoms come together, they form bonding and antibonding bands. This method is called Linear Combination of Atomic Orbitals (LCAO). Thus, from those two bands, the valence band is the top part of bonding states, and antibonding states form the conduction band 8,1. In metals, the bands overlap while in semiconductors and insulators there is a forbidden region, which is called the band gap as shown in figure 1.2.b. The band gap is the distance between the upper limit of the valence band and the lower limit of the conduction band. It is much larger in insulators and smaller in semiconductors (less than 4 ev). Midway in this band gap is the Fermi level, which is usually defined as a level, above which are unoccupied electron states and blow are

16 16 occupied electron states, in the ground state 7. For semiconductors (as well as insulators), the conduction band is completely emptied of electrons and the valence band is completely filled, at low temperatures. At a finite temperature in an intrinsic semiconductor, an electron in the valence band may be excited to the conduction band where it is free to move, and leave behind a positive charge, a hole. Another method to introduce free electrons (holes) is by doping a semiconductor. This method creates an impurity level in the band gap. When the doping is from a pentavalent element, called a donor, this produces n-type semiconductors due to an extra electron. Indeed, the Fermi level will move near the conduction band. However, if the doping is from a trivalent element, called an acceptor, this produces p-type semiconductors, and the Fermi level will be near the valence band. Semiconductors are often doped with both types 8, which in this case called p-n junction. Those types are applied to several applications in electronic devices such as transistors and solar cells. a b Figure 1.2. a) Bonding and antibonding orbitals in a single atom and a crystal (after Grosvenor) 11. b) Band structure of metals, semiconductors and insulators (after Christopher) 12.

17 17 Electrons (holes) in the conduction (valence) bands of bulk semiconductors are free to move in three dimensions. A simple approximation for the band structure of a bulk semiconductor, near the top of the valence band or the bottom of the conduction band, is a spherical, parabolic dispersion relation. With this approximation, the density of states, which is defined as the number of states per unit volume per energy, is: (1.1) where is the charge carrier s effective mass, which describes how the particle moves inside the crystal in different fields (electric and magnetic fields) as if it is a free particle, and E is the total energy of charge carrier 13.

18 18 CHAPTER 2: PHYSICS AND GROWTH OF HETEROSTRUCTURES 2.1. Heterostructures: In the previous chapter, the energy bands and the band gap were discussed. The crystal potential is complex; nevertheless, in the effective mass approximation for a bulk crystal, the Schrödinger equation of a free electron or hole wave function is simply: (2.1) However, when two semiconductors with different band gaps, but similar crystal structures, and not too different lattice constants, such as GaAs and GaAlAs 1, join to form a heterojunction, a potential term (V) will be added to this Schrodinger equation because of the discontinuity in both the conduction and the valence bands. (2.2) Thus, if a thin layer of a smaller band gap s semiconductor is sandwiched between two layers of another semiconductor of the bigger band gap, the charge carrier of the thin layer are treated as a particle in a box. When the layer gets to a quantum scale or size, a single quantum well, wire, or dot would be obtained depending on the degree of freedom of the electron or hole. Moreover, when multiple heterojunctions come together, heterostructures are formed 13.

19 19 Table 2.1 The number of the confinement D c and of degrees of freedom D f in electrons motions for the four basic systems. (After Harrison) 13. System D c D f Bulk 3 Quantum Well 1 2 Quantum Wire 2 1 Quantum Dot 3 Figure 2.1. Densities of states of bulk, quantum well, quantum wire, and quantum dot (After Saleh and Teich) Quantum Well: In a single quantum well, the charge carriers are confined to a one-dimensional potential well, but they are free to move in two dimensions. The density of states in quantum wells depending on the two degrees of freedom of the charge carriers is: (2.3) where is the Heaviside step function. The density of states in quantum wells, which is defined as the number of states per unit area per energy, is a constant within each quantized energy level 13.

20 2 There are various systems or types of band alignments between two semiconductors. The type-i system, as shown in figure 2.2 and 2.3 a, is when the conduction band of one semiconductor is higher (lower) in energy, and the valence band is lower (higher), than the second semiconductor s conduction bands such as GaAs/GaAlAs. The second alignment is type-ii, when the conduction and valence bands of one semiconductor are both higher or lower in energy than the neighboring material, this system is called the type-ii staggered alignment such as CdSe/ZnTe, as shown in figure 2.3 b. The other system, when the conduction band of one semiconductor lies entirely below the valence band of the second semiconductor, is called the type-ii misalignment such as InAs/ CaSb, as shown in figure 2.3 c. The final type is type-iii alignment when one semiconductor has a zero band gap or is semimetallic such as HeTe/CdTe 1, 13. Those three types are different in their role in tunneling from one quantum confined region to another and in the lifetime of the charge carrier in quantum wells 1. In addition, the three types occur in quantum wires and dots. Figure 2.2. InAs quantum well structure in GaAs substrate; the black circle represents an electron and the empty circle represents a hole.

21 21 Figure 2.3. Two types of band alignments between two semiconductors a) Type I. b) Type II staggered. c) Type II misaligned. (After Seeger) 1. The discovery of quantum wells and superlattices (multiple quantum wells) have improved various novel optical, electronic, and optoelectronic devices such as laser diodes, light emitting diodes (LED), solar cells, photodetector modulators, high-speed field effect transistors, low noise high-electron-mobility transistors and high-speed heterojunctions bipolar transistors 2, 15. In this thesis we will be looking at Type-I band alignments, present in InAs/GaAs heterostructures Quantum Wire: The charge carriers in quantum wires have one degree of freedom and a twodimensional confining potential. The density of states for one dimension of an electron s motion, which is defined as the number of states per unit length per energy, is 13 : (2.4)

22 22 Nanowires and nanotubes are often modeled as quantum wires. They have different sizes and shapes depending on the growth such as rectangular with rectangular or square cross-section and cylindrical with circular cross-section 13. Those shapes and the density of states of the quantum wires often improve the semiconductor heterostructures device properties compared to quantum well devices. Similar to quantum well applications, quantum wires, especially coupled quantum wires, have many applications such as laser diodes, light emitting diodes (LED) 2, photochemical devices, solar cells, batteries, chemical sensors, and electrochemical catalysts 16. An interesting study of one type of quantum wire is carbon nanotubes (CNTs) 16. CNTs provide several applications such as DC motor brushes, multifunctional nanobrushes, and supercompressible springs Quantum Dot: In quantum dot systems, the electrons or holes are confined in all three dimensions resulting in a totally discrete energy spectrum; hence, their motions are zero dimensional. The density of states in quantum dots (QDs) with zero degrees of freedom is a series of delta-function spikes at the quantized energy levels 1, 4, 18,19, 2 : (2.5) where the two takes to the account the spin of the electron. QDs are similar to free atoms in exhibiting sharp optical line spectra; however, they are also similar to solids because of their large volume density. Hence, they are called artificial atoms, even though they consist of atoms. The confinement

23 23 results in envelope wavefunctions that have an s-like ground state and a p-like excited state in both conduction and valance bands 4, 18, 21. Similar to quantum wires, quantum dots also have different sizes and shapes depending on the growth such as a box with rectangular cross-section, cubic, spherical shape with circular cross-section, pyramid, truncated pyramid, lens, and disc with cylindrical shape 13, 15. One advantage is that quantum dots have quantized energy levels which are tunable depending on size. The increase of QDs size decreases the energy level spacing resulting from quantum confinement. Therefore, with different sizes of QD, there are different colors. For example the big size of quantum dot emits red; however, the small QD has blue color 22, 23. In the QDs studied in this thesis the energy separation between levels is typically on the order of tens of mev. Because of QDs optical and electronic properties, several applications have been improved including biotechnology, display screens, photovoltaic cells and lighting fields. Quantum dot laser diodes provide high output power and have a long lifetime compared with quantum well laser diodes. They include a higher characteristic temperature of threshold current, higher modulation bandwidth, lower threshold currents, and narrower linewidth. Also, the QD diodes play a substantial role in optical communication, compact disks and related optical data storage applications, displays, and lighting 24. Quantum dot light-emitting diode (QD-LED) is one of the most advanced applications in electroluminescent displays and lighting areas. After QD-LEDs incorporate with organic light-emitting diodes (OLEDs), the screen devices improve color purity, improve lifetime, add flexibility, provide brighter images, and produce lower power consumption.

24 24 In addition, QD-LEDs, which are used in solid state lighting, provide high quality of brightness, have a long lifetime, lower global environmental impact, and operate at lower cost 25. Another application, QD solar cell, operates at a higher efficiency with a lower cost, and produces clear environmental implications. The properties of QDs have wide implementation in biotechnology and medicine including drug discovery and diagnostics, photodynamic therapy, flow-cytometry, tracking cell migration, eliminating cancer cells, tissue mapping and demarcation 25, DNA chips, biosensors, and microfluidic devices 22. There are many other QDs applications and promising applications in the future, especially in coupling between QDs such as quantum computation Growth Techniques: After discussing the properties of three types of semiconductor heterostructures in the previous section, the growth techniques, especially quantum dots, will be discussed in this section from the perspective of fabrication. Historically, over the last decade, various techniques of nanostructure fabrication have been reported such as the lithographical and epitaxial growth methods 18, 24, 27. The last method will be focused on in this thesis. In the late 197s, the study of nanostructures began when thin epitaxial layers were produced for the first time 24. Two sophisticated machines, which use the epitaxial growth method, are molecular beam epitaxy (MBE), which will be discussed in section and metalorganic chemical vapor deposition (MOCVD), also known as metalorganic vapor phase epitaxy (MOVPE) 4, 27. Epitaxy, which technically means to arrange upon, is used here to mean above the substrate. The substrate is a thin film of semiconductor used as seed to deposit additional semiconductor

25 25 above it. Hence, if two semiconductor films are similar, the growth is called homoepitaxial; on the other hand, if the films are different, it is called heteroepitaxial 28. There are three different heteroepitaxial growth modes depending on interface energies and lattice constant mismatch as shown in figure 2.4. One of them is the Frank-van der Merwe (FM), which is layer-by-layer growth. Another mode is Volmer-Weber (VW), which is island growth. Finally, Stranski-Krastanow (SK) is layer-by-layer growth in addition to island growth; it will be discussed in section , 27. After using the growth techniques to make the nanostructures, there are also several methods to observe them by direct imaging methods such as scanning tunneling microscopy (STM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) or by diffraction methods such as reflection high energy electron diffraction (RHEED), which will be mentioned in section 2.2.1, and X-ray diffraction 1, 4. Figure 2.4. Different heteroepitaxial growth modes for strained layer growth (After Shchukin et al.) 4.

26 Molecular Beam Epitaxy (MBE): Molecular beam epitaxy (MBE) is one of the most important crystal growth techniques for producing thin films of a variety of materials such as metals, semiconductors, insulators, and superconductors. The most common use of it in the past decade was in the field of semiconductor heterostructures, to produce electronic and optoelectronic devices. Historically, there were several attempts at semiconductor thin films growth prior to the 197s. The discovery of MBE in the late 196s was because of studying the surface vapor interactions with a new compact mass spectrometer, not from finding crystal growth. In 1968, John R. Arthur and John Le Pore were studying the reflection of III-V compound molecular beams in ultrahigh vacuum, and in 1969, Alfred Y. Cho published his results of MBE growth for the first in situ observations using high energy electron diffraction 28. As shown in figure 2.5, the growth chamber in MBE includes effusion cells, shutters, substrate and its holder, an electron gun, a fluorescent screen, a quadrupole mass spectrometer, and cryopanels. The Knudsen effusion cell includes a crucible and a resistive heater. The inner crucible, which is made of reactor-grade graphite or pyrolytic boron nitride, contains the material sources, such as Ga, In, As and Al in III-V growth, and p- and n-type doping materials. In effusion cells, the materials evaporate up to 14 C to shoot them toward the substrate surface as atoms or molecular beams with an appropriate angular distribution, which is important for the thickness of MBE film. Not all elements come out as atom beams because of the stick coefficient in the substrate surface. For example, in III-V growth, group V does not stick to the surface in the

27 absence of group III atoms, so it depends on group III flux arriving. Thus, it comes out as tetratomic or dimeric molecule beams such as As 4 or As 2. In front of each effusion cell, the shutter works as a switch to control the beam from reaching the substrate, which occurs less than.1 second compared with a growth time of 1 monolayer per second. The substrate, such as GaAs (1) wafer in III-V growth, is mounted on a rotatable holder to ensure its required position, and it is heated up to 1 C. The MBE system uses an ultrahigh vacuum environment (UHV), in which the pressure reaches 1-11 torr. In addition, UHV ensures that the growth surface is clean and pure. Surrounding the effusion cells and the substrate are cryopanels, which are cooled with liquid nitrogen. The cryopanels also condense stray and unused beams. The job of the mass spectrometer is to determine the atomic beam, distinguish between various vapor species, and detect problems with the vacuum. Finally, the electron gun shoots high-energy electrons (5 5 kev) to the growth crystal by angles (1-3 ); then the fluorescent screen receives a reflection of the high-energy electron diffraction (RHEED) patterns. These RHEED patterns provide a measure of the crystal growth rate, monitor the crystal during the growth, and give information about the crystal surface geometry such as roughness. In summary, the process in the MBE system impinges atoms and molecules on the substrate surface. Some of them (atoms and molecules) adsorb atoms, dissociate molecules, and incorporate them into the substrate lattice site, or desorbs the not-incorporated atoms to 4, 28,29,3. cryopanels 27

28 28 Figure 2.5. Schematic diagram of the growth chamber in molecular beam epitaxy process (after M. Cukr and V. Novák) 31. The growth chamber is the important chamber in the MBE. There are also two different systems that can be used in the growth chamber: vapor phase epitaxy (VPE) or liquid phase epitaxy (LPE). The first one uses gas materials in effusion cells, and the second uses liquid materials. VPE uses a rapid growth method with manufacturing volume but in high temperatures, which enhance bulk diffusion. LPE produces pure crystal, but not uniform thickness 28. In addition to the growth chamber in MBE, there are often one to two other chambers in modern systems. The interlock chamber installs the substrate to transport devices by opening the valve between the growth and the interlock chambers after pumping down the system. Then, the preparation or analyzing chamber cleans the substrate and measures the crystal before and after growth by using surface

29 29 characterization such as Auger Electron Spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS) 28, Stranski-Krastanow Technique: Stranski-Krastanow or Stranski-Krastanov (SK) growth first was described in 1937 by Stranski and Krastanow. In 1985 it was observed in an InAs/GaAs superlattice by Goldstein et al. 1, 1, 32. This growth forms what is called self-assembled QDs. As mentioned before, there are three growth modes, and their growth styles depend on lattice mismatch and the interface energies of two materials (the substrate and the adlayer). If the sum of the epilayar surface energy and the interface energy is less than the substrate surface energy, the Frank-van der Merwe (FM) mode will occur. However, if the substrate surface energy is less than the sum of the epilayer surface energy and the interface energy, the Volmer-Weber (VW) growth occurs 1, 4, 27. In the case of the Stranski-Krastanow mode, such as in InAs/GaAs, the deposited material wets the substrate forming a two-dimensional layer, which is called the wetting layer, via Frank-van der Merwe growth. After creating a pseudomorphic layer (few monolayers 1-5 ML) with the lateral lattice compression, the strain energy increases with a growing epilayer thickness as shown in figure 2.6. In critical thickness, the epilayer film will not accommodate a two-dimensional layers system. Hence, the coherent islands (three-dimensional QDs) will be formed to reduce the strain energy because the elastic relaxation energy depends on the island shape and volume. Further increasing of the epilayer growth leads to dislocated islands, which is illustrated by Vanderbilt and Wickham theory 4, 18, 33.

30 3 Figure 2.6. Diagram of Stranski-Krastanow growth mode (After Eberl et al.) Droplet Epitaxy: Another fabrication method, which forms a self-assembly of high-quality nanostructures, is droplet epitaxy (DE) growth. Droplet epitaxy also forms various types of nanostructures such as quantum dots, disks, dot molecules, and rings. The last form, rings, will be discussed in chapter three. DE growth does not require lattice mismatched or strain energy as Stranski-Krastanow growth does. In addition, the absence or appearance of the wetting layer can be controlled in this technique by changing the growth condition 35, 36, 37. The first step of droplet epitaxy (DE) method of III-V growth is applying group III elements under the absence of group V elements on the substrate such as GaAs at temperatures higher than their melting points to create a liquid metal droplet, which is explained by the Volmer Weber growth mode. The second step is shooting a molecular

31 beam of group V such as As4 for the crystallization of the group III droplet into an III-V nanostructure 35,38, Figure 2.7. Diagram of droplet epitaxy growth for GaAs quantum nanostructure (after Sanguinetti) 35.

32 32 CHAPTER 3: QUANTUM RINGS In the last section of chapter 2, I mention that different types of nanostructures may be formed using the droplet epitaxy growth technique. One of those nanostructures is the quantum ring; the quantum ring could also be grown by Stranski -Krastanow methods 39. Each method depends on growth conditions such as temperature, materials, etc., to form ring- shaped nanostructures or other kind of nanostructures. The quantum ring (QR) structures that I will be investigating result in a pair of quantum dots, which form on a ring or thin nanowire (wetting ring); in addition, there is a hole in the center of the system, as shown in figure 3.1. Figure 3.1. a) 2 x 2 µm two- dimensional AFM image of QRs in the sample. b) Threedimensional AFM image of a QR magnify. c) Details of one QRs dimensions along [1-1] and [11] directions from part b (after Liang et al.) Fabrication Approach of Quantum Rings: This thesis will focus on QRs grown at the University of Arkansas by droplet epitaxy (DE), using molecular beam epitaxy (MBE) system for InGaAs ring-shaped

33 33 nanostructures on semi-insulating GaAs (1) substrate. As shown in figure 3.2 in ref. 39, at the beginning, the surface oxide was desorbed and a.5 µm GaAs buffer layer was grown for 1 minutes at 6 C. Then, the GaAs substrate was cooled to 35 C at ~ torr. After reaching the growth temperature, an indium flux of 1 monolayer was applied to the GaAs surface to form In liquid droplets (figure 3.2a), which were hemispherical-shaped due to equilibrium configuration. The density for In droplets is around cm -2, which will be the density of InGaAs QRs. Subsequently, the In droplets were exposed for 2 minutes to highly over-pressured As 4 molecular flux, arsenic beam equivalent pressure (BEP) of Torr to crystallize the In droplets into InAs nanocrystals. During the crystallization or arsenization process, there is another process, which is called nanodrilling. The nanodrilling process is intermixing or exchanging between In droplets and Ga on GaAs substrate during an additional annealing time, during which a hole develops in GaAs substrate. At the same time, there is diffusion from the top to the bottom of In droplets edges, which is called downward-slope diffusion process. During the three processes (figure 3.2c), atoms traveled out from InAs nanocrystals to the substrate or to edges until the original InAs nanocrystals changed to InGaAs quantum rings, as shown in figure 3.1b and figure 3.2d. After that, the sample was capped with ~ 7 to 1 nm GaAs layer at ~ 5 to 6 C, which also has an effect on the final QRs shape 39, 4.

34 34 Figure 3.2. Schematic of the formation process of the InGaAs quantum rings (after Lee et al) 39. As shown in figure 3.1b, c and 3.2e, the shape of a quantum ring was asymmetric along [11] and [1-1] directions because of the highly anisotropic nature of the diffusion process on GaAs substrate surface 39. In addition, the two QDs in a QR are not identical in the height and the width 39, 4. The shape of QRs or their dimensions in the experiment were different from one QR to another in the same sample, while some of the formations 39, 4, were not completely quantum ring or quantum dot but between them in their shapes 41. Also, by changing temperature growth or controlling the opening of the As valve, different average-sizes of QRs occurred with different densities 39. In addition, in some different growth conditions, the double quantum rings occurred rather than single quantum rings; however, they could be transformed into single quantum rings 42.

35 Energy Transfer in a Quantum Ring: After discussing the fabrication of a QR and knowing its shape, which contains two QDs and a thick wetting ring, the goal of this thesis is to study the behavior of the charge carriers in such a QR and to find out whether a lateral coupling between the two quantum dots in a single quantum ring will occur and under what conditions. In general, there are different types of coupling that may occur between a QD pair. One type of coupling is tunneling between two QDs. The concept of this type of coupling comes from quantum tunneling theory, where the wave function of a particle, such as an electron, goes through the barriers of a potential box from one side to the other side, when the energy of the particle is less than the potential barrier ( E < V o ) as shown in figure 3.3 a. In the case of two QDs, the charge carriers of the first quantum dot s wave function, which we consider as being the incoming particle wave function, penetrates the barrier between the QDs. The probability of charge carriers wave function, which passes through the barrier, was reduced but not the energy. From the solution of Schrödinger equation, the transmission and reflection coefficient is given by equations, respectively 43 : (3.1) (3.2) where a is the distance between the two QDs and m is the mass of the charge carriers 43. This type has a short range of coupling because the probability of tunneling decreases exponentially for taller and wider barriers 44. Another type of coupling is emission and absorption, as shown in figure 3.3 b. Simply, when a photon from the first QD is emitted, the second QD absorbs this photon. This type has a long range of coupling. The third type

36 36 of coupling is Förster resonant energy transfer, which is also known as dipole - dipole interaction. When the laser excites the first QD, the energy of the electron transfers to the electron in the second QD, and then a photon may be emitted from the second QD. This transfer process may be a phonon-assisted mechanism as shown in figure 3.3 c. Also, this type has a long range of coupling, and is proportional to, where R is the distance between the QD pair 45.

37 37 a) b) c) Figure 3.3. Three types of coupling between a QD pair. a) Tunneling (after Zettili) 43. b) Emission and absorption. c) Förster resonant energy transfer.

38 38 In reference 4, Liang et al. claimed that transfer energy occurs between the two quantum dots, separated by 19 nm, in a quantum ring grown by droplet epitaxy at low density ~ 1 6 cm- 2. Due to the different sizes between the two QDs, the excitation in the narrower QD, which has the higher energy ground state, may relax to the broader QD, which has the lower energy ground state, as shown in figure 3.4 a below. They (Liang et al.) performed three experiments to test this theory. First, by using photoluminescence (PL) measurement, which will be discussed in detail in chapter four, the ensembles of QRs were exposed to a yttrium aluminum garnet (YAG) laser, 532 nm, with an excitation intensity of.1 W/cm 2 at 8 K. As a result, a PL emission around 1.29 ev was observed from the two QDs. The two peaks of a QD pair were separated by 15 mev, which is smaller than the separation between ground and excited states expected for QDs of these dimensions, therefore it is believed that these arise from the ground states of two separate QDs. Additionally, the Gaussian fitting of the PL spectrum displays a bimodal distribution behavior in their height distributions and subsequently their PL spectrums. The wetting ring may reduce the height of the barrier between the QDs, which makes coupling between them a possibility. In the data from reference 4, the population of excited states was avoided with variation of the excitation laser intensity to give the normalized QD PL spectra in low pump regime. Two additional peaks observed at 1.45 ev and 1.39 ev, were attributed to wetting layer (quantum well) and wetting ring (quantum wire), respectively. A second investigation looked at what happened when an individual QR was exposed to Ti: sapphire laser emitting at 72 nm focusing by lens at 4 K in micro - photoluminescence measurement. As shown in figure 3.4 b, two groups of

39 39 PL lines at and ev came from the two QDs, separated in energy by ~17 mev, similar to what was observed in the first investigation. Again, two other peaks observed at 1.45 ev and 1.37 ev, which were attributed to wetting layer and wetting ring, respectively. Finally, as shown in figure 3.4 c, when the excitation intensity increases, due to multiexciton and charged exciton recombination, more peaks appear in the of region X 1 (1.246 ev) and X 2 (1.269 ev) in addition to the two QDs ground state excitons. Another observation in figure 3.4 c, the excitation intensities that accompanied the two QDs are different although they were exposed to the same laser power. The lower energy (larger) QD showed a larger intensity at low laser power than the higher energy (smaller) QD, and with different pump intensities, the PL results would not change. This could be an indication of energy transfer from the high to low energy QD. However, it is possible that this happened because of spectral variation in the carrier relaxation coming from the wetting ring.. In fact, due to the large separation between QDs (~19 nm) and short lifetime of the excitons, it is not expected that coupling would occur. However, it could be that the wetting ring has a positive effect on the cross-talk between the QD pair in a QR by building a channel for the change carrier to transfer energy between them. Therefore, the system of coupling of lateral quantum dots in quantum ring or quantum dot molecule (QDM), in general, could be different from a vertical quantum dot pair 4.

40 4 a) b) c) Figure 3.4. a) The conduction band schematic of the two quantum dots of quantum ring. b) Micro- PL spectrum of two QDs in QR. c) Varying the excitation intensities of micro- PL spectrum of two QDs in QR (after Liang et al.) 4. The objective of this thesis is to further investigate the possibility of coupling between the QDs in this system. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra, which will be discussed in detail in chapter four, will be the methods to test whether there is coupling between the two quantum dots in a nanoring.

41 41 CHAPTER 4: EXPERIMENTAL MEASUREMENT AND SETUP In this chapter, the methodology used to study the potential coupling between a quantum dot pair in a single quantum ring will be explained. The concept of two experimental measurements and setup to study a single ring nanostructure and the coupling in it will be discussed in the first section, while the sample and lab equipment and setups will be described in the second section Experimental Measurement: As mentioned in chapter two, a single quantum dot has characteristic physical properties such as electrical and optical. By using the luminescence emitted from a QD, these properties are observed and measured from electroluminescence, cathodoluminescence, thermoluminescence, or photoluminescence 46. In our lab, we focus on optical properties of QD or coupled QDs (as in case of QR) by using the photoluminescence (PL) and photoluminescence excitation (PLE) techniques Photoluminescence Spectra: Photoluminescence (PL) is the process of absorption of a photon from a bulk or heterostructures semiconductor. As shown in figure 4.1, an electron in the valance band is excited from a light source such as a laser, across the band gap into the conduction band (wetting layer) leaving behind a hole. Then, the electron and the hole typically relax to ground states by emitting phonons. After that, they interact by Coulomb attraction leading to an electron-hole pair. This bound electron-hole pair is called an exciton (X), which is can recombine resulting in a photon 13, 47.

42 42 Figure 4.1. Photoluminescence process in a single QD. In addition, when high intensity lasers are used, a QD may emit multiple exciton complexes such as biexciton and triexciton. There are other kinds of bound complexes such as a negative trion (X -1 ), which is two electrons and a hole, and a positive trion (X +1 ), which is two holes and an electron 47. Because of the delta function density of states of QDs, the PL line is a narrow spectral line 48. When using continuous wave (CW), a number of PL lines appear in our experiment. However, to study the lifetime of excitons and energy levels of a single QD (resonant excitation), a pulsed laser is typically used. Non-resonant excitation occurs when a laser excites the electron from the valance band to continuum states in the conduction band (wetting layer or higher). However, resonant excitation occurs when a laser excites the electron from the valance band to discrete states in the conduction band (energy level of a QD) as shown in figure 4.3.

43 a) b) Figure 4.2. a) Photoluminescence spectra from a single QD shown emission from X, X -1, X -2, X -3, X -4 and X -5 excitons (after Warburton et.al.) 2. b) Power dependent Photoluminescence spectra from a single QD isolated shown exciton and biexciton in s- shell and p-shell (after Zrenner)

44 Photoluminescence Excitation Spectra: Photoluminescence spectrum is obtained in a fixed laser wavelength and usually in a non-resonant excitation. However, photoluminescence excitation (PLE) spectrum is obtained by varying laser wavelengths while monitoring a single PL line 46. During the scanning of the laser excitation, non-resonant and resonant excitations occur as shown in figure 4.3. Sometimes in PLE resonant excitation, the incident photon from the laser may not be absorbed. In addition, the charge carriers may not relax to the lowest state and may directly recombine. Those processes are described in this relation: (4.1) where and are emission intensity and the excitation intensity, respectively. are the probability of the laser absorption, the probability of charge carriers relaxations and the probability of exciton recombination after relaxation, respectively 46. Figure 4.3. Photoluminescence excitation process in a single QD.

45 Experimental Setup: Sample: Two samples (ARK_QRings_SC695 and ARK_QRings_SC694) with different growth temperatures were measured. As mentioned in chapter three, these samples have been provided by collaborators at the University of Arkansas. InGaAs quantum ring samples grew by droplet epitaxy (DE) using molecular beam epitaxy (MBE) system on semi-insulating GaAs (1) substrate. The first sample (ARK_QRings_SC695) was grown at a temperature of 45 o C, while the other sample (ARK_QRings_SC694) was grown at 35 o C. The quantum rings were distributed randomly in these samples as shown in figure 4.4. By using three of T-Cube DC Servo Motor Controller by Thorlabs Instrument, we moved the sample in horizontal and vertical directions, in addition to controlling the focus in Z direction. Those stepper motors could move ~ (.1 mm) per step to determine the right position for a single quantum ring. Figure 4.4. Position (Horizontal, Vertical, Focus) of an isolated QR.

46 Lab Description: The lab contains different equipment (figure 4.5) to study optical properties of nanostructures such as quantum wires, dots, and rings. The main equipment for our study of QRs is Tsunami system (Mode-Locked Ti: Sapphire laser) manufactured by Spectra- Physics instruments. It delivers continuously tunable output over a broad range of near infrared (IR) wavelengths from 69 nm to 18 nm. In addition to a continuous wave (CW), it produces a pulsed mode from 8 picoseconds to less than 5 femtoseconds. The maximum power beam from the Tsunami system is 2 W [User Manual, Tsunami system Mode-Locked Ti: Sapphire laser, Spectra-Physics the Solid-State Laser Company, June 22]. In the forefront of Tsunami system is Millennia Pro, which also is manufactured by Spectra-Physics instruments. A diode source in this system is pumped to high power up to 1 W of solid state CW green laser in one single pump, which goes through the Tsunami system after that [Manual, Laser Forefront The Millennia Pro Series: A New Standard of Reliability Spectra-Physics lasers and photonic, 24]. Another system used in the lab, which detects PL spectra from the sample, is Trivista spectrometer made by Princeton Instruments. Spectral resolution in Trivista system can reach 4 pm in the VIS spectral range (5 nm). It has three stages, which we are able to be used in various combinations of one, two, or three stages, and triple grating in each (3, 75, and 11 groove/mm). Two modes can be used: additive and subtractive modes (see Appendix A). In the end of the final stage, a liquid-nitrogen-cooled silicon, charge-coupled device (CCD) captures lights or photons after a PL spectrum goes through a Trivista spectrometer. It has 13 X 4 pixels [Manual, Trivista System, Princeton Instruments,

47 47 a division of Roper Scientific, Inc., September 25]. To keep the samples in low temperature, one of two cryostats was used: closed cycle cryostat by Advanced Research Systems Instruments or optical Super Tran cryostat by Janis Research Company. The closed cycle cryostat cools down the sample to ~ 2 K, and helium gas is used in this system. The second cryostat cools down to ~ 4.7 K or, in most of our experiment, to 7.5 K, and in this system, liquid helium was used. In both cryostats, the samples are in vacuum. In addition to the main equipment, the power in our experiment was in the range of ~ nw to mw; the power was controlled by the Laser Power Controller (LPC), manufactured by BEOC (Brockton Eletro-Optics Corps), and also could be reduced by neutral density filters. In addition to the power controller, a temperature controller by Lakeshore Instrument was used to control and read the samples temperature. Also, there are two reading devices: Optical Power and Energy Meter by Thorlabs Instrument (power reader), and a pulsed laser wavelength meter by Bristol Instruments (absolute laser wavelength reader). Most of this equipment is connected and controlled by Labview program by National Instruments. Most of this program was written by Professor Eric Stinaff, as shown in figure 4.6. Also, the data analyses and plots were used from Labview and Origin programs.

48 48 Figure 4.5. Lab and equipment setup. Figure 4.6. Three Labview programs; on the right side is the main system controlling the spectrometer, laser, CCD and show PL spectra. In the middle is PLE measurement of the Labview program. On the left is a QR position program Photoluminescence Setup: In the PL setup, two methods were used to expose the CW laser to the sample: oblique incidence or normal incidence, (figure 4.7). In both methods, one stage or three stages (subtractive mode) of TriVista System could be used to detect PL. However, in the normal incidence, a retarder (.5) and linear polarizer were set on excitation side, and

49 49 linear polarizer was set on detection side to reduce a laser background. Most of the power for PL was between a few nanowatts to hundreds of microwatts, and a laser wavelength between 78 nm and 82 nm was used. Typically a grating density of 3 groove/mm was used. The front slit width was 1 µm or 5 µm, and the binning of the CCD pixels was less than 3 and larger than 6 in most of measurements. Figure 4.7. Schematic of photoluminescence setup in oblique or normal incidences by adjusting the mirror to right position Photoluminescence Excitation Setup: In the PLE setup, three methods were used to expose the CW laser to the sample, as shown in figure 4.8. First, oblique incidence was used with one stage of the TriVista System. A retarder and linear polarizer in excitation side were set to rotate the laser polarization perpendicular to the polarizer in the detection side, to reduce a laser background. The second method was also oblique incidence, but used the three stages of TriVista System in subtractive mode. In this setup, the emission from the last quantum

50 5 dot in a single quantum ring was monitored by controlling the two side slits of the first two stages of the spectrometer, which act as a high rejection notch filter to eliminate the laser background. The third method was similar to the second method, but with normal incidence. In addition, a retarder (.5) and linear polarizer were set on excitation side, and a linear polarizer was set on detection side. The power was less than 3 microwatts in the first method, around 2 milliwatts or less in the second method, and less than 2 milliwatts in the third method. In the first method, the laser wavelength scanned from 78 nm to 98 nm. However, in last two methods the laser wavelength was usually scanned from 9 nm to longer wavelengths just before the laser entered to the monitored reign defined by the notch filter created by the first two stages. In addition, the front slit width was 1 µm, and the bin less than 3 and larger than 6 in most of measurements. Typically, the grating used was 3 groove/mm in the first method and 11 groove/mm in the last two methods. Figure 4.8. Schematic of photoluminescence excitation setup in oblique or normal incidences by adjusting the mirror to the right position for one and three stages of TriVista system.

51 51 CHAPTER 5: DATA AND RESULT In this chapter, results of our experiment are highlighted. Different emissions were observed from QRs in PL data and analyzed depending on the various parameters (power, energy, etc.). In addition, the coupling in a single QR was studied in PLE data Photoluminescence Data: Result 1: Shown in table 5.1 and figure 5.1 are different QRs in a single sample (ARK_QRings_SC695) with different bins, slit widths, filters, and gratings which were excited with laser wavelength 78 nm and power in nanowatts (microwatts per square centimeter) with room lights turned off and monitors turned on. In addition, the two setups: normal and oblique incidences and two cryostats: Closed cycle and Janis cryostats were used. Three main emissions were observed in ranges between 84 nm to 86 nm (1.476 ev to 1.44 ev), between 89 nm to 913 nm (1.393 ev to ev), and between 94 nm to 965 nm (1.319 ev to ev). Those emissions are identified as coming from wetting layer, wetting rings, and QD pairs, respectively, which are almost at the same PL energy position described in ref. 4. The peak around 85 nm, which may represent the wetting layer, is broad and almost has a Gaussian shape; moreover, a small broad peak nears the wetting layer at 835 nm, which is from the GaAs substrate. From our observation, the wetting rings have from one to two narrow groups of peaks, while QD pairs have from two to four narrow groups of peaks. There are additional peaks to those main emissions at different wavelengths. Some of them come from the room light or monitors, another comes from laser pumping, and others are random peaks from

52 52 cosmic rays. To define some of those peaks, the monitors ware turned on and off at the same QR as shown in figure 5.2. At approximately 815 nm, 915 nm, 925 nm, 97 nm, and 115 nm several peaks appeared when the monitors were turned on and did not when the monitors were turned off. Those peaks are certainly from the monitors. However, the peaks around 985nm (figure 5.2 a) and around 975 nm (figure 5.2 b) remain and may therefore be from QRs, charged excitons, defects, impurities or the laser. Because of those multiple possibilities, we did experiments described in the following section to reduce the number of those possibilities. Table 5.1 Different QRs in ARK_QRings_SC695 sample with laser wavelength 78 nm at room lights off and monitors on. Slit Laser Power Gratin temperature QR bin Width (nw) \ incidence Filter g (gv) (µm) [µw/cm 2 (K) ] \ [2] 7.5 Normal \ [2] 21.4 Oblique none \ [.4] Oblique \ [2.7] 7.5 Normal \ [1.3] 7.5 Normal \ [2] 7.5 Oblique \ [2.7] 7.5 Normal 8

53 53 ARK_QRings_SC695, λ Laser : 78nm, Room lights OFF, monitors ON. PL Energy (ev) PL Intensity (count/ s) QR1_ QR2_ QR3_ QR4_ QR5_ QR6_ QR7_ PL Wavelength (nm) Figure 5.1. Different QRs from table 5.1 in ARK_QRings_SC695 sample with laser wavelength 78 nm shown three main groups of emissions: wetting layer, wetting ring, and a quantum dot pair.

54 54 Sample: ARK_QRings_SC695 Janis cryostat, T : 7.5 K. normal incidence, Grating: 3 gv, bin: 26, silt with: 1 µm P laser ~ 135 nw (2.7 µw/cm2). λ Laser :78nm. Room lights OFF, filter 85. Monitors ON_ Monitors OFF_ PL Intensity (count/s) PL Wavelength (nm) ARK_QRings_SC695, Janis cryostat, T : 7.5 K. Normal incidence, Grating: 3 gv, bin: 22, silt with: 1 µm P laser ~ 6.5 nw (132.6 nw/cm 2 ). λ Laser :78 nm. Room lights OFF, filter 8. 7 a) Monitors ON_ Monitors OFF_ PL Intensity (count/s) PL Wavelength (nm) b) Figure 5.2. The emission from the same two QRs at different conditions: monitors ON or OFF.

55 Result 2: The first result displays almost the whole emissions from the sample for an isolated QR in general. By reducing the power to the lowest power possible for different QRs and focusing at QR ranges as shown figure 5.3, two groups or two main peaks were observed, which were also observed in ref. 4, as shown in figure 3.4 b. Those two peaks differed in the wavelength for different QRs; however, the wavelengths are still between 94 nm to 965 nm. In addition, the separation between them varies from one QR to another. Those differences are because of varying sizes or shapes from one to another. In general, those two peaks may emit from two ground states of a QD pair, which also differ in the energy levels because of the different sizes between the two QDs. Although the power was reduced to the lowest power, more than two peaks were observed from some QRs. Those multiple lines may emit from multiple localization centers. This means that when the QRs were grown, some of them may not have a final shape of two perfect QDs as what we assumed before. They may form multiple islands instead of a QD pair. It also may be from a charge or impurity in the sample. Again, to reduce the possibility of knowing which of those peaks may come from the QR or not, we did experiments described in the following section.

56 56 ARK_QRings_SC695, closed cycle cryostat, T ~ 21.3 K. Oblique incidence, Grating: 3gv, Slit Width:1 µm, bin: 6 and 9 λ Laser :78 nm, P laser ~ 2 nw (4.8 nw/cm 2 ), Filter : 9, Monitors ON PL Energy (ev) QR1_ QR2_ PL Intensity (count/s) QR3_ QR4_ QR5_ QR6_ QR7_ PL Wavelength (nm) a)

57 57 ARK_QRings_SC695, closed cycle cryostat, T ~ 21.3 K. Oblique incidence, Grating: 3gv, Slit Width:1 µm, bin: 6 and 9 λ Laser :78 nm, P laser ~ 2 nw (48 nw/cm 2 ), Filter : 9, Monitors ON PL Energy (ev) QR1_ QR2_ PL Intensity (count/s) QR3_ QR4_ QR5_ QR6_ QR7_ PL Wavelength (nm) b) Figure 5.3. Two groups of peaks emit from QD pairs in QRs at lower power a) 2 nw (4.8 nw/cm 2 ). b) 2 nw (48 nw/cm 2 ).

58 Result 3: From the first and second results, the two peaks between 94 nm to 965 nm were identified as emitting from two quantum dots in a single quantum ring and the peaks between 89 nm to 913 nm emit from the wetting ring. To be sure of that identification, we used normal incidence setup, where the laser spot size is very small, around 1 micrometer, and moved the sample horizontal by micrometers, using the horizontal T- Cube DC Servo Motor Controller. As shown in figure 5.4, the peaks from the QDs and wetting ring gradually disappeared by moving small horizontal steps. The small peaks at 815 nm, 915 nm, 925 nm and 97 nm do not change or disappear during the moving, which means that those peaks are not from QRs. Also, in figure 5.4 a, at around nm, a peak behaves as the peaks of the QR, which means that it may come from the QR, although this peak is far from them. Similarly, in figure 5.4 b, the two peaks around nm and nm behave as the peaks of the QR. Those peaks are most likely not from the laser pumping, charge, or impurity in the sample because they should not slowly disappear when the sample slowly moved. We are still working to identify the origin of these peaks.

59 59 ARK_QRings_SC695, Janis cryostat, T : 7.5 K. Normal incidence, Grating: 3 gv, bin: 22, slit with: 1 µm P laser ~ 6.5 nw (132 nw/cm 2 ). λ Laser :78 nm, filter 8 Room lights OFF. Monitors ON. QR_ Moved by -.2 mm_ Moved by -.28 mm_ Moved by -.31 mm_ Moved by -.33 mm_ Moved by -.35 mm_ Moved by +.2 mm_ PL Intensity (count/s) PL Wavelength (nm) a)

60 6 ARK_QRings_SC695, Janis cryostat, T : 7.5 K. Normal incidence, Grating: 3 gv, bin: 22, slit with: 1 µm P laser ~ 1 nw (2 µw/cm 2 ). λ Laser :78 nm, filter 8 Room lights OFF. Monitors on _moved by.37 mm _moved by.15 mm _moved by -.1 mm _moved by -.22 mm _moved by -.5 mm _ QR 8 7 PL Intensity (count/s) PL Wavelength (nm) b) Figure 5.4. Moving the sample small horizontal steps from QR in Normal incidence setup to show the peaks that emit from witting ring and a QD pair.

61 Result 4: Another interesting result was that by increasing the power for the same QR in the same conditions of laser wavelength exposed, temperature, grating, bin, slit width, filter, and incidence setup, as shown in figure 5.5, additional peaks appear beside the two main groups of peaks that come from a QD pair were observed. This result was also observed in ref.4 as shown in figure 3.4 c in chapter three. Indeed, under very high power those narrow peaks come together to form almost one broad peak. One possibility of those additional peaks is that they are coming from biexcitons in s-shell and from excitons in P-shell. In figure 5.5 a, at the lowest power 1 nw (24 nw/cm 2 ), two peaks around 98 nm and 983 nm may be from QDs in excitation state.

62 62 ARK_QRings_SC695,closed cycle cryostat, T : 21.4 K. Oblique incidence, Grating:11gv, Slit Width:5 µm, bin 26 λ Laser :78 nm, Filter : none PL Intensity (count/s) uw_ mw/cm 673 uw/cm 2 33 uw_ uw/cm 2 1 uw_ uw/cm 2 3 uw_ uw/cm 2 1 uw_ uw/cm nw_ uw/cm 2 1 nw_ nw/cm 2 33 nw_ nw/cm PL Wavelength (nm) 1 nw_ a)

63 63 ARK_QRings_SC695, closed cycle cryostat, T : 21 K. Oblique incidence, Grating:11gv, Slit Width:1 µm, bin: 7 λ Laser :78 nm, Filter : none,monitors OFF µw/cm 2 2 nw _ PL Intensity (count/s) nw/cm 2 2 nw_ nw/cm 2 2 nw_ PL Wavelength (nm) b) Figure 5.5. PL depending on the varying power wavelength and fixing the laser wavelength and other conditions for the same QR Photoluminescence Excitation Data: Result 5: By using the first method, the oblique incidence in one stage of the TriVista System, changing from narrow to broad QDs spectra and the opposite were observed by varying the laser wavelengths; in addition, the PL spectra of QDs disappeared at almost nm and again appeared at 83 nm until 845 nm or almost 85 nm with less intensity as shown in figure 5.6. In figure 5.6 b, c and d, the whole range from 78 nm to 1 nm was displayed, and as shown in result one, there are peaks from the QD, wetting

64 64 layer, and wetting ring, which all disappeared at laser wavelength 845 nm. At almost 99 nm in figure 5.6 c, and at 99 nm and 995 nm in figure 5.6 d may be peaks from QDs, as discussed in result three, because they also disappeared at laser wavelength 845 nm. In addition, the peaks at 815nm, 915 nm, 925 nm and 97 nm did not behaved as QRs emission and disappeared at a laser wavelength of at 845 nm, which also means that those peaks are not from QRs. With this method, we did not find any result of coupling between two QDs in a QR even though the power reached to 3 µw (612 µw/cm 2 ) as shown in figure 5.6 d. ARK_QRings_SC695, Closed cycle cryostat, T~ 21K Oblique incidence, Grating:75gv, Slit Width: 2, bin 21 P laser ~14 nw (2.8 µw/cm 2 ), Intensity fixed at 8 Room lights ON, Monitors ON. Filter :none PL Wavelength (nm) Laser Wavelength (nm) a)

65 65 PL Wavelength (nm) ARK_QRings_SC695, Janis cryostat, T~ 5.9K Oblique incidence, Grating: 3gv, Slit Width: 5 µm, bin 16, Filter :none P laser ~255 nw (5.2 µw/cm 2 ) Room lights ON, Monitors ON. Laser Wavelength (nm) b)

66 66 PL Wavelength (nm) ARK_QRings_SC695, Closed cycle cryostat, T~ 21K Oblique incidence, Grating: 3gv, Slit Width: 5 µm, bin 13, Filter :none P laser ~7 µw (142 µw/cm 2 ) Room lights ON, Monitors ON. Laser Wavelength (nm) c)

67 67 PL Wavelength (nm) ARK_QRings_SC695, Closed cycle cryostat, T~ 21K Oblique incidence, Grating: 3gv, Slit Width: 5 µm, bin 13, Filter :none P laser ~ 3 µw (612 µw/cm 2 ) Room lights ON, Monitors ON. Laser Wavelength (nm) d) Figure 5.6. PLE spectra of three QRs shown the changing of QDs peaks between narrow and broad.

68 Result 6: Using the oblique and normal incidences of PLE in subtractive mode and three stages of the TriVista System, among several experiments (more than 45 QRs) which did not show any coupling as shown in figure 5.7 a and b, we found in three QRs the possibility of coupling between a QD pair in a QR as shown in figure 5.7 c, d and e. In most of those experiments, two optical phonon modes associated with GaAs crystal (Raman spectra of GaAs) were observed during the laser scanning. When the laser (photons) interacts with GaAs molecular vibrations and scatters them, the photons lose energy and phonons (atoms oscillations) carry away the same amount of lost energy. Because the laser was scanning or moving, the optical phonons emission were moving and were usually observed in laser wavelength ranges between 935nm to 945nm depending on where the laser scatters the GaAs lattice. The optical phonon energy in GaAs lattice is.35 ev 49.

69 69 PL Wavelength (nm) Laser Wavelength (nm) ARK_QRings_SC695 Janis cryostat, T ~ 8.51 K Grating: 3gv- 3gv- 3gv, Slit Width:1 µm, bin: 1 Oblique incidence. Filter : 85. P laser~ 17 mw (.34 W/cm 2 ), Intensity fixed at 8 Room Light on. Monitors On. monitoring is at this particular wavelength (Bristol reading nm) 27 a)

70 7 PL Wavelength (nm) Laser Wavelength (nm) ARK_QRings_SC695 Janis cryostat, T ~ 7.5 K Grating: 11_11_11 gv, bin: 15, slit with: 1 µm Normal incidence. Filter : 85 P laser ~ 1.2 mw (24.4 mw/cm 2 ), Intensity fixed at 6 Room lights ON. Monitors on monitoring is at this particular wavelength ( Bristol reading nm) b)

71 71 PL Wavelength (nm) Laser Wavelength (nm) ARK_QRings_SC695, Closed cycle cryostat, T ~ 21.38K Grating:3-3-3gv, Slit Width:1 µm, bin: 17 Oblique incidence P laser ~4.5 mw (91.8 mw/cm 2 ), Intensity fixed at 1 Room Light on. Filter :none. monitoring is at this particular wavelength 989 nm (Bristol reading nm) c)

72 72 PL Wavelength (nm) Laser Wavelength (nm) ARK_QRings_SC695 Janis cryostat, T ~ 7.5 K Grating: 11_11_11 gv Slit Width:1 µm, bin: 11 Normal incidence. Filter : 85 P laser ~6 µw (12.2 mw/cm 2 ), Intensity fixed at 6 Room lights ON. Monitors On. monitoring is at this particular wavelength 965 nm ( Bristol reading nm) d)

73 73 PL Wavelength (nm) Laser Wavelength (nm) ARK_QRings_SC695 Janis cryostat, T ~ 7.5 K Grating: 3_3_3 gv, bin: 18, slit with: 1 µm Oblique incidence. Filter : 85 P laser ~ 6 mw (.12 W/cm 2 ), Intensity fixed at 8 Room lights ON. Monitors ON At this particular wavelength 959 nm (Bristol reading nm) e) Figure 5.7. PLE data where the second QD monitored and laser scanned to it. a) and b) No sign of coupling in QR. c) and d) possibility of coupling in QRs.

74 74 In figure 5.7 c, a PL peak at wavelength nm appeared when the laser wavelength at almost 975 nm (Bristol at nm), while a PL peak at wavelength nm appeared during laser scanning. The next day, we returned to the same QR and did PLE with the Monitor Wavemeter on (taking Bristol reading) and observed that when the laser wavelength at nm in Bristol reading, a PL peak at wavelength nm appeared again, as shown in figure 5.8 a. In figure 5.8 b, a PL spectrum for the same QR at a laser wavelength 87 nm with a high power 39 µw (7.9 mw/cm 2 ) has multiple narrow peaks distributed in three main groups. We noticed that the separate energy between the peak ( nm), where we believe the laser energy is, and the peak (954 nm), where a photon emission was observed, is almost 22.5 mev.

75 75 PL Wavelength (nm) Laser Wavelength (nm) ARK_QRings_SC695, Closed cycle cryostat, T ~ 2.94K Grating:3-3-3gv, Slit Width:1 µm, bin: 19 Oblique incidence P laser ~11.2 mw (228 mw/cm 2 ),,Intensity fixed at 7 Room Light on. Filter :none. monitoring is at this particular wavelength 989 nm (Bristol reading nm) a)

76 76 ARK_QRings_SC695, Closed cycle cryostat, T : K. Oblique incidence, Grating: 3 gv, bin: 17, slit with: 1 µm P laser ~ 39 µw (7.9 mw/cm 2 ). λ Laser : 87 nm, filter: 9 Room lights OFF. Monitors ON nm nm 3 PL Intensity (count/s) Wavelength (nm) b) Figure 5.8. a) PLE for the same QR in figure 5.7 c focusing in part of where a photon was observed when laser wavelength at nm in Bristol reading. b) PL spectrum for the same QR shown where two wavelength of the laser and photon emission. In figure 5.7 d, a PL peak at wavelength nm appeared when the laser wavelength was at almost nm. To investigate more, we returned to the same laser wavelength (almost nm, while Bristol read at nm) and did a quick PL measurement; we noticed an emission at nm, as shown in figure 5.9 a. Then the middle side slit was opened and the laser was measures to be at a wavelength of 949 nm. In addition, as shown in figure 5.9 b, by changing the power hitting this QR at laser wavelength 82 nm, we monitored the two wavelength lines (949 nm and nm). At

77 77 the lowest power 6 nw (122 nw/cm 2 ), three broad peaks (948 nm, 951 nm and 954 nm) emitted and with increasing the laser power the three peaks become multiple narrow peaks. The peak at nm appeared at laser power 7 nw (1.42 µw/cm 2 ); the other peaks at 949 nm appeared at laser power 288 nw (5.87 µw/cm 2 ). The energy separation between these two peaks is almost 9 mev, while almost 7.9 mev is the energy separation between the first peak (948 nm) and the last peak (954 nm), which were emitted from QR. ARK_QRings_SC695, Janis cryostat, T ~ 7.5 K Grating: 11_11_11 gv, Slit Width:1 µm, bin: 11 λ Laser : nm, (Bristol at nm ). the middle slit open to see laser line of reading PL in Blue PL the middle slit close Normal incidence. Filter : 85, Room lights ON. Monitors On 949. nm 2 laser line_ uw_ PL Intensity (count/s) nm mw/cm PL Wavelength (nm) a)

78 78 ARK_QRings_SC695, Janis cryostat, T ~ 7.5 K Grating: 11_11_11 gv, Slit Width:1 µm, bin: 11 λ Laser : 82 nm, (Bristol at nm ). P laser : from 6 nw to 19.2 µw (122 nw/cm 2 to 391 µw/cm 2 ) Normal incidence. Filter, Room lights OFF. Monitors On uw/cm 2 72nW_ uw/cm uw_ uw/cm 2 7 nw_ uw/cm uw_ uw/cm 2 5 nw_ uw/cm uw_ PL Intensity (count/s) nw/cm 2 4 nw_ nw/cm nw/cm nw/cm 2 36nW_ nW_ nw/cm 2 9 nw_ nw_ PL Wavelength (nm) 12 uw/cm uw/cm nw_ uw/cm 2 144nW_ uw/cm 2 6 nw_ nw_ uw/cm 2 8 nw_ b) Figure 5.9. a) Two PL spectra for QR in figure 5.7 d shown where two wavelength of the laser and photon emission. b) The same QR with different powers at laser wavelength 82 nm, in addition, two wavelengths of the laser and photon emission were shown.

79 79 At figure 5.7 e, three PL peaks at around wavelength 961 nm appeared when the laser wavelengths were at nm, 95.8 nm, and nm (Bristol at nm, nm, and nm, respectively), while two PL peaks at wavelength 958 nm and 964 nm appeared during laser scanning. Again, we returned to the same QR and did PLE with the room lights and monitors turned off to see whether the two lines at 958 nm and 964 nm disappeared, but they did not. To investigate more, we returned to the same laser wavelength (955.2 nm, while Bristol read at nm) and did a quick PL measurement; we noticed an emission at 961 nm, as shown in figure 5.1 b. Then, the middle side slit was opened, and the laser was measured to be at a wavelength of nm. We did the same thing when the laser was at 95.8 nm and nm, which showed the laser at wavelengths nm and nm. In addition, as shown in figure 5.1 e, by changing the power to this QR without closing the side slits at laser wavelength 82 nm, we monitored the four wavelength lines (961 nm, nm, nm and nm). At the lowest power 6 nw (122 nw/cm 2 ), two narrow peaks ( nm and nm) emitted and when we increased the laser power additional peaks appeared similar to result four. The two peaks at 961 nm and appeared above laser power 24 nw (4.8 µw/cm 2 ); the other peaks at nm appeared at laser power 12 nw (244 nw/cm 2 ). However, the peak at nm is near the line of ground state of one QD at power 6 nw (122 nw/cm 2 ). The second line of the other ground state s QD at nm near to 958 nm peak that appeared in PLE along the laser scanning. The separate energy between the first peak ( nm) and the last peak ( nm), which emitted from the QR, is almost 13.3 mev. The energy separations between the laser wavelength

80 8 lines nm, nm, and nm and the peak 961 nm are 17.4 mev, 22.2 mev, and 25.8 mev, respectively. PL Wavelength (nm) Laser Wavelength (nm) ARK_QRings_SC695 Janis cryostat, T ~ 7.5 K Grating: 3_3_3 gv, bin: 18, slit with: 1 µm Oblique incidence. Filter : 85 P laser ~ 6 mw (122 mw/cm 2 ), Intensity fixed at 8 Room lights OFF. Monitors OFF At this particular wavelength 959 nm (Bristol reading nm) a)

81 81 ARK_QRings_SC695, Janis cryostat, T ~ 7.5 K Grating: 3_3_3 gv, Slit Width: 1 µm, bin: 18 λ Laser : nm, (Bristol at nm ). the middle side slit open to see laser line of reading PL in Blue PL the middle side slit close Oblique incidence. Filter : 85, Room lights ON. Monitors On nm Laser line_ mw_ PL Intensity (count/s) nm mw/cm PL Wavelength (nm) b)

82 82 ARK_QRings_SC695, Janis cryostat, T ~ 7.5 K Grating: 3_3_3 gv, Slit Width: 1 µm, bin: 18 λ Laser : 95.8 nm, (Bristol at nm ). the middle side slit open to see laser line of reading PL in Blue PL the middle side slit close Oblique incidence. Filter : 85, Room lights ON. Monitors On nm 961 nm laser line_ mw_ PL Intensity (count/s) mw/cm 2 Phonon PL Wavelength (nm) c)

83 83 ARK_QRings_SC695, Janis cryostat, T ~ 7.5 K Grating: 3_3_3 gv, Slit Width: 1 µm, bin: 18 λ Laser : nm, (Bristol at nm ). the middle side slit open to see laser line of reading PL in Blue PL the middle side slit close Oblique incidence. Filter : 85, Room lights ON. Monitors On nm laser line_ mw_ PL Intensity (count/s) mw/cm nm Phonons PL Wavelength (nm) d)

84 84 ARK_QRings_SC695, Janis cryostat, T ~ 7.5 K Grating: 3_3_3 gv, Slit Width:1 µm, bin: 18 λ Laser : 82 nm, (Bristol at nm ). P Laser : from 6 nw to 6 µw (122 nw/cm 2 to 122 µw/cm 2 ) Oblique incidence. Filter: 85, Room lights OFF. Monitors On uw/cm 2 16 nw_ uw/cm 2 1 nw_ uw/cm 2 6 uw_ uw/cm uw_ PL Intensity (count/s) uw/cm 2 8 nw_ uw/cm nw/cm nw/cm 2 6 nw_ nw/cm 2 4 nw_ nw/cm 2 24 nw_ nw_ nw_ PL Wavelength (nm) 24.4 uw/cm uw_ nw_ uw/cm 13.2 uw/cm 2 65 nw_ nw_ uw/cm 2 38 nw_ uw/cm 4.8 uw/cm 2 24 nw_ e) Figure 5.1. a) PLE for the same QR in figure 5.7 e focusing in part of where a photon was observed when laser wavelengths at nm, 95.8 nm and nm when room light and monitors ON. b), c) and d) Two PL spectra for the same QR shown where three wavelengths of the laser and photon emission. e) The same QR with different powers at laser wavelength 82 nm, in addition, three laser wavelengths lines and a photon emission line were shown.

85 85 CHAPTER 6: DISCUSSION AND CONCLUSIONS The photoluminescence and photoluminescence excitation results presented in chapter five will be summarized and discussed in this chapter. From result one and five, three main regions: wetting layers (between 84 nm to 86 nm), wetting rings (between 89 nm to 913 nm), and quantum dot pairs (between 94 nm to 965 nm) were observed from different QRs in varying conditions of temperatures, incidences, etc. Excluding the peaks that emit from the monitors or room lights (as shown in result one, three, and five), some peaks appeared in different wavelengths above the 975 nm. Those peaks (the last ones I mentioned) have different possibilities where they come from, for example, QRs (excitons or biexcitons), charge, defect, or impurity in the sample or also laser pumping. However, from result three and five, some of those peaks behaved as the emissions that come from QRs; hence, they may emit from QRs as excitons, biexcitons or multi-exciton. As a function of power, the two peaks or groups of luminescent lines may emit from two ground states of a QD pair in a single QR at the lowest power as shown in result two, while the additional peaks, which appeared by increasing the power as shown in result four, may emit from biexcitons in ground states and from excitons in excited states. Some of the QR data did not show two groups of lines at the lowest power, but three (as in figure 5.9 b) to four. Thus, some QRs may not have two perfect QDs; they may have three, four or multiple islands. At this point, we are not sure where those lines (the line wavelengths above the 975 nm and the lines beside the two groups at lowest power) come from, but there are some experiments that we will try in the future such as photon correlation to investigate them more. Also, our CCD is not having enough sensitivity

86 86 above 1 nm wavelength, so we will examine this region with an additional detector in the future to see if there is emission in that longer wavelength region. In addition to the sample (ARK_QRings_SC695), which were where most of our results are from, we will examine more the other sample (ARK_QRings_SC694), where we expect that the QD pair emission will be different from the emission in the first sample because of the difference of two sample growths. Moving to PLE spectra in result five, as a function of the laser, the spectra of a QD pair changed from one broad line to two narrow lines and again from two narrow lines to one broad line during laser scanning from 78 nm to 85 nm, with the wavelengths lower than the wetting layer wavelength. We are still not sure why this happened; however, it may be that when the spectrum is one broad line, the laser is absorbed more efficiently at that wavelength, and when the spectra are two narrow lines, the laser is absorbed less efficiently at that wavelength. In other words, although the power is fixed when the laser is scanning, the power absorbed may change depending on laser wavelength. From results one to five, we tried to study the spectra that emitted from a single QR and focused at QD pair s region, while the possibility of coupling or energy transfers between the QD pair in a single QR are displayed in results five and six. However, in result five (PLE), we did not find any sign of coupling because the power may be too low, while in result six only three QRs showed signals of a possibility of coupling among almost 5 QRs examined. The failure to find the coupling between those amounts of data may be because of the experiment s environment in the lab or physical properties of QRs. One of the challenges in the lab was that the sample drifted and vibrated over time. It

87 87 may be that the separation between the two QDs is too big to allow energy transfer to happen. The PLE signal in three QRs may be coupling or may not. Also, the coupling may happen in some QRs which have special conditions. For example, instead of two QDs there may be three or more QDs in a single QR; as a result, the separations between those QDs are small, as what happened in second QR (figure 5.9 b). By comparing the second and third QRs in result six as a function of power, we noticed that two peaks from two QRs appeared at a power above 1 nw (2 µw/cm 2 ), and may be from excited states of QDs. Finally, we need to do more experiments in those QRs after we exclude the lab challenges, which I mentioned above. In addition, we will try photon correlation experiment for those three QRs. We want also to apply an electric field to the sample to see if additional information can be gained from the Stark effect.

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92 92 APPENDIX A: SUBTRACTIVE MODE IN TRIVISTA SYSTEM In the triple subtractive mode, the three stages of the TriVista system are used as shown in figure A.1 a. The mission of this mode is stray light rejection with CCD detector. As shown in figure A.1 b, through the entrance slit (S 1 ), first the polychromatic light enters the first stage. Then, it is dispersed by the first grating (G 1 ). Some of the light in current wavelengths pass the first side slit (S 1,2 ); those dispersed lights are recombined by the second grating (G 2 ) to pass the second side slit (S 2,3 ). Finally, the last grating (G 3 ) directs the passing light to CCD detector (D). Here, the first and second stages work as a band pass filter and G 1 and G 2 should be the same, so they cancel each other [Manual, Trivista System, Princeton Instruments, a division of Roper Scientific, Inc., September 25]. a) b) Figure A.1. a) Schematic of what inside the three stages of TriVista system. b) Subtractive mode process. a) and b) both from [Manual, Trivista System, Princeton Instruments, a division of Roper Scientific, Inc., September 25].

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