Ion channeling studies of defect formation in gan and related materials

ION CHANNELLING STUDIES OF DEFECT FORMATION IN GaN AND RELATED MATERIALS MUKHTAR AHMED RANA (M.Sc. PHYSICS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2005 With the name of Allah, the most Gracious, the most Merciful Like for others what you like for yourself - Prophet Muhammad (peace be upon him) Acknowledgements The kind help of our group members at Center for Ion Beam Applications is acknowledged with special thanks to my supervisors Thomas Osipowicz and Mark Breese and Director of the Center Frank Watt for their guidance. Many thanks to engineer Theam Choo for his technical help during experiments. The guidance of Leszek Wielunski, Andrew Bettiol, Jeroen van Kan, Ms Minqin Ren, Shao Peige and Istvan on several occasions is thankfully acknowledged. The help of Huang Long and Markus Zmeck during experiments is also appreciated. Contributions from our collaborators, Dr Ian Watson at Institute of Photonics, University of Strathclyde, UK. Prof S.J. Chua, Dr Anthony Choi, Dr Chen Peng at Center for Optoelectronics, Faculty of Engineering, NUS. Mr Y.Y. Liu and Assoc Prof Thong Leong at Centre for Integrated Circuit Failure Analysis and Reliability, Faculty of Engineering, NUS. Assoc Prof Andrew Wee at Surface Science Laboratory, Faculty of Science, NUS. Assoc Prof Shen Xiang and Dr Wang Sun at Department of Physics, Faculty of Science, NUS. is gratefully acknowledged. Very thankful to Peter Smulders at University of Groningen, Netherlands, who computer code FLUX used for ion channelling simulations presented in the thesis. Special gratitude to my favorite poet Allama Muhammad Iqbal whose poetry refined my vision in general. My thankfulness to my parents, wife Shamila and kids Saad and Abuzar for their patience, which gave me courage to continue this research work. Summary In recent years, GaN and its alloys have played a major role in blue, green and ultraviolet light emitting devices, which are essential components of full-color displays, high density data storage systems and range of other applications. Defects in such materials control basic processes and affect electronic and optical properties. RBS/channeling, channelling contrast microscopy and ionoluminescence techniques were used to study defect formation in sapphire-coherent and lateral growth of GaN. Thermal stability of GaN is investigated, quantitatively, over a wide range of temperature 500-1100 o C using RBS/channelling with depth resolution of 5-20 nm. Structural and optical properties of InGaN, used as light emitting medium in GaN based light emitting diodes and laser diodes, are also studied. For this, RBS/channelling, x-ray diffraction spectrometry and photoluminescence were used. All the defects found in crystals can ultimately be resolved into lattice translations and rotations. Monte Carlo simulations were used to study the effects of lattice translations and rotations on ion channelling, the major technique used for defect analysis of crystals. The conditions of magnitude and depth of lattice translations are determined under which channelling and dechannelling are enhanced. A condition of super-channelling along (110) planar channels in Si crystal produced due to a single interface rotation is determined. Contents Abstract Acknowledgements Contents List of Tables List of Figures 10 Introduction 16 1.1 General introduction . . . . . . . . . . . . . . . . . . . . . . . . 16 1.1.1 GaN and lighting technology . . . . . . . . . . . . . . . . 16 1.1.2 Ion channelling . . . . . . . . . . . . . . . . . . . . . . . 18 1.2 Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.3 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 1.4 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 GaN and related materials 24 2.1 A brief historic review of GaN research . . . . . . . . . . . . . . 24 2.2 Physics of GaN and related compounds . . . . . . . . . . . . . . 26 2.2.1 Crystal structure . . . . . . . . . . . . . . . . . . . . . . 26 2.2.2 Electronic band structure . . . . . . . . . . . . . . . . . 26 2.2.3 Properties of GaN and related materials . . . . . . . . . 26 2.3 2.4 Growth and device fabrication processes . . . . . . . . . . . . . 27 2.3.1 Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.3.2 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.3.3 Plasma etching . . . . . . . . . . . . . . . . . . . . . . . 32 Defects in GaN and related compounds . . . . . . . . . . . . . . 32 Experimental facilities and analytical techniques 3.1 3.2 34 Experimental facilities . . . . . . . . . . . . . . . . . . . . . . . 34 3.1.1 General layout . . . . . . . . . . . . . . . . . . . . . . . 34 3.1.2 The 3.5 MeV Singletron Accelerator . . . . . . . . . . . . 35 3.1.3 Nuclear Microprobe . . . . . . . . . . . . . . . . . . . . . 36 3.1.4 Goniometer . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.1.5 Scanning system . . . . . . . . . . . . . . . . . . . . . . 38 3.1.6 Data acquisition system . . . . . . . . . . . . . . . . . . 38 Analytical techniques . . . . . . . . . . . . . . . . . . . . . . . . 38 3.2.1 RBS/channelling . . . . . . . . . . . . . . . . . . . . . . 39 3.2.2 Channelling contrast microscopy . . . . . . . . . . . . . . 40 3.2.3 Ionoluminescence . . . . . . . . . . . . . . . . . . . . . . 42 Analysis of defect structures using ion channelling 43 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.2 Ion channelling . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 4.2.1 Channelling theory . . . . . . . . . . . . . . . . . . . . . 45 4.2.2 Energy loss under channelling conditions . . . . . . . . . 46 Dechannelling by defects . . . . . . . . . . . . . . . . . . . . . . 47 4.3.1 Point Defects . . . . . . . . . . . . . . . . . . . . . . . . 48 4.3.2 Dislocations . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.3.3 Stacking Fault . . . . . . . . . . . . . . . . . . . . . . . . 51 4.3.4 Twins . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.3 4.4 Depth distribution of defects . . . . . . . . . . . . . . . . . . . . 52 Stoichiometric and structural alterations in GaN thin films during annealing 55 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 5.3.1 Random RBS measurements . . . . . . . . . . . . . . . . 58 5.3.1.1 Gallium measurements . . . . . . . . . . . . . . 58 5.3.1.2 Nitrogen and oxygen measurements . . . . . . . 60 5.3.2 Channelling measurements . . . . . . . . . . . . . . . . . 64 5.3.3 Decomposition reactions . . . . . . . . . . . . . . . . . . 68 5.4 Interpretation and discussion . . . . . . . . . . . . . . . . . . . . 70 5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Coalescence of epitaxial laterally overgrown GaN fronts 76 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 6.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 6.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . 80 6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 InGaN alloys 86 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 7.2 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 7.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . 88 7.3.1 RBS/channeling Results . . . . . . . . . . . . . . . . . . 88 7.3.2 PL and XRD results . . . . . . . . . . . . . . . . . . . . 88 7.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 7.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Planar channelling and lattice disorder 93 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 8.2 Simulation details . . . . . . . . . . . . . . . . . . . . . . . . . . 96 8.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . 97 8.3.1 Lattice translations . . . . . . . . . . . . . . . . . . . . . 97 8.3.1.1 Intensity oscillations . . . . . . . . . . . . . . . 97 8.3.1.2 Oscillation wavelength . . . . . . . . . . . . . . 101 8.3.2 8.4 Lattice rotations . . . . . . . . . . . . . . . . . . . . . . 105 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . 107 Channelling movies 108 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 9.2 Method and discussion . . . . . . . . . . . . . . . . . . . . . . . 108 9.2.1 Planar channelling movies . . . . . . . . . . . . . . . . . 109 9.2.2 Axial channelling movie . . . . . . . . . . . . . . . . . . 111 List of Tables 2.1 Important properties of GaN and related materials. Experimental values are given in brackets. . . . . . . . . . . . . . . . . . . 4.1 Important parameters used for discussion regarding depth distribution of defects. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 29 54 The areal atomic density (×1015 /cm2 ) of gallium, nitrogen and oxygen present in as-grown and annealed GaN samples, determined using MeV proton backscattering spectra fitted with SIMNRA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 63 Composition of 100 nm surface layer of as-grown and annealed GaN samples determined using a MeV proton beam backscattering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 7.1 Peak values of PL measurements on InGaN samples. . . . . . . 89 7.2 Peak values of XRD measurements on InGaN and GaN substrate. 89 List of Figures 1.1 Number of papers published in refereed journals on GaN from 1988 to 2004. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Schematic showing planar channels (a), axial channels (b) and random view (c) of a typical crystalline structure. . . . . . . . . 2.1 17 19 (a) Clinographic projection of the hexagonal wurtzite (GaN) structure and (b) schematic representation of GaN arrangement on sapphire (0001) surface, updated from ref. [6]. . . . . . . . . 27 2.2 Band structure of GaN [27] . . . . . . . . . . . . . . . . . . . . 28 2.3 (a) A terrace-step-kink (TSK) model of growing crystal surface showing the possible conditions. . . . . . . . . . . . . . . . . . . 2.4 28 Schematic showing principle (a) of two-flow MOCVD [6] and generally adopted procedure (b). TMG in the figure stands for trimethyl gallium. . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.5 GaN growth on sapphire after Hiramatsu et al. [105] 31 3.1 Schematic diagram of 3.5 MeV Singletron accelerator and three . . . . . . beam lines dedicated to different applications. Inset shows the photograph of the facilities (accelerator in background and beam lines in foreground. This figure is adopted from ref. [36]. . . . . 3.2 35 Schematic of a nuclear microprobe. The symbol α represent the divergence half angle of the beam set by collimator into the lenses. 36 10 9.2.1 Planar channelling movies In planar channelling movies, MeV protons are incident at (1) 0.00o and (2) 0.06o (almost one half of the critical angle). Phase-space maps were generated using simulation code mentioned above at varying depth with an interval of 100˚ A, except first 100˚ A interval, which is divided into smaller steps. Movies use these maps to show protons travelling along planar channels. They show change in phase and space coordinates of channelled protons during their trajectories along (110) planes. An ion channelled in a crystal remains channelled as long as its transverse energy does not exceed the critical limit, specific to each planar or axial channel. When a proton enters a channel, it receives an amount of transverse energy depending upon its phase and space coordinates. angle to {110} plane (mrad) angle to {110} plane (mrad) 1000 Å 1Å -3 -3 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 4.0 0.5 1.5 2.0 2.5 3.0 3.5 4.0 3.5 4.0 angle to {110} plane (mrad) angle to {110} plane (mrad) 1.0 position across {110} plane (Å) position across {110} plane (Å) 3000 Å 6000 A -3 -3 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 position across {110} plane (Å) 0.5 1.0 1.5 2.0 2.5 3.0 position across {110} plane (A) Figure 9.1: Selected maps from planar channelling movie for incident angle of 0.00o . In the case of 0.00o incidence angle, protons oscillate with two symmetric arms as clear from the corresponding movie. In this case, the contribution of the incidence angle to the initial transverse energy is zero and it is due to their distance from atomic planes at the entrance. As the incidence space coordinates 109 of protons are uniformly distributed at the entrance of the planar channel, the phase-space distribution oscillates with symmetric arms and changes in both arms of the distribution are the same during the channelling trajectory. Fig. 9.1 shows the selected maps out of those used in the movie of planar aligned case. In 0.06o incidence angle case, the initial transverse energy of a proton has two components. One is the spatial coordinate at the entrance of the channel, which is different for each proton, and other is the incidence angle which is same for all. The effect of the incidence angle breaks the symmetry of the phase-space distribution arms and most protons approach near to planar walls during oscillations due to their higher transverse energy. Fig. 9.2 shows the selected maps in case of 0.06o incident angle. angle to {110} plane (mrad) angle to {110} plane (mrad) 1000 A 1A -3 -3 -2 -2 angle to {110} plane (mrad) angle to {110} plane (mrad) position across {110} plane (A) position across {110} plane (A) 3000 A 6000 A -3 -3 -2 -2 position across {110} plane (A) position across {110} plane (A) Figure 9.2: Selected maps from planar channelling movie for incident angle of 0.06o . As is clear from these movies, there is significant difference in the coherence of channelled protons in these two incidence angle cases. The difference in intensity oscillations of transmitted protons passing through a schematic set of 110 lattice translations (stacking fault) in the last chapter (Figs. 8.4 and 8.5) for different incidence angles is the level of difference in asymmetry of phase-space distribution arms. Peaks and (dips) in intensity are observed when more and (less) dense arm reaches the defect. 9.2.2 Axial channelling movie The axial channelling movie shows MeV protons travelling into [110] axial channels in cubic Si crystal. These protons enter the channel aligned with the axis. During axial channelling, protons have two spatial degrees of freedom. So, this movie includes dimensional spatial maps of axially channelled protons with depth intervals of 50 and 100˚ A. It shows actual spatial distribution of protons in the axial channel during their travel into the crystal. Fig. 9.3 shows the selected maps from axial channelling movie. In this figure, Si atoms are located at centers of white circles. 7.68 y (A) y (A) 7.68 10 A x(A) 5.43 100 A x(A) 5.43 x(A) 5.43 7.68 y (A) y (A) 7.68 500 A x(A) 5.43 2000 A Figure 9.3: Selected maps from axial channelling movie. 111 The atomic rows bounding the axial channel produce a repulsive field, which dechannel a fraction of protons and focus remaining around the channel axis. The radius of the white circles (with atomic rows passing through their centers) in the movie is due to their integrated repulsive effect. Protons on channel axis have minimum potential energy, so protons travel along helical trajectories around channel axis with reducing orbit radius. The dechannelled fraction and focussing around the channel axis increase with depth. The focussing rate is initially high and decreases to a very small value at large depths, as clear from the movie. 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Meth B 159, 248 (1999). 124 [...]... channelled ion beam, this process provides information about presence of defects 1.2 Aim The aim of the project was to understand defect formation during growth of GaN, especially in lateral growth modes, annealing of GaN over a wide range of temperature (500-1100 o C) and InGaN alloys The relationship between structural and optical properties of this material is also investigated RBS /Channeling and related. .. energies and splitting of A-, B- and C- valence bands are mentioned in Fig 2.2 The band gap of wurtzite GaN is higher than that of the zincblende structure 2.2.3 Properties of GaN and related materials In this section, structural and electronic properties of GaN and related materials are reviewed Basic properties of wurtzite GaN and related compounds (AlN, 26 (a) 2 hcp sublattices Ga or N c N or Ga stacking... mismatching is caused due to tilt and unequal width of coalescing wings Chapter 7: InGaN layers are used as active medium in green and blue LEDs and LDs, despite their large defect density The major fraction of defects are threading dislocations which originate at the interface between substrate (sapphire or GaAs) and the supporting GaN layers underneath InGaN High light emission efficiency of InGaN is... the formation of quantum dot-like structures during its growth This chapter describes 2 MeV He ion RBS/channelling, x-ray diffraction spectrometry and photoluminescence 21 of InGaN layers Chapter 8: Ion channelling can be used to determine structure of defects in crystals This chapter describes Monte Carlo simulations of planar channelling in a crystal containing depth distribution of lattice translation... Channelling of MeV Protons Through Thin Crystals” Physical Review Letters Vol 93, 105505 (2004) 23 Chapter 2 GaN and related materials GaN and its alloys (AlGaN and InGaN) provide the possibility of producing ultraviolet to red LEDs and LDs using the same set of materials which is attractive for integration of these devices for full-color displays and other applications In this chapter, the history of GaN. .. media in the above-mentioned devices, with a high dislocation density The structure of phase segregation in InGaN and its role in exciton confinement is also not understood Ultraviolet LDs using AlInGaN with an emission wavelength of 365 nm and lifetime of 2000 hours were fabricated by Masui et al in 2003 [26] 25 2.2 2.2.1 Physics of GaN and related compounds Crystal structure GaN and its alloys (InGaN and. .. sudden and surprising success After this, interest in GaN research grew rapidly as clear in Fig 1.1 and interest is being maintained due to essential position of GaN and its alloys in solid state lighting The number of papers shown in this figure includes word ”gallium nitride” (or GaN) in their abstracts Data was collected from the Web of Knowledge database [8] A variety of publications documenting GaN. .. 7.4 88 Random and channelled RBS spectra of InGaN samples with 9% indium 7.3 84 90 Emission wavelength as a function of In incorporation in InGaN GaN and InGaN (6 and 9%) are wavelengths whereas other are taken from [6] 8.1 91 A schematic representation of phase-space coordinates of channelled protons under planar channelling conditions Ellipse... laterally grown GaN substrate with extended lifetime of 10000 hours and 2 mW output power [24] Ultraviolet InGaN/AlGaN LEDs with an external quantum efficiency of 7.5% and an output power of 5 mW operating at an emission wavelength of 371 nm were fabricated in 1998 [117] Still, GaN is one of the most investigated materials due to the lack of understanding of the mechanism of light emission from InGaN, which... an n-type dopant in GaN The n-type character of un-doped GaN has been a great hindrance in the development of p-type GaN We have measured the intensity of oxygen incorporation in GaN at high temperatures up to 1100 o C using RBS and the results are presented in chapter 5 31 2.3.3 Plasma etching Along with annealing, plasma etching is a another step in the processing of GaN photonic and electronic devices . nGaN samples containing 6% (S59) and 9% (S61) indium. . . . . . . . . . . . . . . . . . . 90 7.4 Emission wavelength as a function of In incorporation in InGaN. GaN and InGaN (6 and 9%) are wavelengths. 1.1 and interest is being maintained due to essential position of GaN and its alloys in solid state lighting. The number of pap ers shown in this figure includes word ”gallium nitride” (or GaN) in. ION CHANNELLING STUDIES OF DEFECT FORMATION IN GaN AND RELATED MATERIALS MUKHTAR AHMED RANA (M.Sc. PHYSICS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL