Muonium Defect Centers in Aluminum Nitride
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MUONIUM DEFECT CENTERS IN ALUMINUM NITRIDE AND SILICON CARBIDE by HISHAM BANI-SALAMEH, M.S. A DISSERTATION IN PHYSICS Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Approved Roger L. Lichti Chairperson of the Committee Charles Myles Carl David Lamp Stefan K. Estreicher Accepted John Borrelli Dean of the Graduate School May, 2007 Copyright 2007, Hisham Bani-Salameh Texas Tech University, Hisham Bani-Salameh, May 2007 ACKNOWLEDGMENTS I would like to express my deep gratitude toward those who helped me throughout this experience. First and foremost, I would like to thank “Allah” for everything I managed to accomplish in my life so far, and for giving me the ability and the strength to get through this amazing learning experience, and giving me the chance to work with one of the best professors in the world, Dr. Roger L. Lichti. For you Dr. Lichti, I can’t find enough words to express my thanks to you for believing in me and for your support and patience; without you, none of this would have been accomplished. My father Nahar Bani-Salameh, my mother Rasmieh Bani-Salameh, my six brothers (Khalid, Jamal, Abdulsalam, Gazi, Mohamed and Ahmed) and my two sisters (Ahlam and Nesreen), I couldn’t have wished for better family, thank you very much for everything you’ve done for me and may “Allah” give me a chance to pay you back. My wife Kolthoom Alkofahi, I will never forget your unlimited support and patience; thank you for your sacrifice and I hope I’ll be able to help you fulfill your dreams and accomplish your goals in this life. My kids Layth and Layan, thank you for making me smile in the worst of days and keeping me fit by chasing you around. Please forgive me if one day I was too busy to give you the attention you need. I would like to thank the U.S. National Science Foundation and the Robert A. Welch Foundation for supporting this work. Last but not least, I would like to give a special thanks to Dr. Steve F.J. Cox for his significant contribution to the progress of the μSR research and for complimenting this work after reading through this thesis. ii Texas Tech University, Hisham Bani-Salameh, May 2007 TABLE OF CONTENTS ACKNOWLEDGEMENTS ii TABLE OF CONTENTS iii ABSTRACT v LIST OF TABLES vi LIST OF FIGURES vii 1 INTRODUCTION AND MOTIVATIONS 1 2 FUNDAMENTALS OF μSR TECHNIQUES 7 2.1 Muons 7 2.2 μSR Techniques 11 2.3 Data Collection 15 3 THEORETICAL DESCRIPTION 18 3.1 Isotropic Muonium 23 3.2 Time Evolution of the Muoniun Spin Polarization 24 3.2.1 Longitudinal Field Case 29 3.2.2 Transverse Field Case 31 3.3 Anisotropic Muonium 32 3.4 Another Method to Calculate the Polarization 35 3.5 Muonium Dynamics 36 3.6 Charge exchange and Spin exchange processes 42 4 ALUMINUM NITRIDE EXPERIMENTAL DATA AND DISCUSSION 45 4.1 AlN Structure. 45 4.2 Hyperfine Decoupling Results. 47 4.3 Site Assignments for Mu0 in AlN. 50 4.4 Dynamics of Mu0 in AlN. 54 4.5 Discussion. 63 5 SILICON CARBIDE EXPERIMENTAL DATA AND DISCUSSION 71 5.1 Temperature Dependence of Hyperfine Constants in SiC 74 iii Texas Tech University, Hisham Bani-Salameh, May 2007 5.2 Dynamics of Muonium States in 4H and 6H SiC 80 5.3 Discussions and conclusions 93 6 SUMMARY AND CONCLUSIONS 98 REFRENCES 102 iv Texas Tech University, Hisham Bani-Salameh, May 2007 ABSTRACT We report the results of μSR measurements on Aluminum Nitride (AlN) and Silicon Carbide (SiC). The importance of studying muonium states comes from its analogy to atomic hydrogen making it an excellent source of information on isolated hydrogen impurities in various materials. Neutral muonium exists in AlN to high temperatures, a large hyperfine constant of ~4450 MHz with a small temperature- dependent dipolar contribution indicating weak anisotropy is obtained from decoupling curves. Tentative site assignments and results on the diffusion of these Mu0 centers along with the associated conversion rates are presented. The low-energy location of neutral muonium in AlN lies off-axis in the unblocked c-axis channels at sites anti-bonding to Aluminum. Motion of Mu0 at low temperatures is due to tunneling and is dominated by thermally activated processes at high temperatures. Diffusion-limited conversion out of the mobile Mu0 state is observed in both low and high temperature regimes. All electrical types, high-resistivity, n-type and p-type, of the hexagonal 4H and 6H polytypes of SiC were studied. Two isotropic Mu0 states were found in 4H-SiC and a total of four Mu0 states were seen in the 6H-SiC samples. Temperature dependence of the hyperfine constant (AHF) for each state is discussed. Data on the hyperfine interactions imply isotropic atomic-like states with no hint of any bond-centered Mu0 species in SiC. Temperature and field dependences of signal amplitudes and relaxation rates were studied. Tentative assignments for locations and some of the dynamical characteristics of the muonium centers have been reached; however, more work is needed to fully understand the nature of these centers in SiC. v Texas Tech University, Hisham Bani-Salameh, May 2007 LIST OF TABLES 2.1 Muon and Proton Physical Properties. 2 4.1 Aiso and D values at different temperatures in AlN. 48 4.2 Fit results of the temperature-dependent hop rate and conversion rate in AlN. 61 5.1 Hyperfine Constant Parameters in SiC. 79 5.2 Activation energies from the diamagnetic amplitudes in 6H-SiC. 84 vi Texas Tech University, Hisham Bani-Salameh, May 2007 LIST OF FIGURES 2.1 Decay of pions into a muon and a muon neutrino. 9 2.2 The angular distribution of emitted positrons from the muon decay. 11 2.3 Schematic of the position of the four positron counters. 13 2.4 Raw Time-dependent asymmetry data. 16 0 3.1 Breit-Rabi diagram for MuT in Si. 24 0 3.2 Field-dependent polarization of MuT in Si. 30 4.1 Part of the AlN wurtzite structure showing bond lengths and angles. 46 4.2 Experimental repolarization curves in AlN. 49 4.3 The Wurtzite structure of AlN showing site assignments for Mu0. 52 4.4 Simulated zero-time longitudinal asymmetry for possible Mu0 sites in AlN. 53 4.5 Muon spin depolarization raw data in AlN. 55 4.6 Longitudinal-field data indicating motion of Mu0 in AlN. 56 4.7 Temperature-dependent conversion rate and hop rate of Mu0 in AlN. 59 4.8 A segment of AlN wurtzite structure showing possible paths for motion. 64 4.9 Zero-field data on AlN. 65 4.10 LCR data on AlN. 68 5.1 The stacking sequence for 4H and 6H polytypes of SiC. 72 5.2 The muon spin precession spectra for SiC. 76 5.3 The temperature dependence of the hyperfine constant for the Mu2 in SiC. 78 5.4 A 3-D representation of the diamagnetic signal in n-type 4H-SiC sample. 81 5.5 The diamagnetic amplitude in n-type 6H-SiC sample. 82 5.6 Temperature-dependent amplitudes and relaxation rates in p-type 6H-SiC. 83 5.7 The full temperature-dependent diamagnetic signal amplitude in 6H-SiC. 85 5.8 Relaxation rates of Mu1 and Mu2 in p-type 6H-SiC. 86 5.9 TF data on the high-resistivity 6H-SiC sample taken at 15 gauss. 87 5.10 Muon magnetic resonance data of the three neutral signals in hr 6H sample. 89 5.11 The diamagnetic amplitude and phase in hr 4H-SiC sample at 80 Gauss. 91 vii Texas Tech University, Hisham Bani-Salameh, May 2007 5.12 Amplitudes of the diamagnetic, Mu1 and Mu2 signals in p-type 4H-SiC sample. 92 5.13 A 3-dimentional representation of the 4H structure. 96 viii Texas Tech University, Hisham Bani-Salameh, May 2007 CHAPTER 1 INTRODUCTION AND MOTIVATIONS Because of their importance in modern electronics, semiconductors have been studied extensively. Impurities (whether intentional through doping or unintentional during growth or during processing) play a critical role in determining the properties of the semiconductor. The doping process usually results in an extra energy level in the bandgap that will modify the electronic properties of the material; therefore, as long as we have control over what kind and what amount of impurities are introduced, this process can be very useful. Unfortunately, some impurities (like Hydrogen) will enter the material unavoidably, most likely during the growth process or during various device processing steps. Since the early 1980’s, following the discovery that atomic hydrogen can passivate shallow donors and acceptors in Si, the structure and properties of hydrogen in semiconductors have received increasing attention. More intensive research in the following years revealed that hydrogen has the same important role as a passivating agent in a wide range of semiconductors [1-4]. Hydrogen will form complexes with dopants and other impurities in the host material forcing the corresponding energy level of that defect to move into or out of the bandgap modifying the host material’s electrical and optical properties as a result. With hydrogen being the most abundant element on earth, its presence in semiconductors is very difficult to avoid. Once inside, hydrogen can exist in three different charge states, H0, H+ and H- depending on the Fermi energy with respect to the hydrogen-related levels. Hydrogen is very reactive and very mobile and once inside the material, it will quickly tie up dangling bonds and form bound states with the existing defects.