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Isolated Hydrogen in II–VI Zinc–Chalcogenide Widegap Semiconductors Modelled by the Muon Analogue

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Isolated Hydrogen in II–VI Zinc–Chalcogenide Widegap Semiconductors Modelled by the Muon Analogue Rui C¶esarVil~ao Isolated hydrogen in II{VI zinc{chalcogenide widegap semiconductors modelled by the muon analogue Faculdade de Ci^enciase Tecnologia Universidade de Coimbra Coimbra { 2007 ii Rui C¶esarVil~ao Isolated hydrogen in II{VI zinc{chalcogenide widegap semiconductors modelled by the muon analogue [Hidrogenio¶ isolado em calcogenos¶ de zinco da fam¶³lia II-VI com propriedades semicondutoras e intervalo largo de energias proibidas, estudado atraves¶ da analogia com o muao]~ Disserta»c~aode doutoramento em F¶³sica, especialidade de F¶³sicaExperimental, submetida `aFaculdade de Ci^enciase Tecnologia, Universidade de Coimbra Coimbra - 2007 iv Aos meus Pais A` Susana vi Abstract We have investigated the behaviour of isolated hydrogen in II{VI zinc chalco- genide widegap semiconductors, by means of the positive-muon analogue. A broad program of muon-spin rotation, relaxation and resonance measurements was under- taken for monocrystalline samples of ZnSe, upon adequate characterization (parti- cularly on the electrical transport properties, by means of Hall-e®ect and resistivity measurements). Two compact paramagnetic muonium states MuI and MuII were identi¯ed and the interconversion process was characterized. Capture of a second ¡ electron by MuII, forming the negative ion MuII was observed as well. This charged state becomes thermally unstable above 60 K and we relate this to possible ioni- zation to the conduction band and estimate the corresponding acceptor level. In adition to ZnSe, monocrystalline samples of ZnS and ZnTe were investigated. Only one paramagnetic state is observed in ZnS, whose behaviour is shown to be much similar to that of MuII in ZnSe. In ZnTe, only a diamagnetic state is observed, which we suggest corresponds to a deep donor in this material. Finally, a global con¯guration and energy level model is presented for muonium/hydrogen in II{VI semiconductors. viii Resumo Neste trabalho investigou-se o comportamento, atrav¶esda analogia com o mu~ao positivo, do hidrog¶enioisolado em materiais calc¶ogenosde zinco com propriedades semicondutoras e intervalo largo de energias proibidas, da fam¶³liaII-VI. Foi rea- lizado um programa amplo de medidas experimentais de rota»c~ao,relaxa»c~aoe res- son^anciado spin do mu~aoem amostras monocristalinas de ZnSe devidamente car- acterizadas (particularmente do ponto de vista das propriedades de transporte el¶ectrico,atrav¶esde medidas de efeito de Hall e resistividade el¶ectrica).Identi¯cou- se dois estados paramagn¶eticoscompactos de mu¶onio,MuI e MuII, e caracterizou-se o processo de interconvers~ao.Observou-se ainda a captura de um segundo electr~ao ¡ pelo estado MuII, formando o i~aonegativo MuII. Este estado negativamente car- regado ¶einst¶avel para temperaturas superiores a 60 K, possivelmente ionizando para a banda de condu»c~ao,tendo sido determinado o presum¶³vel n¶³vel aceitador correspondente. Para al¶emde ZnSe, investigou-se tamb¶emamostras monocristali- nas de ZnS e ZnTe. Observa-se apenas um estado de mu¶onioparamagn¶eticoem ZnS, cujo comportamento se veri¯cou ser muito semelhante ao do estado MuII existente em ZnSe. Em ZnTe observa-se um estado diamagn¶eticoque sugerimos corresponder a um dador profundo neste material. Finalmente, apresenta-se um modelo global para as con¯gura»c~oese n¶³veis de energia do mu¶onio/hidrog¶enionos semiconductores da fam¶³liaII-VI. x Acknowledgements A work such as the one now presented is necessarily a team work. The input and collaboration of many people is present at all stages, from the initial ideas and proposals, through the beamline days and discussion at all levels of the ex- perimental data and respective signi¯cance, up to the ¯nal redaction. It is to me a privilege to make part of a team such as the one which helped to complete this work. A scienti¯c privilege, certainly, but a personal privilege as well. I am very happy to express now my public thanks: - to Prof. Dr. Jo~aoCampos Gil, whom I owe a most dedicated, friendly, present guidance, in the big lines as in the little details. - to Prof. Dr. Nuno Ayres de Campos, whose counsel, encouragement and friendship helped me looking farther with con¯dence. - to Prof. Dr. Alois Weidinger, whose profound physical insight, shared friendly and patiently in intense interaction and discussions, brought me immense bene¯ts. - to Dr. Helena Vieira Alberto, a quotidian source of science, humanism and friendship. - to Jo~aoPedro Duarte, comrade of many scienti¯c and non-scienti¯c ¯ghts, always fought with wit and joy. - to Prof. Dr. Stephen Cox, Prof. Dr. Roger Lichti and Dr. Kim Chow, who have contributed to this investigation with their eminent knowledge, as well as with many days of hard work. I would also like to gratefully acknowledge the kind help, during this work, of Dr. Benilde Costa, Dr. Francisco Gil, Dr. D. Siche, Dr. K. Irmscher, Marco Peres, Prof. Dr. Teresa Monteiro, Prof. Dr. Ermelinda Eus¶ebio,Dr. Manuela Silva, Victor Hugo Rodrigues, Prof. Dr. Jos¶eAnt¶onioPaix~ao,Dr. Ana Maria Matos Beja, Dr. Bassam Hitti, Dr. Stephen Cottrell, Dr. James Lord, Dr. U. Zimmermann, Dr. Robert Scheuermann, Dr. Alex Amato, Dr. Hubertus Luetkens, Dr. Dierk Herlach, Dr. Philip King, as well of my colleagues at the Physics Department of the University of Coimbra. xii Contents 1 Hydrogen in semiconductors 1 1.1 Introduction . 1 1.2 Hydrogen as a passivating/activating impurity . 2 1.3 Isolated hydrogen in semiconductors . 3 1.3.1 Hydrogen as a compensating amphoteric centre . 4 1.3.2 Hydrogen as a source of conductivity . 8 1.4 Isolated hydrogen in zinc II-VI compounds . 9 1.4.1 Zinc II-VI compound semiconductors for optoelectronics . 9 1.4.2 Hydrogen in zinc chalcogenides . 13 2 The ¹SR techniques 19 2.1 Introduction . 19 2.2 ¹+ as a light pseudo-isotope of hydrogen . 20 2.3 Muonium . 22 2.3.1 Preliminary remarks . 22 2.3.2 Hyper¯ne structure of the muonium atom . 24 2.3.3 Muonium dynamics . 37 2.4 Production of polarized muon beams . 42 2.4.1 Muon decay . 42 2.4.2 Polarized muon beams . 45 2.5 Muon implantation and thermalization . 46 2.6 Muon spin rotation, relaxation, resonance . 51 2.6.1 Basic experimental setup . 51 xiii xiv CONTENTS 2.6.2 Muon spin rotation . 55 2.6.3 Muon spin relaxation . 58 2.6.4 Muon spin resonance . 59 2.6.5 User facilities and spectrometers . 66 3 Sample characterization 71 3.1 Introduction . 71 3.2 Samples . 72 3.2.1 ZnSe . 72 3.2.2 ZnS and ZnTe . 82 3.3 Resistivity and Hall e®ect in ZnSe . 83 3.3.1 Ohmic contacts . 83 3.3.2 Ohmic contacts to ZnSe . 87 3.3.3 Resistivity and Hall e®ect measurements . 91 3.3.4 Results . 101 4 Experimental results and discussion I: ZnSe 105 4.1 Introduction . 105 4.2 High-transverse ¯eld spectroscopy . 107 4.2.1 Novel deep muonium center in ZnSe . 107 4.2.2 Temperature dependence of the paramagnetic states: AA sample . 110 4.2.3 Temperature dependence of the paramagnetic states: Ctec sample . 119 4.2.4 Discussion . 122 4.3 Low-transverse ¯eld spectroscopy . 124 4.3.1 AA sample . 124 4.3.2 Ctec sample . 128 4.4 Final-state analysis experiments . 135 4.4.1 Discussion . 138 4.5 Further experimental information . 148 4.5.1 Probing Mu dynamics with LF . 148 CONTENTS xv 4.5.2 Development of doping-dependent studies . 152 4.5.3 High temperature investigations . 155 4.6 Conclusive remarks . 161 5 Experimental results and discussion II: ZnS and ZnTe 163 5.1 ZnS . 164 5.1.1 High transverse ¯eld spectroscopy . 164 5.1.2 Low transverse ¯eld spectroscopy . 168 5.1.3 Final-state analysis of the diamagnetic fraction . 171 5.1.4 Probing Mu dynamics with LF . 173 5.2 ZnTe . 174 5.2.1 Spectroscopic measurements . 174 5.2.2 Final-state analysis of the diamagnetic fraction . 178 5.2.3 High-temperature spectroscopic incursion . 182 5.2.4 Probing Mu dynamics with LF . 184 5.3 Conclusive remarks . 186 6 Conclusions and perspectives 187 6.1 Towards a ¯nal synthesis . 187 6.1.1 Con¯guration model . 188 6.1.2 Energy levels . 190 6.2 Future developments . 192 6.2.1 Open questions . 193 6.2.2 Related investigations . 195 Appendix A 199 Bibliography 201 xvi CONTENTS Chapter 1 Hydrogen in semiconductors Embarque vossa do»cura que c¶anos entenderemos... Tomareis um par de remos, veremos como remais; e, chegando ao nosso cais, n¶osvos desembarcaremos.1 Gil Vicente Auto da Barca do Inferno 1.1 Introduction Impurities play an essential part in the physics of semiconductors. The extreme sensibility of the electrical and optical properties of the members of this class of materials with respect to the presence of impurity atoms may be regarded as one of its most de¯ning characteristics [See82, Yu99]. Hydrogen occupies a special role among all impurities. As the simplest atom, one might expect hydrogen to have a specially simple behaviour in semiconductors, upon which generalizations to other impurities could be inferred. Notwithstanding the current state of hydrogen research in semiconductors depicts an extremely complex impurity which assumes a variety of forms, from several possible atomic con¯gurations to molecular states 1Free translation: "Climb your sweetness aboard and we shall go along... You shall take a pair of oars, we shall watch you rowing; and, when we arrive to our pier, we shall step you o®." 2 CHAPTER 1. HYDROGEN IN SEMICONDUCTORS and complexation with other impurities and defects [Est95, Mye92, Nic98, Nic99a, Pan91, Pea94, VdW05, VdW06]. Much interest is deposited in the knowledge of the hydrogen impurity per se, due to its omnipresence in semiconducting materials science.
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