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Growth and Characterization of Znse by Metalorganic and Gas Source Molecular Beam Epitaxy

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Growth and Characterization of Znse by Metalorganic and Gas Source Molecular Beam Epitaxy Growth and Characterization of ZnSe by Metalorganic and Gas Source Molecular Beam Epitaxy By Christopher Alan Coronado B.S. Physics, University of Washington, 1984 M.S. Electrical Engineering, University of Michigan, 1987 Submitted to the Department of Materials Science and Engineering in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY IN ELECTRONIC MATERIALS at the Massachusetts Institute of Technology May, 1994 © Massachusetts Institute of Technology, 1994. All rights reserved. 2,zt n I Signature of Author _ /' r / Department of Materials Science and Engineering April 29, 1994 Certified ll, 101w- - . I.- 1.- -- - by _._ - I- I j Leslie A. Kolodziejski Associate Professor of Electrical Engineering Thesis Supervisor Accepted by. ___ _ V Carl V. Thompson II Professor of Electronic Materials Chair, Departmental Committee on Graduate Students Science MASSACHiISETTSINSTITUTE Or Tc pJyLGY AUG 18 1994 I3RARMRE Growth and Characterization of ZnSe by Metalorganic and Gas Source Molecular Beam Epitaxy by Christopher Alan Coronado Submitted to the Department of Materials Science and Engineering on April 29, 1994, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract ZnSe was grown using the gaseous source epitaxial methods of metalorganic molecular beam epitaxy (MOMBE) and gas source molecular beam epitaxy (GSMBE). A nitrogen plasma source was used during GSMBE to incorporate the p-type dopant nitrogen into ZnSe epitaxial layers. The first part: of this work was the growth of undoped ZnSe by MOMBE. Diethylzinc and diethylselenium were the primary metalorganic source gases used. The MOMBE growth rate was less than 400 A/hr when a diethyl metalorganic was the source of Zn and/or Se. The low growth rate is postulated to be the result of surface passivation of metal incorporation sites by chemisorbed ethyl radicals. A parallel study of the effects of laser illumination and electron-beam irradiation during MOMBE growth was also conducted. Laser illumination and electron-beam irradiation were observed to alter the growth rate under appropriate growth conditions. An increase in the growth rate by a factor of 15 over the unilluminated growth rate was observed using laser illumination. The second part of the study was the growth of ZnSe by GSMBE, and experiments to dope ZnSe p-type using a radio frequency (RF) nitrogen plasma source. High quality epitaxial films were produced under a variety of growth conditions as indicated by intense low temperature photoluminescence dominated by free- and donor-bound exciton features. Employing the RF plasma source, nitrogen has been incorporated into ZnSe epitaxial layers in concentrations as high as 6x1018 N atoms/cm 3 , as measured by secondary ion mass spectroscopy. Hydrogen incorporated in the ZnSe:N grown by GSMBE. Passivation of the nitrogen acceptors by hydrogen greatly reduced the electrically-active nitrogen concentrations; the highest net-acceptor concentration measured by the capacitance-voltage technique was - lx1017 cm- 3 . Thesis Supervisor: Leslie A. Kolodziejski Title: Associate Professor of Electrical Engineering 2 Contents Abstract ........................................................................................................................ 2 Acknowledgements...................................................................................................... 14 Glossary of Acronyms Used in Thesis ......................................... 15 1 Introduction ......................................... 17 1.1 Background .................................................................................................... 17 1.1.1 Progress Toward Blue-Green Injection Lasers ...................................... 17 1.1.2 ( Zn,Mg)(S,Se) and (Zn,Cd)(S,Se) Material Systems ............................. 19 1.1.3 Material Properties of ZnSe ......................................... 20 1.1.4 Technological Applications ........................................ 22 1.1.5 Central Materials Issues ......................................... 24 1.2 Dissertation Overview .................................................................................... 25 1.2.1 Epitaxy System and Experimental Procedures ....................................... 25 1.2.2 MOMBE and GSMBE . .........................................................................25 1.2.3 Acceptor Compensation in ZnSe . ..........................................................26 1.2.4 ZnSe:N ................................................................................................. 26 1.2.5 Conclusions and Future Work ............................................................... 27 2 Epitaxy System and Experimental Procedures . .....................................................28 2.1 II-VI Chamber Description ............................................................................. 30 2.2 Gas Delivery System .......................................................................................33 2.3 Sample Preparation............................................................. 34 2.3.1 Sample Holder ...................................................................................... 34 2.3.2 Wafer Preparation ................................................................................. 35 2.4 Temperature Calibration . ................................................................................36 2.5 Other Growth Procedures ............................................................................... 37 3 3 Metalorganic Molecular Beam Epitaxy of ZnSe ................................................ 38 3.1 Instability in MBE Source Fluxes ................................................ 38 3.2 M etalorganic Gas Sources .............................................................................. 40 3.3 Growth Conditions ................................................ 41 3.4 Low MOMBE Growth Rate ................................................ 44 3.4.1 Addition of Hydrogen Gas ........................................ ........ 45 3.4.2 Mixed Source Experiments ................................................ 46 3.4.3 Site Blockage by Ethyl Radicals ................................................ 48 3.4.4 Other Reported MOMBE Results ................................................ 51 3.5 Laser-Assisted MOMBE ........................... ............................. ..... 53 3.5.1 aser-Assisted Growth Set-Up ........................................ ........ 54 3.5.2 Growth Rate Enhancement ....................................... ............................56 3.5.3 Intensity Dependence................................................ 57 3.5.4 W avelength Dependence ....................................................................... 58 3.5.5 Flow Ratio Dependence ................................................ 62 3.6 MOMBE Using DMZn and Solid Se ................................................ 63 3.7 Electron-Beam Assisted Growth ................................................ 64 3.8 Model for Laser-Assisted Growth ................................................ 65 3.9 ZnSe Film Characterization ................................................ 73 3.9.1 RHEED ................ ................. 7.............................................73 3.9.2 IDCXRD ................................................ 75 3.9.3 Photoluminescence ............................................................................... 77 3.9.3.1 Representative PL of a MOMBE Thin Film ................................. 77 3.9.3.2 Laser Tuning of Stoichiometry ................................................ 78 3.!;,.3.3 MBE PL ................................................ 80 3.10 Summary ...................................................................................................... 81 4 Gas Source MBE of ZnSe ................................................ 84 4.1 GSMBE Growth ................................................ 84 4.1.1 (GrowthConditions ............................................................................... 85 4.1.2 H2Se Cracking ...................................................................................... 86 4.1.3 Estimate of Growth Rate Uncertainty ................................................ 87 4 4.1.4 Growth Rate Dependencies................................................... 88 4.1.4.1 Substrate Temperature ................................................... 89 4.1.4.2 H2Se Flow and Zn Flux ......... ............................................ 90 4.2 Characterization of GSMBE films ................................................... 91 4.2.1 Structural Characterization ........................................ ........... 91 4.2.1.1 RHEED ......... ......... ................................................. 92 4.2.1.2 Nomarski and Scanning Electron Microscopy ............................. 93 4.2.2 Photoluminescence ................................................... 96 4.2.3 Electrical Measurements ........................................ ........... 99 4.3 Effect of Surface Stoichiometry ................................................... 99 4.5 Summary ................................................... 101 5 Acceptor Compensation in P-Type ZnSe ........................................ ........... 103 5.1 Early Compensation Model: Native Defects .................................................. 105 5.2 Recent Compensation Models ................................................... 106 5.2.1 Lattice Relaxation
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