Optimization of MBE of CdTe/CdTe: Refinement in Structural Quality Evaluation of MBE Grown (111) CdTe
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OPTIMIZATION OF MBE OF CdTe/CdTe: REFINEMENT IN STRUCTURAL QUALITY EVALUATION OF MBE GROWN (111) CdTe M. B. LEE-, T. FANNING", D. DI MARZIO*, L. G. CASAGRANDE*, AND M. DUDLEY** Grumman Corporate Research Center, Bethpage, NY 11714; Dept. of Materials Science & Engineering, SUNY at Stony Brook, NY 11794. ABSTRACT A detailed optimization of MBE growth of CdTe on CdTe (111)B using white beam synchrotron x-ray topography, and x-ray rocking curve analysis has been carried out. Defect structures in (111)B substrates and epilayers are imaged with white beam topography and, for the first time, direct observation of stress induced microtwin growth in CdTe epilayers has been made. X-ray rocking curve analysis of CdTe substrates is shown to be relatively insensitive to defect structures consisting of dislocations and inclusions. On the other hand rocking curve analysis of (111) epilayers reveals a sensitivity to the presence of microtwin. The defect microstructure of a CdTe (111)B epilayer with the narrowest rocking curve FWHM value published to date (9.4 arc-sec) will be discussed. INTRODUCTION MBE growth of CdTe layers is technologically important because: 1) a CdTe buffer layer is a required preliminary step in MBE growth of HgCdTe, 2) high quality CdTe growth is required for fabrication of many Hg based superlattices and quantum wells, and 3) CdTe is an effective passivant. Because of this relevance, MBE growth of CdTe on CdTe substrates has been extensively studied. However, even though the exact lattice match makes it the simplest of all CdTe epilayer growth, growing high quality CdTe is far from routine. In fact, nearly all published rocking curve results to date indicate a significant broadening of FWHM (full width at half maximum) values of CdTe epilayers when compared to FWHM values of the substrates used [1,2]. In this paper, we report results of our comprehensive, systematic, and non-destructive examination of substrate preparation and MBE growth procedure using two complimentary techniques: white beam synchrotron xray topography and rocking curve analysis. Importance of proper defect microstructure of the substrates as well as the minimization of mechanical stress on substrates for growth of high structural quality CdTe (111) epilayers is discussed. A summary of preliminary results on optimization of CdTe (100) growth will also be discussed. EXPERIMENTAL DETAILS CdTe substrates were obtained from the Grumman crystal growth program and three commercial vendors. Epilayer growth was carried out in a Riber 2300 MBE system. CdTe deposition is carried out with a single CdTe source. White beam x-ray topographic imaging was carried out on the white beam camera on the Stony Brook Synchrotron Topography Beamline (X-19C) at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. Detailed description of the unique sensitivity of the technique can be found elsewhere [3]. Experimental details relevant to rocking curve measurements reported in this paper can also be found elsewhere [4]. RESULTS AND DISCU
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