Defect Reduction in Cd 1-x Zn x Te Epilayers on GaAs Substrates
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DEFECT REDUCTION IN Cdl.xZnxTe EPILAYERS ON GaAs SUBSTRATES Saket Chadda,* Kevin Malloy,* and John Reno+ * Center for High Technology Materials, University of New Mexico, Albuquerque, NM 87131 + Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT Cdo.9lZno.O9Te/CdTe multilayers of various period thicknesses were inserted into Cd0.955Zn0.045Te bulk alloys grown on (001) GaAs. The net strain of the multilayer on the underlying Cdo.955ZnO.045Te was designed to be zero. X-ray diffraction full width at half maximum (FWHM) was used as a means to optimize the period thickness of the multilayer. Transmission electron microscopy of the optimum period thickness samples demonstrated four orders of magnitude decrease in the threading dislocation density. Mechanism of bending by equi-strained multilayers is discussed. INTRODUCTION HgCdTe has been the subject of a large development effort by many research laboratories 1-2. CdTe or related II-VI alloys are the standard substrate materials used for such applications. Since, GaAs substrates are 2 available in large areas with low dislocation densities, several groupslhave studied the growth of CdTe or HgCdTe on GaAs. The large lattice mismatch between CdTe and GaAs (-14.6 %) causes the quality of CdTe epilayers to be poor (dislocation densities 1011-1012 cm-2). Reduction in dislocation densities may be achieved by annealing or insertion of multilayers. In this study the role of equi-strained multilayers on blocking threading dislocation is studied. EXPERIMENTAL Details of the molecular beam epitaxy (MBE) growth have been reported elsewhere 3 . Figure 1 illustrates the structure grown on (001) GaAs. A 30 A ZnTe layerwas grown to initiate (001) growth. Next, a 3 Rtm CdO.955ZnO.045Te layer was grown followed by a 15 period. Cd0.91Zn0.09Te/CdTe multilayer in which the individual layers were equal in thickness. Thickness of the period was varied from 2400 A to 2800 A Mat. Res. Soc. Symp. Proc. Vol. 302. ©1993 Materials Research Society
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with 80 A increments in different samples. On top of the multilayer, a 6 gim cap layer of Cd0.955Zn0.045Te was grown. This particular composition was chosen to match HgCdTe having an absorption edge equivalent to 10 9im. X-ray diffraction measurements were performed using a 4 bounce silicon monochromator with a resolution of 8 arcsecs. Lattice parameter and full width at half maximum (FWHM) were determined for each sample. Crossection transmission electron microscopy samples were prepared by standard techniques. Last stage of sample preparation was Ar+ ion milling in a rotating liquid nitrogen cooled stage. The samples were analyzed in a JEM 2000 FX at a working voltage of 200 KV. Cd Zn Te 0.955 0.045
6 jim
0.91 0.09
Cd
Zn
0.91
15 period SLS
h
CdTe Te
0.09
CdTe
."
Y"
Cd
Zn
Te
0.955 0.045
GaAs (100)
3igm
30oA
"ZnTe Figure 1. Schematic diagram illustrating the structures grown. RESULTS AND DISCUSSION Dependence of x-ray FWHM on the period thickness is illustrated in Fig. 2. The value of FWHM decreases until a critical val
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