Twinning in epilayers of CdTe on <211> Si: Influence of ZnTe buffer layer and substrate misorientation
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479 Mat. Res. Soc. Symp. Proc. Vol. 399 01996 Materials Research Society
thickness have not been fuilly investigated. In this paper, we use three types of x-ray analysis 0/20 diffraction, back reflection Laue, and double crystal diffraction to address these issues. Experimental Procedures Layers were grown by MBE on Si substrates. The method of preparing the surface of the Si wafer, the procedure for performing layer growth, and the excellent x-ray and photoluminescence characteristics often obtained will be reported separately [4]. For the present study, layers whose characteristics were less than optimum were chosen. Three samples were examined. Sample A consisted of a layer of CdTe grown to a thickness of 5 micrometers directly onto a Si substrate oriented to within 0.50 of . Sample B consisted of a layer of CdTe grown to a thickness of 12 micrometers with a 200 angstrom ZnTe buffer layer interposed between CdTe layer and the Si substrate which was oriented to within 0.50 of. Sample C was identical to sample B except that the Si substrate was misoriented 50 from the toward the .
Double-Positioning Twin
Lamellar Twin
Fig. 1. Two Types of Twin Structures in CdTe(21 1) Layer
ZnTe
Sample A
ZnTe
Sample B
Sample C
Fig. 2. Schematic diagram showing layer structure.
Back reflection Laue photographs and both 0/20 and double crystal diffraction spectra were taken for all three samples as grown and on samples B and C at after 3 and 7 micrometers of the initial 12 micrometers had been removed by chemically etching in a solution of bromine in methanol to reveal any depth dependence of structural features. Penetration depth of the copper radiation used for diffraction experiments is estimated to be 5 micrometers.
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Back reflection Laue was used to determine epitaxial orientation. 0/20 diffractometry was performed on Siemens D5000RA and Rigaku DMAX instruments to determine the presence and extent of twins. A Blake double crystal diffractometer ecvuipped with a "four bounce" Si-Ge Bartels type monochromator with a beam size of 0.5 x 1 mm was used to quantify overall quality of the CdTe films. Further, by chemical etching of films B and C, the film quality was measured at several distances from the Si interface. Results and Discussion 0/20 spectra for samples A and B contain the 422 reflection expected from a oriented CdTe epilayer and a 331 reflection characteristic of a twin [5]. The spectrum for sample C contains only the 422 reflection. After etching, spectra for samples B and C contain 422 and 331 reflections. In Fig. 3 are shown the ratio of integrated intensity ratios for 422 and 331 reflections where reflections are separately maximized. The observed decrease in the integrated intensity is taken to be indicative of an increase in twin density.
Fig. 3. Integrated x-ray intensity ratio of CdTe(21 1)layers vs the layer thickness. The apparent twin density difference between samples B and C before and after etching is possibly due to the different angles between the interfaces and the corresponding slip pla
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