Strain Relaxation in Iv-vi Semiconductor Layers Grown on Silicon (100) Substrates
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diameter silicon wafer with the buffer layer structure was cleaved to obtain I x 1 cm substrates and used for subsequent LPE growths. All the substrates obtained from this wafer were designated as W1 13. PbSe/BaF 2/CaF 2 buffer layer structures were grown by MBE on two other silicon wafers as well. The substrates obtained from these were designated as W222 and W245. On W222, the growth procedure was similar to that described above except for the thickness of the CaF 2 and PbSe layers, which were 400 A and 2.2 jtm, respectively. For W245, following the growth of a 570 A thick CaF 2 layer, a 2000 A thick BaF 2 layer was grown. Annealing was performed at 900*C for 3 minutes after which a 7000 A thick BaF2 layer was grown at 700 0 C. The thickness of the PbSe layer grown at 280'C was 8100 A. The melt solution for LPE growth of PbSe was prepared by combining weighed amounts of Pb and PbSe according to the relations derived using the molecular weights of the respective constituents [7]. The chalcogen concentration was chosen to be 0.2 wt. % based upon previously published phase equilibria data for PbSe [8]. After loading the melt constituents into one of the wells of a graphite boat, the furnace temperature was increased to 650'C and maintained at this temperature for about an hour to allow homogenization. The furnace temperature was then reduced to about 30 degrees above the expected nucleation temperature. The surface of the melt was observed with an optical microscope while cooling the furnace at the rate of 2°C/minute, and the temperature at which nucleation was observed on the melt surface was recorded. The furnace was then cooled to room temperature and the substrate was placed in the recess provided on the graphite slider. This time, the furnace temperature was kept below 500'C in order to minimize the thermal stress to which the MBE-grown PbSe layer was subjected. The same cooling ramp of 2°C/minute was initiated, and the graphite slider was pulled to position the substrate under the growth solution well at about 2 to 3 degrees above the measured nucleation temperature, which was typically in the 460'C range. After about 80 degrees of cooling, the slider was pulled to position the substrate away from the melt thereby terminating the growth. Pbl.xSnxSelyTey and Pbl.,SnxSe layers were grown by LPE on Si (100) substrates using the same procedure as outlined above. Based on previously published data for (Pbl.xSnx)l.z(Sel. yTey)z [7], the chalcogen concentration in the growth solution, z, was chosen to be 0.25%. The tin content in the liquid growth solution for the PblxSnxSel-yTey layer was 5%, while those for the ternary Pbl-,SnxSe layers were equal to 3%, 5%, 6%, 7%, and 10%. The LPE-grown PbSe, Pbl-,Snx Sel-yTey, and PblxSnxSe layers were studied using optical Nomarski microscopy. Residual strain in the LPE-grown PbSe layer was calculated using a Philips X-ray diffractometer. RESULTS AND DISCUSSION Figure 1 is an optical Nomarski micrograph showing the surface morphology of a PbSe layer (WI 13-H9) grown by LPE on S
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