Ge_1-ySn_y (y = 0.01-0.10) alloys on Ge-buffered Si: Synthesis, microstructure, and optical properties

Novel hydride chemistries are employed to deposit light-emitting Ge_1-ySn_y alloys with y≤0.1 by Ultra-High Vacuum Chemical Vapor Deposition (UHV-CVD) on Ge-buffered Si wafers. The properties of the resultant materials are systematically compared with similar alloys grown directly on Si wafers. The fundamental difference between the two systems is a fivefold (and higher) decrease in lattice mismatch between film and virtual substrate, allowing direct integration of bulk-like crystals with planar surfaces and relatively low dislocation densities. For y≤0.06, the CVD precursors used were digermane Ge₂H₆ and deuterated stannane SnD₄. For y≤0.06, the Ge precursor was changed to trigermane Ge₃H_8, whose higher reactivity enabled the fabrication of supersaturated samples with the target film parameters. In all cases, the Ge wafers were produced using tetragermane Ge₄H₁₀ as the Ge source. The photoluminescence intensity from Ge_1-ySn_y/Ge films is expected to increase relative to Ge_1-ySn_y/Si due to the less defected interface with the virtual substrate. However, while Ge_1-ySn_y/Si films are largely relaxed, a significant amount of compressive strain may be present in the Ge_1-ySn_y/Ge case. This compressive strain can reduce the emission intensity by increasing the separation between the direct and indirect edges. In this context, it is shown here that the proposed CVD approach to Ge_1-ySn_y/Ge makes it possible to approach film thicknesses of about 1μm, for which the strain is mostly relaxed and the photoluminescence intensity increases by one order of magnitude relative to Ge_1-ySn_y/Si films. The observed strain relaxation is shown to be consistent with predictions from strain-relaxation models first developed for the Si_1-xGe_x/Si system. The defect structure and atomic distributions in the films are studied in detail using advanced electron-microscopy techniques, including aberration corrected STEM imaging and EELS mapping of the average diamond-cubic lattice.