Germanium/gallium arsenide alloys grown by molecular-beam epitaxy

Metastable germanium/gallium arsenide alloys have been grown by molecular-beam epitaxy on (100)-oriented GaAs substrates. Double-crystal diffractometry shows that the layers have crystalline perfection comparable to that of good crystals of GaAs. Electron mobilities in the range 500–1500 cm2 V−1 s−1 are observed from epilayers with electron concentrations in the range 1017–1019 cm−3. Hole mobilities were relatively small at <40 cm2 V−1 s−1 with concentrations in the range 3×1017–3×1020 cm−3. We find substantial nonstoichiometry of the GaAs component with As/Ga atom ratios ranging from 0.94 to 1.17. As-deficient through moderately As-rich alloys exhibit a positive deviation from Vegard behavior that changes to a negative deviation with further increase in the As content. The closeness of lattice match with GaAs substrates allows the epilayer/substrate combination to retain a coherent interface with epilayer thicknesses up to at least 1.5 μm. This is demonstrated by measurements of tetragonal distortion and of curvature that are in excellent agreement with predictions for an interface without misfit dislocations. We have not observed any indication of phase separation in layers grown at temperatures up to 580 °C.