The Thermodynamics and Kinetics of Phase Separation in III-V Semiconductor Alloys

Cross-sectional transmission electron microscopy (XTEM) has been used to analyze phase separation in the III-V semiconductor alloy InGaAs grown by molecular beam epitaxy (MBE) on (001) InP substrates with a 2 ° offcut, as well as InGaP grown upon on-axis (001) GaAs. Both films were deposited using a time of 20 seconds for each substrate rotation, with a deposition rate of 0.278 nm/s. Phase separation resulting in lateral composition modulation (LCM) along the [110] direction was analyzed via XTEM for InGaAs grown at the temperatures 400, 450, and 500 °C. An Arrhenius analysis of InGaAs XTEM LCM measurements, based on the phase separation model of Malyshkin and Shchukin, yields the activation energy of 0.54+/-0.05 eV associated with cation adatom surface diffusion, which compares favorably with the activation energy of 0.55+/-0.02 eV determined by in-situ MBE In droplet epitaxy surface diffusion measurements. XTEM analysis of vertical composition modulation (VCM) for InGaP grown upon an on-axis (001) GaAs substrate reveals a wavelength of 5.3 nm, that comes close to matching 5.5 nm, which is product of the rotation time and the deposition rate. However, for InGaAs grown on an off-axis InP (001) substrate, the VCM wavelengths observed by XTEM deviates significantly from the expected 5.5 nm and instead ranges from 7-9 nm, due to the step-bunching promoted by deposition on the off-axis InP substrate. Both the effects of LCM and VCM phase separation in InGaAs can be understood in terms of the greater surface diffusivity of In adatoms relative to Ga adatoms as revealed by droplet epitaxy measurements. Trends in surface diffusion in ternary alloys are also applied to quaternary alloys, leading to a more accurate theoretical estimate of the critical temperature for phase separation.