Interdiffusion in III-V opto-electronic materials.

Photoluminescence coupled with repetitive thermal annealing has been used to study the thermal interdiffusion processes in both InGaAs and GaAsSb grown on GaAs substrates. The effect of growth temperature and group-V to group-III flux ratio were studied in InGaAs MQW wafers. The wells in the MQW stack were grown at different temperatures, one of which was grown well below the standard growth temperature to act as a vacancy source in the MQW stack. Due to this vacancy source we have been able to simultaneously measure the In-Ga interdiffusion coefficient, diffusion coefficient for group-III vacancies in GaAs, and the background concentration of these vacancies. It was shown that the interdiffusion at all temperatures is governed by a constant background concentration of vacancies in the material and that this background concentration is the concentration of vacancies in the substrate material. The measured background concentration of vacancies is about 1017 cm-3. This result showed that the vacancy concentrations in GaAs are not at thermal equilibrium concentrations as has been widely assumed. Rather it has been shown to have a value which is "frozen-in", probably at the GaAs crystal growth temperature. The activation energy found for intermixing of InGaAs/GaAs is shown to be governed solely by the activation term for vacancy diffusion which is calculated to have an activation energy of 3.4+/-0.3 eV. These results provide a mechanism to explain the reduced diffusion coefficient under the Ga rich conditions widely reported. The flux ratio was found not to play any significant role in the interdiffusion process. These results can be explained by the diffusion in all cases being governed by a single vacancy-controlled second-nearest-neighbour bopping mechanism. The interdiffusion process on the group-V sublattice in the GaAsSb material system was shown to obey Pick's second law with an activation energy of 2+/-0.3 eV. This result contradicts earlier reports of non-linear diffusion process in this system