In situ studies of strain relaxation during III-V semiconductor heteroepitaxy

In situ stress measurements have been taken during MBE growth of InGaAs/GaAs and InAlAs/GaAs in order to gain insight into the relaxation behavior of thin films grown on mismatched substrates. In the materials studied, relaxation proceeds via the formation and glide of dislocations. Film stress evolution is monitored by a multibeam optical stress sensor, which measures the substrate curvature induced by a strained thin film. Continuously recording the wafer curvature during growth allows us to study the dynamic evolution of strain; this method offers information that would be unavailable by ex situ techniques. The effect of temperature, growth rate, and growth interruptions on the relaxation rate has been explored, in the hope of developing a stronger understanding of the factors influencing dislocation glide and the density of threading dislocations. Additional relaxation gained by interrupting and annealing during InGaAs growth has shown that relaxation is initially kinetically limited. In thicker layers, the same magnitude of residual strain is attained regardless of growth conditions.{09}This residual strain is much larger than predicted by models of dislocation formation and insertion. We have attributed this offset to the effect of the spatially non-uniform misfit dislocation configuration in real films. There is a significant, temperature-dependent enhancement of the strain relaxation rate in the presence of a growth flux, with a larger enhancement at lower temperatures and faster growth rates. We have explained this observation with a model in which the activation energy for dislocation single kink nucleation is lowered in the presence of a supersaturation of adatoms. The predictions of this model are in excellent agreement with our measurements in terms of temperature and growth rate dependence. A method has been developed for monitoring and analyzing the stress evolution during the deposition of metamorphic buffer layers. Real-time monitoring was used to adjust buffer layer recipes during growth to achieve thinner layers with the same residual strain and final lattice parameter. Together, these investigations have furthered our understanding of strain relaxation in semiconductor thin films.