Disorder-induced intervalley scattering in photoexcited AlGaAs

Alloy semiconductor structures are important in bandgap engineered devices because of the tunability of the bandgap and lattice constant. Transport in such alloys is inherently affected by disorder scattering arising from the random placement (in this case) of Al and Ga atoms on the cation sublattice. Intervalley scattering involves large k-vector components which must exist in the disorder-scattering potential in order for these to cause intervalley transitions. Most previous studies of disorder scattering involve averaging of the entire Brillouin zone to obtain a long-range effective scattering potential. Here, a general k-dependent formulation of the disorder-scattering potential, based on the coherent potential approximation, is made. Disorder-induced intervalley scattering rates are computed from the imaginary self-energy and subsequent integration over partial densities of states. We find a scattering potential (= square root (x(1-x))V₀ in the long-range model) of 18 meV at the L point and 38 meV at the X point in the Brillouin zone. Using an ensemble Monte Carlo simulation, the phonon-induced intervalley scattering and disorder-induced intervalley scattering transitions are compared for femtosecond photoexcited Al_0.6Ga_0.4As. We find that L-X scattering is affected slightly by the presence of disorder scattering. The disorder-induced scattering lifetimes for L-X scattering are found to be three to five times longer than the phonon-induced processes.