Control of low-temperature-grown GaAs for ultrafast switching applications

The dynamic properties of low-temperature-grown GaAs (LT-GaAs) depend critically on growth and annealing conditions, such as substrate temperature during the MBE process (T_s), annealing temperature (T_a) and duration (τ_a). Previous empirical models based on carrier rate-equations introduce parameters, such as carrier lifetime, that cannot be directly correlated with growth and annealing conditions. The Schockley-Read-Hall model we use here introduces deep donor (N_DD) and acceptor (N_A) concentrations, instead of lifetimes. In LT-GaAs, deep donors are in majority constituted by an As-antisite related defect. The acceptors, in absence of doping, are most likely constituted either by Ga vacancy defects or by the residual doping during MBE growth. The samples studied are grown on GaAs semi-insulating substrates at different T_s and annealed under different conditions. For each sample, we first measure N_DD using X-ray diffraction analysis. Then we fit both continuous photoconductivity and pump-probe reflectometry measurements using N_A as the only adjustable parameter. From the set of data obtained, we can relate T_s, T_a and τ_a with N_DD and N_A. This gives us a way to predict LT-GaAs dynamics from growth and annealing conditions. This approach has been used to fabricate ultrafast photoconductive switches showing high sensitivity and good insulation.