Optical nonlinearity in low-temperature-grown GaAs: Microscopic limitations and optimization strategies

We have quantitatively measured the linear and the nonsaturable absorption as well as the absorption modulation and its recovery time in as-grown and annealed low-temperature (LT) GaAs. Correlation of the optical data with As antisite $({\mathrm{As}}_{\mathrm{Ga}})$ defect densities yields the absorption cross section and the saturation parameter of the dominant ${\mathrm{As}}_{\mathrm{Ga}}$ to the conduction-band defect transition. We show that this defect transition is mainly responsible for the large nonsaturable absorption in as-grown LT GaAs with fast recovery times. Reducing the ${\mathrm{As}}_{\mathrm{Ga}}$ density by annealing yields an optimized material with small nonsaturable absorption, high absorption modulation, and fast recovery times.