Photoluminescence studies of heteroepitaxial GaAs on Si.

We present a systematic study of the photoluminescence of undoped GaAs layers deposited by MOCVD on Si substrates. The study includes an examination of substrate and layer thickness effects in thin GaAs layers, and a detailed investigation of the stress effects on the intrinsic band-edge transitions in thicker samples. For sample thickness t = 0.5micron, we observe strong midgap emission bands associated with defects close to the interface. These bands depend strongly on the nature of the Si substrate. The crystal quality improves with sample thickness, and for t >= 0.5micron the emission is dominated by lines in the band edge region which are relatively independent of substrate preparation. Photoluminescence excitation spectra reveal that the highest energy line is due to an intrinsic exciton transition, and that a splitting of this line observed for t > = 2micron reflects the presence of two different regions of strain in the material. The magnitude of the strain is estimated from the shift of the exciton lines relative to unstrained GaAs, and is found to be consistent with an upper limit provided by the thermal expansion mismatch and allowing for varying degrees of relaxation during cooling from the growth temperature.