Study of deposition dependent characteristics of gold on n-GaAs by photoreflectance spectroscopy

We used the noncontact electric field modulation technique of photoreflectance to study the effect of gold on GaAs grown by two different metalization methods. A semitransparent 7 nm gold overlayer was grown on low doped n-GaAs (2 x 10(16) cm(-3)) using metalization by evaporation and by sputtering. Our experimental results indicate that the photoreflectance lineshape depends on the metalization method in a characteristic way. We present evidence of a previously unreported modulation mechanism in photoreflectance. We employed simulation of photoreflectance lineshape based on a multilayer model to reproduce characteristic features of experimental lineshapes. For sputtered Au/GaAs samples the best simulated lineshape was obtained through an unusual modulation, which is based on a strongly pined surface electric field. This produced a lineshape somewhat similar to a third derivative functional form with severely suppressed Franz-Keldysh oscillations (FKO). Simulation with the same set of parameters and a parallel field modulation restored FKO and reproduced the experimental lineshape of the evaporated Au/GaAs. The parallel field modulation spontaneously reproduced the below-band-gap feature, which is often observed in electromodulation spectroscopy. We believe the change in FKO is correlated with the density of interface states as they influence the extent of Fermi-level pinning. (C) 1997 American Institute of Physics.