SURFACE AND BULK MODULATION IN PHOTOREFLECTANCE FROM UNDOPED GAAS

Plasma discharge is used to alter the surface Potential in undoped GaAs. The results determine the origin and the magnitude of photomodulation in photoreflectance from undoped GaAs. We examined photoreflectance for 30-70-kV/cm surface electric fields in undoped GaAs grown on heavily doped n- or p-type GaAs underlayers, UN+ and UP+ samples, respectively. Photoreflectance for 2-500-muW/cm2 laser modulation intensity- was analyzed using separate electro-optic functions to determine the magnitude of the built-in electric field in the dark and the laser illuminated samples. Before plasma discharge, photomodulation of the built-in electric field in UP+ samples was about three times that in UN+ samples at 0.2 mW/cm2. The large modulation in UP+ samples appears to have come from surface modulation effects, possibly from poor pinning of the surface Fermi energy. A hydrogen plasma discharge lasting 4 s produced long term effects in UP+ sample. It increased the magnitude of the electric field in UP+ samples from approximately 40 kV/cm to approximately 60 kV/cm, st 0.2 mW/cm2. The discharge more than doubted the amplitude of UP+ photoreflectance signal and increased the number of Franz-Keldysh oscillations. After discharge, the photomodulation voltage for UP + samples was reduced from 0.32 to 0. 14 V at 0.2 mW/cm2, and became comparable with bulk photovoltage from (p-i-n) structures and UN+ structures.