SURFACE AND BULK MODULATION IN PHOTOREFLECTANCE FROM UNDOPED GAAS

Plasma discharge is used to alter the surface Potential in undoped GaA s. The results determine the origin and the magnitude of photomodulati on in photoreflectance from undoped GaAs. We examined photoreflectance for 30-70-kV/cm surface electric fields in undoped GaAs grown on heav ily doped n- or p-type GaAs underlayers, UN+ and UP+ samples, respecti vely. Photoreflectance for 2-500-muW/cm2 laser modulation intensity- w as analyzed using separate electro-optic functions to determine the ma gnitude of the built-in electric field in the dark and the laser illum inated samples. Before plasma discharge, photomodulation of the built- in electric field in UP+ samples was about three times that in UN+ sam ples at 0.2 mW/cm2. The large modulation in UP+ samples appears to hav e come from surface modulation effects, possibly from poor pinning of the surface Fermi energy. A hydrogen plasma discharge lasting 4 s prod uced long term effects in UP+ sample. It increased the magnitude of th e electric field in UP+ samples from approximately 40 kV/cm to approxi mately 60 kV/cm, st 0.2 mW/cm2. The discharge more than doubted the am plitude of UP+ photoreflectance signal and increased the number of Fra nz-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 b ecame comparable with bulk photovoltage from (p-i-n) structures and UN + structures.