Space-charge layers in semi-insulating GaAs: Photoexcited two-dimensional electron systems

Resonant Raman spectra of photoexcited semi-insulating GaAs and Fe/GaAs show features characteristic of two-dimensional electron plasmas. The results are ascribed to the presence of a space-charge layer at the surface, originating in a slight mismatch of Fermi-level positions at the vacuum (or metal) interface and in the bulk. Calculations using values of intersubband transition energies from the data give an estimated shift of ～0.04 eV for the Fermi-level position at T = 85K.