Our photo reflectance (PR) spectroscopy measurements of the delta-doped GaAs film at 300 K reveal many Franz-Keldysh oscillations (FKOs) above the valence band edge, E(0) and the spin-orbit split energy, E(0)+Delta(0) which enables us to determine the electric field strength from periods of FKOs provided reduced masses of the electron and holes are known. The reduced masses can be determined unambiguously at E(0)+Delta(0), but not at E(0), at which the heavy- and light-hole transitions are degenerate. However, the ambiguity at E(0) can be resolved by applying the fast Fourier transform to the PR spectrum to separate the contributions from the heavy and light holes. (C) 1996 American Institute of Physics.
