Intrinsic Optical Absorption of Gallium Phosphide between 2.33 and 3.12 eV

Single ribbons of gallium phosphide have been synthesized which are very thin (down to ∼0.35 μ) and yet possess relatively large cross-sectional areas with uniform thickness so that high-resolution optical-absorption measurements can be extended well above the lowest energy direct interband electronic transition (Γ15→Γ1). The Γ15→Γ1 exciton energy gap is 2.873 eV below 25°K, and the internal binding energy of the direct exciton is about 5 meV. Direct transitions involving hole states in the split-off valence band can be clearly seen at low temperatures, and the spin-orbit splitting is 82±1 meV. Weak structure corresponding to indirect transitions at the Γ15→X3 energy gap is also observed at low temperatures, and the separation of the X3 and X1 conduction-band minima is 0.29±0.01 eV, in excellent agreement with the value obtained from interconduction-band absorption spectra. The temperature coefficient of the Γ15→Γ1 direct energy gap is − (5.2±0.1)×10−4 eV(°K)−1 near 300°K, more than twice the value for the Γ15→X1 indirect gap.