Spectral ellipsometry of GaSb: Experiment and modeling

The optical constants ε(E)[=ε₁(E)+iε₂(E)] of single crystal GaSb at 300 K have been measured using spectral ellipsometry in the range of 0.3--5.3 eV. The ε(E) spectra displayed distinct structures associated with critical points (CP's) at E₀ (direct gap), spin-orbit split E₀+Δ₀ component, spin-orbit split (E₁,E₁+Δ₁) and (E₀^',E₀^'+Δ₀^') doublets, as well as E₂. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CP's plus excitonic and band-to-band Coulomb enhancement effects at E₀, E₀+Δ₀, and the E₁, E₁+Δ₁ doublet. In addition to evaluating the energies of these various band-to-band CP's, information about the binding energy (R₁) of the two-dimensional exciton related to the E₁, E₁+Δ₁ CP's was obtained. The value of R₁ was in good agreement with effective mass/k·p theory. The ability to evaluate R₁ has important ramifications for recent first-principles band-structure calculations, which include exciton effects at E₀, E₁, and E₂ [M. Rohlfing and S. G. Louie, Phys. Rev. Lett. 81, 2312 (1998); S. Albrecht et al., Phys. Rev. Lett. 80, 4510 (1998)]. Our experimental results were compared to other evaluations of the optical constants of GaSb.