Polariton Reflectance and Photoluminescence in High-Purity GaAs

The low-temperature optical reflectance and photoluminescence of high-purity GaAs have been studied in the spectral energy region near the direct fundamental gap. Strong free-exciton reflectance structure is seen. This structure is strongly influenced by both spatial dispersion and polariton effects. The general shape and strength of this reflectance structure are well accounted for by polariton theory with the unknown surface-barrier thickness as the only adjustable parameter. The sharp spike in the reflectance data provides an accurate value $E_L=1.51515\mathrm{}\pm 0.00015$ eV for the longitudinal exciton energy at 2 K. The longitudinal-transverse exciton splitting falls within the range 0.1-0.25 meV. Two structures in the luminescence spectrum are interpreted, respectively, as lower (LPB) and upper polariton branch (UPB) luminescence. The LPB luminescence occurs at the transverse exciton energy as expected. The UPB luminescence, previously identified as the "free-exciton" structure, has the rather unusual behavior that it always occurs above, and has its low-energy "onset" at, the longitudinal exciton energy. Uniaxial stress and temperature-dependence results are presented to support the interpretation of the data. The over-all energy splittings and polarization behavior seen in both reflectance and luminescence spectra agree with theoretical results. The stress behavior of the reflectance provides the value $j=-0.05\mathrm{}\pm 0.05$ meV for the exciton exchange energy. An anticrossing seen in the piezoluminescence data is attributed to an interaction, arising from the valence-band anisotropy, of the UPB with an optically forbidden triplet exciton state. Possible alternative explanations of the luminescence data are discussed and comparisons are made with earlier work.