Indirect exciton fine structure in GaP and the effect of uniaxial stress

The technique of high-resolution wavelength-modulated absorption has been used to study the indirect exciton in GaP, both with and without uniaxial stress. The existence of a camel's back in the exciton dispersion curves is confirmed, and its height at zero stress is found to be 2.4±0.2 meV for the lower ($X_7$) exciton ground state and slightly larger for the upper ($X_6$) exciton. The valley-anisotropy splitting between these states at zero stress is found to be 1.9±0.2 meV at the minima of the dispersion curves. The observed changes in exciton binding energy with stress are treated approximately using perturbation theory. The following deformation potentials are found: $b=-1.8\mathrm{}\pm 0.2$ eV, $d=-4.5\mathrm{}\pm 0.5$ eV, ${\mathcal{E}}_2=6.5\mathrm{}\pm 0.5$ eV, ${\mathcal{E}}_1-a=1.6\mathrm{}\pm 0.2$ eV. The exciton parameters obtained rest mainly on the results for [001] stress, as the other stress directions (particularly [111]) give complex line shapes which cannot be understood with a simple model. The observation of an excited state in pure material is found to imply a mean group-state binding energy of 20.5 meV, in good agreement with recent theoretical calculations, but higher than many experimental estimates. The binding energy is confirmed to be of this magnitude from level-crossing effects in the stress measurements.