Stress dependence of the sulfur-bound excitons in GaP

We present uniaxial-stress experiments performed on the sulfur-bound exciton spectrum of GaP. The data have been obtained at pumped-liquid-helium temperature for the stress directions [111], [100], and [110], respectively. First we analyze the stress dependence of the exciton ground state. Most results are satisfactorily accounted for by a simple model which neglects the "camel's-back" structure of the conduction band: the deformation potentials which describe the bound-exciton complex directly reflect the band-edge behavior. Unexpected data have been collected for the high-energy components. They disagree with the predictions of the simple model and are not presently understood. Next we deduce the magnitude of the valley-orbit interaction. We find $E\left({\Gamma }_{12}\right)-E\left({\Gamma }_1\right)=12\mathrm{}$ meV. Taking account of the sulfur-donor ionization energies ${E_D}_1=107\mathrm{}$ meV and ${E_D}_{12}=54\mathrm{}$ meV, we get for the ratio $\frac{E_{\mathrm{Bx}}}{E_D}$ the values 0.18 and 0.14, ${\mathrm{respectively}.\mathrm{}}^{12}$ These values appear somewhat larger than the predicted ratio ∼10% from the "Hayne rule."