Abstract
Raman-scattering experiments performed on n-type GaAs samples, doped with Te, Si, and S impurities, as a function of hydrostatic pressure in the indirect-gap range 4--9 GPa, reveal the bistable character of the related impurity level under optical illumination. A numerical simulation of our results shows that the negative-U model of Chadi and Chang is valid for reproducing the experimental data at low pressure. However, at very high pressures beyond 5--6 GPa, a positive-U description involving a small lattice relaxation is necessary to reproduce results for Te- and Si-doped samples. This demonstrates that the DX defect evolves under pressure, and that a transition from negative to positive U can occur at about 5--6 GPa.
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