Raman scattering experiments were performed on n-type samples, which were doped with Te, Si, and S impurities, as a function of hydrostatic pressures which ranged from 4 to 9GPa. This revealed the bistable nature of the related impurity level under optical illumination. Numerical simulations of the results showed that the Chadi-Chang negative-U model could reproduce the experimental data for low pressures. However, at pressures above 5 to 6GPa, a positive-U description which involved a small lattice relaxation was necessary in order to reproduce the results for Te- and Si-doped samples. This demonstrated that the DX defect evolved under pressure, and that a transition from negative to positive U could occur at 5 to 6GPa.

J.Zeman, M.Zigone, G.Martinez: Physical Review B, 1995, 51[24], 17551-60