Shallow to deep transformation of Se donors in GaSb under hydrostatic pressure

We have observed that highly doped GaSb:Se, which is opaque to far IR radiation, becomes transparent at hydrostatic pressures above 9.8 ± 2 kbar. We discuss how this behavior may be explained by the transformation of Se shallow donors into Se-DX (where DX is the unknown donor or X donor) centers in GaSb. Under this assumption the position of the Se-DX energy level at zero pressure is calculated to lie 80±30 meV above the conduction band at atmospheric pressure. The onset of transparency allowed us to observe several multiphonon absorbance features. We assign six of them to two-phonon absorptions. From the measured pressure dependence of the TO phonon, the Grüneisen parameter for this compound is calculated to be γTO = 1.23 ± 0.18. No persistent photoconductivity is observed for these Se-DX centers, a fact that may be explained by the expectation that the optical energy necessary to transform them back into the shallow form is larger than the band-gap energy of GaSb at all pressures examined, although it may be also an indication that the Se shallow donors change to deep donors associated with the L₁ minima of ionization energy larger than 90 meV.