BARRIER BETWEEN LOCALIZED AND SHALLOW NEUTRAL DONOR STATES IN GAAS-GE

The photoconductivity (PC) of GaAs:Ge under hydrostatic pressure shows another giant PC caused by a neutral, localized donor state with A(1) symmetry in addition to the well-known persistent PC due to the DX st ate. We find that the top of the barrier for electron recapture to the Al state is pinned to the conduction-band edge and that the capture c ross section sigma(T-->infinity) is surprisingly small. In agreement w ith recent theoretical calculations, the observed shallow-deep donor i nstability is attributed to the change of the short-range part of the donor potential due to interaction with acoustic phonons.