Unusual Behaviour of the DX-Centre in GaAs:Ge

Magnetotransport measurements have been performed as a function of hydrostatic pressure and temperature to study the effect of deep donor levels on the electron concentration and mobility in bulk GaAs heavily doped with Ge. For the first time, the coexistence of the metastable DX centre and the non-metastable localized A1 level has been unambiguously observed in GaAs. Furthermore, we have determined the energy positions and pressure dependences of both resonant levels (0.07 eV and -7 meV/kbar) for the A1 level and 0.1 eV and -10 meV/kbar for the DX centre) and the energy barrier for electron emission from the DX-centre (0.29 eV). The latter appeared to be pressure independent. From ' the mobility behaviour during depopulation of the DX-centres, an elegant proof for negative U character of DX centres and the positive U character of deep A1 levels has been obtained.