COEXISTENCE OF 2 DEEP DONOR STATES, DX- AND DX0, OF THE SN DONOR IN GA1-XALXAS

We present results of correlated electron-paramagnetic-resonance and photoconductivity measurements on the tin donor in Ga1-xAlxAs (0. 3 < x < 0. 7), which demonstrate that the photoionization of the DX center leads to the formation of a localized paramagnetic donor state, DX0(Sn), according to the reaction DX- --> DX0 + e-, in agreement with negative- U models for the DX ground state. The DX0 state, which has been previously attributed by us to the A1ab antibonding state, is observed only in a limited alloy range 0. 3 < x < 0. 5, where the DX0/+ level is not resonant with the conduction band. Our results provide a simple explanation for the origin of the intermediate state in the electron-capture process of the single donors, the existence of which has been postulated previously from an analysis of the electron-capture kinetics of the DX center as well as from low-temperature photo-DLTS (deep-level transient spectroscopy) measurements: the one-electron A1ab state. As A1ab States are a fundamental property of substitutional donors, these results are also relevant for the analysis of DX-center properties of the other substitutional donors Si,Se,Te, and S in Ga1-xAlxAs.