Coexistence of the DX center with nonmetastable states of the donor impurity in Si-doped AlxGa1-xAs: Effects on the low-temperature electron mobility

The coexistence of the DX center with nonmetastable bound states of the ordinary substitutional configuration of the donor impurity is extensively investigated in Si-doped AlxGa1-xAs samples having different x AlAs molar fractions in the direct gap region. The occupation of nonmetastable states is evidenced by comparing the n(CV) ''electron density,'' as derived from capacitance-voltage measurements in Schottky diodes, with the n(H) Hall density. In samples of compositions not far from the direct-to-indirect gap transition and with doping levels in the 10(18)-cm(-3) range, a nonmetastable state SX, degenerate in energy with the conduction band, can reach a significant occupancy when the saturated persistent photoconductivity condition is approached during low-temperature photoionization of DX centers. On the other hand, when the free-electron density is smaller than a critical density of a few 10(16) cm(-3), electrons freeze out into a localized ST stare, or into a low mobility impurity band, linked to the Gamma conduction-band edge. A finite occupancy of the SX or S Gamma state gives rise to a significant N-D0 density of substitutional donors in the neutral D-0 charge state having a strongly correlated spatial distribution. Electron capture into donor states, DX or not, has a spatially selective character, as can be evidenced by low-temperature mobility data under conditions where impurity scattering dominates. For DX centers, this is demonstrated by the hysteretic behavior of low temperature (T-0) mu(H) VS n(H) data, where any given n(H) value is reached through DX center photoionization steps or through electron capture via a proper thermal cycling. When N-D0 is negligible the hysteresis amplitude is maximum. However, whenever N-D0 reaches significant values, either at the To temperature or during the thermal cycle, the hysteresis amplitude vanishes. This is systematically observed in all the cases where the occupation of SX or Sr states is independently demonstrated through the analysis of n(CV) and n(H) data. Two diverse complementary effects are proposed to explain the observed vanishing of the hysteresis amplitude.