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 (Formula presented)(Formula presented)As samples having different x AlAs molar fractions in the direct gap region. The occupation of nonmetastable states is evidenced by comparing the (Formula presented) “electron density,” as derived from capacitance-voltage measurements in Schottky diodes, with the (Formula presented) Hall density. In samples of compositions not far from the direct-to-indirect gap transition and with doping levels in the (Formula presented)-(Formula presented) 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 (Formula presented), electrons freeze out into a localized SΓ state, or into a low mobility impurity band, linked to the Γ conduction-band edge. A finite occupancy of the SX or SΓ state gives rise to a significant (Formula presented) density of substitutional donors in the neutral (Formula presented) 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 ((Formula presented)) (Formula presented) vs (Formula presented) data, where any given (Formula presented) value is reached through DX center photoionization steps or through electron capture via a proper thermal cycling. When (Formula presented) is negligible the hysteresis amplitude is maximum. However, whenever (Formula presented) reaches significant values, either at the (Formula presented) temperature or during the thermal cycle, the hysteresis amplitude vanishes. This is systematically observed in all the cases where the occupation of SX or SΓ states is independently demonstrated through the analysis of (Formula presented) and (Formula presented) data. Two diverse complementary effects are proposed to explain the observed vanishing of the hysteresis amplitude.
|Number of pages||13|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - Jan 1 1996|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics