The coexistence of the DX center with non-metastable bound states of the ordinary substitutional configuration of the donor impurity was investigated in Si-doped samples with various AlAs molar fractions. The occupation of non-metastable states was deduced by comparing the so-called electron density, that was estimated from capacitance-voltage measurements of Schottky diodes, with the Hall density. In samples with compositions that were not far from the direct-to-indirect gap transition and which had dopant levels of the order of 1018/cm3, a non-metastable state (SX) could attain a significant occupancy when a saturated persistent photoconductivity condition was approached during the low-temperature photo-ionization of DX centers. When the free-electron density was lower than a critical density of some 1016/cm3, electrons froze out into a localized S state, or into a low-mobility impurity band that was linked to the  conduction-band edge. A finite occupancy of the SX or S state gave rise to a significant density of substitutional donors in the neutral charge state having a strongly correlated spatial distribution. Electron capture into any donor states had a spatially selective nature, as shown by the low-temperature mobility under conditions where impurity scattering predominated. In the case of DX centers, this was demonstrated by an hysteretic behavior of low-temperature data.

A.Baraldi, P.Frigeri, C.Ghezzi, A.Parisini, A.Bosacchi, S.Franchi, E.Gombia, R.Mosca: Physical Review B, 1996, 53[16], 10715-27