We report the temperature dependence of the Hall mobility in the dark and in the persistent-photoconductivity (PPC) states of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As:Te and ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As:Si samples, which incorporate buffer layers separating the epilayer from the substrate. The mobility of the Si-doped sample is higher in the PPC state than in the dark, whereas for the Te-doped sample the mobility in the dark is higher. The results for the Si-doped sample can be explained equally well by the positive-U and the negative-U models of the DX center and hence are not suitable for drawing definitive conclusions about the charge state of the DX center. On the other hand, the results for the Te-doped sample are shown to be conclusively in favor of the neutral charge state (positive-U) model of the DX center.

• Received 5 April 1993

DOI:https://doi.org/10.1103/PhysRevB.48.8757