Photoionization of the DX centers in Te-doped AlxGa1-xAs: Absence of two-step photoionization of DX centers.

Abstract

Photoionization studies have been performed for Te-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As to investigate the properties of the theoretically predicted intermediate neutral relaxed state of the DX center, i.e., ${\mathit{DX}}^{0}$. The expected effect of the ${\mathit{DX}}^{0}$ state on the photoionization process in the neutral charge region, where the recapture of an electron by ${\mathit{DX}}^{0}$ is possible, was calculated from the rate equations for a two-electron system. The photoionization measurements were made in both the depletion region and the neutral charge region. It is found that the photoionization rates of DX centers in the two charge regions are identical, indicating no significant effect of the ${\mathit{DX}}^{0}$ state on the photoionization of DX(Te) centers. Thus, the intermediate ${\mathit{DX}}^{0}$ state, if it indeed exists, must be highly unstable, with an activation energy of less than 20 meV. These results contradict an earlier report which claimed to find direct evidence for a two-step photoionization of DX(Te) centers. The results of the present study, together with a detailed analysis of the experimental method used in the earlier study, indicate that the earlier claim is unsubstantiated.