Direct evidence for two-step photoionization ofDX(Te) centers inAlxGa1−xAs

A detailed analysis of the photoionization process of DX(Te) centers in ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As (0.25<x<0.55) for different temperatures and light intensities revealed, for the first time, that the process goes through two steps. The only possible interpretation of the phenomenon is by means of a negative-U character of the defect; i.e., the center binds two electrons in the ground state, forming a ${\mathit{DX}}^{\mathrm{-}}$ energy state. An intermediate state for the process is a one-electron localized state of the defect.