DIRECT EVIDENCE FOR 2-STEP PHOTOIONIZATION OF DX(TE) CENTERS IN ALXGA1-XAS

A detailed analysis of the photoionization process of DX(Te) centers in AlxGa(l-x)As (0.25 < x < 0.55) for different 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 DX- energy state. An intermediate state for the process is a one-electron localized state of the defect.