FAR-INFRARED SPECTROSCOPY OF SILICON DONORS IN Al//xGa//1// minus //xAs.

We report first measurements of the 1s-2p transition of shallow Si donors in Al//xGa//1// minus //xAs. For AlAs mole fraction x APP GRTH 0. 25, the transition is observed only after the material is briefly exposed to visible or near visible light. If we interpret this persistent photoeffect in terms of the D-X center model for deep donor states in Al//xGa//1// minus //xAs, our results indicate that the photoionized D-X centers act as shallow donors.