DX CENTERS VERSUS SHALLOW D- CENTERS IN ALYGA1-YAS-BASED QUANTUM-WELLS

We report on a far-infrared magneto-optical study of AlyGa1-yAs/AlxGa1 -xAs multi-quantum-well structures made of the AlGaAs alloy both in th e quantum wells and in the barriers in the direct-gap regime of this a lloy. These structures have been selectively doped with Si donors in o rder to achieve in some of them a stable population of shallow D- stat es (shallow neutral donors having trapped an additional electron). Inc reasing step by step the Al contents allows one to obtain progressivel y DX as the ground donor state and to obtain interesting information a bout DX centres versus shallow neutral and negatively charged donors. In particular, it is shown that the same donor ion can give rise to tw o different negative-donor states: the shallow D- state and the deep D X state. The one or the other is populated, depending on the 'history' of the sample. The energy of the DX2 level (DX with two Al neighbours ) is determined with high accuracy.