DX centres versus shallow centres in -based quantum wells

We report on a far-infrared magneto-optical study of multi-quantum-well structures made of the AlGaAs alloy both in the quantum wells and in the barriers in the direct-gap regime of this alloy. These structures have been selectively doped with Si donors in order to achieve in some of them a stable population of shallow states (shallow neutral donors having trapped an additional electron). Increasing step by step the Al contents allows one to obtain progressively DX as the ground donor state and to obtain interesting information about DX centres versus shallow neutral and negatively charged donors. In particular, it is shown that the same donor ion can give rise to two different negative-donor states: the shallow state and the deep DX 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.