DIRECT EVIDENCE OF THE 2-ELECTRON CHARACTER OF DX CENTERS BASED ON CO-DOPING WITH SHALLOW DONORS AND APPLICATION OF HIGH-PRESSURE

A short review is given of some recent experiments specially planned and performed to unambiguously solve the problem of the charge state of the DX center. We mainly concentrate on experiments performed on co-doped samples and at high pressure. We show that at least for some DX centers (e.g. Ge-DX) it is certain that they are occupied by two electrons. However, such double occupation does not necessarily mean that for every DX center the effective Hubbard correlation energy, U, is negative. Moreover, we show that not all of the so-called X-like centers must be occupied by two electrons.