Optical spectroscopy of negative-U centers

Abstract
The identification of strongly correlated double electron local sites in semiconductors usually requires a nonequilibrium situation in order to observe a single occupied state of a local site. We present a way of a direct observation of negative-U centers by means of a single photon optical transition from band states into local states provided controlled occupancy of local levels via doping the semiconductor or via change of the gate voltage. The photoabsorption spectrum shows single and double electron transitions as the consequence of the hybridization of local states and band electron states. The value of the correlation energy in localized states determine positions and intensities of peaks while the occupancy of local levels reflects itself in the line shape. A qualitative comparison with available data on installed atoms as possible double electron centers in semiconductors is made.