METASTABLE STATES OBSERVED BY OPTICAL-ABSORPTION OF DX CENTERS IN ALXGA1-XAS-TE

With the use of thick n-type AlxGa1-xAs:Te crystals (x=0.29, 0.37 and 0.46) between 10 K and 300 K, optical absorption of deep levels, so called DX centers, has been measured in sequence under (a) thermal equilibrium and (b) non-equilibrium states induced by the photoionization of the deep DX center. Absorption found at the expense of the deep DX center absorption has a peak at 0.56 eV when x=0.46: the band is related to the metastable persistent photoconductivity. The intensity correlation of the two absorptions indicates the presence of another metastable state which is optically inactive. This state is interpreted as a neutral donor state with reference to a negative U model of DX centers.