MAGNETOOPTICAL PROPERTIES OF THE DX CENTER IN AL0.35GA0.65AS-TE

Magneto-optical absorption spectra of a 0.4 mm thick, single-crystal Al0.35Ga0.65As:Te sample give evidence for two bleachable absorbers, one of which is identified as the DX centre. The bleached-state absorption coefficient and magnetic circular dichroism (MCD), measured from 0.66 to 2.2-mu-m at 1.7 K, are adequately described by the Drude free-electron model, in terms of which a value for the electron effective mass is obtained. Cooling the sample in darkness leads to transmission transients, from which the absorption coefficient and optical-conversion cross section for the bleachable deep DX ground state are derived. The MCD at the beginning of each transient is identified also with the DX ground state, and its temperature dependence reveals that the bulk of it has a non-paramagnetic origin. We conclude that the paramagnetic contribution to the MCD from the DX ground state is very small, being less than 0.004% of its peak absorption coefficient. This provides strong support to the diamagnetic ground-state, negative-U model of Chadi and Chang. The origin of the second bleachable absorber (threshold approximately 1.6 eV) has not been established.