MAGNETIC CIRCULAR-DICHROISM OF THE DX CENTER IN AL0.35GA0.65AS-TE

Magneto-optical absorption spectra of 0.4-mm-thick, single-crystal Al0.35Ga0.65As:Te give evidence for two bleachable absorbers, one of which is identified as the DX center. 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. Cooling the sample in darkness leads to transmission transients, from which ground-state absorption coefficients and optical-conversion cross sections for the bleachable absorbers are derived. The conversion threshold is 0.6 eV for the DX and 1.5 eV for the second bleacher, and the recovery behavior is significantly different for the two. The MCD at the beginning of each transient is identified with the ground state of each absorber, and temperature dependence reveals that the bulk of the initial MCD has a nonparamagnetic 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 at T = 1.7 K and B = 2 T. This provides strong support to the diamagnetic-ground-state, negative-U model of Chadi and Chang. The origin of the second bleachable absorber has not been established.