Interband Faraday Effect in Aluminum Antimonide, Germanium, and Gallium Antimonide

The Faraday rotation at the absorption edge corresponds to interband transitions. Data for both Ge and GaSb exhibit a reversal in sign of the rotation at photon energies approaching that required for direct transitions at the zone center. The rotation for AlSb shows no reversal which indicates indirect transitions. Measurements were made at 77 and 296^∘{}K. In the case of GaSb and Ge, the Faraday rotations show large shifts with temperature as the edge is approached associated with direct transitions, while this shift is small in AlSb where the rotation is associated with indirect transitions. The spectral dependence of the Faraday rotation at the direct energy gap is proportional to (ωg-ω)^-1/2. For frequencies small compared to the gap frequency ωg, it is of the form const ω². The resultant direct energy gaps calculated from our measurements are: Ge, (0.80±{}0.01) eV at 296^∘{}K, (0.89±{}0.01) eV at 77^∘{}K; GaSb, (0.74±{}0.02) eV at 296^∘{}K, (0.82±{}0.02) eV at 77^∘{}K. The temperature dependence of the indirect gap in AlSb is found to be -3.5×{}10^-4 eV (^∘K)^-1. The temperature dependence of the rotation in GaSb suggests that the effective indirect gap increases with temperature.