Impurity and Exciton Effects on the Infrared Absorption Edges of III-V Compounds

The fundamental absorption edge of each of several III-V compounds has been examined for structure due to excitons and impurities using high resolution and sample temperatures down to 1.4°K. The effects of an applied magnetic field were studied. The absorption edge of GaSb shows three sharp peaks ($\alpha$, $\beta$, $\gamma$) and a low-absorption tail. Under a magnetic field, the peaks ($\alpha$, $\beta$, $\gamma$) shift and split in a manner close to the expected behavior of the exciton. The peak $\alpha$ is shown to be the free-exciton peak and the peaks ($\beta$ and $\gamma$) are attributed to impurity-exciton complexes. The tail is found to be associated with electron transitions to the conduction band from impurity levels near the valence band. Absorption associated with impurity-valence-band transitions is also observed at low photon energies. Analysis of these results gives the energy gap, the electron $g$ factor, and the ionization energies of impurity levels. The absorption edge of InSb shows a step which is found to be due to ionized acceptors. Under a magnetic field, two peaks develop from the step absorption which behave like the impurity-exciton peaks in GaSb. The optical transitions involved appear to be related to the emission observed in the InSb laser. The electron $g$ factor and the ionization energies of impurity levels have been estimated from these results. Some studies on the impurity absorption near the intrinsic edge of other III-V compounds are reported.