Absorption edge characteristics of GaAs, GaSb, InAs, and InSb

The physical characteristics of the fundamental absorption edge of semi-insulating GaAs and unintentionally doped GaSb, InAs, and InSb are examined using spectroscopic ellipsometry. A five parameter model is developed to describe the key characteristics of the absorption edge. Among these parameters are the bandgap energy, the characteristic energy of the Urbach tail, and the absorption coefficient at the bandgap energy. The results indicate that the Coulomb interaction strongly influences the shape of the band edge with progressively less influence as the bandgap energy decreases. The energy dependence of the optical transition strength is observed to be nearly constant in narrow bandgap InSb.