Far infrared transmission spectroscopy of binary semiconductors

Abstract

The far infrared optical properties of a selection of binary semiconductors have been studied by the technique of dispersive Fourier transform spectroscopy. A commercial modular Michelson interferometer has been rebuilt in a single pass dispersive mode for this work. The performance of the instrument has been substantially improved by mounting the moving mirror on a pneumatically controlled precision linear slide to provide a smooth travel. The interferogram was sampled internally by monitoring the interference fringes derived from a secondary He-Ne laser channel. Precision alignment maximises the throughput signal enabling a relatively small specimen to be studied. Direct measurements of the amplitude and phase transmission spectra of GaP, GaAs, InSb, InAs and ZnSe have been performed at room temperature and 100K for the first time. The single-pass configuration has enabled new optical constant data to be obtained accurately on either side of the reststrahlen band. In most cases, these are the first reported results at the two temperatures. In each case the absorption coefficient and the complex dielectric response functions have also been calculated. Prominent features in the spectra are assigned as phonon-combination bands with the aid of critical point phonon frequencies derived from a lattice dynamical model. The magnitude of the imaginary part of the anharmonic self-energy function of the zone-centre transverse optical phonon required to account fully for the observed absorption has been estimated. The results give a clear indication of the range of validity of the anharmonic mechanism and the onset of lattice absorption due to non linear terms in the dipole moment expansion.