Optical properties of Ge : Sb : Te ternary alloys

The complex dielectric function of the three stoichiometric compositions Ge Sb4Te7, GeSb2Te4, and Ge2Sb2Te5 has been measured by spectroscopic ellipsom etry in the energy range of 1.4-5.1 eV. Each composition was measured in th e amorphous and in the two crystalline states, the metastable face-centered -cubic and the stable hexagonal phases. The complex dielectric function sho ws strong differences between the amorphous and the crystalline samples, th ese differences are responsible for the drastic contrast in the reflectivit y spectra. The ellipsometric spectra are fitted with models which fulfillin g the Kramers-Kronig relations. Thus the Forouhi-Bloomer model was used for the amorphous samples and the Lorentz harmonic oscillator model for the cr ystalline samples. The temperatures at which the phase transformations are achieved were determined by measuring the ac electrical resistance at 100 H z while the samples were heated. The temperature derivative of the electric al conductivity shows two well defined peaks at critical temperatures where the phase transformation occurs as is revealed by the x-ray data. (C) 1999 American Vacuum Society.