Optical and Electrical Properties of Phase Change Materials

Abstract

Optical and electrical properties of the phase change material Ge 2 Sb 2 Te 5 are reviewed for its three phases. Implications of these data for the energy distribution of the density of electron states in the vicinity of the band edges are described. Near-band edge optical data obtained from ellipsometry and optical absorption spectra have been fitted with dispersion equations to determine values of ~ 0.7 eV for the optical bandgap of the amorphous phase, and ~ 0.5 eV for the highly conducting fcc and hexagonal phases. Infrared absorption spectra exhibit the effects of free carrier absorption in both crystalline phases. Conductivity at ~ 300 K is dominated by holes in all three phases; hole concentrations and mobili-ties are 10 20 cm-3 and ~ 30 cm 2 /V-s in hexagonal, and 10 20 cm-3 and ~ 1 cm 2 /V-s in fcc. Temperature dependence of the sheet resistance of thin film Ge 2 Sb 2 Te 5 documents the dramatic decreases in electrical resistance at the amorphous-to-fcc and fcc-to-hexagonal phase transitions. Comparison of the temperature dependence of the electrical properties in each of the three phases of Ge 2 Sb 2 Te 5 provides additional insights concerning their conduction mechanisms.