Optical properties and structure of tellurium–germanium– bismuth–antimony compounds with fast phase-change capability

Abstract

Quaternary TeGeBiSb amorphous films, 100 nm in thickness, were prepared by rf sputtering onto either a Si (100) or polycarbonate substrate. Crystalline films were obtained by subsequent annealing at 300 °C for 10 min. At least two phases with hexagonal and rhombohedral structures were identified by x-ray diffractometry in the quaternary system. Their compositions are close to formulas Te5Ge4Bi4Sb and Te5Ge4Bi7Sb, with c/a ratios of 5.5 and 9.4, respectively. The Te5Ge4Bi4Sb film shows a reasonable reflectivity contrast ranging from 20% to 25% in the whole visible spectrum, while the reflectivity contrast of Te5Ge4Bi7Sb film shows a sharp lowering tendency with decreasing wavelength and turns negative at wavelengths smaller than 470 nm. Results of thermal analyses show that the films crystallize at around 236–266 °C with corresponding activation energies 2.67–2.88 eV. The change in reflectivity contrast is explained based on the variation in optical band gaps of the TeGeBiSb materials with either Bi content or annealing temperature. These materials may find applicability in reversible type phase-change optical recording.