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Investigation of the Structural and Optical Properties of Ge-doped SbTe Films with Various Sb:Te Ratios

In this study, we investigated the structural and optical properties of Ge-doped SbTe Ge–ST thin films with three differing compositions: Ge0.06Sb0.77Te0.17 Ge–STH ,G e 0.05Sb0.70Te0.25 Ge–STM, and Ge0.05Sb0.64Te0.31 Ge–STL, grown on Si substrate by radio-frequency sputtering method. The films were annealed at 250°C for crystallization and their crystal structures were examined by X-ray diffraction. Compared to the X-ray diffraction spectra of the undoped SbTe, the Ge–ST thin films had a hexagonal structure with large stacking periods. Using Raman spectroscopy, we investigated the shift of the phonon mode frequencies A1g and Eg of the films with varying Sb:Te ratios. We compared the dependence of the phonon frequencies of Ge–ST on the Sb content to those of the corresponding undoped SbTe. The composition dependence of the A1g phonon frequency could be explained in terms of the linear extrapolation of Sb and Sb2Te3 crystals. By using spectroscopic ellipsometry, we measured the dielectric function of the thin films in the near-IR, visible, and ultraviolet spectral regions. The optical energy gaps and bandgaps of the amorphous and crystalline phases, respectively, were determined using linear extrapolation of the absorption coefficient. The optical gap energies of the amorphous Ge–ST films were determined to be about 0.5–0.6 eV, whereas the indirect bandgap energies of the crystalline films shrank substantially to about 0.15–0.2 eV.