Fabrication and structural analysis of amorphous GeTe4, Sb2Te3 and Ge20SbxTe80-x thin films prepared by thermal evaporation

Abstract

Amorphous Ge₂₀Te₈₀, Sb₄₀Te₆₀, and Ge₂₀Sb_xTe_80-x (x = 1, 2, 5, 10) compounds were synthesized and deposited as thin films (40–80 nm) on glass substrates via thermal evaporation under high vacuum (10^–5 Torr). Their structural characteristics were examined using X-ray diffraction, scanning electron microscopy, and atomic-force microscopy. Applying the void–cluster model, the quasi-period of density fluctuations (r), correlation length (L), and packing fraction (L/r) were estimated from the first sharp diffraction peak in the XRD patterns. Partial substitution of Te by Sb in Ge₂₀Te₈₀ led to a reduction in L and L/r, indicating increase in structural disorder. Raman spectroscopy revealed vibrational modes associated with short-range ordering in Ge₂₀Te₈₀ and Sb₄₀Te₆₀, aiding interpretation of Ge₂₀SbₓTe₈₀₋ₓ films. In Ge₂₀Te₈₀, dominant Raman peaks at 121.5 and 139 cm⁻¹ were assigned to GeTe₄₋_nGe_n (n = 0, 1) tetrahedra, while bands at 154–159 cm⁻¹ and 174–177 cm⁻¹ corresponded to Te chains and defective octahedral units. For Sb₄₀Te₆₀ films, peaks at 124.5 and 142 cm⁻¹ were attributed to Sb–Te vibrations in pyramidal SbTe₃ units and distorted octahedral Sb coordination. Additional peaks at 97 and 152 cm⁻¹ were associated with Te–Te bonds, while the band at 113.5 cm⁻¹ corresponds to Sb–Te stretching modes. The Raman intensity at 121.5, 139 and 157 cm⁻¹ in Ge₂₀Sb_xTe_80-x exhibits a non-monotonic dependence on Sb concentration, reaching a maximum at x = 2. A low- frequency boson peak (< 100 cm⁻¹) observed in Sb₄₀Te₆₀ suggests a high density of low-energy vibrational states, characteristic of disordered structure.