Enhancement of Structure and Optical Aspects by Thermally Induced Amorphous-Crystalline Transition in As30Te55Ga15 Films

The synthesis of As-Te-Ga glass films with a higher Ga ratio is essential to improve their structural and optical properties for practical applications. Herein, 150 nm thick films of As₃₀Te₅₅Ga₁₅ chalcogenide were prepared utilizing the thermal evaporation technique from their bulk melted quench glass. The influences of annealing temperature (\(\:{T}_{a}\)) performed through the amorphous-crystalline region on the crystal structure and optical characteristics of As₃₀Te₅₅Ga₁₅ films were investigated. The structure of As₃₀Te₅₅Ga₁₅ films was investigated using differential scanning calorimetry and X-ray diffraction techniques, while the optical properties were accomplished utilizing a double-beam spectrophotometer. The As₃₀Te₅₅Ga₁₅ glass exhibits one glass transition and crystallization phenomenon, implying its homogeneity and purity. Both as thermally evaporated and annealed As₃₀Te₅₅Ga₁₅ films at \(\:{T}_{a}\) < 433 K are noncrystalline, whereas the thermal-treated films at \(\:{T}_{a}\) ≥ 433 K are semicrystalline. The annealed As₃₀Te₅₅Ga₁₅ films are composed of hexagonal Ga₇Te₁₀ at \(\:{T}_{a}\) = 433 K, monoclinic As₂Te₃ and hexagonal Ga₇Te₁₀ at \(\:{T}_{a}\) = 473 K, and monoclinic As₂Te₃, hexagonal Ga₇Te₁₀ and hexagonal Ga₂Te₅ at \(\:{T}_{a}\) = 513 K. The change in optical constants and dispersion parameters showed two opposite trends associated with the amorphous and crystalline nature. For example, the band-gap energy demonstrated both direct and indirect optical transitions that dropped as \(\:{T}_{a}\) was elevated to 433 K, but increased again for films annealed at \(\:{T}_{a}\) = 473 K. The control of crystallite size and induced imperfections by annealing contributes to the improvement in optical characteristics.