Optical properties of amorphous and crystalline Sb-doped SnO₂ thin films studied with spectroscopic ellipsometry: Optical gap energy and effective mass

We investigated the optical properties of amorphous and crystalline antimony (Sb)-doped tin dioxide (SnO₂) thin films grown using the co-sputtering deposition method at room temperature. We used undoped and Sb-doped (8 wt. %) SnO₂ targets. Varying the relative power ratio of the two targets, we controlled the Sb-composition of the SnO₂:Sb thin films up to 2.3 at. % of Sb contents. Through annealing, the as-grown amorphous SnO₂:Sb thin films were transformed to crystalline thin films. Dielectric functions were obtained from the measured ellipsometry angles, Ψ and Δ, using the Drude and parametric optical constant models. We determined the absorption coefficients and optical gap energies of the SnO₂:Sb thin films from the dielectric functions. We found increasing optical gap energy with increasing Sb composition. Increases in the Drude tail amplitudes, a signature of free carrier concentrations, were found in annealed, crystalline thin films with increasing Sb composition. The increase in the optical gap energy with increasing Sb composition was mainly attributed to the Burstein-Moss effect. Using Hall effect measurements, we obtained Hall carrier concentrations (N_Hall) and electron Hall mobilities (μ_Hall). The carrier concentrations and mobilities increased from 2.6 × 10¹⁹cm⁻³ and 1.0 cm²/(V s) to 2.0 × 10²⁰cm⁻¹ and 7.2 cm²/(V s), respectively, with increasing Sb contents. This result suggests that the nominally undoped SnO₂ films are unintentionally n-type doped. Assuming that the N_Hall and optical carrier concentrations (N_opt) were the same, we obtained the effective masses of the SnO₂:Sb thin films with increasing Sb compositions. The effective masses of the SnO₂:Sb thin films increased from 0.245 m₀ to 0.4 m₀ with increasing Sb doping contents, and the nonparabolicity of the conduction band was estimated. We discussed the relation between the optical (μ_opt) and Hall (μ_Hall) mobilities as a function of Sb contents and grain sizes.