Optical constants of amorphous Sb2Se3 thin films in the spectral range 2.5 to 15 µm

Sb2Se3 plays an important role as high-index layers in mid-infrared edge filters used in thermophotovoltaics (TPVs) to enable the spectral control performance. However, the information on its mid-infrared optical constants is still insufficient. In our investigation, Sb2Se3 thin films were deposited on Ge wafers at different substrate temperatures using electron beam evaporation. The compositions, surface morphologies and mid-infrared spectral transmittance were characterized using the energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectrometer, respectively. The crystallographic structure was also identified using X-ray diffraction (XRD). The mid-infrared optical constants were obtained using the classical Lorentz oscillator models as the dispersion equation in the fitting to the measured spectral transmittance. It was revealed that thin films were amorphous deposited at a low substrate temperature less than 150 °C, which has an approximately ideal stoichiometric ratio and the lowest free carrier absorption, being preferential for achieving the optimal optical constants. In the spectral region from 2.5–15 µm, the amorphous thin films have a refractive index in the range of 3.00–3.18, and an extinction coefficient less than 0.04.