Optical and low-temperature thermoelectric properties of phase-pure p-type InSe thin films

Abstract

Polycrystalline phase-pure p-type InSe thin films were deposited on glass substrates by reactive evaporation at an optimized substrate temperature of 473 ± 5 K and pressure of 10⁻⁵ mbar. The as-prepared InSe thin films were analyzed by X-ray diffractometry, energy-dispersive X-ray spectroscopy, atomic force microscopy, UV–Vis–NIR spectroscopy, electrical conductivity and Hall measurements. The lattice parameters, particle size, dislocation density, number of crystallites per unit area and the lattice strain of the prepared InSe thin films were calculated and found as a = 4.00 ± 0.002 Å and c = 16.68 ± 0.002 Å, 48 ± 2 nm, 4.34 × 10¹⁰ lines cm⁻², 15.37 × 10¹⁰ cm⁻² and 1.8 × 10⁻³, respectively. The as-deposited InSe thin films showed a direct allowed transition with an optical band gap of 1.35 ± 0.02 eV and high absorption coefficient of about 10⁵ cm⁻¹. The oscillator energy (E _o) and dispersion energy (E _d) were calculated using the single-oscillator Wemple and DiDomenico model. The p-type conductivity and photosensitivity of the as-prepared InSe thin films confirmed their potential application in photovoltaic devices. The mean free path, relaxation time, density of states, Fermi energy and effective mass of holes in the film were determined by correlating the results of thermopower and Hall measurements. The sudden and sharp increase in thermopower from 80 to 37 K was explained as due to the effect of phonon drag on charge carriers.