Optical and electrical properties of Cl-doped SnO₂ thin films prepared by ultrasonic spray pyrolysis

Cl-doped SnO₂ (CTO) films were prepared utilizing ultrasonic spray pyrolysis technology, where SnCl₂ was the single source for both tin and chlorine. The effect of substrate temperature was systematically studied on the crystalline state, surface morphology, thickness, optical and electrical properties, and thermal stability. Phase analysis indicated that the critical substrate temperature for crystallizing the CTO film is about 450 °C. Furthermore, the preferred orientation of the films evolved from (200) orientation to a dual orientation of (211) and (301) with increasing substrate temperature. All the film deposited at higher temperature (≥ 500 °C) presented pyramidal surface morphology with clear grain boundary. we deduced that the pyramidal morphology is primarily attributable to the crystal structure of SnO₂ (containing Sn–O₆ octahedra) and the anisotropic growth behavior (i.e., preferred orientation), rather than to any specific crystallographic orientation. Moreover, the Cl concentration, denoted by the ratio of [Cl⁻]/(2 × [Sn⁴⁺], decreased markedly from 9.53% to 0.14% with increasing substrate temperature. From the correlation between the bandgap and the Cl concentration, we deduced the solubility limit for Cl doping in SnO₂ to be about 0.80 at.% (i.e., [Cl⁻]/(2 × [Sn⁴⁺]) = 0.80%). The film deposited at 500 °C exhibited the lowest resistivity (1.11 × 10⁻³ Ω·cm) and outstanding electrical stability, showing no degradation in electrical conductivity after one week of aging at 150 °C and even a slight improvement. All in all, our study reveals that the CTO film has great promise for optoelectronic applications and deserves more attention.