Structure, microstructure and optical properties of Sn-doped ZnO thin films

Transparent and conductive Sn-ZnO films were successfully doped up to higher concentration of 10 wt% by spray pyrolysis on glass substrates. Structural, microstructural, and optical characterizations were carried out. All the films crystallise within the würtzite hexagonal structure, confirming successful Sn doping, but with different preferred orientation depending on Sn concentration. Despite these differences, the thin films show enhanced crystalline properties up to 8% of Sn, in agreement with SEM images that reveal grains with regular shape and sharp edges. In disagreements with reported data, the increase in Sn concentration improves the crystalline properties and increases the grain size, i.e. 20-200 nm. It is important to note that only at high doping level of 10%, the crystalline properties are slightly degraded. In line with this observation, the Urbach tail energy of 10% film reaches a value of 145 meV which is attributed to a disorder in the film structural and crystalline properties. Finally, the optical band gap was found to increase with increasing Sn up to 6% followed with a slight decrease at higher Sn concentration. The shift in the bad gap energy with Sn doping is discussed in terms of widening due to Burstein-Moss effect and narrowing due to many-body effect. It is found that 4% seems to be the optimal doping concentration for Sn-doped ZnO films, with enhanced crystalline and structural properties, and with a transmittance similar or even higher than un-doped ZnO film.