Structural, optical and light scattering properties of post etched RF sputtered ZnO:Al thin films deposited at various substrate temperature

ZnO thin films have become prominent material because of its application in solar cells as front contact. The light scattering capacity of the front contact is important in achieving high efficiency of the solar cells. Here ZnO:Al films deposited by reactive RF magnetron sputtering of the ZnO target doped with 2% Al2O3 by varying the substrate temperature is presented. Decrease in dislocation density with increasing substrate temperature indicates the improvement in crystalline nature of films. Optical band gap, Urbach energy and refractive index of films are correlated with structural changes (grain size and strain) in films with increasing substrate temperature. The as deposited films (at different substrate temperatures) are smooth giving rise to no scattering of light. After the films were etched in 0.5% diluted HCl for 15 s the light scattering capacity improved. The morphological studies of as-deposited and etched films show that substrate temperature has a strong effect on the developed surface morphology, and hence on the light scattering properties. The film haze, which is a measure of light scattering capability, increased from 0.1 to 0.66 with increasing the substrate temperature from room temperature to 300 °C. The etched film deposited at 5 × 10−4 mbar deposition pressure and 200 °C temperature shows a maximum haze value of 0.8 at 400 nm. The increased light scattering ability of films explained in terms of Urbach energy which is a measure of structural disorder in film surface.

► ZnO:Al films deposited outside the erosion area have better crystallinity and lower resistivity.
► Decrease in dislocation density of ZnO:Al Films with increasing substrate temperature.
► Band gap and refractive index are decreased for etched films with increasing substrate temperature.
► Urbach energy is increased for etched films deposited with increasing substrate temperature.
► The film haze increased from 0.1 to 0.66 with increasing the substrate temperature.