Pretreatment of glass substrates by Ar/O2 ion beams for the as-sputtered rough Al doped zinc oxide thin films

Textured TCO surfaces are required in silicon thin film solar cells to gain efficient light trapping. Nowadays, magnetron sputtered ZnO:Al films are usually etched in HCl solution to obtain textured surface. This study introduces a method to achieve as-grown rough ZnO:Al films by ion beam pretreatment of the glass substrate. The reference ZnO:Al films deposited on untreated glass are composed of well aligned columnar grains. In contrast, in the as-grown rough films, additional large conical grains are observed. The large grains exhibit faster growth rate than the surrounding columnar grains, and therefore overgrow the columnar grains gradually and finally cover the whole surface. In order to investigate the ZnO:Al film structural properties, transmission electron microscopy and X-ray diffraction are employed. The crystal orientations of these two types of grains are further analyzed by selected area diffraction patterns. The columns in the as-grown rough ZnO:Al are similarly textured as the reference ZnO:Al film on untreated glass. The c-axis is well aligned nearly perpendicular to the substrate surface, but a tilt of 10° was observed with respect to the growth direction. The large conical grains show no strong out of plane texture and random in plane orientation.