Textured zinc oxide by expanding thermal plasma CVD: the effect of aluminum doping

Aluminum doped ZnO films are deposited on glass substrates at a temperature of 200°C by expanding thermal plasma CVD. Surface texture, morphology and crystal structure have been studied by AFM, SEM and XRD. A rough surface texture, which is essential for application as front electrode in thin film solar cells, is obtained during deposition. The addition of aluminum as a dopant results in distinct differences in film morphology, a transition from large, rounded crystallites to a more pyramid-like structure is observed. The structure of films is hexagonal with a preferred crystal orientation in the faces (002) and (004), indicating that films are oriented with their c-axes perpendicular to the substrate plane. In addition, spectroscopic ellipsometry is used to evaluate optical and electronic film properties. The presence of aluminum donors in doped films is confirmed by a shift in the ZnO band gap energy from 3.32 to 3.65 eV. In combination with reflection and transmission measurements in the visible and NIR ranges, film resistivities have been obtained from the free-carrier absorption. These results are consistent with direct measurements. Resistivities as low as 6.0 10-4 Ωcm have been obtained.