Relations between texture and electrical parameters of thin polycrystalline zinc oxide films

Aluminium doped zinc oxide (ZnO:Al) films, deposited by sputtering methods, are widely used as transparent conducting electrodes, especially for window and contact layers in heterojunction thin film solar cells. In this paper the influence of the discharge parameters (excitation mode, target material, oxygen partial pressure) on the texture of the ZnO-films is studied, XRD-texture measurements of aluminium doped ZnO-films prepared by reactive magnetron sputtering by DC and RF excitation show a strong influence of the discharge conditions (especially the oxygen partial pressure) on the preferential orientation and the strain in the films. This structural information can be related to the electrical properties of the ZnO-layers. Films with the lowest resistivities and the highest mobilities show the largest grains (d(g) approximate to 40 nm) and narrow texture distributions around the c-axis. For RF-sputtered films from a metallic target the half width of the texture distribution is as low as chi(1/2) = 4.5 degrees DC-sputtering from a Zn:Al-target increases the half width to chi(1/2)=6.8 degrees. These findings lead to a better understanding of the resistance minima which have been observed for reactive magnetron sputtering of ZnO or ZnO:Al as a function of the oxygen partial pressure. The electrical properties of Al-doped ZnO-films are caused by bulk and not by surface properties.