Enhancement of crystallinity in ZnO:Al films using a two-step process involving the control of the oxygen pressure

The ZnO:Al films were prepared using a two-step process through the control of oxygen pressure by DC-pulsed magnetron sputtering. The seed layers were prepared with various Ar to oxygen pressure, and the bulk layers were deposited under pure Ar. At the seed-layer condition of Ar/O₂ = 24/1 (Ar rich), the seed layer showed grains that led to the formation of hillock, and the grains of seed layer were seem to act as nucleation sites for columnar growth of bulk layer. While, at the seed-layer condition of Ar/O₂ = 4/1 (oxygen rich), grains show large lateral growth with a nonuniform distribution, and a number of dislocations are shown in grains. As oxygen pressure during the deposition of seed layer increased, the decrease of local strain (or the increase of crystallinity) was observed. The etched surface showed the crater-like structure and the abrupt morphology change appeared at high oxygen pressure, which was likely due to nonuniform grain structure and/or the decrease in the number of chemical attack sites by the drastic decrease of grain boundary. The haze values increased with increasing oxygen pressure, which was explained by the change of crater size, as shown in the AFM image.