Modified Structure Zone Model to Describe the Morphological Evolution of ZnO Thin Films Deposited by Reactive Sputtering

The morphological evolution of zinc oxide (ZnO) thin films deposited by magnetron sputtering is described by the use of a Structure Zone Model. A modified Structure Zone Model was revealed, in which the boundaries between zones with specific features are shifted toward lower homologous temperatures (T/T_m) than in the classical models. The range of homologous temperatures for this study were 0.13 < T/T_m < 0.43. The promotion of the formation of high temperature structures at relatively low temperatures is a consequence of the energetic species generated during the sputtering process that bombard the growing film. The reduction of the shadowing effect, along with the substrate heating that increases the surface diffusion, led to the suppression of zone T. Two new subzones of zone II were identified, IIa and IIb. The film in subzone lia displays pronounced faceting. The film in subzone lib has a characteristic smooth surface due to enhanced surface diffusion at higher substrate temperatures during deposition, although pitted due to still incomplete surface diffusion.