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

The morphological evolution of 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 towards lower homologous temperatures (T/Tm) than in the classical models. The range of homologous temperatures for this study were in the range of 0.13 T/Tm 0.43. The promotion of formation of "high temperature" structures at relatively low temperatures is a consequence of the energetic species generated during the sputtering process which bombard the growing film. The reduction of shadowing effect, along with the substrate heating that increases the surface diffusion, led to suppression of zone T. Two new sub-zones of zone II were identified , IIa and IIb. The film in sub-zone IIa displays a pronounced faceting. The film in sub-zone IIb has a characteristic smooth surface due to enhanced surface diffusion at higher substrate temperatures during deposition, although pitted due to still incomplete surface diffusion.