Study on thermal solid-phase crystallization of amorphous ZnO thin films stacked on vanadium-doped ZnO films

Thermal solid-phase crystallization (SPC) of an amorphous ZnO film stacked on a vanadium-doped ZnO (VZO) film was investigated. ZnO films were deposited on 30-nm-thick amorphous VZO films on c-face sapphire substrates at room temperature by RF magnetron sputtering. Stacked film was subsequently calcined at 800 °C in a nitrogen atmosphere. ZnO film grew in an amorphous state up to about 150-nm thick on the amorphous VZO film, but self-orientation occurred in a thicker layer. Any secondary phase such as Zn2VO4 was not formed in the case of total film thickness (ttotal) ≥100 nm. V concentration decreased by thermal diffusion of V through the ZnO layer from the VZO film, and thereby the formation of secondary phase was effectively avoided. The amorphous ZnO layer was crystallized from highly-aligned initial thin layer of VZO film when ttotal ≤200 nm and crystal orientation of the stacked film was superior to single VZO film. However, the c-axis orientation was deteriorated drastically at ttotal ≈400 nm due to SPC affected by the tilted regions existed in the as-deposited ZnO film. Therefore, it is suggested that careful selection of ZnO film thickness is necessary to obtain the high-quality ZnO films in this method.