Effects of ZnO addition on electrical and structural properties of amorphous SnO₂ thin films

Amorphous Zn-Sn-O (ZTO) thin films with relative Zn contents (= [at.% Zn]/([at.% Zn] + [at.% Sn])) of 0, 0.08 and 0.27 were fabricated by co-sputtering of SnO₂ and ZnO targets at room temperature. Changes in structural, electrical and optical properties together with electron transport properties were examined upon post-annealing treatment in the temperature range from 200 to 600 °C in vacuum and in air. Characterization by XRD showed that an amorphous ZTO thin film crystallized at higher temperatures with increasing Zn content. Crystallized ZTO films with a relative Zn content of 0.27 might not contain a single SnO₂ phase which is observed in the films of the other compositions. Amorphous ZTO films showed decreasing electrical resistivities with increasing annealing temperature, having a minimum value of 1 × 10^{- 3} Ω cm. Upon crystallization, the resistivities increased drastically, which was attributed to poor crystallinity of the crystallized films. All the ZTO films were found to be degenerate semiconductors with non-parabolic conduction bands having effective masses varying from 0.15 to 0.3 in the carrier concentration range of 6 × 10 ¹⁸ to 2 × 10²⁰ cm^{- 3}. As for a ZTO film with a relative Zn content of 0.27, the degree of non-parabolicity was much lower compared with films of the other compositions, leading to a relatively stable mobility over a wide range of carrier concentration.