RF diode reactive sputtering of n- and p-type zinc oxide thin films

We present the relationship between parameters of reactive RF diode sputtering from a zinc oxide (ZnO) target and the crystalline, electrical and optical properties of n-/p-type ZnO thin films. The properties of the ZnO thin films depended on RF power, substrate temperature and, particularly, on working gas mixtures of Ar/O2 and of Ar/N2. Sputtering in Ar+O2 working gas (up to 75% of O2) improved the structure of an n-type ZnO thin film, from fibrous ZnO grains to columnar crystallites, both preferentially oriented along the c-axis normally to the substrate (〈0 0 2〉 direction). These films had good piezoelectric properties but also high resistivity (ρ≈103 Ω cm). ZnO:N p-type films exhibited nanograin structure with preferential 〈0 0 2〉 orientation at 25% N2 and 〈1 0 0〉 orientation for higher N2 content. The presence of nitrogen NO at O-sites forming NO–O acceptor complexes in ZnO was proven by SIMS and Raman spectroscopy. A minimum value of resistivity of 790 Ω cm, a p-type carrier concentration of 3.6×1014 cm−3 and a Hall mobility of 22 cm2 V−1 s−1 were obtained at 75% N2.