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

Abstract

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/O 2 and of Ar/N 2. Sputtering in Ar+O 2 working gas (up to 75% of O 2) 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≈10 3 Ω cm). ZnO:N p-type films exhibited nanograin structure with preferential 〈0 0 2〉 orientation at 25% N 2 and 〈1 0 0〉 orientation for higher N 2 content. The presence of nitrogen N O at O-sites forming N O–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 3.6×10 14 cm −3 and a Hall mobility of 22 cm 2 V −1 s −1 were obtained at 75% N 2.