Influence of the reactive N2 gas flow on the properties of rf-sputtered ZnO thin films

Nitrogen (N)-doped ZnO thin films were RF sputtered with different N2 volume (ranging from 10% to 100%) on sapphire (001) substrates. The influence of N2 vol.% on the properties of ZnO films was analyzed by various characterization techniques. The X-ray diffraction studies showed that the films grow along the preferential (002) crystallographic plane and the crystallinity varied with varying N2 vol.%. The films sputtered with 25 vol.% N2 showed better crystallinity. The transmittance was decreased with increasing N2 volume until 25% and was almost constant above 25%. A maximum optical band gap (2.08 eV) obtained for 10 vol.% N2 decreased with increasing N2 volume to reach a minimum of 1.53 eV at 100%. The compositional analysis confirmed the incorporation of N into ZnO films, and its concentration increased with increasing N2 volume to reach a maximum of ∼ 3.7 × 1021 atom/cm3 at 75% but then decreased slightly to 3.42 × 1021 atoms/cm3. The sign of Hall coefficient confirmed that the films sputtered with ≤ 25 vol.% N2 possess p-type conductivity which changes to n-type for > 25 vol.% N2.