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.