Tunable electrical and optical properties in composition controlled Hf:ZnO thin films grown by atomic layer deposition

Hf:ZnO thin films doped with various Hf contents were prepared at 200°C by atomic layer deposition and assessed as transparent conductive oxides. Low Hf contents (≤6.7 at%) resulted in highly conductive polycrystalline thin films; high Hf contents reduced both crystallinity and conductivity due to the limited solubility of Hf in the ZnO matrix. The lowest electrical resistivity of 6 × 10 ^-4 Ω · cm and high electron density of 3 × 10 ²⁰ cm ^-3 were shown by the sample with 3.3 at% Hf. All the thin films showed ca. 80% transmittance in the visible region. The films' optical band-gaps increased from 3.29 to 3.56 eV with increasing Hf content up to 6.7 at%; further increases resulted in deviation from the Burstein-Moss effect and excess Hf incorporation induced two band edges due to phase separation, which was correlated with X-ray photoelectron spectroscopy and photoluminescence results.