Characterization of Hf/Mg co-doped ZnO thin films after thermal treatments

• Thin films are sputtered from HfxMg0.05Zn0.95 − xO targets at room temperature.
• Bandgap increases with Hf content but decreases with post-annealing temperature.
• Bandgap narrowing after annealing partly results from the relaxation of stresses.
• Bandgap narrowing partly results from quantum confinement effect by nanomaterials.
• Hf doping increases resistivity due to the lattice disorder and enlarged bandgap.

Rf-sputtered Mg0.05Zn0.95O thin films become amorphous/nanocrystalline with the addition of hafnium oxide. All films (thickness: ~ 100 nm) sputter-deposited from HfxMg0.05Zn0.95 − xO targets are highly transparent (> 80%) from 400 to 800 nm. The Tauc bandgap ΔE (eV) increases with the Hf content. However, the bandgap decreases after thermal treatment. The reduction in the bandgap is positively correlated with the Hf content and annealing temperature. The residual stresses of films sputtered from Mg0.05Zn0.95O and Hf0.025Mg0.05Zn0.925O targets are determined based on X-ray diffraction (XRD) data using a bi-axial stress model. The residual stresses of as-deposited films are compressive. As the annealing temperature increases, the residual stresses are relaxed and even become tensile. The bandgap narrowing after thermal treatment is attributed to the stress relaxation that changes the repulsion between the oxygen 2p and zinc 4s bands. Slight grain growth may also result in bandgap reduction because bandgap modification caused by the quantum confinement effect becomes significant in amorphous/nanocrystalline materials. The amorphous thin films reveal good thermal stability after 600 °C annealing for up to 2 h, as evidenced by the XRD and transmission spectra.