Effect of the RF sputtering power on microstructural, optical and electrical properties of Al doped ZnO thin films

• ZnO:Al thin films have been deposited by RF sputtering technique.
• The effect of the sputtering power on film properties has been investigated.
• Electrical, optical and structural characterization has been carried out.
• PL analysis revealed lattice defect chemistry variation at the increase of RF power.
• At high RF power VZn defects decrease and effective Al doping is promoted.

ZnO:Al (AZO) thin films have been deposited by radio frequency (RF) magnetron sputtering and RF power applied to the target has been varied in the range 600–1200 W. RF power effect on structural, electrical and optical properties was investigated and the relationship existing between these properties and the film lattice defect distribution was discussed. At the increasing of the RF power it was found that AZO films, having a preferential growth orientation along (002) direction, showed a decrease of the lattice distance indicating a less defected structure. Furthermore, at the increase of the RF power a higher optical absorption by free carriers, coupled with an increase of the band gap value, was observed. Resistivity varied from 1.1 × 10− 3 Ω cm at 600 W down to a minimum value of 5.6 × 10− 4 Ω cm at 1200 W, whereas the carrier density increased up to 1 × 1021 cm− 3.Lattice defect variation of AZO films was analyzed by photoluminescence (PL) measurements. Presence and amount of different lattice defects were evaluated for AZO films deposited at different RF powers. At 600 W the film structure was dominated by zinc vacancies (VZn), whereas for higher RF power the PL band associated with VZn decreased and interstitial oxygen (Oi) band remarkably increased. PL analysis revealed that extrinsic Al doping is the dominant effect on the conductivity enhancement. It was hypothesized that at higher RF power a more effective diffusion phenomenon can give more effective Al doping and less amount of zinc vacancies. As a consequence, Al atoms are more effectively trapped into the structure.