The mechanisms of negative oxygen ion formation from Al-doped ZnO target and the improvements in electrical and optical properties of thin films using off-axis dc magnetron sputtering at low temperature

Transparent conducting aluminum-doped zinc oxide (AZO) films have been prepared on glass substrates by dc magnetron sputtering using ceramic ZnO with 2 wt% Al₂O₃ target. The mechanism of negative oxygen ion generation on an AZO target surface and its influence on the conductivity of films were discussed. The negative ion generation on an AZO target was contributed by the surface ionization leading to the spot emission from Al atoms adsorbed on the AZO target surface. The contribution of negative ions' current was mainly from the erosion area of the target due to its higher temperature. To reduce the damage caused by negative ion bombardment to film growth, an off-axis sputtering system was proposed, where the substrates were placed perpendicular to the target. The effects of distance (d) on the electrical properties of films were experimentally verified in detail. A low resistivity of 3.7 × 10^-4 Ω cm, an average transmittance above 85% in the visible range (300-800 nm) and reflectance higher than 85% in the infrared range (2500-4000 nm) were obtained for the films deposited at d = 2.5 cm. The overall analysis revealed that the generation of negative ions on the AZO target has a great influence on film growth, especially in the ultra-low pressure deposition process. Our work demonstrates the feasibility of reducing the negative effects of ion bombardment on the quality of films, which would be of great merit for industrial applications.