Structural, electrical and optical properties of aluminum doped zinc oxide films prepared by radio frequency magnetron sputtering

Aluminum doped zinc oxide (AZO) films are prepared by rf magnetron sputtering on glass or Si substrates using specifically designed ZnO targets containing different amount of ${\mathrm{Al}}_2{\mathrm{O}}_3$ powder as the Al doping source. The structural, electrical, and optical properties of the AZO films are investigated in terms of the preparation conditions, such as the ${\mathrm{Al}}_2{\mathrm{O}}_3$ content in the target, rf power, substrate temperature and working pressure. The crystal structure of the AZO films is hexagonal wurtzite. The orientation, regardless of the Al content, is along the $c$ axis perpendicular to the substrate. The doping concentration in the film is 1.9 at. % for 1 wt % ${\mathrm{Al}}_2{\mathrm{O}}_3$ target, 4.0 at. % for 3 wt % ${\mathrm{Al}}_2{\mathrm{O}}_3$ target, and 6.2 at. % for 5 wt % ${\mathrm{Al}}_2{\mathrm{O}}_3$ target. The resistivity of the AZO film prepared with the 3 wt % ${\mathrm{Al}}_2{\mathrm{O}}_3$ target is ${\text{∼4.7×10}}^{\text{−4}}$ $\Omega$ cm, and depends mainly on the carrier concentration. The optical transmittance of a 1500-Å-thick film at 550 nm is $\sim$ 90%. The optical band gap depends on the Al doping level and on the microstructure of the films, and is in the range of 3.46–3.54 eV. The optical band gap widening is proportional to the one-third power of the carrier concentration.