High mobility ZnO:Al thin films grown by reactive DC magnetron sputtering

ZnO:Al thin films with high mobilities were grown by reactive DC magnetron sputtering from a Zn:Al target in Ar:O₂ at a substrate temperature of 450°C under optimized sputtering conditions. Doping of the films has been modified by varying the oxygen partial pressure. Films with a thickness ranging from 150 nm to 2000 nm show an increasing optical scattering effect. Hall measurements show that the mobility improves with thickness (at constant carrier concentration), 16 cm²/Vs for a 150 nm film and 53 cm² /Vs for a 1200 nm film. The latter is one of the highest values of mobility for doped ZnO:Al thin films reported in literature. A maximum conductivity of σ=3.7*10³ 1/Ω cm is obtained. SEM cross sections indicate that improvement of mobility is correlated to improvement of crystallinity. High optical transmission and the increase of optical path length due to optical scattering together with the good electrical properties therefore makes the films most suitable for solar cell applications