Heavily-doped ZnO:Al thin films prepared by using magnetron Co-sputtering: Optical and electrical properties

Abstract

Photovoltaic applications require transparent conducting-oxide (TCO) thin films with high optical transmittance in the visible spectral region (380 − 780 nm), low resistivity, and high thermal/chemical stability. The ZnO thin film is one of the most common alternatives to the conventional indium-tin-oxide (ITO) thin film TCO. Highly transparent and conductive ZnO thin films can be prepared by doping with group III elements. Heavily-doped ZnO:Al (AZO) thin films were prepared by using the RF magnetron co-sputtering method with ZnO and Al targets to obtain better characteristics at a low cost. The RF sputtering power to each target was varied to control the doping concentration in fixed-thickness AZO thin films. The crystal structures of the AZO thin films were analyzed by using X-ray diffraction. The morphological microstructure was observed by using scanning electron microscopy. The optical transmittance and the band gap energy of the AZO thin films were examined with an UV-visible spectrophotometer in the range of 300 − 1800 nm. The resistivity and the carrier concentration were examined by using a Hall-effect measurement system. An excellent optical transmittance > 80% with an appropriate band gap energy (3.26 − 3.27 eV) and an improved resistivity (~10 −1 Ω·cm) with high carrier concentration (1017 − 1019 cm −3) were demonstrated in 350-nm-thick AZO thin films for thin-film photovoltaic applications.