Hydrogen roles approaching ideal electrical and optical properties for undoped and Al doped ZnO thin films

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (H O ) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm 2 V −1 s −1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 10 20 cm −3 carrier concentrations for H Al co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering. • Experimental and computational results show that H O is the main factor controlling electron mobility & carrier concentration. • Hydrogen supports Al 3+ ions substitution for Zn 2+ ions increasing carrier concentration. • Hydrogen at zinc vacancy sites reduces ionized scattering centers, improving crystal quality and electron mobility. • Hydrogen substitution for V O sites affects the effective electron mass m∗, controlling electron mobility. • The highest mobility obtained in this study is 61 cm 2 V −1 s −1 for Al H co-doped ZnO thin film.