The physical properties of Al-doped zinc oxide films prepared by RF magnetron sputtering

Abstract

Al-doped zinc oxide (AZO) films are prepared by RF magnetron sputtering on glass and Si substrates with specifically designed ZnO targets containing different amounts of Al 2O 3 powder as a doping source. The physical properties of the AZO films are investigated in terms of the preparation conditions, such as Al 2O 3 content in the target, RF power ( P RF), substrate temperature ( T s) and working pressure ( P w). The crystal structure of the AZO film is hexagonal wurtzite, and all the films show the typical crystallographic orientation, with the c-axis perpendicular to the substrate. The growth rate increases with increasing P RF, but decreases with increasing T s and P w. Films 1500 Å thick with the lowest resistivity (ρ of 4.7 × 10 −4 ω cm and the transmittance over 90% at the visible region are prepared by using norminal 3 wt.% Al 2O 3 target at T s = 150°C, P w = 2 mTorr and P RF = 150 W. Optical transmittance measurements show that AZO films are degenerate semiconductors with direct bandgap. The optical energy bandgap for undoped ZnO film is ∼ 3.3 eV and those for AZO films increase as the carrier concentration ( n e) in the film increases. The blue shift in the AZO films is proportional to one third power of n e.