Investigation Of ZnO: Al Thin Films Prepared By RF Magnetron Sputtering

Aluminum-doped zinc oxide (AZO) thin films with low resistivity and high transparency in visible range are promising as alternatives to ITO for transparent conducting oxide (TCO), sicne indium is a very expensive and scarce material, while AZO, the source materials of which are inexpensive and non-toxic, is the best candidate. Nowadays, various technologies have been reported to produce AZO thin films, and magnetron sputtering with high deposition rate and uniformity is considered to be one of the most important methods for preparing AZO thin films.In order to being compatible to the lift-off technology in semiconductor fabrication process, AZO thin films were prepared by RF magnetron sputtering using a zinc oxide target doped with Al₂O₃ (2 wt%.) on quartz glass substrate at room temperature. The effect of RF power, Ar pressure, film thickness and thermal annealing on structural, compositional, electrical and optical properties of AZO thin films were investigated in detail. The structural properties were characterized by XRD, AFM and SEM, while the compositional characteristics were investigated by AES and XPS, and the electrical and optical properties were studied by Hall system and spectrophotometer, respectively.The crystal structure of the AZO films is hexagonal wurtzite and show the typical c-axis crystallographic orientation, and due to low temperature deposition, films are loose and porous. All samples are nonstoichiometric with oxygen enrichment which is attributed to the O atoms chemisorbed at the grain boundary. The lowest resistivity 1.83×10^-3Ω·cm and the transmittance over 80% in visible range were obtained at rf power of 250W and Ar pressure of 1.2 Pa .It was found that the main impediment for improving the electrical properties of AZO thin films deposited at room temperature is the O atoms absorption in the grain boundary, which would decrease the carrier concentration and mobility. And the O atoms absorption also destroy the Al chemical activity as donor in the ZnO matrix after rapid thermal annealing in N₂ atmosphere for 1min at 500℃～800℃. The c-aixs of the AZO films changed from perpendicular to the substate to inclined to the substrate with increasing the films thickness, which is mainly due to the energetic oxygen bombardment on the tapered (002) plane.