The properties of heavily Al-doped ZnO films before and after annealing in the different atmosphere

A radio frequency power (r.f.) of 200 W was supplied to ZnO target, and a direct current (d.c.) power of 40 W was supplied to Al target for the preparation of heavily Al-doped ZnO (ZnO:Al) films. The advantage of this kind of deposited method is that the Al content could be changed in a wide range. The ZnO:Al films before and after annealing in N2 or O2 all developed a columnar structure. The crystal sizes were nearly the same (approx. 20 nm) for ZnO:Al films before and after annealing in N2 or O2. The as-deposited ZnO:Al films had polycrystalline structure and low resistivity (8.52×10−3 Ω cm). After annealing in N2 or O2, ZnO:Al films exhibited poor crystallinity, and the resistivity increased substantially. The lower optical energy gap (3.29 eV) could be obtained as ZnO:Al film annealing in O2. However, the weak absorption in the visible region of the spectrum terminated at shorter wavelengths with the onset of the ultraviolet absorption edge for all samples. Annealing in N2 or O2 did not improve the properties of ZnO:Al films.