Effect of annealing ambient on the structural, optical and electrical properties of (Mg,Al)-codoped ZnO thin films

Nominal 1 at.% Al-doped Zn1-xMgxO (x=0-8 at.%) thin films are prepared by the sol-gel method. The codoping films retain the hexagonal wurtzite structure of ZnO, and show preferential c-axis orientation. The effect of annealing ambient (in vacuum and nitrogen) on the optical and electrical properties of (Mg,Al)-codoped ZnO films is investigated. The transmittance of the codoping films is obviously degraded to 50-60% by vacuum annealing, but is enhanced to 70-80% after nitrogen annealing. Both the carrier concentration and the Hall mobility decrease, and the resistivity increases with the widening of the band gap in films treated in vacuum. However, the carrier concentration of films annealed in nitrogen displays fluctuations with increasing x, and the carrier concentrations of films with x=2 and 4% are higher than those of other films. The initial band gap increase of nitrogen-annealed films might be attributed both to Mg doping and to the Burstein-Moss effect due to the NO-(N2)O complex donors. In view of the transmittance and conductivity, nitrogen annealing might be a more appropriate post-annealing method than vacuum annealing for (Mg,Al)-codoped ZnO films to fulfil the requirements of transparent conducting oxide.