| ZnO is a II-VI semiconductor with a wide and direct band gap (Eg～3.2-3.4 eV at 300 K), excellent chemical and thermal stability, and specific electrical and optoelectronic property of having a large exciton binding energy (~ 60 meV). It has recently gained much interest because of its potential use in many applications, ranging from transparent conductive contacts, solar cells, laser diodes, ultraviolet lasers, thin film transistors, optoelectronic and piezoelectric applications to surface acoustic wave devices. For this reason the study on PL of Al doped ZnO is a very active field of research. However, Un-doped ZnO usually contains various intrinsic defects such as Zn vacancies, interstitial Zn, O vacancies, interstitial O, and antisite O. These intrinsic defects form either acceptor level or donor level in the band gap that would greatly affect the luminescent properties of ZnO. By introducing extrinsic dopant Al, the defect environment is changed whether the Al atom substitutes the zinc atom or it occupies the interstitial site. Al doping in ZnO has been reported to be able to change the electrical and optical properties of ZnO thin films. However, the microstructure and visible light emission of Al-doped ZnO films need to be further studied.In this paper, Al doped ZnO thin films with c-axis preferred orientation were prepared on glass and Si substrates by radio frequency magnetron sputtering technique. And effect of Al-doping contents, substrate and annealing treatment on the structural and optical properties of ZnO were studied. In this way some experimental data and the theoretical basis are provided for the application of ZnO films. The results are summarized as follows:1,Influence of Al-doping on the structural and optical properties of ZnO films: The results showed that the crystallization of the films was promoted by appropriate Al concentrations; the photoluminescence spectra (PL) of the samples were measured at room temperature. Strong blue peak located at 437 nm (2.84 eV), two weak green peaks located at about 492 nm (2.53 eV) and 524 nm (2.37 eV) were observed from the PL spectra of the four samples. The origin of these emissions was discussed. In addition, absorption and transmittance properties of the samples were researched by UV spectrophotometer; the UV absorption edge shifted to a shorter wavelength first as Al is incorporated, and then to a longer wavelength with the increasing of Al concentrations. The optical band gaps calculated based on the quantum confinement model are in good agreement with the experimental values.2,Effect of substrate and annealing on the structural and optical properties of Al doped ZnO films: The results showed the full width at half maxima (FWHM) of the Si-substrate ZnO:Al (002) peaks decreased evidently and the grain size increased. However, a strong blue peak from glass-substrate ZnO:Al film whose intensity became weak when deposited on Si substrate. Finally, we discussed the influence of annealing temperature on the structural and optical properties of Si-substrate ZnO:Al films. After annealing, the crystal quality of Si-substrate ZnO:Al thin films was markedly improved and the intensity of blue peak (～445 nm) increased noticeably. This observation may indicate that the visible emission properties of the ZnO:Al films are dependent more on the film crystallinity than on the film stoichiometry.3,Structural and photoluminescence properties of Al doped ZnO films deposited on Si substrate: X-ray diffraction (XRD) measurements showed that the crystallinity of the films was promoted by appropriate Al content (0.75 wt.%). Then the ZnO:Al film with Al content of 0.75 wt.% was annealed 600℃in vacuum at different time. XRD patterns revealed that the crystal quality of ZnO:Al thin films (Al content is 0.75 wt.%) was markedly improved after annealing at 600℃for 0.5 h. Four main emission peaks located at 390, 445, 490 and 530 nm were observed from PL spectra. As Al content was increased to1.14 wt.%, the intensity of the blue peak (～445 nm) and the green peak (～490 nm) increased and FWHM decreased, and blue centre blue shifted to 429 nm. A similar result can be obtained after annealing 600℃for 0.5 h. This is attributed to Al doping and annealing treatment lead to the change of contents of antisite oxygen and interstitial Zn related defects in the films.
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