Growth, Structure And Properties Of ZnO:Al And ZnO:Fe Thin Films

ZnO is a wide band-gap semiconductor with band gap of 3.37eV and exciton binding energy of 60meV at room temperature. ZnO transparent conducting films have a high transmittance in the visible and a high reflectance in the infrared region. Its resistivity can be reduced by proper doping of donors. These properties make ZnO thin films important applications in photo-electronic, piezoelectric and solar cells. In recent years, ZnO transparent conducting films have attracted much attention. Different deposition techniques such as evaporation, sputtering, CVD and spray pyrolysis and so on have been established.In the work, radio frequency magnetron sputtering was used to prepare pure and doped ZnO thin films. High quality ZnO thin films were obtained by optimizing growth parameters such as sputtering power, the base vacuum, working pressure, substrate temperature and so on. The optimized parameter collocation for ZnO thin film growth using JPGF-400B-G r.f. magnetron sputtering system are: Zn target power 150W, dopant target power 80W, substrate temperature 150℃, base pressure < 1.2×10^-3Pa, the working gas pressure 0.8Pa, the ratio of O₂ and Ar partial pressure 1:3.XRD,AFM,EDS,UV-Vis absorption spectroscopy and four point probe were used to the structure and property study of ZnO:Al thin films. It is shown that the deposited films are polycrystallines with hexagonal (wurtzite) structure and c-axis orientation (perpendicular to the substrate surface). The average grain size of ZnO:Al thin films is about 20-40nm. The crystalline size increases with the substrate temperature and the film thickness. Only (002) peak was found in the XRD diffraction patterns of ZnO: Al thin films. The grain size of ZnO:Al thin films is larger with respect to that of pure ZnO thin films under the same deposition conditions.The resistivity changes of the ZnO:Al films under different growth conditions were analyzed. The resistivity of ZnO:Al films changes sensitively with the substrate temperature and sputtering time. It is lower than un-doped ZnO films under the same conditions. The resistivity of ZnO: Al films is high when the sputtering time is less than 15 minutes because of the discontinuity of the films. The crystallite sizes become larger and the crystallinity of the resulting films improves with the increasing of the sputtering time and the film thickness. These decrease the scattering by the grain boundaries and enhance the carrier lifetime and hall mobility, leading to the decrease of the resistivity. The lowest resistivity of the films is about 2.4×10^-4Ω.cm. Heat treatment in hydrogen reduces the resistivity of the pure ZnO films by more than 5-6 orders of magnitude while heat treatment in vacuum reduces the resistivity of the ZnO and ZnO: Al thin films by more than 4 and 0.5 orders of magnitude.The band gap of ZnO thin films is 3.37eV, which is bigger than the maximum energy of the visible light (3.1eV). Irradiation of visible light can not arouse inbeing excitation, so this material is transparent for visible light. The highest transmission over the visible range exceeds 90%. There is no obvious changes in optical transmission for annealed ZnO:Al thin films with respect to pure ZnO thin films. The aluminium content in the ZnO:Al thin films influence the transmittance significantly. The transmittance over the visible range decreases with the increasing of aluminium percentage.Undoped ZnO films show two emission peaks in the UV and green regions, respectively. The UV peak corresponds to the emission due to free excitons, while the green emission is mainly attributed to different intrinsic defects such as the zinc vacancies and oxide antisite defects O_Zn forming local deep levels in the band gap. ZnO:Al films do not show any emission similar to undoped samples. However, all Al-doped samples show a broad red emission band centred around 700nm (1.77e) which does not present in undoped ZnO films. With the increase of the Al concentration, there is a blue shift of the red band and an increase of its intensity. Despite the intensity and width of the red band vary proportionally to the Al concentration in the samples, we think that this red emission band is probably related to the defects caused by the incorporation of Al into the ZnO lattice.Fe doped ZnO thin films were also prepared and the properties were studied. The XRD peaks of ZnO:Fe films appear at about 34.42°, 36.26°and 72.56°, corresponding to the ZnO (002), (101) and (004). The intensity of the (002) peak is much stronger than that of the others, showing that the ZnO:Fe thin fims is also c-axis oriented. The resistivity of the ZnO thin films doped with Fe decreases by more than 6 orders of magnitude. The changes of the resistivity in ZnO:Fe thin films were analyzed. A blue shift of the transmittance with the increasing of the Fe target power was observed. The average transmittance is over 80% in the visible.