Deposition Of Al Doped ZnO Thin Films And Its Applicationsin Thin Film Transistors

ZnO-TFT are generally considered to be one of the promising candidates for the next-generation LCD displays which need to meet the demand of high resolution, higher refresh rates and lower power consume.So far, Several deposition techniques of aluminum doping on zinc oxide (AZO) films have been developed, including physical methods such as RF sputtering, Metal-organic Chemical Vapor Deposition (MOCVD), chemical bath deposition (CBD) and electrodeposition. Compared with chemical methods, physical methods help to achieve flow production since the electrode and gate layers are generally deposited by sputtering or evaporation, moreover the physical deposition do not discharge wastewater.In this thesis, Al doped ZnO polycrystalline thin films are prepared by electron beam evaporation (EBE). A top-gate prototype TFT based on AZO films were successfully fabricated by EBE technique and shadow masks for the further research. To improve the performance of the TFT devices, the characteristics of thin films and TFTs are closely investigated. Main contents are listed as follows:Pure ZnO films were deposited by EBE. The structural characteristics of ZnO films annealed at various temperatures and atmospheres were studied by the X-ray diffraction (XRD) pattern. The ZnO films are polycrystalline, exhibiting a preferred orientation along the (002) plane after annealed at400℃. The optical band gap values of ZnO films increase slightly with the increasing annealing temperature and the band gap of ZnO films annealed in air is larger than that annealed in N2,in the range of3.0-3.2eV. The sheet resistivity of ZnO films under different annealing conditions are measured, the films annealed300℃in N2exhibit the lowest resistivity of189ohm per square.AZO films with different Al doping content were successfully deposited on glass by EBE, the transmission spectrum were studied by spectrophotometer. The average optical transmittances of all the AZO films in visible spectrum are almost above80%. The optical band gap broadens with the increment of Al doping concentration, in the range of3.24-3.42eV. Hall Effect measurements show the carrier mobilities increase as Al content increasing. The largest carrier mobility (about18.0cm2/Vs) of20%Al doped ZnO films were observedTop gate TFT devices based on AZO films were fabricated. The typical transfer characteristics were observed for all these prototype TFT devices based on AZO films with different Al content. The carrier mobility, threshold voltage, subthreshold swing and the ratio of on-to-off current were calculated from the Ids-Vgs curve. The TFT based on ZnO with20%Al doping concentration exhibits the best performance.