Optical And Electrical Properties Of Nitrogen Doped ZnO And Application Of It To Thin-Film Transistors

As-grown ZnO thin film typically shows n-type conductivity, with relatively high electron concentration. For the realization of ZnO based transparent thin-film transistor, the carrier concentration of ZnO must be decreased to an appropriate level (lower than 1017 cm-3). In this work, N doping process was employed to reach the goal. N-doped ZnO thin films were deposited on amorphous glass substrates by a radio-frequency magnetron sputter system, with NH3 as an N dopant source. The X-ray diffraction results showed that the N-doped ZnO thin films grown at room temperature still maintained the (001) orientation preffered wurtzite structure.The process of carrier recombination in N-doped ZnO thin films was studied by room temperature photoluminescence spectroscopy. It was found that the photoluminescence intensity of ZnO exhibited a sharp decrease upon N doping, which was interpreted as due to a transformation of radiative recombination mechanism from free-exciton to donor-acceptor-pair transition.Multiphonon resonant Raman scattering process in N-doped ZnO thin films has been studied, and enhancement of resonant Raman scattering and longitudinal optical (LO) phonon overtones up to the sixth order were observed at room temperature. The resonant Raman scattering intensity of the 1LO phonon in N-doped ZnO increased to 3 times as strong as that of undoped ZnO, which mainly arose from the defect-induced Raman scattering caused by N doping. The nature of the 1LO phonon at 578 cm-1 was interpreted as a quasi-mode with mixed A1 and E1 symmetry because of the defects formed in the ZnO lattice. In addition, the previously neglected impurity-induced two-LO-phonon scattering process was clearly observed in N-doped ZnO.For the fabrication of thin-film transistors using N-doped ZnO as an active channel layer, the influence of N doping, with NH3 as an N dopant source, on the electrical properties of ZnO thin films was studied. Interestingly, with the NH3 flow rate increasing, variations in resistivity values of the N-doped ZnO showed a trend like the italic character ' N':the resistivity value increased by 2 orders of magnitude with a low NH3 flow rate (≤1.0 SCCM); with the NH3 flow rate ranging from 1.0 to 3.0 SCCM, it began to decrease and reached the minimum; then it grew again as the NH3 flow rate increased further. The phenomena can be attributed to the incorporation of both N and H ions into the ZnO lattice, where shallow acceptors and donors were formed and compensated for each other. Moreover, high density impurities might be formed in the ZnO lattice as NH3 flow rate increased further, which could induce degradation of crystal quality causing the increase in the resistivity. So high quality N-doped ZnO thin film should be deposit with a low NH3 flow rate. Thin-film transistors were fabricated on amorphous glass employing N-doped ZnO as an active channel layer, which demonstrated typical characteristics of n-type depletion mode field effect transistors.