The Study of ZnO and MgZnO Thin Films and Their Applications in Transparent Thin Film Transistors

The thesis focused on the study of mechanical, optical and electrical properties of Zinc Oxide (ZnO) & Magnesium Zinc Oxide (MgZnO) thin films, and their applications in thin film transistors (TFTs). The thin films were deposited by radio frequency magnetron sputtering on various substrates, such as glasses, p-type Si wafers, polyimide (PI) and polyethylene terephthalate (PET). We then investigated film properties by tensile test, XRD, SEM, and optical transmittance measurement. The optical transmittances of >60% and >80% were obtained in the ~μm-thick ZnO films and ~ Å -thick MgZnO films in the visible light wavelength range as they are deposited on both glass and PET substrates. The ultraviolet absorption edge moved to a shorter wavelength, corresponding to a wider band gap, with the increase of magnesium contents. We investigated the electrical properties of the dielectric layers through metal-oxide-semiconductor (MIS) experiments. The dielectric constants of ~2000 Å -thick Mg0.1Zn0.9O, Mg0.2Zn0.8O and Mg0.3Zn0.7O thin films were unable to be determined because of large leakage currents. Relative dielectric constants of 7 or above were obtained for Mg0.4Zn0.6O(O2:Ar=1:6, 5mtorr), Mg0.5Zn0.5O(O2:Ar=0、1:9、1:6、1:3, 5、10 mtorr) and MgO(O2:Ar=1:4, 3mtorr). As Mg content increased, the leakage current of MgZnO thin films reduced significantly. Among all the thin films tested, Mg0.5Zn0.5O deposited at 10 mtorr from gas composition of O2:Ar=1:9 showed smallest hysteresis in the C-V measurement, which indicated the amount of positive mobile charges was smallest. In addition, we observed that the leakage current density of Mg0.5Zn0.5O thin films was not correlated to the oxygen partial pressures during deposition. We fabricated TFTs with various active layers, such as MgxZn1-XO (x=0、0.01、0.05、0.1、0.2), and various gate dielectric layers, such as film stack of silicon nitride (SiNx) and silicon dioxides (SiO2), Mg0.5Zn0.5O and MgO. We observed "overshoot" at the onset of the saturation region in the output characteristics of TFTs with film stack of SiNx and SiO2 as gate dielectric layer. Because of the interface defects between SiNx and ZnO or MgZnO, the TFTs exhibited large threshold voltages of ~ 20V, low mobilities of ~10-3 cm2V-1s-1 and on/off ratios of ~ 10-3. TFTs with Mg0.5Zn0.5O gate dielectric exhibited large gate leakage currents, which might result from the poor insulating of the Mg0.5Zn0.5O. From experimental results, we found that the off currents of TFTs with active layers deposited at small amount of oxygen flow or deposited from targets containing small amount Mg were reduced significantly. However, when excess oxygen flow or Mg content was applied, no TFT characteristics could be obtained.