Photoconductive And Semiconductor Properties Of Zno Nano Materials

ZnO is a II-VI compound semiconductor. With a direct bandgap of 3.37eV, the exciton binding energy of 60meV, ZnO is an ideal photoconductive material of short working wavelength, and shows great prospects in the field of UV detectors, which has a wide range of application for both civilian and military purposes.In this thesis, ZnO thin films were prepared by Sol-Gel methods and their UV-sensing properties were analyzed. I-V measurements on two-electrode devices at different temperatures in vacuum were applied to extract the resistivity data, from which the grain-boundary Schottky barrier height of the polycrystalline thin films was deduced. The influence of annealing temperature on the UV sensitivity and the height of the Schottky barriers was then analyzed. Further more, the Zn1-xMgxO thin films with the Mg doping concentration x=0.1 were prepared by Sol-Gel method, and using the orthogonal experiment the optimal parameters for film preparation were found. The I-V characteristics of these devices at different temperatures were also measured in vacuum to extract the resistivity data, from which the grain-boundary Schottky barrier height of the polycrystalline thin films with Mg doping could be deduced. Finally, the defects in ZnO thin films were analyzed from the perspective of electronic states, and their impacts on the energy-band structure of the Zn1-xMgxO with different Mg concentration were analyzed.Experimentally, ZnO thin films were deposited on quartz substrates by Sol-Gel synthesis using Zinc Acetate Dihydrate as raw material, and the surface morphology of the films were studied with Atomic Force Microscope. The films were processed into two-electrode devices, on which UV sensitivity was measured at 10V bias and under 1.24mW/cm2 UV illumination. The grain-boundary Schottky barrier height of the polycrystalline thin films was also obtained on these devices. The highest UV sensitivity was achieved at the thermal-treatment temperature of 650℃, with the films being uniform and dense, and the polycrystalline dimension was determined to be in the range of 20～30nm. The grain-boundary Schottky barrier height of the aforementioned films were determined to be 0.079eV when measured in dark, and decreased to 0.011eV when the UV was turned on. The UV sensitivity of the films is then related to the observed change in the grain boundary barrier height. The grain-boundary barrier height of Zn1-xMgxO thin films was similarly measured, and their UV-sensing mechanism, lattice defects and the role of trap centers were analyzed within the theoretic framework of the semiconductor physics.Using the first-principles ultrasoft pseudopotential method based on density-functional theory in the generalized gradient approximation, we calculated the band structure of wurtzite Zn1-xMgxO with different concentration of Mg. Based on the result of this numerical calculation, the change in the energy-band structures, mainly the Partial Density of States near the bandedges, were discussed. By comparing the result with the one of pure and stoichiometric ZnO, the mechanism of the resulting bandgap widening were discussed. The impacts of oxygen vacancy (VO) and interstitial zinc (Zni) on the band structure were found out to be similar to that of the impurities. The existence of MgZn affect the electronic state of both neiborghing Zn and O, causing the Zn4s and O2p to shift to higher energy. The net effect of these changes can explain the widening of the energy-band gap. All the calculation was progressed in the MS.CASETP software.