Study On The Property Of The ZnO Films Prepared By Electrostatic-enhanced Ultrasonic Spray Pyrolysis

Zinc oxide, a representative II–VI group compound semiconductor with a direct wide bandgap of 3.37 eV at room temperature and a large exciton binding energy of 60 meV, is an important photoelectric material and draws much attention. ZnO is of interest for low-voltage and short wavelength light emitting devices such as light-emitting diodes, diode lasers, and ultraviolet photo-detectors. In this thesis, the ZnO thin films were fabricated by Electrostatic-enhanced Ultrasonic Spray Pyrolysis (EUSP); and the films morphology, composition, structure, and defects were characterized by scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence spectra, and Raman spectra; we also studied the influence of doping on the optical, electrical, and gas-sensing properties of ZnO films; moreover, we assembled the ZnO nanostructures on the Si substrate, on where, ZnO buffer layer was grown by EUSP; and then, the as-prepared ZnO nanorods were used to fabricate the gas ionization sensor, at last, we explored the function and principle of the gas ionization sensor.ZnO films have been fabricated using EUSP. The ZnO films had a polycrystalline hexagonal wurtzite type structure. The influence of preparing conditions on ZnO films morphology were explored by SEM, the dense and smooth films were prepared using the precursor within the concentration of 0.01⁰.005M, at the substrate temperatures between 400⁵00oC, with carrier gas at the flow rate between 200⁵00sccm. The influence of preparation conditions on photoluminescence was discussed, photoluminescence and Raman spectra of ZnO show that blue and green luminescence observed in ZnO films can be attributed to Zni and VZn. After annealing at 600 oC, only the UV emission can be seen from the ZnO sample.We explore the influence of substrate temperatures and carrier gases on the ZnO electrical properties by EUSP. The ZnO films structure and composition were characterized by XRD and XPS. The substrate temperatures of 360-480oC produced a p-type ZnO structure, whereas a temperature of 520oC produced an n-type ZnO structure. Hall-effect measurements indicated that prepared at 440oC exhibited the lowest resistivity of 9.99?cm-1 associated with a high carrier mobility of 141 cm2/Vs. Low-temperature photoluminescence PL spectra illustrate acceptor states in the ZnO films attributed to zinc vacancies and nitrogen substituting for oxygen defects. The scanning capacitance microscopy images and annealing the p-type ZnO indicate that the absorbed oxygen in the grain boundary (GB) aroused the p-type conductivity, and the high Hall mobility of the p-type ZnO film own to the holes accumulation layer, which was induced by the negatively charged interface states in the GBs.We have prepared the Ag-doped and N-doped p-type ZnO thin films trying to find out the best amount of doping, and we also studied double-acceptor theory, revealed that it was feasible. The p-type ZnO was realized by dual-doping with nitrogen and silver via EUSP. The structural, electrical, and optical properties were explored by XRD, Hall-effect and optical transmission spectra. The resistivity of ZnO:(N,Ag) film was found to be 56 ?cm-1 with the high mobility of 76.1cm2/Vs. Compared with the ZnO:N film and the ZnO:Ag film, the ZnO:(N,Ag) film exhibited the higher and the more stable optical transmittance.ZnO nanowires were grown on the Au-deposited Si substrate. The SEM images at different stages indicate that the growth of nanowires follow VLS mechanism. ZnO nanowires were also assembled on the Si substrate with the ZnO buffer layer, which was deposited by EUSP. We compare the effect of buffer layers deposited by EUSP and magnetron sputter, the ZnO film with EUSP shows the better buffering effect. The ZnO nanostructures with different morphology were achieved on Si by choosing Zn or Cu as the catalyzer, the wedge-liked ZnO nanostructure was gained with Cu-catalyzed. The HRTEM lattice fringes image and fast Fourier transform crystal lattice from the sample illustrate that the wedge-liked ZnO nanostructure grew along [011₀] orientation with±(0001) as the upper and lower surface, and owned the lower dislocations and stacking faults. The ZnO with Zn-catalyzed are of wire-shapes with of about 50³00nm in diameter and hundreds of micron in length, the nanowires are along the[011₀] orientation. The PL spectra of ZnO nanostructures indicate that the Au-catalyzed ZnO has the highest luminous intensity at UV wavelength, while Zn-catalyzed ZnO own the weakest UV emission, and the Cu-catalyzed ZnO is moderate-intensity; this is due to the use of different catalyst caused the unequal defects number and defect type.The gas ionization sensors were fabricated with as-prepared ZnO nanostructure. By measuring their field ionization current, the sensors can fingerprint and distinguish different gas species. We employed finite element simulations to demonstrate the field enhancement effect near the tip top of the ZnO nanorod, the result show a maximum field enhancement factor ofβ= 187 was obtained for our model. Responses of the gas sensor to N2 mixed with toluene and acetone were investigated; the precise discharge current provides the'fingerprint'for the gas to be identified. In addition, the influence of the morphologies of ZnO nanorods on the response of the gas sensor to N2-organic gas mixture was also discussed. From the gases discharge current and breakdown voltages, the device can be a clear distinction between toluene and acetone. We measured the response of devices to the NOx compounds, the experiments show that at the lower concentrations, the concentration of NOx response to current with a similar linear relationship, and the device shows the response speed of 17 40s, with the sensitivity of 0.045±0.007 ppm/pA. We also measured the response to benzene, isopropyl alcohol, ethanol, and methanol, the device shows the high selectivity on the gases with the larger static polarizability and the lower ionization energy.The NO2-sensing properties of the ZnO films, which were prepared under the different substrate temperature, were investigated. The experiments show that the powder-like ZnO film deposited at 550oC is more sensitive to NO2, and the film illustrate the good response- restoration properties. The result shows that the sensitivity of the film deposited at 400oC increase with thickness reduction; while as to the film deposited at 550oC, the sensitivity increases with the thickness of the film, until the thickness reach to a certain degree, and then, the sensitivity decreased with the thickness raised. We explore the influence of doping on the NO2-sensing properties of the ZnO films at the work temperature of 260oC, the ZnO:Al film with the Al content of 0.4mol% show the best sensitivity of 74.8 to 40ppm NO2, while the ZnO:Ag film, which contents 3mol % Ag, exhibits the higher NO2 sensitivity; and we find the optical transmittance of the ZnO:Ag film increase with the NO2 concentration. The dynamic response to NO2 were tested using the as-prepared ZnO:Al film, ZnO:Ag film, and undoped ZnO. The experiments show that the ZnO: Al film is of the highest sensitivity to NO2, but required a longer time to achieve steady-state; the sensitivity of ZnO: Ag film is low, and the film is not easy to return to the initial state after the NO2 exhausted; the undoped ZnO is the middle sensitivity, but the fast response.