The Preparation And Investigation Of ZnO:Al Films And ZnO Based Device

ZnO is a II-VI compound semiconductor with a wide direct bandgap of 3.37 eV at room temperature, exciton binding energy of 60 meV, and a hexagonal wurtzite structure of space group P63me. Its latice parameters are a = 0.3249 nm and c=0.5206 nm。Due to their excellent physical and chemical properties, ZnO films have many practical and potential applications such as surface acoustic wave devices , planar optical waveguides, transparent electrodes, ultraviolet photodetectors, piezoelectric devices, varistors, gas sensors, UV/violet/blue-LEDs (light emitting diodes) and -LDs (laser diodes), etc. To achieve the requirements of different applications, many techniques, such as molecular beam epitaxy, magnetron sputtering, metalorganic chemical vapor deposition,and pulsed laser deposition, spray pyrolysis, sol-gel process, reactive deposition and thermal oxidation of Zn films have been used to deposit ZnO films. Each of these techniques has its merits and demerit.The key point to the application of ZnO films is high quality for the purpose, and the cost must be considered. In this thesis, the Al-doped ZnO (AZO) films were prepared by reactive frequency magnetron sputtering at the pure Ar gas. The properties of AZO films with the substrate temperature and working pressure was studied. By adding O₂ into the working gas, we successfully made the LEDs on p-Si and n-GaAs substrate. In order to further improve the quality AZO films, H2 was added to the working gas. The influence of H2 flux on the transparent and conductive properties of the AZO films was investingated. ZnO nano particles was also made by sol-gel method. The sensors were made by ZnO powder sintered at different temperature and the sensitivity to VOC gas was tested. The details are as follows:(1)The AZO films were grown on glass substrate by magnetron sputtering with AZO (98 wt.% ZnO and 2 wt.% Al2O3) ceramic target in Ar ambient. With the substrate temperature increasing, the resistivity dercased first and then increased, the grain size increased. With the Ar pressure increasing (1 Pa-3 Pa), the crystallinity and grain size decreased and the resistivity increased. It was because that the substrate temperature and the Ar pressure could affect the ZnO particles'energy state, which definitely determined the quality of AZO film. By adding O₂ into the working gas, LED were successfully made on p-Si and n-GaAs substrate. The light was so obvious that it could be seen in the dark room by naked eyes. EL spectrum of AZO/n-GaAs was obtain at room temperature.(2)In order to improve the conductivity of the AZO film without impairing transmittance, H2 was added into the working gas. It was found that there was H-doping saturation stage in AZO film. The optical and electrical properties of AZO films could be improved at this stage. The resistivity of 4.15×10-4 ?.cm and the average transmittance of more than 90 % in the visible range were obtained with the optimal H2-flux of 1.0 sccm at a relatively lower temperature of 100℃. The method of further improving the electrical properties of AZO films prepared at low growth temperature can be especially useful for some low-melting point photoelectric devices and substrates.(3)Transparent ZnO gel was prepared by sol–gel process for getting fine powder of ZnO with the pre-firing at 300℃. In order to purify the final products and obtain different grain size, ZnO powder was further calcined at different temperature of 500℃, 700℃and 900℃. The sensor was prepared by coating the powder on alumina tubes and post-firing at 450℃, which was then tested for the sensitivity to VOC( volatile organic compounds ) gases. The average crystallite sizes of the power obtained at 500℃,700℃and 900℃were 42 nm, 46 nm and 52 nm, respectively. Since small grain size gives high surface area to bulk, high surface free energy and high surface activity, the sensor made from the ZnO powder obtained at 500℃showed the maximum sensitivity to different VOC gases at the optimal working currents. TiO₂ doped ZnO sensor was also investigated. By adding 2 % TiO₂ into ZnO powder, the optimal working current to ethanol decreased and sensitivity increased. TiO₂ reacted with ZnO along grain boundaries and form the Zn2TiO4 phase. Considering the ionic radius matching between Ti⁴⁺ ions and Zn²⁺ ion, Ti⁴⁺ will diffuse into the grain bulk, where it substitutes for Zn²⁺ ions, thereby increasing the activity of ZnO in virtue.