Preparation Of ZnO Thin Films And Study Of Their Properties

In recent years, wide band-gap semiconductor material zinc oxide (ZnO) attracted more and more attention as a kind ofⅡ-Ⅵcompound semiconductors. Similar to GaN, ZnO exhibits a hexagonal structure with a direct band gap of 3.37 eV at room temperature, while the exciton binding energy of ZnO (60 meV) is much larger than that of GaN (25 meV) and the thermal energy of 26 meV at room temperature. In addition, ZnO has high melting point (at 1975℃), high thermal and chemical stability. ZnO single crystal thin film can be obtained at a temperature under 500℃, which is much lower than those for GaN and other wide band gap semiconductors, so it can greatly reduce the defects formed in high temperature. Furthermore, ZnO is abundant, cheap, innoxious and easy to be prepared with potential commercial value. As an important candidate of short-wave optoelectronic devices and UV detector, ZnO has become a hotspot in the area of semiconductor optoelectronic devices.To realize the application of ZnO in optoelectronic devices, excellent epitaxy growth of ZnO thin films are necessary. Especially the hetero-epitaxy of ZnO film, that is a very important work. Si is a nice substrate because of its low cost. However, to grow higher crystal quality ZnO films on Si substrate is rather difficult because of the mismatch of the crystal lattices between ZnO and Si.In this thesis, ZnO thin films were deposited by the method of radio frequency (RF) magnetron sputtering. All the ZnO films have high orientation along c axis. We investigated the structure, surface morphology, optical and electrical properties of ZnO films. The results of this work are summarized as follow:1. We deposited ZnO films on different kinds of substrates by the method of RF magnetron sputtering. All the ZnO films have high orientation along c axis. The formation of the c-axis orientation ZnO films is a self-assembled process. When grown on Si, it will preferential nucleate firstly; whily in the case of glass, an amorphous ZnO layer initially forms. Then with the increase of thickness, ZnO films have orientation along c axis. 2. Using reactive RF magnetron sputtering, ZnO films with strong c-axis orientation have deposited on p-Si(111) substrates. We have studied the influence of the power of radio-frequency, the ratio of Ar/O2 and annealing temperature on the ZnO thin films'properties. The results show that the sputtering technical parameters have obvious influence on the properties of ZnO thin films. When the power is 200 W, the sputtering gas pressure is 1.0 Pa and the substrate temperature is 200℃, the c-axis orientation and the quality of ZnO thin films get better when the ratio of Ar/O2 is high. The average transmittance of visible light is up 80%. The resistance is smallest when the ratio of Ar/O2 is 1:1. The quality of the ZnO films was improved by annealing. The as-deposited ZnO film experiences the tensile stress along the c-axis orientation, while the compressive stress exits in the annealed ZnO films. When the annealing temperature rises up, the tensile stress along the c-axis orientation decreases, while the compressive stress increases. At the same time, the grain size of the films is increased. The resistance is smallest when the annealing temperature is 800℃.3. We investigated the contact properties between Al and ZnO. ZnO can solve in acid and alkali solution, so we made the Al electrode on the surface of ZnO thin film by lift-off processing. The results showed good ohmic contact between Al and ZnO.4. We investigated the properties of n-ZnO/p-Si heterojunction. n-ZnO/p-Si heterojunction was fabricated by the processing of depositeing Al-doped ZnO film on the p-Si (111) surface. The electrical junction properties were investigated by current-voltage (I-V) measurement, which reveals that the heterojunction shows typical rectifying behavior. Under reverse bias conditions photocurrent caused by the ZnO surface exposing in room light is obviously larger than the dark current. That shows that the heterojunction has response to the light, which results in the potential of ZnO as the material to fabricate the photoelectrical detector.