Investigations On The Preparation And Properties Of NiO/ZnO Based Heterojunction And MgNiO Solid Solution Thin Films

As a new type of wide direct band-gap optoelectronic semiconductor material, ZnO has a band gap of 3.37eV, and high exciton binding energy of 60meV, so, ZnO has been recognized as one of the most promising materials for blue and ultraviolet (UV) optoelectronic devices, but it is still difficult to grow highly active, stable, reliable and reproducible p-type ZnO. NiO is a typical p-type semiconductor with a wide direct band-gap of 3.7eV, and it is a suitable material for fabricating ZnO-based heterojunction optoelectronic devices. Solar-blind UV detectors have many applications in military and civil fields. AlxGa_1-xN and Mg_xZn_1-xO based materials are the two most promising candidates for the fabrication of solar-blind UV detectors, but the shortcomings of those two materials hinder the further development of the photodetectors. Due to the same crystalline structure and very similar lattice constants of NiO and MgO, Mg_xNi_1-xO solid solution thin films with high property can been prepared, and the band gap of the films can be tuned continuously.In this thesis, NiO and ZnO thin films were synthesized by radio frequency (RF) magnetron sputtering and researches have done on them. Suitable parameters were chosen to prepare NiO/ZnO based heterostructure, energy band structure of the heterostructure has been studied, and theⅠ-Ⅴcharacteristics of the heterostructure have been measured. Besides, Mg_xNi_1-xO thin films with different Mg contents were fabricated and studied. Main research works are as follows:(1) Fabrication and researches on NiO,ZnO thin films and NiO/ZnO based heterostructure. P-type NiO and n-type ZnO thin films were fabricated by RF magnetron sputtering technique, and the influence of growth parameters on the film properties has been studied. The researches indicate that 350℃is a optimal growth temperature to obtain p-type NiO thin films with good electrical properties. Under low oxygen content, the electrical conductivity of NiO films is proportional to 1/4 power of partial pressure of oxygen. ZnO thin films grown at 550℃has the best electrical properties, while, ZnO thin films grown at 450℃has best crystalline properties. X-ray photoelectron spectroscopy was used to measure the valence band offset of the NiO/ZnO heterostructure, the valence band offset (ΔEV) is determined to be 1.47eV, the conduction band offset (ΔEC) in the structure was calculated to be 1.8eV, and it has a type-Ⅱband alignment.Ⅰ-Ⅴcharacteristics measurement shows that the heterostructures have obvious rectifying behavior, indicating a p-n heterojunction has been made successfully.(2) Mg_xNi_1-xO (x=0.25-0.56) thin films were fabricated by RF magnetron sputtering on quartz substrates. The influences of sputtering power,substrate temperature and Mg contents on the properties of Mg_xNi_1-xO thin films have been studied. XRD measurements indicate that the Mg_xNi_1-xO films are of cubic NiO structure with (111)-preferred orientation. UV-visible transmission spectra show that the absorption edges of thin films shift to short wavelength with increasing Mg content. The band-gap is already in the solar-blind region when Mg content in the film is 46%. XPS results showed that typical solid solutions of Mg_xNi_1-xO have been prepared without obvious phase separation. The result implies that Mg_xNi_1-xO is a promising candidate material for solar-blind UV detection.