Research On Luminescent And Electrical Properties Of Zinc Oxide Thin Films And Related Devices

Zinc Oxide (ZnO) is a semiconductor with a wide direct band-gap of 3.37 eV and has a large exciton binding energy of 60 meV, which makes it a potential candidate for high-efficience light-emitting devices and low-threshold laser diodes. As one of the most promising and attractive aspects in the optoelectronics, ZnO has witnessed a number of milestone progresses in recent decade. However, stable, repeatable and high quality p-type ZnO films always are the imperative key scientific problems. The reasons are as follows: (â…°) the self-compensation and asymmetric doping effects; (â…±) the deep acceptor level in band gap leads to difficulty in the ionization of the acceptor, and (â…²) the high formation energy results in low solid solubility of acceptor elements in ZnO, which has encountered great difficulties, becoming the bottle neck, blocking its further application. Based on the present research difficulties on ZnO, in this thesis, we pay attention to systematically investigate structural, electrical and optical properties of non-polar and polar plane ZnO, Ag doping and ZnO/NiO heterojunction as well as related optoelectronic devices. The major work and interesting results we obtained are listed below:(1) The polar and non-polar ZnO thin films were fabricated on c-Al₂O₃, quartz cubic MgO (111) and (100) substrates by plasma-assisted molecular beam epitaxy (P-MBE). XRD measurements show the [0001] preferential orientation for the ZnO films grown on oAI₂O₃, quartz and MgO (111) substrates and [1010] orientation for the one grown on MgO (100) substarate. Comparing with the c-plane ZnO film, the nonpolar m-plane ZnO film shows smaller surface roughness and slower growth rate. The stronger band tail absorbance of the m-ZnO film was observed, indicating that high defect density in m-ZnO films. The high defect density and anisotropy of mobility are responsible for low Hall mobility in eletrical property measurement with Van der Pauw configuration.(2) Ag-doped ZnO (ZnO:Ag) thin films were sputtered on quartz substrates by radio frequency (rf) magnetic sputtering technique. The influence of oxygen partial pressure on structural, electrical and optical properties of ZnO:Ag films has been investigated. ZnO:Ag films gradually transform from n-type into p-type conductivity with increasing oxygen partial pressure. X-ray photoelectron spectroscopy measurement indicates that Ag substitutional Zn site (Agzn), as acceptors, occur in ZnO film and is responsible for the formation of p-type conductivity. The I-V curve of p-ZnO:Ag/n-ZnO homojunction shows a rectangle characteristic with a turn-on voltage of-7V.(3) n-ZnO/p-NiO heterojunction was fabricated by magnetron sputtering and P-MBE. Valence-band offset (VBO) of n-ZnO/p-NiO heterojunction was investigated by x-ray photoelectron spectroscopy (XPS). It was found that n-ZnO/p-NiO heterojunction has a type-â…¡band alignment and its velence band offset (VBO) is determined to be 2.60±0.20 eV, and conduction-band offset (CBO) is deduced to be 2.93±0.20 eV. Furthermore, n-ZnO/p-NiO heterojunctional ultraviolet detector was fabricated on sapphire substrate. Response wavelength from ZnO (381 nm) and NiO (311 nm) was altered by applying different bias voltage on p-n junction. Our results illustrate a facile route to realize UV photodiode based on ZnO thin films.