Fabrication And Characteristics Of Mixed-phase ZnMgO Thin Films With Single Absorption Edge And Their Ultraviolet Photodetectors

Zn Mg O-based materials possess more excellent material characteristics over narrow bandgap and other wide bandgap semiconductors, including wide range of tunable bandgap(3.37 e V to 7.8 e V), high electron saturation velocity, high radiation hardness, lattice-matched bulk cystal substrates(Zn O and Mg O), low defect density, easy fabrication, non-toxic and harmless, abundant resources and environmental friendly, which makes them become one of the candidates for ultraviolet(UV) photodetecting. Low dark current and high responsivity are always expected in Zn Mg O UV photodetectors. Wurtzite Zn Mg O(w-ZMO) UV photodetectors usually show relatively high responsivity, while the dark current is also high. Cubic Zn Mg O(c-ZMO) UV photodetectors usually have relatively low dark current, while the responsivity is also low. It is anticipated that mixed-phase Zn Mg O(m-ZMO) UV photodetectors have both advantages mentiond in w-ZMO and c-ZMO UV photodetectors. Currently, the responsivity of m-ZMO UV photodetectors is usually high, while the dark current remains high. More importantly, the devices have dual detecting bands and uncontrollable cut-off wavelength due to the films possessing two distinct absorption edges. These problems limit the deeper development of m-ZMO UV photodetectors. For the problem of dual detecting bands and high dark current in m-ZMO UV photodetectors, detailed researches have been done and some breakthrough results were achieved in this thesis. The main contents were as follows:(1) For the problem of the m-ZMO having two distinct absorption edges, m-ZMO thin films with single absorption edge were realized on a-Al2O3 by MBE, whose lattice-mismatch is very small with both Zn O and Mg O, and their absorption edges were tuned from 3.9 e V to 4.8 e V.(2) Metal-semiconductor-metal(MSM) structured m-ZMO UV photodetectors were fabricated on m-ZMO thin films with the single absorption band and tunable absorption edge by photolithography and a wet etching procedure and their cut-off wavelength were tuned from 325 nm to 275 nm. The devices have relatively low dark current(40 V, the dark current is 8 p A ~ 130 p A) and high responsivity(40 V, the peak responsivity is 9 A/W ~ 410 A/W). Moreover, the rejection ratio of UV and visual is 103 and the response time is about 37 ms ~ 1 s. It is related with inherently high-resistance c-ZMO and large amounts of heterojuntion grain boundaries between w-ZMO and c-ZMO that the devices have relatively low dark current. It is related with large amounts of heterojuntion grain boundaries between w-ZMO and c-ZMO and deep defects in thin films acting as the traps of holes and prolonging the lifetime of holes that the devices have relatively high responsivity.(3) Our m-ZMO devices have better comprehensive properties from the comparision of related performances of other single-phase and mixed-phase ZMO UV photodetectors, namely, relatively low dark current, relatively high responsivity, relatively high rejection ratio and relatively fast response time. The achievement of high-performance m-ZMO device will have huge directive significance for its development and application o in UV detecting region.