Preparation And Application Of Verticalβ-Ga₂O₃ Nanostructured Arrays

As an emerging ultra-wide bandgap semiconductor,β-Ga₂O₃ has been widely studied in optoelectronic materials and power devices,because of its wide bandgap（E_g=～4.9 e V）,high breakdown electric filed,favorable thermal and chemical stability.The nanostructuredβ-Ga₂O₃materials have both the advantages of ultra-wide bandgap semiconductor and the characteristics of nanomaterial,which can also exhibit a variety of novel optical and electrical functions not available in theβ-Ga₂O₃ film.So they have great application potential in gas sensors,UV photodetectors,miniature optoelectronic devices and flexible electronic devices.In recent years,some progress has been made on the preparation ofβ-Ga₂O₃nanostructure arrays,but the challenges in controllable dimension,good reproducibility,simple process and low cost still remain.As an important technology for semiconductor device preparation,etching process has been widely used in substrate treatment,structure making,pattern transfer and electrode making.It is very useful to broad the application ofβ-Ga₂O₃ based device.So far,only a few reports have appeared on the etching ofβ-Ga₂O₃ which mainly reported the etch parameters and etch rate ofβ-Ga₂O₃ single crystals and thin films,but little of the etching of nanostructuredβ-Ga₂O₃.In this dissertation,the etching properties ofβ-Ga₂O₃ film and the preparation ofβ-Ga₂O₃ nanowire/nanotube arrays and Ga N@β-Ga₂O₃ core-shell heterostructured nanowire arrays have been systematically investigated.On this basis,a solar-blind UV photodetector（PD）based on verticalβ-Ga₂O₃ nanowire arrays and the photocatalystic activity of Ga N@β-Ga₂O₃ core-shell heterostructured nanowire arrays have also been studied.The main research includes the following five aspects.1.Vertically aligned and uniformly distributedβ-Ga₂O₃ nanowire arrays have been successfully prepared by inductively coupled plasma（ICP）etching using self-aggregated Nickel nano-mask onβ-Ga₂O₃ epitaxial film.Theβ-Ga₂O₃ epitaxial film has been grown by a homebuilt halide vapor phase epitaxy（HVPE）system,and the self-aggregated Ni nano-mask has been formed through rapid thermal annealing.The preparedβ-Ga₂O₃ nanowires are essentially uniform in morphology and scale,and their etched sidewalls are nearly vertical,which exhibits good etching anisotropy.The effects of various parameters like initial Ni film thickness,annealing conditions and ICP etching conditions on the surface morphologies and structural characteristics ofβ-Ga₂O₃ nanowire arrays have been investigated in detail.On this basis,the ICP etching mechanism ofβ-Ga₂O₃ nanowire arrays has also been discussed.2.The metal-semiconductor-metal type solar-blind UV PD based on vertically alignedβ-Ga₂O₃ nanowire arrays has been prepared by using a physical vapor deposition（PVD）system.The photoelectric characteristics of the fabricated UV PD based on verticalβ-Ga₂O₃ nanowire arrays have been analyzed and compared with the UV PD based onβ-Ga₂O₃ film.The results show that theβ-Ga₂O₃ nanowire PD possesses a photocurrent of 2.3×10^-5 A and a photoresponsivity of 0.122 A/W at 5 V bias,which are～10 times higher than those of the correspondingβ-Ga₂O₃ film PD.Furthermore,theβ-Ga₂O₃ nanowire PD also exhibit a high photocurrent to dark current ratio of～9375 and a UV/visible rejection ratio of～3461.5 along with millisecond-level photoresponse times.3.In order to achieve optimum device performance,the influence of the device electrode structural parameters and metal nanoparticles（NPs）on the performance optimization of MSM type UV PD based onβ-Ga₂O₃ film have been investigated.The findings show that theβ-Ga₂O₃ film device with 400μm interdigital electrode width and 100μm interdigital electrode spacing have a larger photocurrent and photoresponsivity,but also have increased dark current.On the other hand,gold and silver nanoparticles have been prepared on theβ-Ga₂O₃ film by vacuum evaporation combined with rapid thermal annealing process.The effects of metal NPs onβ-Ga₂O₃film have been studied and compared with theβ-Ga₂O₃ film UV PD without metal nanoparticles.The results show that the introduction of gold and silver NPs effectively improve the photoresponsivity by around 4.5 times and 16.4 times.4.Vertically aligned Ga N@β-Ga₂O₃ core-shell nanowire arrays have been prepared by thermally oxidizing Ga N nanowire arrays fabricated via ICP etching,and their photocatalytic activity has been investigated.The effects of different oxidation temperatures and times on the surface morphologies and microstructural characteristics of core-shell nanowire arrays have been systematically investigated.The findings show that all the oxidized samples can exhibit a morphology of vertically aligned nanowires and the single nanowire possesses a uniform core-shell heterostructured morphology.The thickness ofβ-Ga₂O₃ shell increases with rising oxidation temperature or extending oxidation time.When the oxidation temperature reaches 1000℃,the Ga N nanowire arrays are completely transformed intoβ-Ga₂O₃ nanowire arrays.In the research of photocatalytic property,it is found that compared with pure Ga N orβ-Ga₂O₃ nanowire arrays,Ga N@β-Ga₂O₃ core-shell nanowire arrays show better photocatalytic efficiency for the degradation of organic dye.The enhanced photocatalytic performance is related to the enhanced light absorption and the separation of photogenerated carrier pairs caused by the unique core-shell heterostructure.5.The preparation and characterization of vertically alignedβ-Ga₂O₃ nanotube arrays have been investigated.Based on the difference in plasma etching rates between Ga N andβ-Ga₂O₃,a method to prepare verticalβ-Ga₂O₃ nanotube arrays by thermal oxidation and ICP etching has been proposed.During the ICP etching process,the Ga N core in the thermally oxidized core-shell nanowire arrays is easily removed by using appropriate etching parameters,while theβ-Ga₂O₃ shell avoids being etched away.The obtainedβ-Ga₂O₃ nanotube arrays show high density and uniform size,and the wall thickness can be controlled by adjusting the thermal oxidation conditions.This technique provides a novel,low-cost and simple method for the preparation ofβ-Ga₂O₃nanotube arrays and will be applicable for the preparation ofβ-Ga₂O₃ based nano-optoelectronic devices and flexible electronic devices.