| Information and energy technology requires high performance semiconductor materials and highly efficient photoelectric conversion devices.Wide bandgap semiconductors,e.g.gallium nitride(GaN),are characterized by large bandgap,high electron saturation speed,low dielectric constant,strong field breakdown and good thermal conductivity,with extensive application in national defense,communication,energy and lighting industry,new energy vehicles,smart grid and many other areas,which have stimulated intensive international competition.Based on the above research background,we studied and explored some fundamental properties of GaN in this dissertation.We found that 1)the preparation of GaN nanostructured surface for regulation of wetting behviors of GaN surface;2)UV-induced hydrophilic behavior of n-type GaN;3)finite element simulation analysis of plasmonic nanostrucrtes on GaN,and 4)the polarization field of GaN can greatly improve the injection efficiency of hot electrons,thus effectively improving the efficiency of photoelectric detection.The dissertation contains five chapters.They are "Chapter 1,Introduction;Chapter 2,Preparation of GaN micro/nanostructures and optical properties simulation of localized surface plasmons;Chapter 3,The effects of micro/nanostructures on GaN wettability;Chapter 4,The effects of micro/nanostructures on GaN photoelectric responsivity;Chapter 5,Conclusions and prospects",respectively.Detailed research contents are as follows:Chapter 1:We have reviewed GaN structural characterastics,physical properties and the preparation methods of surface micro/nanostructures in detail.Then,the theory of surface wettability and the photoelectric detection are reviewed detailedly.Chapter 2:1.We have studied the preparation and the formation mechanisnm of etched rough structures on GaN with different polarities.Main conclusions are:(1)The gully like structures was prepared on Ga-polar GaN by HF without adding any oxidants.Then,we have studied the etching mechanism and it is the hydroxyl radicals which can form by Au catalyzing under UV illumination that oxidize the GaN.(2)The pyramid structure was prepared on N-polar GaN by KOH and we have studied the effect of K2S2O8 on the morphology.2.The Au and Ag localized surface plasmons were prepared by photochemical method as well as sputtering and post annealing process.Then,we have studied the the kinetic characteristics of the reaction.Main conclusions are:(1)During the growth process of Au nanoparticles by photochemical method,with the illumination time increases,the particle size increases and the density increases.When the irradiation time is 10 min,the average particle size is?128.7 nm and the density is?18.3/pm2.(2)In the process of localized surface plasmons preparation by sputtering and post annealing process,with the sputtering time and annealing temperature increases the size of Au nanoparticles increases and the density decreases.For Ag nanoparticles,with the sputtering time increases the size of Ag nanoparticles decreases and the density increases.However,the effect of annealing on the size of Ag nanoparticles isn't obvious.3.The near field optical properties of the metal localized surface plasmons were simulated by using Comsol Multiphysics.Main conclusions are:(1)The increase of nanoparticle size will result in the increase of light scattering and the red shift of the resonance peak.For an Au nanoparticle in air,when the radius increases from 5 nm to 50 nm,the plasmon resonance peak can be red shifted by 20 nm.Moreover,the smaller the particles,the larger intensity of the surrounding electric field.The intensity of the electric field around particles with radius of 5 nm is 103 times larger than the particles with radius of 50 nm.(2)The increase of the medium refractive index around the nanoparticles will result in the increase of light scattering.When the radius of an Au nanoparticle is 25 nm,the light scattering in GaN at plasmon resonance is larger than the absorption.Moreover,the medium with a large refractive index around the nanoparticles will result in the redshift of the resonance peak.For the Au nanoparticles with a radius of 25 nm in GaN,the resonance peak will red shift 120 nm compared with that in the air.Chapter 3:1.We have studied the wetting behavior of both Ga-polar and N-polar GaN effected by the surface micro/nanostructures.Main conclusions are:(1)The micro/nano structures on Ga-polar and N-polar surface will make GaN more hydrophilic,with the minimum contact angle 21.8° on Ga-polar face and 42.1° on N-polar face.However,after lauric acid modification,the micro/nanostructures will make GaN more hydrophobic,with the maximum contact angle 138.5° on Ga-polar face and 129.5° on N-polar face.(2)After lauric acid modification,both Ga-polar and N-polar GaN can show high contact angle hysteresis.In our experiments,the maximum difference between advancing and receding angles can reach 40° on the Ga-polar GaN,while can reach 50° on the N-polar GaN.2.The effect of UV irradiation on the wettability of GaN was studied.Main conclusions are:(1)UV irradiation can lead to the contact angle of n-type GaN reduces from the original 61.3° to 10°;and the contact angle can recover by keeping the sample in dark for a few days.However,it has no effect on p-type GaN.(2)The UV-induced hydrophilicity of n-type GaN is due to the decrease of surface chemisorbed oxygen during UV illumination,the contact angle is greatly reduced by the formation of hydroxyl groups which is from the water molecules dissociation at the oxygen vacancies.However,it has no effect on p-type GaN due to the low oxygen content.(3)When n-type GaN shows UV induced hydrophilicity,the luminescence intensity of the yellow band is greatly increased.It is because the radiation recombination probability increases which is caused by the decrease of surface band bending and depletion region width.However,it has no effect on p-type GaN.Chapter 4:1.We have studied the effect of etched rough structures on GaN photoelectric responsivity.Main conclusions are:The photocurrent of etched GaN is about 1.65 times larger than that of the blank sample.The photoelectric responsivity at 365 nm is about 1.5 times larger than that of the blank sample.Response time of etched GaN is shorter than the blank sample.2.We have used sandwich-structured Au-Cu2O to verify that the optical properties of metal localized surface plasmons can enhance the photoelectric responsivity of semiconductors.Main conclusions are:(1)The nucleation process of Cu2O is instantaneous during the electrochemical deposition.(2)The increased light absorption of Au localized surface plasmons can increase the photo-to-dark current ratio by 199.7%(about 3 times larger than the blank sample)and improve the photoelectric conversion efficiency by 54.3%.3.We have studied the effect of GaN polarity on the photoelectric responsivity enhanced by metal localized surface plasmons.Main conclusions are:(1)The metal nanoparticles can greatly improve the GaN photoelectric responsivity.Under UV and green light illumination,Au nanoparticles can enhance the photoelectric responsivity 54 times and 64 times than that of the blank sample,respectively.While the Ag nanoparticles can only enhance the photoelectric responsivity in UV region,about 18 times larger than that of the blank sample.(2)The simulation results show that under UV and green light illumination,the increased light absorption of Au nanoparticles can enhance the GaN photoelectric responsivity by 4.4 and 3.1 times,respectively.It is in accordance with the results on N-polar GaN but much smaller than the results on Ga-polar GaN.(3)The polarization of GaN plays a very important role in improving hot electrons injection.The enhancement of responsivity in Ga-polar GaN under UV light illumination dues to the hot electrons injection from metal localized surface plasmons to semiconductor promoted by the polarization field.While the enhancement of responsivity under green light illumination dues to the promoted ionization of defects by coupling with localized surface plasmon resonances.In summary,we first fabricate the etched rough structures and the metal localized surface plasmons on GaN.At the same time,the optical properties of the localized surface plasmons were studied by finite element method.Then,we studied the effects of the above two types of miro/nanostructures on GaN wettability and photoelectric responsivity.Finally,taken account of the simulation results,the role of polarization field on photoelectric responsivity enhanced by localized surface plasmons was investigated.The innovations of this dissertation include:(1)The characterastics of GaN photoelectric etching was studied and we have realized the etching of Ga-polar GaN without adding any oxidants.Then,we have studied the wetting behavior of GaN with different polarities and have prepared the GaN with both high contact angle and high contact angle hysteresis.(2)The UV induced hydrophilicity of n-type GaN was found and it relates to the chemisorbed oxygen on the surface.(3)The sandwich-structured Au-Cu2O was used to verify that the optical properties of metal localized surface plasmons can enhance the semiconductor photoelectric responsivity.(4)Under both UV and green light illumination,the GaN photoelectric responsivity was enhanced by Au localized surface plasmons.It was proved that the GaN polarization field can promote the hot electrons injection. |
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