Study On Hetero-interface Modification And UV Photodetector Fabrication Of GaN Nanorods

GaN-based ultraviolet（UV）photodetectors have many advantages,such as high quantum efficiency,strong radiation hardness,high physical and chemical stability,low power consumption and visible light noninterference,which make them a wide range of application prospects in civil and military fields,such as environmental monitoring,biomedicine,astronomical research,missile warning/tracking,highly confidentiality UV communication,etc.UV photodetectors based on GaN one-dimensional（1D）nanorods arrays（NRAs）could take advantage of the unique quantum size effect of 1D nanostructures.The large specific surface area of 1D nanoarrays could increase the light absorption,and the increased number of surface states can effectively prolong the lifetime of photogenerated carriers.The reduced nano size can increase the effective active area of limited carriers and reduce the carrier transit time.Therefore,the studies of GaN NRAs-based UV photodetector are expected to further promote the development of GaN UV photodetector towards“low noise,fast speed and high sensitivity”.However,the following key technologies are still difficult to break through.First,the effective integration of each nanorod in GaN NRAs to provide independent parallel photogenerated carrier transmission channels is challenging.Second,how to suppress the interface reaction between GaN NRAs and growth substrate,so as to avoid the problem that large interface resistance hinders the efficient transport of carriers.Third,how to construct high-efficiency p-n heterojunction of GaN NRAs,in order to quickly separate photogenerated carriers at the interface,and to realize high-performance self-powered UV photodetection without external bias.To solve the above problems,this work focuses on the van der Waals（vd W）hetero-interface modification of GaN NRAs on two-dimensional（2D）materials and the fabrication of UV photodetector.The main research results are shown as follows:1.Based on 2D graphene,from the perspective of integration of GaN NRAs,a new structure of integrated hybrid graphene/（In）GaN NRAs/graphene UV photodetector has been demonstrated.To solve the problems of poor morphology and poor crystal quality of GaN caused by the coalescence of nanorods grown on Si substrate,graphene has been transferred onto the Si substrate to control the nucleation of（In）GaN NRAs.The self-assembled quasi van der Waals epitaxy（Qvd WE）of vertically aligned（In）GaN NRAs on graphene has been realized by plasma assisted molecular beam epitaxy（PA-MBE）.The microfeatures,crystal qualities and compositions,interfacial properties and optical properties of graphene/（In）GaN NRAs have been studied.The mechanism of the effect of graphene on the nucleation of（In）GaN NRAs is revealed by experimental research and theoretical calculation.Owing to the dangling bond-free structure of 2D graphene and the small lattice mismatch between graphene and（In）GaN,the consistent orientation and high crystal quality NRAs is obtained.To effectively integrated all nanorods,freestanding monolayer graphene as top contact electrode by transferring onto the top of nanostructured NRAs.The（In）GaN NRAs is sandwiched between the transparent top and bottom graphene contact electrodes to form a“sandwich structure”double Schottky junction,providing countless parallel photogenerated carrier channels.The integrated UV photodetector exhibits ultrafast response speed and superhigh photosensitivity.The response time is～50μs and the maximum responsivity（R_λ）can reach～10⁵ A W^-1.The device also shows superior photoresponse repeatability and stability.2.Based on 2D transition metal dichalcogenides（TMDs）,from the perspective of hetero-interface modification between GaN NRAs and growth substrates,the controllable preparation of 1D/2D vd W hetero-interface of GaN/TMDs has been demonstrated.To solve the problems of a great deal of dislocations and large interface resistance induced by serious interface reaction between GaN and Si substrate,2D Mo S₂,WS₂ and Mo Se₂ flakes have been inserted into the GaN/Si hetero-interface,and the Qvd WE growth of GaN NRAs has been realized.The microfeatures,distribution and crystal quality of GaN NRAs grown on TMDs and on Si have been investigated.The interface characteristics and formation mechanism of GaN/TMDs are researched by experiments and simulations.The introduction of TMDs not only suppresses the interface reaction,but also greatly reduces the subsequent dislocations caused by lattice mismatch of GaN/Si,improving the crystal quality of GaN.The PL emission peak intensity of GaN enhances by 40%.The self-powered UV photodetector has been fabricated based on the vertical 1D/2D vd W GaN/TMDs/Si heterostructure.These three TMD heterostructures all show excellent self-powered photodetection abilities,which are far greater than the performance of GaN/Si heterostructure,demonstrating that the R_λis improved by40～55 times.The self-powered UV photodetector based on GaN/Mo S₂/Si heterostructure has a R_λof 10.1 A W^-1,a detectivity（D^*）of 2.3×10¹³ Jones and a rise/fall time of 0.5/4.2 ms without any external bias.Moreover,the proposed type-II band alignment structure and carrier transport mechanism of GaN/TMDs hetero-interface have been researched experimentally and theoretically.3.Based on 2Dα-Ga₂S₃,from the perspective of band modification of p-n junction of GaN NRAs,constructing GaN NRAs on Ga₂S₃ nanosheets to form a high-performance p-n heterojunction-based self-powered UV photodetector have been demonstrated.To solve the problems that it is difficult to obtain p-type materials in GaN NRAs heterojunction-based self-powered photodetectors,a new method for synthesizing 2D p-typeα-Ga₂S₃ nanosheets has been proposed in this work.The structural properties,chemical compositions and optical properties of as-synthesized large-sizeα-Ga₂S₃ nanosheets are systematically investigated.The phototransistors based onα-Ga₂S₃ nanosheets show excellent photoresponse performance and field effect properties.The electron mobility of the p-channel conductivity transistor has been calculated and analyzed.The mechanical flexibility and stability of Ga₂S₃ nanosheets on flexible substrate are further studied.The self-powered UV photodetector is prepared based on the GaN/Ga₂S₃ p-n heterojunction with type-II band alignment.The R_λand D^*can reach13.9 A W^-1 and 1.17×10¹⁴ Jones,respectively,under zero-bias.The device also exhibits a high spectral selectivity for UVA region,and the UV-to-visible rejection ratio R_{365 nm}/R_{400 nm} is of 2.6×10³.Moreover,the device shows an ultrafast response speed,and its rise/fall time is of 81/84μs.The self-powered UV photodetector in this work exhibits great application potential in environmental monitoring,UV space communication and other fields.In conclusion,this work adopts 2D materials:graphene,TMDs andα-Ga₂S₃ to realizes the vd W hetero-interface modification of 1D GaN NRAs,and finally obtains the self-powered UV photodetector with low noise,high photosensitivity and fast response speed.The research results of this work provide an important guidance for realizing the high-performance nanoscale self-powered optoelectronic devices.