Research On Photodetectors Based On Graphene And Wide Bandgap Semiconductors

Ultraviolet detection technology is widely used in environmental monitoring,UV communications,flame detection,etc.Wide-bandgap semiconductors are traditional photodetector materials,which have many advantages in UV detection,such as intrinsic detection,radiation resistance,high conversion efficiency,small size,and low operating voltage.Graphene is an emerging two-dimensional material with favorable electrical conductivity,light transmittance and thermal conductivity.With the combination of the two,the photodetectors with graphene and wide-bandgap semiconductor composite structure can make full use of the high detection efficiency of semiconductors and the high carrier transport capacity of graphene.The introduction of nanostructures and p-n junctions in the detectors can increase the light absorption area of the detection material and enhance the electric field intensity in the depletion layer.In this dissertation,to realize high-performance UV detectors,the preparation of AlGaN andβ-Ga₂O₃materials,the optimization of nanostructures and heterojunctions,and the performances of the photodetectors based on the composite structures of these two wide-bandgap semiconductors and graphene are systematically explored.Research work accomplished and the results obtained include the following aspects.（1）High-quality GaN and AlGaN/GaN nanowire arrays were prepared based on molecular beam epitaxy（MBE）equipment.The grown nanowires were characterized,and excellent crystal quality was demonstrated.By combining the nanowire array with graphene,a photodetector based on graphene/GaN nanowire array was prepared.The device test results show that a fine Schottky junction was formed between graphene and GaN.The structure not only greatly improves the responsivity in the 360 nm of GaN UV band,but also significantly improves the responsivity of graphene in 1540nm IR band.Therefore,the structure realizes UV-IR dual-band detector.In order to realize solar-blind UV detection,AlGaN/GaN nanowire arrays with an Al content of 40%were further prepared.The combination of graphene and the AlGaN/GaN nanowire array and verification of the Schottky contact between them were fulfilled,and the detection responding in the near IR band was realized.（2）The growth conditions ofβ-Ga₂O₃nanowires were optimized based on metal organic chemical vapor deposition（MOCVD）equipment,andβ-Ga₂O₃nanowire arrays were prepared.The effect of nanowire density on the optical field localization was calculated and analyzed by finite difference time domain method.The graphical substrate technology was employed to increase the density of the nanowire arrays.Graphene was successfully transferred onto the surface of theβ-Ga₂O₃nanowire array.A solar-blind UV detector based on the nanowire array was prepared and achieved.These lay a foundation for subsequent research of high-performanceβ-Ga₂O₃solar-blind UV detectors.（3）Using MOCVD equipment,the effects of growth temperature,reaction chamber pressure and gas flow rate on the growth ofβ-Ga₂O₃films were explored,and the growth conditions were optimized.A disordered nanoporousβ-Ga₂O₃film was realized by the self-etching method of Ga droplet,and a solar-blind UV detector based on the graphene/disordered nanoporousβ-Ga₂O₃film was prepared.The influence of the parameters of ordered nanopore arrays on the performance of the device was analyzed.The structure of the nanopore array was optimized and gaseous Ga was introduced to repair plasma etching damage.A solar-blind UV detector based on graphene/ordered nanoporousβ-Ga₂O₃film was fabricated,which has better responsivity than the disordered nanoporousβ-Ga₂O₃film detector.（4）The pn junction ofβ-Ga₂O₃/p-GaN was prepared by growing a high-quality thinβ-Ga₂O₃film on a p-GaN film using a two-step growth method.The energy band difference of theβ-Ga₂O₃/p-GaN heterojunction was analyzed by XPS.The photodetector based on graphene/ordered nanoporeβ-Ga₂O₃film/p-GaN was prepared by combining the p-n heterojunction structure with nanopore technology.Compared with the graphene/ordered nanoporeβ-Ga₂O₃film detector,responsivity,detection efficiency,quantum efficiency and response time have all been improved.Dual-color UV detection of 254 nm and 360 nm was realized.In this dissertation,in-depth and systematic study has been carried out on theory and process practice of wide-bandgap semiconductor UV photodetectors.Beneficial exploration has been fulfilled for the performance improvement of the detectors of nanostructured wide-bandgap semiconductors combined with graphene.A series of meaningful research results have been obtained,which may be valuable references for the development of such detectors.