Study On The Preparation And Performance Of Gallium Oxide-based Photodetectors

Due to the complete absorption of ultraviolet radiation from 200 nm to 280 nm in the ozone layer,this band almost does not exist in the atmosphere,so we call it"solar blind"band.Due to the advantages of low background noise and low false alarm rate,the ultraviolet detector based on this band has a wide application prospect in the military fields,such as:ultraviolet guidance,ultraviolet space early warning and missile warning,and in the fields of people's livelihood,such as:high response fire warning,corona detection,atmospheric environment monitoring and so on.Among many broad band gap semiconductors,?-Ga₂O₃ is a new type of ultrawide band gap semiconductor material.Due to its unique chemical and physical properties,wide band gap(?4.9 e V)and low cost and simple preparation process,?-Ga₂O₃is the best natural candidate for solar blind ultraviolet photodetectors.In recent years,although great progress has been made in the research of?-Ga₂O₃ based photodetectors,the poor quality of epitaxial?-Ga₂O₃ and the relatively single device structure make the detector have lower spectral responsivity and slower spectral response speed,which cannot meet the commercial needs.Therefore,it is still necessary to make continuous efforts to achieve high response and high sensitivity detectors based on high quality epitaxial?-Ga₂O₃films.Based on the above research background,the process optimization of high-quality epitaxial?-Ga₂O₃ film and the research of high-performance solar blind ultraviolet photodetectors based on?-Ga₂O₃ thin film are carried out in this paper.Starting from improving the quality of epitaxial?-Ga₂O₃ film,optimizing the device structure and modifying the device surface,it is expected that?-Ga₂O₃ thin film materials with high crystallinity and low oxygen vacancies can be realized,and?-Ga₂O₃ based ultraviolet photodetectors with high response and high sensitivity are expected.The main research content and main conclusions of this paper are as follows:1)Based on the pulsed laser deposition technology(PLD),the process optimization of the sapphire substrate epitaxial?-Ga₂O₃ film and the subsequent film quality improvement were carried out.The effects of substrate temperature,annealing temperature and annealing environment on the crystallization quality,surface morphology and optical band gap of?-Ga₂O₃ were systematically studied.The results show that the appropriate increase of substrate temperature can provide enough energy for the deposition particles to migrate on the surface of the substrate,so that the?-Ga₂O₃ thin film can be transformed from amorphous state to multi-directional nanocrystalline state;annealing at high temperature is conducive to the secondary crystallization of the film,which makes the?-Ga₂O₃ film change from nanocrystalline state to high crystalline single crystal state;however,the annealing temperature is higher than 900°C,Al atoms in the substrate diffuse into the?-Ga₂O₃ film,which results in the increase of oxygen vacancy concentration and the conversion of more Ga³⁺to Ga¹⁺,resulting in the destruction of the crystal structure of?-Ga₂O₃.Annealing in oxygen,the oxygen atom compensation reduces oxygen vacancy concentration in?-Ga₂O₃and improves surface roughness,but the introduction of interstitial oxygen atoms causes the optical band gap to become smaller.Annealing in nitrogen,the grain size and thin film crystallization quality of?-Ga₂O₃were increased by nitrogen doping,but the surface roughness was increased by larger grain size.Annealing in the air,more OH^-is produced under the influence of impurities in the air,resulting in an increase of defects such as oxygen vacancy in?-Ga₂O₃.In summary,the optimal process of epitaxial?-Ga₂O₃ thin films with high crystallinity and low oxygen vacancy concentration is as follows:substrate temperature at 600°C and annealing at oxygen atmosphere and 800°C.2)Based on the optimized?-Ga₂O₃ film,a?-Ga₂O₃ based MSM solar-blind ultraviolet detector was developed.Under a bias of 15 V,the light-dark ratio of the detector reached699.6,the spectral response speed reached 0.64 s/1.12 s,and the spectral response reached6.03A/W.It is proved that optimizing the oxygen vacancy concentration in?-Ga₂O₃ can effectively improve the performance of the detector.In order to further reduce the dark current of the detector,we proposed an MSM(a-MSM)detector with asymmetric electrode geometry.Under illumination,the existence of the asymmetric Schottky barrier makes the?-Ga₂O₃-based a-MSM detector exhibit self-powered characteristics.Under 0 V bias,the spectral responsivity of the a-MSM detector reached 0.487 A/W,and the detectivity reached6.13×10¹⁰ Jones;under-15 V bias,the dark current reached 21.9 n A,which was reduced to the original 10.4%,but its spectral responsivity is also reduced to 58.5%of the original.In order to further optimize the spectral responsivity of the a-MSM detector,metal Pt nanoparticles were introduced to modify the surface of ?-Ga₂O₃-based a-MSM detector.Under light irradiation,Metal Pt nanoparticles produce localized surface plasmon effect,which enhances light absorption and improves the photoelectric properties of the device.Under the bias voltage of 0 V,the responsivity of the detector reaches 1.532 A/W,and the detectivity reaches 2.18×10¹¹Jones,which are increased by 315%and 354%.It is proved that the synergistic effect of asymmetric electrodes and metal Pt nanoparticles is an effective method to improve the photoelectric characteristics of photodetectors.3)Aiming at the problem that the self-heating effect of?-Ga₂O₃ due to the lower thermal conductivity affects the performance of the device,this paper selects p-type 4H-Si C with excellent thermal conductivity as the substrate,and introduces(Al_xGa_1-x)₂O₃buffer layer to reduce the defect density between the substrate and epitaxial?-Ga₂O₃,the?-Ga₂O₃/4H-Si C pn heterojunction self-driven ultraviolet photodetector was successfully prepared.Under a bias voltage of 0 V,the light-dark ratio of the detector exceeds 10³,and the spectral responsivity reaches 10.35 m A/W.At the same time,the spectral response speed of the detector is increased to the millisecond level,and the performance index of the detector is better than that of other detectors with similar structure.4)Aiming at the problem of the existence of many interface states due to oxygen vacancies at the?-Ga₂O₃-based heterojunction interface,which deteriorates the performance of the device.In this paper,,P-type oxide NiO is selected to make the O in NiO compensate the oxygen vacancy at the interface and reduce the interface defects.The?-Ga₂O₃/NiO all-oxide heterojunction self-driven ultraviolet photodetector was successfully prepared.Under 0 V bias,the spectral responsivity,detectivity and spectral response speed of the detector reached0.245 m A/W,1.8×10⁸ Jones and 12 ms/8 ms,respectively.In order to further optimize the performance of the heterojunction device,metal Pt nanoparticles were introduced for the first time to modify the heterogeneous junction surface.The research results show that,under 0 V bias,the spectral responsivity,detectivity and spectral response speed of the detector reach 4.27 m A/W,4.23×10⁹ Jones and 4.6 ms/7.6 ms,respectively.The important application of Pt nanoparticles in improving the photoelectric performance of?-Ga₂O₃-based heterojunction detectors is demonstrated for the first time.