Study On Heterostructured Ultraviolet Photodetectors Based On Wide Band-Gap Oxide Semiconductors

High-performance ultraviolet（UV）photodetectors have a wide range of applications in many fields such as military and civilian.With the continuous development of semiconductor technology,UV photodetectors based on wide-bandgap semiconductors have gradually attracted attention from both researchers and industrialists.Due to their wide forbidden band width,they naturally do not absorb visible light,and the prepared optoelectronic devices do not require additional filtering structures,while the devices have high quantum efficiency and good stability.However,devices based on wide-band materials are affected by the low carrier mobility and many defect traps of the materials,and thus face the problems of high dark current and low photoresponsivity.In this paper,we focus on the shortcomings of wide-band oxide semiconductor UV photodetectors,improve the photoelectric performance of UV detectors by constructing heterostructures,investigate the effect of the self-polarized electric field of BaTiO₃ on the distribution of the carrier concentration in TiO₂ films,find that the composite of CuI and TiO₂ can produce self-powered performance,and explore the mechanisms by which two-dimensional and MOFs-derived materials improve the optoelectronic performance of Ga₂O₃-based UV detectors,the specific research is as follows:We first prepared an UV photodetector based on a heterostructure of TiO₂/BaTiO₃,which utilizes the self-polarization effect of BaTiO₃ to introduce a perpendicular electric field,which in turn regulates the distribution of the electron concentration inside TiO₂ in the dark state.The heterogeneous film was prepared by a one-step sol-gel method and a pair of fork-finger Au electrodes were deposited on it by magnetron sputtering.In the dark state,due to the self-polarized electric field generated by the BaTiO₃ film with a strength of 2.4 V/cm,the electrons on the TiO₂ surface move toward the interface between TiO₂ and BaTiO₃ to balance the self-polarized electric field generated by the BaTiO₃ film,which resulted in the decrease of the concentration of electrons on the surface of TiO₂,and the reduction of the electron concentration on the TiO₂/BaTiO₃ heterostructured film device at a bias voltage of 5 V.The dark current of the TiO₂/BaTiO₃ heterostructured film device decreases to 0.13 nA,while the dark current of the single TiO₂ device is 1.12 nA.In the case of light illumination,a large number of photogenerated carriers are generated within the TiO₂ film,and the self-polarized electric field generated by the BaTiO₃ film and the applied electric field will promote the separation of the photogenerated electron-hole pairs,resulting in the increase of the electrical conductivity of the TiO₂ film,and the heterostructured film device exhibits a high photoresponse,and the peak responsivity of the device at 280 nm is 13.1 A/W.In addition,simulations were carried out by Silvaco software to investigate the effect of the self-polarized electric field generated by the BaTiO₃ film on the spatial distribution of the carrier concentration in the TiO₂ film under the dark state.In the third chapter of this dissertation,p-CuI/n-TiO₂ heterostructured UV detectors were prepared.Firstly,TiO₂ film was prepared on the surface of FTO substrate by sol-gel method,and CuI film was deposited on TiO₂ film by solution method,followed by Au electrode on the heterogeneous film by vacuum evaporation.Under the light condition,with the help of the type II pn junction formed by CuI and TiO₂,the photogenerated electron-hole pairs at the interface can be separated and transported efficiently by the space charge region and photocurrent is generated,and the heterogeneous film device can be operated without applied bias voltage.The device with the best optoelectronic performance has a photogenerated bias of about 17 m V under UV illumination.In the self-powered operating mode,the heterogeneous film device has a photocurrent of 1.25μA,a light-to-dark suppression ratio of 522,a peak responsivity of 28.48 m A/W at 330 nm,response recovery time of 35 ms and 40 ms,and good repeatability.Day-blind UV detectors are widely used because of their low background noise and high detection sensitivity.The absorption range of Ga₂O₃ material basically covers the day-blind wavelength band,which is an ideal material for the preparation of day-blind UV detectors.In Chapter 4,β-Ga₂O₃ nanofilms were prepared on ITO substrate by hydrothermal method,and the forbidden band width of the prepared Ga₂O₃ film was found to be 4.9 e V by UV-Vis characterization,and thenAg electrodes were deposited on it,and it was found that the device had a dark current of 11.9 nA at 5 V reverse bias voltage,and a responsivity of 20 A/W at 250 nm by testing.In order to further improve the optoelectronic performance of the detector,a Nb₂C layer of MXene material was prepared on Ga₂O₃ film by spin-coating method to construct a Nb₂C/Ga₂O₃heterostructure,which has a higher work function compared with Ag electrode,and has strong transmittance and high carrier mobility.It is found that the dark current of the Nb₂C/Ga₂O₃ heterostructured film device is significantly reduced to 1.4 nA compared with the original device.The peak responsivity of the heterostructured thin-film device at 250 nm is 28 A/W,and the response recovery speed is improved due to the heterostructured structure formed by Nb₂C and Ga₂O₃ to accelerate the separation of photogenerated carriers.In Chapter 5 of this thesis,MOFs-derived In₂O₃ materials were synthesized using a hydrothermal method and In₂O₃ layers were spin-coated on Ga₂O₃ films prepared in the previous chapter to fabricate In₂O₃/Ga₂O₃ heterostructured UV detectors.In the dark state,a depletion layer is formed on the surface of In₂O₃ due to the presence of adsorbed oxygen,and a barrier region is formed at the interface of In₂O₃ and Ga₂O₃.The presence of the depletion layer and the barrier region increases the resistance of the device in the dark state,which reduces the dark current of the heterostructured device,which is only 0.68 nA at a reverse bias voltage of 5 V.In the light state,the introduction of the MOFs-derived materials with a large specific surface area increases the light uptake of the device,and the absorbed oxygen complexes with the photogenerated holes,leaving a large number of unpaired electrons inside the material,which improves the photoresponse of the heterostructured device.The device has a peak responsivity of29.3 A/W at 250 nm and a high light-to-dark suppression ratio of 4.2×10³.