Preparation And Properties Of Self-powered Solar-blind Photodetectors Based On Ga₂O₃

Gallium oxide（Ga₂O₃）,with a band gap of 4.9 e V,is a new type of ultra-wide bandgap oxide semiconductor and a natural solar-blind photodetector material.Solar-blind photodetectors based on the Ga₂O₃ have attracted great interest due to their broad potential applications in military and civilian fields,owing to their high photosensitivity and low false alarm rate.Most of the current studies focus on the photoconductive structure,while it shows some disadvantages,such as slow photoresponse,small photosensitive area,and extra bias.Self-powered photodetectors based on the photovoltaic effect can detect optical signals without external bias,eliminating the need for a power unit and meeting the carbon peaking and carbon neutrality goals.In this academic dissertation,we have constructed all-inorganic heterojunction,organic-inorganic hybrid heterojunction,phase junction,as well as Schottky junction type Ga₂O₃-based self-powered solar-blind photodetectors and explored the intrinsic physical mechanisms.The main results are as follows:（1）Growth of Ga₂O₃ epitaxial film.Ga₂O₃ epitaxial film is the critical factor in achieving high-performance Ga₂O₃-based solar-blind photodetectors.We used a commercial metal-organic chemical vapor deposition（MOCVD）system to grow Ga₂O₃epitaxial films on（0001）-sapphire（c-A1₂O₃）substrates.Theβ-Ga₂O₃ epitaxial films obtained at 790°C exhibits a selective orientation along with the（（?）01）plane.Although the as-grown Ga₂O₃ epitaxial films are suitable for solar-blind photodetectors,the high growth temperature,as well as expensive equipment and raw materials,hinder their application.Plasma-enhanced chemical vapor deposition（PECVD）equipment is relatively inexpensive and can grow epitaxial films under a lower deposition temperature with the help of plasma.We used low-cost metal gallium as a reaction source and studied the effects of oxygen rate,growth temperature,radio frequency power,as well as the distance between gallium source and substrate on the crystallization quality and growth rate of epitaxial films.A high growth rate of～0.58μm/h and a single（（?）01）plane orientation film with full width at half maximum（FWHM）value of 0.86°was obtained when grown on the c-A1₂O₃ substrate under the growth temperature of 820℃.（2）All inorganic type self-powered solar-blind heterojunction photodetectors based on Ga₂O₃.The inorganic semiconductor has high carrier concentration and stable performance.In this work,we built self-powered solar-blind photodetectors by constructing the Sr Sn O₃/β-Ga₂O₃ and Cu MO₂/β-Ga₂O₃(M=Ga³⁺、Cr³⁺)nn/pn type heterojunction,respectively.Both the photodetectors can operate without an external power supply,demonstrating self-powered characteristics.Under 0 bias,the Sr Sn O₃/β-Ga₂O₃ nn photodetector shows a high I_dark/I_photoratio of 4.3×10⁶ and a high detectivity of1.3×10¹³ Jones under the irradiation of a 254 nm light while the Cu Ga O₂/β-Ga₂O₃ and Cu Cr O₂/β-Ga₂O₃ pn photodetectors show the I_dark/I_photoratio of 2.3×10⁴/3.5×10⁴ and high detectivity of 0.9×10¹¹/4.7×10¹¹Jones.The photo-generated electron-hole pairs in the space charge region at the heterojunction interface can be effectively separated owing to theⅡtype band alignment between the Sr Sn O₃/β-Ga₂O₃ and Cu MO₂/β-Ga₂O₃heterojunctions.（3）Organic-inorganic hybrid heterojunction type self-powered solar-blind heterojunction photodetector based on Ga₂O₃.The bandgap of inorganic semiconductors with high carrier concentration is generally narrow,and there is a risk of response to non-solar blind light.Some organic small molecule materials have the characteristics of high hole mobility and natural solar-blind filter,which is suitable for constructing organic-inorganic hybrid self-powered photodetector with Ga₂O₃.We used four triphenylamine（TPA）type small molecule hole transport materials（SMHTMs）to construct a typeⅡheterojunction photodetector with Ga₂O₃ using a simple solution-processed method.Thanks to the filter effect of the SMHTMs films,all the photodetectors exhibit a self-powered character with extremely low dark current and high I_dark/I_photoratio.Among them,the TAPC/β-Ga2O3 heterojunction photodetector exhibits the best photoelectricity performance with an ultralow dark current of about 20 f A,a large I_dark/I_photoratio of5.9×10⁵ and a superhigh detectivity of 10.2×10¹² Jones at 0 V with illumination of 1000μW/cm² 254 nm light.Taking advantage of its high wavelength selectivity,we further designed a solar-blind communication system.（4）Phase junction type self-powered solar-blind heterojunction photodetector based onα/β-Ga₂O₃.Despite the excellent performance of the all-inorganic and organic-inorganic hybrid heterojunction photodetectors,it is difficult to fabricate devices or maintain the solar-blind response region due to the mismatch between the lattice and bandgap of the heterogeneous materials.In view of Ga₂O₃ having a variety of crystal and phase transformation characteristics,we creatively proposed the construction of Ga₂O₃phase junction structure.Smaller lattice mismatch and similar bandgap can solve the problems existing in the heterojunction.We constructed a high-performance solar-blind photodetector based onα/β-Ga₂O₃ junction nanorods with graphene-silver nanowires（Ag NWs）top electrode.The matched band structure ofΠtype formed at the interface of phase junction will promote the automatical separation of photogenerated carriers and transfer them to the corresponding electrodes.The phase junction photodetector demonstrates an enhanced photoelectric performance with a high responsivity of 0.26m A/W,a high I_dark/I_photoratio of 2.1×10²,a large detectivity of 2.8×10⁹ Jones,and a super high rejection ratio(R_{254 nm}/R_{365 nm})of 2.7×10³ under 0 V.（5）Schottky junction type self-powered solar-blind heterojunction photodetector based on Ga₂O₃.Although the spectral selectivity of the phase junction detector is excellent,the barrier difference between different crystal types Ga₂O₃ is small,and the photoelectric performance of the photodetector is not unsatisfactory.Schottky junction often has a large built-in electric field.We fabricated self-powered flexible photodetectors by using silver nanowires（Ag NWs）and laser-induced graphene electrodes（LIG）with amorphous Ga₂O₃（a-Ga O）films,respectively.A Schottky junction can be formed between the a-Ga O film and the Ag NWs electrode,so the photoelectron-hole pair can spontaneously separate without additional bias.Under the self-power model,the response speed of the Ag NWs/a-Ga O photodetector is 0.24 s,and the I_dark/I_photo is 120.However,the responsivity of the photodetector is only 23μA/W.In order to improve the performance of the photodetector,we used LIG electrode to prepare the LIG/a-Ga O photodetector.By adjusting the laser frequency or speed,an asymmetric Schottky junction is constructed.The I_dark/I_photo is 200,and the responsivity is 0.6 m A/W at 0 V.