Design And Performance Study Of Ultraviolet Photodetectors Based On Van Der Waals Heterojunction

Since the discovery of graphene,two-dimensional?2D?layered nanomaterials with atomic layer thickness,stacked with vander Waals force and free of surface chemical dangling bonds have exhibited a series of excellent optoelectronic properties,and have been widely used in various electronic and optoelectronic devices.The emergence of both structural and electronic variety in van der Waals?vdW?-bonded layered materials has opened new avenues for fundamental scientific studies and applied device designs,and provides an ideal research platform for exploring novel physical phenomena and internal mechanisms.In a variety of 2D material systems,2D transition metal chalcogenides?TMDs?are ideal materials for the fabrication of optoelectronic devices due to their good chemical stability,high carrier mobility and layer-dependent tunable band-gaps.Among them,molybdenum disulfide?MoS₂?is one of the most widely studied 2D materials.When the layers of MoS₂ decreased to monolayer,it will transform to a direct-gap with 1.9 eV from the indirect-gap with1.2 eV?bulk?.In addition,as a newly discovered precious metal TMDs,platinum selenide?PtSe₂?has a wider tunable band-gap with a band-gap of 1.2 eV for single layer,0.21 eV for double layer,and decrease to zero with increasing layers.These 2D materials with excellent optoelectronic properties have great potential for high performance photodetectors.At present,2D material photodetectors based on different structures and detection mechanisms have been implemented,which have demonstrated good detection performances and broadband detection from ultraviolet,visible and infrared to terahertz.Despite these advantages,2D layered nanomaterials and photodetectors have some shortcomings.For example,2D materials have a lower optical absorption coefficient;the presence of significant exciton effects greatly prevents the separation of photogenerated electron-hole pairs.In addition,some 2D materials still present challenges in large-area preparation.To overcome these disadvantages,the design of a 2D/3D mixed-dimension van der Waals heterojunction device is an effective solution to achieve high performance photodetectors.However,the total optical absorption and spectral selectivity are limited by the ultrathin nature and availability of 2D materials in all 2D materials based photodetectors.3D semiconductors can provide a larger spectral response range since they are abundant,and improve the optical absoption as well as the device function,furthermore,the dangling bonds free surface of 2D materials allow them to be easily integrated with 3D substrates beyond the lattice matching restriction.Thus,the construction of 2D/3D mixed-dimension van der Waals heterojunctions can lead to stronger light absorption and separation of photo-generated carriers,resulting in higher responsivity and faster response speed.Meanwhile,due to the great significance in communications,chemical analysis,and military warning,many efforts have been devoted to developing high-performance ultraviolet?UV?photodetectors.Because of the high absorption in the ultraviolet region and good shielding for visible light and infrared light,the wide band-gap semiconductor materialshave gradually become the ideal materials for developing UV detection.Among them,as one of third-generation semiconductor materials,gallium nitride?GaN?has a band-gap of 3.4 eV,which has good radiation hardness,high thermal conductivity and chemical stability,and is one of the hot materials for preparing UV photodetectors.In order to realize the detection of solar-blind UV light,it is necessary to find a semiconductor material with a larger band-gap.The?-Ga₂O₃ with a band-gap of 4.9 eV,a large light absorption coefficient,high chemical and thermal stability,is an ideal material for constructing a solar-blind photodetector.In summary,the large-area 2D MoS₂ and PtSe₂ films were synthesized by a thermal decomposition and magnetron sputtering-selenization,respectively.By integrating with three-dimensional GaN and?-Ga₂O₃,the mixed-dimension van der Waals heterojunctions ultraviolet photodetectors were fabricated.Furthermore,their photodetection performances were investigated.The main results are as follows:1.The large-area 2D MoS₂ films were synthesized by a thermal decomposition method in a CVD furnace.The number of MoS₂ layers can be adjusted by changing spinning speeds.The as-synthsized MoS₂ films were characterized by XRD,Raman,AFM and XPS.The results show that the large-area and high-quality MoS₂ thin films with controllable thickness were obtained.2.The large-area 2D PtSe₂ films were synthesized on SiO₂/Si and Si substrates by selenium method.And PtSe₂/GaN heterojunctions were fabricated by in-situ fabrication of PtSe₂ film on GaN substrates.The thickness of PtSe₂ layers were controlled by the thickness of Pt film.The PtSe₂ films were characterized by XRD,Raman,AFM,XPS and TEM,which presents the large-area and high-quality 2D PtSe₂ films with controllable thickness were synthesized.3.Self-powered MoS₂/GaN p-nheterojunction UV photodetectors were constructed,which exhibited high sensitivity to DUV light illumination and pronounced photovoltaic behaviours.Further analysis revealed that MoS₂/GaN heterojunction photodetector has a high responsivity of 187 mA/W,a high specific detectivity of2.34×10¹33 Jones,a high linear dynamic range of 97.3 dB and fast response speed of46.4/114.1 ms?5 kHz?under a DUV light of 265 nm at zero bias voltage without an external power supply.Moreover,the MoS₂/GaN heterojunction photodetector could operate with excellent stability and repeatability in a widefrequency range over 10kHz,indicating this device can follow the fast varying UV signals.4.PtSe₂/GaN heterojunction device was in-situ fabricated by synthesis of large-area vertically standing 2D PtSe₂ film on n-GaN substrate.The PtSe₂/GaN heterojunction device demonstrates excellent photoresponse properties under illumination by deep UV light of 265 nm at zero bias voltage.Further analysis reveals that a high responsivity of 193 mA/W,an ultrahigh specific detectivity of 3.8×10¹44 Jones,linear dynamic range of 155 dB and current on/off ratio of¹0⁸ as well as fast response speeds of 45/102?s were obtained at zero bias voltage.Moreover,this device response quickly to the pulse laser of 266 nm with a rise time of 172 ns.Such high-performance PtSe₂/GaN heterojunction UV PD demonstrated in this work is far superior to previously reported results,suggesting that it has great potential for deep UV detection.5.MoS₂/?-Ga₂O₃ heterojunction photodetector was fabricated.The photodetector shows a responsivity of 2.05 mA/W and a specific detectivity of 1.21×10¹11 Jones at zero bias voltage.MoS₂/?-Ga₂O₃ heterojunction photodetector exhibits excellent solar-blind photoresponse properties with a cut-off wavelength of 260 nm and a high rejection ratio of 1.6×10³ under light illumination of 245 nm?20.1?W/cm²?.Such high-performances of this photodetector are superior to other previously reported?-Ga₂O₃ based photodetectors,which indicates that 2D/3D hybrid-dimension Van der Waals heterojunction has potential applications in deep ultraviolet detection.