| Photodetector is defined as a device capable of converting optical signals into electrical signals,and it plays an important role in optical communication,medical imaging,night vision,gas sensing and safety detection.With the expansion demand for high-performance photodetectors,especially for broad-spectrum detectors that can simultaneously cover multi-band responses is increasing.At present,photodetectors based on silicon(Si),?-? and ?-? materials take over the principal market.In practical applications,when detecting different wavelength ranges,it is often necessary to select semiconductor materials with relevant band structure.For instance,GaN,Si and InGaAs are used to fabricate detectors in the ultraviolet(UV),visible and near infrared(NIR)bands,on the basis of their own band gap.With regard to infrared band,narrow bandgap semiconductor materials are required,most typical example of which is HgCdTe semiconductor alloy.Although there are semiconductor detectors for different wavelengths,they are often limited by the energy band characteristics of the material itself,so they need to be selected and switched according to specific occasions and environments,which is inconvenient in practical applications.Therefore,there is a growing demand for the development of photodetectors with broadband response.In addition,the development of photodetectors is also confronted with a series of problems such as:how to make breakthroughs in response speed,detection rate and other performance and how to improve the integration while controlling the cost.Two-dimensional(2D)materials are expected to be a new generation of semiconductor materials because of its excellent physical properties,after Si and compound semiconductors,which provide ideas for the breakthrough of traditional photodetectors.Firstly,the rich bandgap of 2D materials make it possible for wide-band photoelectric detection.Secondly,the thickness of 2D material is atomic layer and the quantum confinement leads to strong exciton binding effect,which improves the optical absorption efficiency.Thirdly,the surface of 2D materials is passivated naturally without hanging bonds,and can be integrated well with other materials without the restriction of lattice matching.Thus,various Van der Waals heterojunctions can be created artificially to achieve functions that a single material does not have.Finally,the bandgap of 2D layered semiconductor is closely related to its thickness.Optical absorption edges can be modulated by simply changing the number of layers.Graphene,as the representative of 2D materials,has remarkable advantages in wide spectrum and fast response due to its zero band gap and high carrier mobility.However,the intrinsic light absorption efficiency of graphene is very limited(2.3%for single layer graphene)and the lifetime of photogenerated carriers is very short,which can not be effectively separated in time,resulting in very small photocurrent.The above-mentioned reasons make the graphene-based photodetectors have larger dark current and lower photoelectric conversion efficiency.Comparatively speaking,the new narrow bandgap 2D semiconductor and topological materials have more advantages in the application of optoelectronic devices.They generally have small band gap,high carrier mobility and high efficiency of light absorption and utilization.They are expected to break through the performance of broadband,high responsivity and fast response.This paper focuses on improving the performance of broadband photodetectors from existing high-performance material and exploring new material.Some problems in the application of photodetectors are solved,to broaden the detection range and improve the performance as well,which lay a theoretical foundation for the future application of broadband photodetectors in real life.The main contents are as follows:(1)Black phosphorus(BP)is a typical 2D semiconductor material with narrow band gap.It is widely used in broadband photodetectors,especially in infrared detectors at room temperature.However,up to now,there are few reports on BP-based terahertz(THz)photodetectors,and there is still much room for improvement in performance.In order to broaden the detection range of BP-based photodetectors and further improve their photoelectric performance in THz.We proceed from material modification,Se-doped mineralizer-assisted growth method was used to enhance the mobility,carrier concentration and stability of BP.As a result,the Se-BP THz detector shows a responsivity of 8.1 V W-1 and a noise equivalent power of 1.08 nW Hz-1/2 at 0.27 THz.High quality images have also been successfully realized.In order to meet the needs of large-scale applications,electrochemical exfoliation was also improved and few layers of BP nanosheets with high yield and crystallinity were prepared.The as-exfoliated BP nanosheets have good optical/photoelectric properties,and thus showing excellent performances in piratical applications including 2 ?m ultrafast mode-locked laser,visible-infrared photodetector and quasi-solid-state micro-supercapacitor(QMSCs).More significantly,the QMSCs,fabricating by the free-standing BP thin films,can be patterned on a single substrate with flexible photodetectors based on same BP thin film to form a self-powered optoelectronic system.These encouraging results make BP thin film promising material for a wide range of penitential applications including flexible micro-electromechanical storage systems and flexible optoelectronic devices.(2)SnTe is a new type of topological crystalline insulator.Previous studies have shown that this material is suitable for the development of broadband detection,but its responsivity is relatively low.We reported a controllable approach for the production of highly crystalline SnTe nanoflakes on Si substrate by a low-cost and simple physical vapor deposition(PVD)process.To further improve the performance of the SnTe photodetectors,the channel length-scaling dependence of the photoresponse was well studied.When the incident light irradiance was fixed,the improved responsivity and response time has a clear inversely proportional dependence on channel length.The device shows a simultaneous ultra-broadband response and high responsivity of 71.11 AW-1 at 254 nm,49.03 AW-1 at 635 nm,10.91 AW-1 at 1550 nm,and 4.17 A W-1 at 4650 nm,respectively.Furthermore,a flexible SnTe-based photodetector was also fabricated on PET substrate.The device can maintain a stable photocurrent under various bending angles and cycles.Considering the high responsivity,broadband photodetection and flexibility,the SnTe-based photodetectors may fnd applications in wearable devices,telecommunication,and biological imaging.(3)We explored a new type of photoelectric detector based on two-dimensional topological semi-metallic material ZrGeSe.It is found that the 2D topological semi-metal based on ZrGeSe had very high carrier mobility at room temperature(RT),the carrier mobility of the field effect transistor(FET)can reach to 1202.2 cm2 V-1 s-1 at RT.It is also found that ZrGeSe was a topological semi-metal with narrow band gap from single layer to multi-layer by band calculation,which was expected to realize wide-band photoelectric detection from UV to THz.Some preliminary results have been obtained in photoelectric detection from UV to mid-infrared(MIR)bands.These results indicate that ZrGeSe,a topological semi-metallic material,shows great potential for wide-band photoelectric detection. |
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