Research On Preparation And Optoelectronic Properties Of Two-Dimensional Semiconductor Materials And Heterojunctions

With the development of photodetectors towards miniaturization and low power consumption,photodetectors based on traditional materials have been unable to meet the growing demand.To achieve high-performance and easy-to-integrate photodetection,it is urgent to explore and develop novel photodetectors.The rapid development of two-dimensional materials provides new opportunities for highperformance photodetectors.Benefiting from their unique layered structure,excellent electrical and optoelectronic properties,outstanding mechanical properties and strong light-matter interaction,two-dimensional materials have proved to be strong competitors for the new-generation of photodetectors.Moreover,using thin layers of two-dimensional materials as channel materials,their electrical and optoelectronic properties can be easily modulated by local fields.In addition,two-dimensional materials can be integrated with various materials through van der Waals force,combining the advantages of both to achieve photodetectors with different structures and functions.Based on the above research ideas,photodetectors based on twodimensional semiconductor materials and their heterojunctions with different structures and functions were designed and fabricated in this thesis.Their electrical and optoelectronic properties were studied and their working mechanisms were described.The main research contents and achievements are as follows:(1)At first,GeSe bulk was grown successfully by chemical vapor transport method and GeSe single crystal thin films were obtained by direct mechanical exfoliation.The good crystallinity was confirmed by characterizations of X-ray diffraction,scanning electron microscopy and Raman spectroscopy.On this basis,the field effect transistors based on GeSe flakes were fabricated and were investigated systematically.The on/off ratio and field-effect mobility of the representative device are 90 and 0.4 cm2V-1s-1.The photoconductive photodetector based on GeSe flakes were designed and fabricated.The representative device has a responsivity of up to 7.6×103 A/W and the response/recovery time is 0.8/1.7 s.In order to further improve the responsivity of the GeSe photodetector,PbS quantum dots were spin-coated on the surface of the device.The responsivity of the device is increased by 2.6 times through the photogating effect.(2)To achieve the fast photoresponse,n-Si/p-GaTe heterojunction devices with type-II band alignment were fabricated.At first,GaTe bulk were grown successfully by chemical vapor transport method and GaTe single crystal thin films were obtained by PDMS-assisted exfoliation.The good crystallinity was confirmed by characterizations of transmission electron microscopy,Raman spectroscopy,photoluminescence spectroscopy,and X-ray photoelectron spectroscopy.On this basis,self-powered photodetectors based on n-Si/p-GaTe vertical heterojunctions were fabricated by PDMS dry transfer.The representative device shows a high responsivity of 5.73 A/W,a fast response time of 20 μs,and a specific detectivity of 1012 Jones,an on-off ratio of 103,a broad spectroscopy response from 300 nm to 1100 nm,and good cycling stability and repeatability.(3)In order to further optimize the optoelectronic properties of the n-Si/p-GaTe heterojunction photodetector,the responsivity of the device is increased from 2.2 A/W to 4.5 A/W and the response/recovery time is decreased from 22/15 μs to 2/5 μs through AlOx interface passivation and thermal oxidation.Photoluminescence spectroscopy,Xray photoelectron spectroscopy and GaTe field effect transistor shows that the defect states and recombination centers at the interface were shielded by AlOx passivation layer leading to the enhancement of the carrier separation at the interface.The p-type doping of GaTe is achieved by thermal oxidation,which strengthed the built-in electrical field of the heterojunction and promoted the separation of carriers resulting in the improvement of the optoelectronic properties.In summary,photoconductive photodetectors based on GeSe single crystal thin films and photodetectors based on n-Si/p-GaTe vertical heterojunctions were fabricated,and their optoelectronic properties were optimized according to different mechanisms,which provide new ideas and approaches for design and optimization of other novel photodetectors.