| Graphene has excellent physical properties,such as near-zero bandgap,high conductivity,high mobility,high transparency and high flexibility.Therefore,graphene films have extensive application in the field of optoelectronic devices.However,the weak light absorption characteristic of graphene make it lack of spectral selective absorption characteristics and optical gain characteristics,so a single graphene-based photodetector cannot obtain better photodetection performance.Germanium quantum dots(Ge QDs),exhibit obvious quantum confinement effects due to their large exciton Bohr radius(approximately 24.3 nm).They are ideal light-absorbing materials that achieve high-efficiency luminescence and photoelectric response of silicon-based photodetectors.However,the low mobility limits the optical gain of Ge QDs,so the research on photodetectors based on Ge QDs is not yet in-depth.In order to solve these problems,this paper combines the graphene with high mobility characteristics and the Ge QDs with adjustable light absorption characteristics to form composite nanomaterials,and studies the effective charge transfer mechanism in the composite system.The composite photodetector with high response rate,high detection rate and high gain is prepared.The research contents and conclusions of this thesis are as follows:Firstly,this thesis studies the preparation and characterization of graphene.Monolayer/bilayer graphene was prepared using chemical vapor deposition(CVD)method and wet chemical transfer method,and then Raman spectroscopy(Raman)test,optical microscope test,transmittance test were employed to characterize and analyzed the obtained graphene.The experimental results demonstrated that the monolayer/bilayer graphene prepared by this method had a large area.Secondly,after photolithography,oxygen plasma etching,high-vacuum vapor deposition electrode and other methods,the obtained graphene could be prepared into graphene-based photodetectors and then 532 nm laser was exploited to test and study its photoelectric properties.Through the analysis and calculation of output characteristic(Id-Vd)curve,transfer characteristic(Id-Vg)curve,light modulation characteristic(Id-Vdunder different optical power density)curve,the mobility of monolayer graphene is 1998cm~2/V·s,the monolayer graphene-based photodetector has a response rate of 0.37 A/W and a detection rate of 6.34×10~5 Jones with optical power density and Vd being 10.4mW/cm~2 and 0.5 V,respectively.The mobility of the bilayer graphene is 1490 cm~2/V·s,while the bilayer graphene-based photodetector has a response rate of 0.523 A/W and a detection rate of 6.56×10~5 Jones when the optical power density and Vd being 1mW/cm~2and 0.5 V,respectively.The experimental results exhibit that the graphene-based photodetector lacks light absorption characteristics,resulting in low light response rate,light detection rate,and photoconductivity gain.Thirdly,the Ge QDs prepared by chemical methods were studied.Ge O2 was used as a precursor to prepare Ge QDs with better dispersion and higher areal density,and the prepared Ge QDs were characterized with transmission electron microscope(TEM),X-ray diffractometer(XRD),fluorescence spectroscopy(PL).The results showed that Ge QDs with a size of about 2.5 nm-15.5 nm were prepared in this experiment,and the light-emitting area of Ge QDs is located in the visible light region,so Ge QDs can be used as an ideal light-absorbing material.Finally,the graphene/Ge QDs composite material was characterized by atomic force microscope(AFM)and X-ray photoelectron spectroscopy(XPS),and the preparation and performance of the composite photodetector were studied.The output characteristic(Id-Vd)curve and the light response characteristic(Id-t)curve of the composite photodetector with or without illumination under the laser irradiation of 532 nm wavelength of the composite device are obtained.For monolayer graphene/Ge QDs composite photodetector,when the optical power density is 10.4 mW/cm~2 and Vdis 0.5V,the response rate is 0.46 A/W,the detection rate and the photoconductivity gain are7.6×10~5 Jones and 3.98×10~6,respectively.For the bilayer graphene/Ge QDs composite photodetector,when the optical power density is 1mW/cm~2 and Vdis 0.5 V,the responsivity is 2.58 A/W,the detection rate and the photoconductivity gain are 1.79×10~6Jones and 5.31×10~5,respectively.The results demonstrate that the addition of Ge QDs improves the light absorption capacity of the graphene/Ge QDs composite photodetector for incident light.Therefore,compared with the graphene-based photodetector,its responsivity,detection rate and photoconductivity gain all have a improvement. |
PDF Full Text Request