Plasma Surface Modification Of Two-dimensional TMDs Heterjunction

Since graphene was successfully discovered in 2004,it has attracted much attention because of its unique physical and chemical properties,especially in the field of infrared photoelectric detectors and high-frequency devices.However,the zero-bandgap structure and low light absorption rate（only 2.3%）severely limit the performance of graphene photodetectors.At this point,as a complementary material to graphene,the graphene-like transition metal dichalcogenides（TMDs）possess unique structure,adjustable band gap（1.06-2.88 e V）with the number of layers,and a much higher optical absorption rate than graphene.Two-dimensional（2D）TMDs are naturally passivated and have no dangling bonds,being free of lattice mismatch problems when they stack with other 2D materials.Therefore,a variety of van der Waals heterojunctions can be prepared by stacking two or more 2D materials,and the excellent properties of various materials can be simultaneously played,which is of great significance to the performance improvement of photodetectors.In addition,the lack of controlled doping means for 2D TMDs materials or their heterojunctions limits their further large-scale application.Therefore,in this thesis,high-quality MoS₂/WSe₂ and MoS₂/Graphene heterojunctions were prepared by mechanical exfoliation and dry transfer method.On the basis of the systematic study of their optical and electrical properties,a controllable doping process was developed based on our laboratory mild plasma technology to achieve non-destructive and controllable doping of two-dimensional TMDs heterojunctions.The electrical and photoelectrical properties of the heterojunction devices are improved,and the doping mechanism is deeply analyzed.The specific research content of this paper is summarized as follows:1.High-quality MoS₂/WSe₂ heterojunctions were prepared by mechanical exfoliation and dry transfer method.The morphology and optical properties of the samples were characterized by optical microscopy,atomic force microscopy（AFM）,Raman and fluorescence spectroscopy.Then,the MoS₂/WSe₂ FET devices were constructed by lithography.Due to the strong built-in electric field of the PN junction,the output characteristics of the devices have a good rectification effect,and the maximum rectification ratio is 93.5 when V_ds=±1 V.In addition,the MoS₂/WSe₂ photodetectors display good optical detection capability for visible light under self-driven conditions(V_ds=0).Under 520 nm laser irradiation with a power of 1.37μW,the responsivity can reach 2.12×10³ A/W,and the corresponding photocurrent and detection rate are 1.48×10^-8 A and 2.33×10¹¹ Jones,respectively.2.The effects of mild ammonia plasma on the electrical and photoelectrical properties of the MoS₂/WSe₂ heterojunctions have been systematically studied.Through a large number of comparative experiments,it is found that the device performance is the best when the plasma treatment parameters are kept at:working pressure of 10 Pa,ammonia gas flow rate of 10sccm,rf power of 50 W,treatment time of 90 s.Due to the N-type doping of MoS₂,the built-in electric field of the PN junction is effectively enhanced,and the Schottky barrier between the semiconductor layer and the metal electrode is reduced to a certain extent.As a result,the output rectification ratio of the heterojunction device increases from 93.5 to 3032.8after such plasma treatment.Meanwhile,under 520 nm laser irradiation with the same power,both the responsivity and detection rate are greatly improved.In particular,the responsivity increases from 2.12×10³ to 6.35×10⁴ A/W,and the detection rate changes from 2.33×10¹¹ to6.89×10¹² Jones.The main mechanism behind the enhancement of photoelectric performance was analyzed by KPFM measurement and band diagram.H free radicals in ammonia plasma can induce the generation of S vacancy in the surface layers of MoS₂,and this can be attributed to the soft plasma treatment.In other words,this indicates that the soft ammonia plasma has no significant effect on the lower WSe₂ layers.The increase of S vacancy leads to n-type doping of MoS₂,resulting in larger band bending at the interface and stronger built-in electric field of the heterojunction.This further improves the separation efficiency of photogenerated carriers,and therefore promote the photoelectric performance.3.The MoS₂/Graphene heterojunctions have been constructed and a series of optical and microstructure characterization tools have been carried out.All the results prove that the heterojunctions are of high quality.The influence of mild ammonia plasma on the electrical and photoelectrical properties of MoS₂/Graphene heterojunction was studied.It is also found that the photoelectric properties of MoS₂/Graphene heterojunction are significantly enhanced after ammonia plasma treatment.The maximum rectification ratio changes from 10.37 to35.27 for an increase of 3.4 times,and the maximum responsivity increases from 2×10³ to3.6×10⁴ A/W.The enhancement of both electrical and photoelectrical properties can be attributed to the self-aligned N-type doping of MoS₂ by mild NH₃ plasma.In addition,the effect of different thickness of graphene on the electrical and photoelectrical properties of MoS₂/Graphene heterojunction was systematically studied.By comparing the 3 nm-,8 nm-and 12 nm-graphene heterojunction devices,it is found that thick layers have a better optical absorption and can generate larger photocurrents than thin layers under the same photoexcitation conditions.As for thin layer graphene devices,due to the semi-vertical heterostructure,thin graphene has a relatively short carrier transport path,and could quickly responds to the external circuit.Therefore,the thin-layer graphene heterojunction devices show a faster response speed of about 10 ms.