Surface Doping of TMDs and Conduction Properties of Its Heterostructures

Author:Luo Wei

Supervisor:qin shi qiao


Degree Year:2017





Graphene has shown great potentials in the field of electronic devices and optoelectronic devices for its unique and excellent physical properties.With a zero band gap,the ON/OFF ratio of graphene FET is relatively low,while the band gap of the two-dimensional transition metal chalcogenides(TMDs)is usually between 1 and 2 eV,the ON/OFF ratio of TMDs FET is relatively high.Based on the characteristics of graphene and two-dimensional TMDs,the surface doping of TMDs and its heterostructures’transport are studied in this paper.The main structure of this paper is as following:1.Based on the basic physical properties of graphene and two-dimensional TMDs,their applications in specific fields are introduced.In order to solve the problems with devices based graphene and TMDs,the doping of TMDs and its heterostructures’systems are proposed.Specially,the surface charge transfer doping of two-dimensional TMDs are emphatically summarized.The fabraication of two-dimensional materials and its heterostructures are discussed as follows,then the characterization of two-dimensional deviced are discriped in the paragraph.2.With mechanical exfolition method to prepare a variety of monolayer and multilayer two-dimensional materials,and using standard electron beam exposure process,graphene,MoS2,MoSe2 and MoTe2 field effect devices(FET)were prepared,the field effect properties of these FETs were tuned by electron beam evaporation of MgO materials onto the surface of the devices.We found that the MgO film is electron dope to graphene and two-dimensional TMDs.The complementary high gain logic device was successfully fabricated on a piece of MoTe2 with surface charge transfer doping of MgO.The voltage gain of the device was as high as 26,and the switch ON/OFF ration of the device was 2.1×104.Moreover,a in-plane p-n junction based on MoTe2 is successfully fabricated by MgO surface charge transfer doping,and the p-n junction region had obvious photoresponse.The results show that the two dimensional TMDs binding oxides have great advantages in integrated circuits.3.Van Der Waals heterostructure such as graphene/WSe2,WSe2/ReS2,graphene/MoSe2/graphene were prepared by Van Der Waals transfer method.The metal electrodes were fabricated by electron beam lithography,and then the heterostructure devices were obtained.In the study of graphene/WSe2 heterostructure,we found that the two interfaces of graphene/WSe2 heterostructure devices have photovoltaic response,and the results show that there exists a structure similar to n-p-n in thedevices.In addition,it is found that the photovoltaic response of heterostructure interface changes with the bias voltage,and the band structure properties under different bias voltage are analyzed.It is found that the change of the electric field at the contact interface of heterostructure will induce the nonlinear change of the photoresponse.In addition,it is also found that graphene/MoSe2/graphene heterostructure has very high photovoltaic response under different back gate control,and the photoresponse characteristics of sandwich heterostructure devices with different layers of MoSe2 are inconsistent.4.The bottom electrode(Ti/Au)and metal Ni wre prepared on SiO2/Si(300 nm/0.5 mm)substrates,the NiOx thin films were obtained by thermal oxidation,then the MoS2 and thermal oxidation of Ni thin films heterostructure was fabraicated,by using electron beam lithography metal electrodes were obtained,then we got the MoS2 and the thermal oxidized Ni film heterostructure device.Electrical characterization shows that the heterostructure has IV property like tunable diode with back gate voltage.The analysis shows that the heterostructure is a combination of a MoS2 FET and a MoS2/NiOx/Ni heterostructure.The characterization shows that the heterostructure has good photovoltaic response,and the responsivity of the device is as high as 5 mA/W.Further photocurrent characterization shows that the MoS2/NiOx/Ni heterostructure has a uniform photoresponse.Because the Ni film can be made into any desired shape,it is found that the heterostructure is easy to be integrated,which is of special significance for the fabrication of array photodetectors.