Surface Charge Transfer Enabled Property Modulation And Photodetector Fabrication Based On Tungsten Selenide

In recent years,two-dimensional（2D）materials,such as graphene,boron nitride and transition metal chalcogenides（TMDs）,have attracted extensive attention because of their high intrinsic mobility,atomic thickness and no danging bonds on the surface.The international roadmap for devices and systems has pointed out that the development of transistors using ultra-thin 2D semiconductors can effectively inhibit the tunneling current and off-state current in short-channel devices,which is expected to overcome the short-channel effect on the device performance.2D materials alternative to achieve ultra-low power consumption and are expected to be applied in electronic devices and optoelectronic devices in the future.However,there are challenges in heterogeneous integration between different functional materials,including bandgap engineering,interface modulation and development of functional devices,as well as the research on transport mechanism.Meanwhile,2D materials are limited by their atomic thickness,traditional diffusion or ion implantation process are difficult to meet the needs of high-efficiency doping.Therefore,it is necessary to explore feasible strategy to accurately modulate the band structure of 2D semiconductors.The abundant surface of 2D semiconductors provides electroactive sites for impurity adsorption.Based on the principle of surface charge transfer,this paper systematically carried out the physical property modulation of 2D tungsten selenide and the construction of logic devices;Thermoelectronic devices based on 2D tungsten selenide are developed,and the fast detection of weak light is realized.The main contents of this paper are as follows:1.The fabrication process of complementary electronic devices based on TMDs usually uses physical transfer technology to construct p-type and n-type channel materials,so it is difficult to realize large-scale integrated circuits.On the other hand,with the change of thickness of tungsten selenide,the edge of energy band has an obvious offset,and leads to thickness-dependent carrier type and concentration,resulting in the reduction of the reliability and performance uniformity of the fabricated devices.Therefore,it is urgent to solve the problem of electrical performance uncertainty caused by thickness variation.Self-limiting oxidation process is used to grow ultra-thin WO_x layer on the surface of WSe₂.By introducing the method of surface charge doping,we can modulate its electrical properties and eliminate the uncertainty of the electrical properties of WSe₂ transistors caused by the change of material layers.After ozone treatment,the hole mobility of WSe₂ transistor increases to 41.4 cm²V^-1s^-1,and the on/off ratio of the device reaches 10⁶.Combined with electron beam lithography and atomic layer deposition process,the high-resolution localized oxidation modification of WSe₂ nanosheet is realized,and the transverse homogeneous p-n junction with a rectification ratio of 10⁴ is obtained.Then,complementary logic electronic devices are developed on an individual WSe₂nanosheet.And the gain of logic inverter is 19.9.It provides an alternative strategy to the construction of 2D complementary logic electronic devices.2.Asymmetric metal electrode contact structure is introduced to construct WSe₂hot-electron transistors,and realize the transformationfrom bipolar to n-type.The rectification ratio of WSe₂ thermionic diode is above 10⁷,far exceeding the previously reported WSe₂ diodes.By depositing a 5 nm thick tunneling layer,the barrier width of reverse breakdown is further increased and the reverse current of Schottky diode is suppressed.In addition,based on the high rectification ratio and excellent ideal factor of WSe₂ thermionic diode,the doubling-frequency device is obtained.The positive half cycle waveform distortion of the output signal is small.The designed devices fully take the advantages of low voltage drop of Schottky diodes and low reverse current of p-n junction diodes.3.The photoelectric characteristics and operation mechanism of Schottky-contacted WSe₂ thermionic diodes are studied.The photodetectors rely on photoelectron absorption and injection excitation,which expands the detection wavelength region and rises the sensitivity of the photodetectors.WSe₂ thermionic diodes present fast response speed,and realize the wide spectrum test response from375 nm to 914 nm.The built-in electric field formed on the material surface and the potential barrier formed by the alumina tunneling layer can effectively suppress the dark current.When the incident light is illustrated,the photogenerated electron-hole pairs will be rapidly separated by the built-in electric field of Schottky junction and reduce the recombination probability.Under the reverse bias voltage,the direction of the external electric field is consistent with that of the built-in electric field.After the superposition of the two electric fields,a strong electric field will be generated,which greatly improves the separation efficiency of photogenerated electron hole pairs.In addition,we can improve the emission energy of hot electrons by using alumina as a barrier tunneling layer,which can not only make WSe₂ hot electron photodiode have a very small dark current（p A）.Moreover,the relatively slow diffusion transport can be replaced by ballistic transport,which greatly shortens the carrier transport time.In general,Schottky-contacted WSe₂ thermionic photodetectorscan realize ultra-sensitive and ultra-fast detection of extremely weak incident light.In addition,hot electrons with high kinetic energy can pass through the potential barrier and produce photocurrent,which can detect the incident light with energy lower than the semiconductor band gap.This type of device expands the detection wavelength range of the detector for weak light detection.