Low Temperature Growth And In-Situ Modulation Of Electronic Properties Of Transition Metal Dichalcogenide Thin Films

With the acceleration of technology and information in modern society,people's demand for new types of multifunctional,portable,and easily integrated electronic devices keeps increasing.Among graphene-like materials,transition metal dichalcogenides?TMDCs?films have received extensive attention from researchers.The atoms within the layers of TMDCs are bonded by strong covalent bonds,and the layers are connected by weak van der Waals forces.TMDCs,with tunable bandgaps and few dangling bonds between the layers,can be easily integrated,thus meet the needs of miniaturization.In addition,TMDCs have rich physical properties such as superconductivity,giant magnetoresistance and charge density wave,and have great application potential in the fields of photodetectors,storage,etc.In this paper,we mainly focused on WSe_2-x,PdSe_2-x,Pt Se_2-x,Ni Se_2-x and WS2TMDCs materials and studied the low-temperature selenization preparation process,the photoelectric properties,and the in-situ modulation of electrical properties of these films.The main results obtained are listed as follows:?1?The process of preparing WSe_2-x films by vacuum selenization method was studied with SiO₂/Si wafer selected to serve as the substrate for the growth of thin films.The selenization temperature dependent carrier concentration,conductivity,mobility,magnetoresistance and photoelectric response of WSe_2-x films were studied through the characterization of the film composition and structure,and electromagnetic testing.The experimental results show that the precursor W film can be fully selenized when the selenization temperature is 400°C and above.The film selenized at 400°C possesses the largest hole mobility of 20.4 cm²V^-1s^-1 whereas the selenized film at 450°C shows the best response sensitivity to the external field,and the relative resistance change??R/R?reaches 33%and 78%with the external magnetic field?B=9T?and light field??=405nm?applied,respectively.An excessive selenization temperature will cause Se vaporization,which can increase the number of Se defects in the film,leading to the decrease of carrier mobility,magnetoresistance and photoelectric response of the samples.Under visible light,the resistance of WSe_2-x thin film decreases significantly as the incident light intensity increases and the WSe_2-x film shows the largest photoelectric response near 773 nm.Under periodic simulated natural light irradiation,the thin film shows efficient and stable optical switching performance,that is,with switching the ON and OFF states exceeds 300 times,the thin film resistance does not show any degeneration.Besides,WSe_1.5film was deposited on PMN-PT?001?single crystal substrate by the pulsed laser deposition method and the in-situ regulation effect of polarization inversion of PMN-PT?001?substrate on the electrical properties of WSe_1.5 film was studied.With an electric field applied to the WSe_1.5/PMN-PT?001?heterostructure,the reversible and non-volatile modulation of WSe_1.5 thin film resistance is realized at room temperature.The relative resistance change of the WSe_1.5 thin film is as high as373214%and is stable during more than 300 times of visible-light ON and OFF states switching.These resistance change behaviors,combined with the Hall effect,in-situ XRD diffraction,and the measurement of the resistance of the WSe_1.5/PMN-PT?111?heterostructure with electric field changes,we found that the ferroelectric field effect and the lattice strain effect induced by the PMN-PT?001?polarization reversal cannot explain the observed resistance change behavior in the experiment.Nevertheless,micro-cracks are generated on the single crystal substrate surface during the 109^oferroelectric domain flipping and their opening and closing can explain the huge change of the film resistance.?2?The vacuum selenization preparation process of PdSe_2-x film on SiO₂/Si substrate was studied.It is found that when the selenization temperature is 300°C,the film which shows p type conductive behavior with a hole concentration of 10¹⁸ cm^-3,possesses the largest carrier mobility of 48.5 cm²V^-1s^-1and room-temperature magnetoresistance of 12%?B=9T?.As the selenization temperature increases,the Se content in the film decreases,and the mobility as well as magnetoresistance decreases.Moreover,a series of PdSe_2-x films were prepared on ferroelectric single crystal PMN-PT?001?substrates by vacuum selenization method and in-situ modulation of the electronic transport properties of PdSe_2-x film by the polarization inversion of the substrate caused by the applied electric field was studied.The results show that the reversible and non-volatile resistance modulation of the film can be achieved through the polarization inversion of the substrate,and the relative change of the film resistance can reach 5868%at room temperature.The mechanism of this modulation is the opening and closing of micro-cracks on the film surface induced by the 109^oferroelectric domain flip in the PMN-PT?001?substrate.When the substrate is in the negative state,the cracks open and the film is in a high resistance state,while in the positive state,the cracks are closed and the film is in a low resistance state.?3?A series of PtSe_2-x films were prepared on SiO₂/Si substrates by vacuum selenization method,and the effects of selenization temperature and Pt film thickness on the electrical transport properties of Pt Se_2-x films were studied.As the selenization temperature increases,the carrier concentration of the film first decreases and then increases.As the thickness of the precursor Pt film increases,the degree of selenization of the Pt film and the percentage of Se decreases.The conductivity type of the Pt Se_2-x film changes from p-type to n-type,the carrier concentration gradually increases,and the magnetoresistance gradually decreases.In addition,the vacuum selenization process of growing Ni Se_2-x film on SiO₂/Si substrate is explored.Under the selenization temperature of 300°C,the film possesses the largest electron mobility,which is 10.5 cm²V^-1s^-1,and the bulk carrier concentration of the film is 2.2×10²¹cm^-3.Finally,a series of WS2 films were prepared on Al₂O₃ substrates by rapid annealing process.It was found that after annealing at 610?,the films show the best the crystalline quality and photoelectric properties.