Growth Of Transition Metal Sulfides And Their Oxidation For Application In Field Effect Transistors

Silicon-based field-effect transistors（FETs）have become the current mainstream devices based on the rich content in the crust,low conductivity of Si,and the large band gap（～9 e V）of its natural oxide Si O₂,good van der Waals interface between Si and Si O₂.However,in recent years,as the transistor density of integrated circuits has increased and the transistor size has decreased,the FET channel length（L）also decreased,which in turn cause problems such as short-channel effect（SCE）and decrease in threshold voltage.Although reducing the thickness of the gate oxide layer can effectively solve the problems such as the threshold voltage drop caused by the SCE.As the thickness of the gate oxide decreases,the gate leakage current and power consumption increase exponentially,even result in gated-induce drain leakage（GIBL）caused by tunneling.Therefore,how to solve the SCE has become a major challenge for FETs.In order to solve the above problems,there are three solutions:replacing the traditional metal-oxide semiconductor field effect transistor（MOSFET）with a new fin field effect transistor（Fin FET）;using a two-dimensional（2D）semiconductor material instead of three-dimensional（3D）silicon crystals;using gate dielectric layers with high dielectric constants（?）.Combined with laboratory conditions,this paper intends to carry out research content based on two ideas:studing the preparation of 2D semiconductors and high-dielectric materials;combining high-dielectric materials with2D semiconductors to build devices with better performance.There are many types of2D high-mobility semiconductors,which can be divided into inorganic semiconductor2D materials,organic semiconductor 2D materials and organic-inorganic hybrid semiconductor materials according to components classify.Inorganic semiconductor2D materials are generally superior to the latter two in terms of thermal st ability and mobility,among which the transition metal sulfides（TMD）,represented by molybdenum disulfide（Mo S₂）and tungsten disulfide（WS₂）,are typical 2D inorganic semiconductor materials with high mobility and adjustable band gap.The high mobility feature of ZrS₂ have been predicted,plus the highκcharacteristics of its natural oxide ZrO₂ has been preliminarily confirmed.Therefore,it is of great significance to prepare Mo S₂,WS₂ and ZrS₂.However,there are still challenges in preparation of high-quality and large-area TMD and dielectric materials.Based on above challenge,the research content of this paper mainly includes following aspects:（1）Growing IVB and VIB group TMD semiconductor materials-WS₂,Mo S₂ and ZrS₂ by chemical vapor deposition（CVD）.Replacing the traditional sulfur precursor-sulfur powder with Na₂SO₄,which has high melting point,to synthesize WS₂ and Mo S₂flakes with low sulfur defects.Transmission electron microscopy（TEM）,Raman spectroscopy,and photoluminescence（PL）characterize the high crystallinity and low sulfur defect density of material.A three-step reaction mechanism was deduced by study the reaction process of the Na₂SO₄-WO₃ system:the thermal decomposition of Na₂SO₄ above 690°C,the products are Na₂S and H₂O;Na₂S,WO₃ and H₂O react with each other to produce liquid Na₂WO₄ and H₂S with a molar ratio of 1:1,in which liquid Na₂WO₄ is partially gasified to provide tungsten source;the synchronous released gasified Na₂WO₄ and H₂S act as metal source and sulfur source respectively to generate WS₂.Therefore,the gaseous tungsten source Na₂WO₄ and sulfur source H₂S,with a molar ratio greater than 1:1,origin from the thermal decomposition reaction of Na₂SO₄.Exhaust gas monitoring experiment confirme that their release continued until the end of growth.Therefore,this source-feeding system not only bridge the mass flux gap between metal and sulfur sources,but also control the source diffusion and provide sulfur sources with a molar ratio higher than 1,which is beneficial to the growth of high-quality and low-defects WS₂.（2）Although the method described in the previous chapter can prepare ZrS₂,the low coverage and difference in growing on silicon wafers hinder subsequent in-situ device fabrication.Therefore,in this part of the work,we used ZrO₂ with high melting point and ZrCl₄ with low melting point as zirconium sources to further optimize the growth of ZrS₂.For the ZrO₂ precursor,the Na₂SO₄-Fe₃O₄-ZrO₂ ternary system is formed by adding fluxes Na₂SO₄ and Fe₃O₄,so that ZrO₂ turns into a liquid state at～850°C（the melting and boiling point is decreased）.In order to push ZrO₂ vapor react on the substrate,OH^-are spin-coated on silicon wafer surface to react with Si O₂,forming molten Na₂Si O₃,which could"capture"gaseous ZrO₂,the sulfur source pushed into furnace at growth stage will react with this captured ZrO₂,forming ZrS₂.This Zrsource could continuously provides ZrO₂ to the ZrS₂ crystal nucleus,facilitating large ZrS₂ growth.For ZrCl₄,K₃[Fe（CN）₆]molecules were spin-coated on the substrate as crystal nucleation sites to solve its surface energy problem,which could achieve large-size ZrS₂ growth.Raman,TEM,XPS,etc.characterizations prove that the material is2D ZrS₂.（3）Based on CVD growth of ZrS₂,highκsingle crystal monoclinic（m）ZrO₂ was prepared using thermal oxidation,then combining Mo S₂ to fabricate FETs.The as-prepared m-ZrO₂ has an equivalent oxide thickness（EOT）of about 0.29 nm,a high dielectric constant of 19,and a breakdown voltage of 7.2 MV cm^-1.Mo S₂ FETs using m-ZrO₂ as dielectric layer show comparable mobility to those using Si O₂ as dielectric layer.Moreover,the transfer curve shows no hysteresis,which is mainly due to charge trapping caused by charge injection at the interface of semiconductor and dielectric layer,hence this work could realize ultra-clean m-ZrO₂/Mo S₂ interface and high crystalline quality m-ZrO₂.Single crystal m-ZrO₂ dielectrics show a good application prospect in digital complementary metal-oxide semiconductor（CMOS）logic FET.（4）Continuing the previous work idea,further preparing large-area zirconia film for its future large-scale CMOS applications.Growing large-area single crystal ZrS₂film by CVD epitaxial growth technology,then conducting thermal oxidization of single crystal film to prepare amorphous uniform ZrO_x film.The epitaxial growth of ZrS₂ was carried out on the surface of c-plane sapphire.The optical picture show that the crystal orientation of the material is consistent,and the SEM magnification show no obvious grain boundary between the hexagonal ZrS₂ domains,which preliminarily proved the single crystal nature of material and the continuity of single crystal film.Raman,TEM,XPS,etc.prove that a large-area single crystal ZrS₂ film was realized.After that,the ZrS₂ single crystal film was oxidized in situ to prepare amorphous ZrO_x,which was conducted at a lower temperature.A top gate FET was constructed in combination with semiconductor WSe₂.In which transfer curve exhibits the characteristics of P-type semiconductors with negligible hysteresis,which preliminarily confirms the potential application of amorphous ZrO_x in 2D FETs.