Study On The Growth And Phase Transition Properties Of Vanadium Dioxide Films Based On Silicon Carbide Substrate

Thermally induced phase change materials have great application prospects in energy storage,photoelectric switch devices and smart coatings,and so on.VO₂ attracts a lot of attention in many phase change materials because of the phase transition temperature?68??near room temperature.In the insulator-metal transition?IMT?process,VO₂ will undergo a monoclinic phase transition to a tetragonal rutile phase with dramatic changes in its electrical,optical,and magnetic properties.As we all know,the phase transition characteristics of VO₂ are closely related to the quality of the thin films.In particular,the continuity and integrity of the surface of the VO₂ films play an important role in the magnitude of resistance changes in the IMT process.A good lattice matching relationship between the substrate and the film is a prerequisite for the preparation of high-quality epitaxial films.In addition,the thermal conductivity of the substrate also plays a significant role in the growth of the film.As the third generation of wide band gap semiconductors,SiC has a wide range of application prospects because of its excellent characteristics such as a amount of crystal forms,high hardness,high thermal stability,high thermal conductivity and high breakdown electric field,and so on.As a result,SiC is considered as the next important semiconductor material to replace silicon.At the same time,it is found that there is a good lattice matching between VO₂ films and SiC with manycrystal forms.Therefore,SiC can be used as a very effective substrate for the epitaxial growth of VO₂ thin films.In addition,another feature of silicon carbide is that graphene can be epitaxially grown by high-temperature annealing in an ultra-high vacuum environment.Graphene has become a new two-dimensional material with great potential for application due to its unique electrical,thermal and optical properties of almost full transmissivity of visible light.Attempt to grow VO₂ thin films by introducing epitaxial graphene as a buffer layer on silicon carbide substrate can introduce interface stress caused by lattice mismatch,and then achieve the purpose of regulating VO₂ phase transition temperature.Therefore,the combination of SiC and graphene with VO₂ not only plays a positive role in crystal growth and phase transformation regulation,but also can greatly expand the application of VO₂ in smart windows,Terahertz devices,photoelectric switches,sensors,and so on.Based on the above descriptions,this article attempts to fabricate VO₂ thin films on silicon carbide substrates by using different preparation techniques.The main research work is as follows:?1?High-quality VO₂ thin films were successfully grown by pulse laser deposition?PLD?on 6H-SiC?0001?substrate.The crystal structure,surface morphology and the change range of resistance during the IMT process were characterized and tested.The influence mechanism of the heated temperature of6H-SiC substrate and the deposition time of VO₂ on the film quality was systematically studied.After analyzing the experimental data,it is concluded that when the substrate heating temperature reaches 500?,it is very conducive to the epitaxial growth of VO₂ film on 6H-SiC?0001?.At this condotion,the substrate surface has enough energy to facilitate the diffusion and bonding of VO₂ molecules deposited on the surface.The continuity and integrity of the surface of VO₂/6H-SiC thin films can be optimized by prolonging the deposition time of VO₂ molecule properly,thereby improving its phase transition characteristics.The excellent phase change characteristics of the sample were mainly reflected in the rapid IMT process,with a very narrow thermally induced width??T=3.3??and a resistance change range of nearly 5 orders of magnitude.Based on the above experimental results,VO₂thin films were grown on the carbon polar surface of 6H-SiC by PLD technology.Combined with parameters such as surface energy and thermal stability,the potential factors leading to the difference in the phase transition characteristics of the VO₂films on the two polar surfaces of silicon carbide were analyzed.The RT curves of the sample showed that the 6H-SiC?0001?was more suitable as a substrate for epitaxial growth of high-quality VO₂ thin film single crystal.?2?Preparation of epitaxial graphene by heating 6H-SiC at high temperature in ultra-high vacuum environment,and achieve the optimized condition of epitaxial graphene with the aid of Pd atoms.The effect of Pd atoms on the quality of epitaxially grown graphene by silicon carbide annealing was studied.The samples were characterized by Raman spectroscopy and showed typical peaks of graphene Raman.Compared with the results of annealing 6H-SiC only,introducing an appropriate amount of Pd atoms before annealing was very beneficial to improve the crystal quality of epitaxial graphene.According to the above experimental results,the preparing technique of MBE was used to fabricate the VO₂ thin films on graphene in situ.The results showed that epitaxial graphene had a certain regulation effect on the phase transition temperature of VO₂.As the thickness of the VO₂ films increases,the interface stress was relaxed due to away from the interface,and resulted in the gradual decrease of the regulation effect.