Exploration Of Laser And Ultrasound Induced Shock Wave Techniques For Wide Bandgap Semiconductor Preparation

Wide bandgap semiconductor materials have become the key materials for developing semiconductor devices with high power,high temperature,high frequency and radiation resistance due to their characteristics of high breakdown electric field,good thermal conductivity,good stability and radiation resistance.The main synthesis methods of wide gap semiconductors include molecular beam epitaxy(MBE),pulsed laser deposition(PLD),metal-organic chemical vapor epitaxy(MOCVD),hydride vapor phase epitaxy(HVPE)and microwave plasma chemical vapor deposition(MPCVD).All of these processes have disadvantages,such as high equipment costs,complex operation,strict deposition requirements,and unintentional doping.Diamond and gallium oxide(Ga2O3)have large bandgap widths,chemical stability properties and complex synthesis methods,which have to some extent hinder their large-scale commercial application.Therefore,it is urgent to explore new methods for wide bandgap semiconductor preparation.Shock wave chemistry studies chemical reaction initiated by the shock wave generated by explosions or high-speed collisions as the excitation source,which can avoid all possible side reaction process in the heating process,and has the advantage of clean environment,low energy consumption,rapid response,convenient operation,flexible control and and the ability to induce chemical reactions at room temperature and pressure.Shock wave chemistry has great advantages in optimizing the preparation of wide bandgap semiconductor materials.In the dissertation,two kinds of wide bandgap semiconductor diamond and gallium oxide are studied.The preparation of widegap semiconductor materials has been systematically and deeply explored by studying the mechanism of laser plasma shock wave and the cavitation dynamics model,and combining with various testing methods,finite element simulation and first-principles calculation.The main research contents and conclusions are as follows:In chapter 1,the research background of this thesis was induced.The structural properties,preparation methods and existing problems of diamond and Ga2O3 are summarized.The mechanism of laser plasma shock wave and ultrasonic cavitation as well as their application in chemistry are reviewed in detail.In chapter 2,AuGa2/α-Ga2O3/Au was synthesized by ultrasonic cavitation.The optical properties and applications of AuGa2/α-Ga2O3/Au were investigated.The optical properties of AuGa2 nanoparticles were calculated based on first principles calculation and finite element simulation.Compared with α-Ga2O3,AuGa2/α-Ga2O3/Au nanocomposites exhibit dual-band optical absorption at 520 and 740·nm due to interband transition and surface plasma resonance.The position and shape of the dband center of Au were adjusted by alloying strategy.The reason of AuGa2/α-Ga2O3/Au increasing rhodamine(RhB)degradation rate was discussed,and the principle of antibacterial activity of AuGa2/α-Ga2O3/Au was also explained.In chapter 3,liquid Ga buffer layer was prepared by ultrasonic cavitation method,and high-quality polycrystalline diamond film with diameter of 50 mm and thickness of 120 μm was successfully grown.The theoretical calculation formula of thermal stress in diamond film changing with temperature is deduced,and the thermal stress of silicon-based diamond film covered with Ga buffer layer during cooling process was calculated by finite element simulation software.Adsorption energy of methyl on Ga surface was simulated based on first-principles calculation.The reason that Ga can promote diamond film nucleation and quality were speculated.The preparation of lowstress diamond films was achieved by SEM,Raman,XPS and other characterization tests to prove that the liquid metal Ga buffer layer can reduce the lattice mismatch as well as thermal mismatch between substrate and diamond film.In chapter 4,the preparation of defective graphene buffer layers by laser plasma shock wave was investigated.Raman and XPS spectrum show that the diamond film prepared with defective graphene as a buffer layer has lower stress.The adsorption energy of methyl on the surface of graphene was calculated based on the first principles,and the principle of defective graphene promoting the nucleation of diamond films was explained.At the same time,defective graphene can interact with the substrate and diamond film through van der Waals forces,relieving the stress caused by lattice mismatch between the substrate and diamond film.In chapter 5,we summarize and analyze the research content of this dissertation,and then draw the conclusion.The innovations of this dissertation lie in:1.AuGa2/α-Ga2O3/Au was synthesized using ultrasonic cavitation method to achieve efficient dye degradation and excellent antibacterial properties.The mechanism was analyzed based on first principles calculations,finite element simulations and dband theory of transition metal.2.Liquid Ga was synthesized using ultrasonic cavitation method.Based on finite element simulations,it is hypothesized that Ga can performed as a seed layer and buffer layer which can reduce the lattice mismatch and thermal mismatch between silicon substrate and diamond film.The components of buffer layer are analyzed.Finally,the nucleation and the growth of low-stress diamond film were achieved.3.Defective graphene was synthesized using laser plasma shock wave.Using it as the seed layer and buffer layer,the preparation of low-stress diamond films on silicon substrates was achieved based on van der Waals heterogeneous interface growth.