Researches On The Growth Of VO₂ And GaN Films And The Characteristics Of N-VO₂/p-GaN Heterojunction

Vanadium dioxide(VO2)is a unique transition metal oxide,which undergoes a reversible semiconductor-to-metal transition(SMT)at a critical temperature(Tc)of 68℃ from low temperature semiconductor phase with monoclinic structure(P21/c)to high temperature metal phase with tetragonal(P42/mnm)structure.With these unique properties,VO2 film has been studied extensively for its numerous potential applicationsin electrical and optical switches devices,memory storage devices as well as smart window coatings.On the other hand,gallium nitride(GaN)has been emerging as the most important wide bandgap semiconductor,due to its wide band gap and some other outstanding properties.GaN devices have been applied to efficient blue and green light-emitting diodes(LEDs)and electronic devices with high frequency and high power for their long life,high stability and low energy consumption.Since the first demonstration of VO2/GaN based nitride-oxide heterojunction in 2012,it has been attractting extensive interest in materials field,as it may provide new opportunities for novel device architectures in solid-state electronics and opto-electronics,which combine desirable functional properties of both classes of materials.However,due to the huge difference in the growth conditions of oxides and nitrides,especially due to the complexity of the vanadium-oxygen correlation system(vanadium in vanadium oxide can exist with more than+2 to+5 valences),the study of high-quality VO2/GaN growth and its phase transition characteristics still faces severe challenges.The thesis focused on a series of researches on the growth of high-quality VO2 film,the interface of n-VO2/p-GaN based oxide/nitride heterojunction and the low-temperature growth of GaN film.The main research contents and results are as follows:(1)Thickness precisely controlled VO2 films were grown on sapphire substrates by RF molecular beam epitaxy(RF-MBE).For VO2 films with different thickness,the SMT behavior is studied by the same heating and cooling processes under exposure to ordinary fluorescent lamps and sunlight simulator AM1.5.Regardless of exposure to the sunlight simulator AM1.5,reversible SMT behavior was observed in all samples with a 5～6℃relaxation widths,which indicates that the VO2 films have high quality and phase purity.But a remarkably conflicting trend in resistance change for extremely thin and thick samples was observed after exposure to the sunlight soaking system.The corresponding mechanism was proposed based on sunlight-induced resistance changes due to the transformation in the electron correlation and structural symmetry.The results might be of special help to accurately control SMT phase transition characteristics to realize reliable device performance.Preferred orientation and high phase purity VO2 thin films with various thicknesses were grown on quartz glass(SiO2)substrates by RF-MBE,the resistance of the thin films varies by 2 orders of magnitude.The dependence of the SMT properties of VO2 film on film thickness may be attributed to strain relaxation with increasing thickness.VO2 films with superior crystalline quality and ideal chemical stoichiometry were grown on TiO2(001)substrate by RF-MBE.An excellent reversible SMT accompanied with an abrupt change in both resistivity and infrared transmittance was observed at room temperature(RT).Remarkably,the Tc deduced from resistivity-temperature curve was well consistent with that obtained from the temperature dependent IR transmittance.Based on Raman measurement and microstructure model,the lowed Tc was mainly attributed to the residual compressive stresses along the c-axis of VO2.(2)To construct high quality VO2/p-GaN heterojunction,VO2 films were deposited on commercial p-GaN/sapphire substrates(which were grown by metal organic chemical vapor deposition,MOCVD)with various methods,including PLD and RF-MBE.For the sample where the VO2 layer was grown by PLD,a distinct reversible SMT phase transition with resistance change up to nearly three orders of magnitude was observed in the temperature dependent electrical resistance measurement,which was comparable to the high quality VO2 film grown directly on sapphire substrates.The XPS analyses confirmed the valence state of vanadium(V)in VO2 films was principally composed of V4+with trace amount of V’+.Meanwhile,n-VO2/p-GaN based nitride-oxide heterojunction with distinct reversible SMT behavior were achieved,where the high quality VO2 films were grown by RF-MBE with precisely controlled thickness on nanometer level.The clear rectifying transport characteristics originated from the n-VO2/p-GaN interface were demonstrated before and after SMT of the VO2 over layer,which were attributed to the pn junction behavior and Schottky contact character,respectively.(3)To realize in-situ growth of VO2/p-GaN based nitride-oxide heterojunction,the low-temperature growth of GaN films was critically imperative.For this purpose,low temperature growth of GaN films was attempted by electron cyclotron resonance plasma enhanced metal-organic chemical vapor deposition(ECR-PEMOCVD)system on ITO glass、ordinary amorphous glass substrates with Cu、Ni、Ti、Ag metal intermediate layer,and Al substrate.Conclusions can be drawn as follows:Different metal buffer layers have a great impact on GaN film,intermediate layers by optimizing the process of metal buffer layer,GaN film samples with better c-axis preferential orientation are obtained by Ti,Ag,Al substrates,which have a larger grain size and a better surface flatness.(4)High-quality p-GaN films were prepared on self-supported AI substrates by ECR-PEMOCVD(Electron Cyclotron Resonance Plasma Enhanced Vapor Deposition)technique.The VO2 films were deposited on p-GaN/Al substrate by magnetron sputtering technique to establish VO2/p-GaN heterostructures.Comparing to the Al2O3 substrate,the integration of VO2/p-GaN heterojunction on metal substrate will help to realize vertical high power varistor switches using the SMT characteristics of VO2 film.A reversible SMT phase transition with resistance change up to nearly three orders of magnitude was observed.The clear rectifying transport characteristics originated from the n-VO2/p-GaN interface were demonstrated before and after SMT of the VO2 film,which were attributed to the pn junction behavior and Schottky contact character,respectively.In a word,several kinds of growth technique were employed for the growth of high quality VO2 thin films on sapphire,SiO2,TiO2(001),p-GaN/Al2O3 and p-GaN/Al substrate,and VO2/p-GaN based nitride-oxide heterojunctions were successfully constructed.The performance factors and related physical mechanisms of VO2 film and VO2/p-GaN heterojunctions were explored through the comprehensive characterization on crystal quality and electric optical properties.