The Mechanism Of Reflection High Energy Electron Diffraction And Precise Control Of Oxide-interfaces

The interface is the device.A variety of novel quantum phenomena exist at the oxide interface offering the potential for possible future devices which are based on the precise control of the interface qualities.Nowadays,with the application of in situ reflection high-energy electron diffraction(RHEED),people can grow very high quality and thickness-controlled oxide films,however,the precise control of termination/interface is still challenging due to the complicated oscillation patterns.Here,we report the unusual phase inversion and frequency doubling observed in the layer-by-layer growth of SrTiO3 using oxide molecular beam epitaxy.In contracts to the common understanding,we also found that the maximum and minimum intensities in the RHEED intensity oscillations are not necessarily corresponding to the complete growth of one layer.Here,we propose a new model to explain the observed phenomena and rules to guide the fabrication of full-controlled termination as well as the ultimate precise interfaces.Following the rules,we can fabricate two dimensional electron gas(2DEG)at the interface of LaAlO3 film and thick homoepitaxial SrTi03 film which has long been a challenge.Furthermore,oxide interfaces can be successfully grown on Si by using these rules,promoting the development of oxide interfaces in electronics applications.We have performed systematic experimental works and have had the following achievements:(1)We observe the strong dependence between RHEED intensity oscillations and the incident angle of electron beams,especially the phase inversion and frequency doubling,during the growth of SrTiO3;(2)Layer-by-layer variations of the mean inner potential is also observed during the growth of SrTiO3 by analyzing the Kikuchi lines;(3)A model including the effects of mean inner potential and surface roughness reveals the underlying mechanism of RHEED intensity oscillations and explains the phase inversion and frequency doubling phenomena;(4)We propose general rules to guide the fabrication of the ultimate precise interfaces.Then obtain 2DEG at the interface of LaAlO3/homoepitaxy SrTiO3(60u.c.)/SrTi03.Besides,we explore the Metal-Insulator transition of strong correlation Mott insulator LaTiO3 which is supposed to be driven by epitaxial compressive strain,however,the oxygen vacancies contribution is not excluded in previously works.Here,we use LSAT as substrate to grow LaTiO3 film due to its smaller lattice constant and difficulty to form oxygen vacancies.Then we grow LaTiO3 film on LSAT substrate under various oxygen growth pressures to explore the effect of strain and oxygen content.We find that controlling the growth oxygen pressure can adjust the valence of Ti atom and then adjust the transport characters of oxide films,which provide a new way to regulate the conductivity of oxide films.Some achievements are as follow,(1)With the increasing of growth oxygen pressure,there is a metallic,insulating and then metallic transition of LaTiOx film;(2)The Metal-Insulator transition of Mott insulator LaTiO3 is not likely to be caused by epitaxial compressive strain.In contrast to the theoretical prediction of the matellice ground state in strained LaTiO3 films,we grew high quality insulating LaTiO3 films grown on LSAT substrates;(3)Oxygen content maybe the possible explanation of the Metal-Insulator transition of Mott insulator LaTiO3.as the valence of Ti atom can vary in the films grown under different oxygen partial pressures.