Preparation Of Layered Perovskite Oxide Thin Films And Characterizations On Interfacial Two-dimensional Electron Gas

In recent years,perovskite-type oxides have been a research hotspot in the field of condensed matter physics and materials science.When perovskite-type oxides become thin films at a lower scale,they can be subjected to stress,size effect,and surface inter-face control,etc.,inducing various physical properties,such as LaAlO3/SrTiO3(LAO/STO)system.At LAO/STO interface,both two dimensional electron gas(2DEG)and superconductivity have been explored,however,the cause of the interfacial 2DEG is still under debated up to now.The existing theoretical models accounting for 2DEG origin include the "polarization catastrophe",oxygen vacancy,and the interface inter-mixing.An in-depth exploration of interfacial property and its mechanism will largely depend on the precise control over sample stoichiometry,but when using molecular beam epitaxy(MBE),its traditional method of flux calibration is complicated and time-con-suming.In this dissertation,we started with growing homoepitaxial SrTiO3(STO)thin films,and developed an improved and efficient method of calibration.Based on the new co-deposition calibration method,high-quality layered oxide(Ruddlesden-Popper type)Srn+iTinO3n+i(STORP)thin films and LAO/STORP heterostructures are fabricated.With changing index n for layer tuning,we further investigated the effects of oxygen vacancies on the interfacial 2DEG.Main achievements are listed below:1)Growth of homoepitaxial STO films using new substrate using the improved calibration method.We utilized reflection high energy electron diffraction(RHEED)spectroscopy to monitor the intensity of the(11)diffraction peaks,and it was found that the RHEED intensity oscillation trend during co-deposition growth can instantly reflect the current stoichiometric ratio:the rising(falling)trend corresponds to Sr(Ti)excess,the magnitude of the trend change corresponds to the current stoichiometric offset.By avoiding possible disturbance on intensity oscillation from glance angle and the extra calibration of oscillation beating pattern,the new co-deposition method bridges effi-ciency and precision:X-ray diffraction(XRD)results showed that the calibrated stoichi-ometric deviation was around 0.2%,and the entire calibration process took only 0.5h.2)Growth of Srn+1TinO3n+1(STORP)thin films.STO was firstly calibrated using new co-deposition method to get a balanced flux ratio as well as shutter times derived from the cycle period.Based on that,a series of recipes were designed corresponding to RP samples' lattice along[001]direction.The XRD and atomic force microscope(AFM)results showed that the(n=1,2,6)STORP films all have high lattice quality as well as flat surface,delivering the superiority of new method in fabricating complex oxides.3)Growth of a series of L AO/STORP heterostructures with different index n.XRD and AFM characterization results demonstrated a high lattice quality and sharp surface.In the subsequent transport measurements,metallic interface was found for n=6 sample,but it recovered to insulating phase after annealing,indicating oxygen vacancies may contribute most to interfacial carriers;in contrast,no hints of two-dimensional electron gas was detected for n=2-5 samples.Our experimental results of LAO/STORP hetero-structures indicate that oxygen vacancies may account for the main cause of 2DEG at the LAO/STORP heterojunction interface.