Study On Wave Optical Theory And Experimental Verification In Low Energy Electron Microscope

Low energy electron microscopy(LEEM)is a cutting-edge technique to characterize the surface,interface and low dimensional materials in situ and in real time with vertical atomic resolution and single-layer sensitivity.It is capable to study the growth kinetics and physical and chemical reactions in both real and reciprocal spaces.Contrast mechanisms in LEEM conventionally fall into two categories: diffraction contrast and phase contrast.Understanding the mechanisms of imaging and contrast in low energy electron microscopy provides theoretical foundations for surface research.With the three-prism aberration-corrected low energy electron microscope in Chongqing University,we first observed the high order Fresnel diffraction of atomic step in LEEM,and first experimentally verified the theoretical prediction of step contrast in Fourier optics and contrast transfer function theory.We analyze the imaging and contrast formation with different wave-optical models,and focus on the phase contrast study by contrast transfer function(CTF)theory.We systematically investigated the influence of contrast aperture size,chromatic envelope function,defocus and source extension on the intensity profile of atomic step in LEEM.Our results can be summarized as following:(1)The high order Fresnel diffraction of atomic step is a universal phenomenon.It can be observed not only within the atomic terraces,but also on both sides of scratches on polycrystalline Cu foil,manifesting itself as the high order Fresnel diffraction fringes.It provides the direct experimental proof of reflection low energy electron holography microscope.(2)There exists a threshold value of cutoff spatial frequency defined by contrast aperture.The chromatic envelope function will broaden the intensity profile and suppress high order Fresnel fringes.High defocusing value will amplify the effect of source extension on step contrast;(3)The experimental data was fitted with CTF theory and the results agree very well,which shows the validity of CTF theory in step contrast analysis in LEEM;(4)The angle spread of our cold field emitter was determined as 0.05 mrad.There is a linear relation between excitation current of objective lens and defocus.We can reconstruct the atomic step height and in principle,we can extract all the aberration coefficients of our instrument by analyzing the experimental near the in-focus conditions.