Preparation And Scanning Tunneling Microscopy Studies Of Bi(110) Films

Due to the novel physical properties produced by the special energy band,the two-dimensional topological insulators became a hot topic in the field of condensed matter physics.Especially,it takes a vital role in the study of possible Majorana fermion in two-dimensional topological insulators.Since Bi（111）films were proved to be topological insulator by theory and experiments,the relative research on the topological properties of Bi（110）films remains to be explored.Here,high quality Bi（110）thin films were prepared by using Molecular Beam Epitaxial technology,and the morphology and electronic properties of the samples were studied by Scanning Tunneling Microscopy.The main research contents and results are as follows:1)Atomically flat Bi（110）films were grown on the substrate of S-wave superconductor NbSe₂ by using Molecular Beam Epitaxy.Combined with the monitoring of Reflection High Energy Electron Diffraction and Scanning Tunneling Microscopy,the optimal growth conditions was determine to be room temperature and deposition velocity（²4℃,⁰.33BL/min）.Based on the morphology of Bi（110）films obtained through Scanning Tunneling Microscopy,the films were found to have a growth model transformation from bilayer to monolayer.Due to the great coverage of Bi（110）film on the NbSe2 substrate,it was almost impossible to find exposed areas of NbSe2 substrate surface in practice.Especially in the case of growing multiple layers,it was hard to directly define the number of film layers.Here,the critical thickness was estimated by controlling the growth time and growth speed combining with the measurements about stage height of the film.And the experiments found that the growth mode transformed from bilayer to monolayer nearly after 4BL films cause by the oscillation of the surface energy.This conclusion also fit well with the previous theoretical calculation about surface energy of Bi（110）thin films.2)The electronic properties of Bi（110）thin films were studied by Scanning Tunneling Spectrum.The measurements showed that the Bi（110）films grown on the NbSe₂ substrate had obvious superconducting properties due to the proximity effect.The superconducting energy gaps of thin films were very close（Δ≈0.5meV）,and no obvious attenuation was found as the thickness increased.This is most likely because,the exponential decay model of superconducting proximity effect changes gently over the initial layers.Also,by comparing the tunneling spectra（d I/dV）on different layers,there was no different local density of states found under either monolayer or bilayer growth modes.The corresponding peak and intensity are almost the same.In other words,the topological edge state was not observed experimentally at Bi（110）films.This result was consistent with previous theoretical calculations by Wada et al.from Tokyo Institute of Technology in Japan.In addition,the periodic quantum well state characteristics of different layers discussed here was worth exploring further.The spectral morphology between BL and ML was similar,and the STS spectrum of the adjacent film has an approximateπphase displacement.These experimental results are similar to those of Pb films grown on Si（111）substrate at low temperature which was produced by Physics Institute of Chinese Academy of Sciences.This discovery provides a new research direction for further exploration of physical properties of Bi（110）thin films.