Fabrication And Electrical Properties Of Tin Islands On Superconducting Substrate NbSe₂

In this thesis,Sn island was successfully grown on superconductor substrate NbSe₂ by molecular beam epitaxy?MBE?method.The morphology and crystal structure of growth and phase transition were studied in detail.The electric properties and superconducting proximity effect of Sn grown on conventional s-wave superconductor NbSe₂substrates have been studied by scanning tunneling spectrum?STS?.The two main experimental results are as follows:1.Flat hexagonal Sn islands was successfully prepared on NbSe₂ substrate by molecular beam epitaxy?MBE?.The growth and surface morphology were studied by scanning tunneling microscope?STM?,and clear atomic resolution images were obtained.Based on the analysis of the lattice structure and the comparison of lattice constants calculated in theory,and according to the height of the island,the face-centered cubic?fcc?structure of Sn is deduced.Two surface structures,1×1 and 2×2,were found on the island surface by atomic resolution images and its fast Fourier transform images.The irregular bright grain structure was observed on the island,and it is supposed to be related to the intersection of two surface structures.By comparing the scanning tunneling spectra between the bright areas and non-bright areas,we have not found any difference in the electronic states.And by scanning the STS across the steps,we compare the spectra of the step edges with those of the wetting layer,and compare the spectra between the adjacent three boundaries,no edge state was observed in this sample.In addition,we detected the superconducting properties of the Sn island under 400 mK.Superconductivity in fcc-Sn islands is very strong.Compared with the superconducting gap of the substrate,the size of the gap obtained on the wetting layer and the Sn island has almost no attenuation.One of the reasons is because the samples are relatively thin?0.8nm?,on the other hand,superconducting proximity effect of the NbSe₂ is relatively strong.2.After the sample were placed in ultra-high vacuum environment at room temperature for a period of time,the samples were scanned again.The obtained surface morphology showed many different height and lager size of Sn islands,and they become rectangle.We have studied it in detail and found that the surface structure is no longer hexagonal,and turns into tetragonal.And a periodic stripe structure appears in the surface.By studying and analyzing the lattice structure of the surface,it is determined that the surface lattice is ?-Sn?001?surface,and the surface lattice becomes larger under the stress.In addition,the stress also causes the subsurface atoms to move upward and they differ in height compared with the surface atoms,forming a overall striped structure.Also at 4.2 K,the electronic structure of?-Sn?001?island at different heights was studied by scanning tunneling spectroscopy?STS?.There are some LDOS peaks on the STS spectrum,and the number of peaks increases with the height of the island,which are quantum well states?QWS?.Superconductivity was also detected on the?-Sn?001?island.