| Tin is a regular metal with multiple phases.Compared to its more stable ? phase,the research on a-Sn is much less due to its low thermal stability.Recently,with the tremendous attention to quantum computing,there has been a fast-growing interest on materials that host topological surface state?TSS?,for example,topological insulators?TIs?and Dirac semimetals?TDs?.TSS is not only a new physical concept,but also new phenomenon and effect in fundamental physics study,and it will play an important role in next generation electronic technology.Recently a-Sn film has been predicted to be either TDS or TIs under a suitable strain,making it is a perfect material platform for TSS research.a-Sn is a metastable phase whose phase transition temperature is only 13.2 ?,and due to the lack of appropriate substrate,it has been challenging to get high quality epi-films.In this study,the focus is to grow single crystalline a-Sn films by molecular beam epitaxy?MBE?and characterize the film properties.The major results and achievements are summarized as follows:?1?Using a low growth temperature,we have performed Sn growths on several substrates,including Si,GaAs and InSb.And finally we have successfully obtained a series of a-Sn films with different thicknesses up to 400 nm.X-ray diffraction?XRD?and reciprocal space mapping?RSM?revealed a high crystalline quality,and RSM on 400 nm thick sample shows the epi-film is still fully strained.?2?Temperature dependent XRD for the 20 nm a-Sn epi-film shows the phase transition temperature has been raised from 13.2 ? to 120 ?,and it is the highest phase transition temperature which has been reported so far.This property makes the following characterizations possible.?3?We have obtained the coefficient of thermal expansion for a-Sn epi-film,to be 7.14×10-6/? from the temperature dependent XRD measurements.This value is higher than the reported 4.7×10-6/? in bulk.It indicates that in-plane thermal expansion of the epi-film is restricted by the substrate,so the thermal expansion is bigger in the out-of-plane direction.?4?Cross-sectional transmission electron microscopy?TEM?,is used to observe the clear interface of the a-Sn epi-film and the substrate and confirms the crystalline quality of the epitaxial film.?5?In electrical transport study,we have observed giant magneto-resistance and superconductivity in our a-Sn epi-films.More measurements and mechanism study are still ongoing.In summary,by appropriate substrate selection and growth condition optimization,we have obtained a-Sn epi-films with different thicknesses up to 400 nm.Structural characterizations confirm the crystalline quality.Through strain engineering,we have successfully raised the phase transition temperature by 100 ?,and obtained the linear coefficient of thermal expansion.In addition,we have observed giant magneto-resistance and superconductivity in a-Sn epi-films for the first time.This study on a-Sn epi-films will play an important role in future research on TSS and electromagnetic properties. |
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