Epitaxial Growth And Characterization Of α-Sn Single Crystalline Films

Tin is a regular metal with multiple phases.Compared to its more stable β 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.