The Study Of La/Si?111? Interfaces And Physical Properties Of Epitaxial LaSi₂ Films

Metal-semiconductor interfaces provide an ideal platform for the study of semiconductor material science,interfacial physicochemistry,low dimensional physics,film growth and superconductivity.The heterojunction formed by metal and semiconductor interfaces is the basis for semiconductor devices.There are many interesting phenomena emerging at the interfaces due to the formation and diffusion of defect,change of electronic structure,chemical reaction,etc.By the variation of metal deposition amount,the quantum phenomena can be observed in the metal systems in different dimensions.Even superconductors,in metal?superconductor?-semiconductor configuration,can exhibit novel physical charateristics,such as quantum size effect,the tunning of superconducting properties,etc.Therefore,metal-semiconductor interfaces are of great significance in the study of novel physics.In this dissertation,we mainly used molecular beam epitaxy,variable temperature scanning tunneling microscopy?STM?and spectroscopy?STS?,and first principle calculations,to study the reaction process and physical properties of the lanthanum?La?and silicon?Si?interfaces,including surface reconstruction,silicide film growth,electronic structures,and superconductivity.By depositing submonolayer La on the Si?111?7×7,which is hold at high temperature,the La reacts with Si,and a series of surface reconstructions form,varing with coverage.When the coverage is less than 0.2 ML(1 ML=7.8×10¹44 atoms/cm^-2,corresponds to the atom density of the bulk-truncated Si?111?surface),5×2reconstruction forms.When between 0.2 and 0.33 ML,?2m+1?×6 reconstrction forms,and when the coverage reaches 0.33 ML,the suface is coverd with 2×3reconstrction.These reconstrctions are quasi-one-dimensional reconstructions formed by Si honeycomb chain-channels?HCCs?or?-bonded Seiwatz chains?SCs?,stabilized by La.They are semiceonducting and satisfy the electron counting model.Increasing the coverage to 0.67 ML,the surface is covered with3^1/2×3^1/2 reconstruction.Through adjusting the substrate temperature,we find that high substrate temperature?630??is prone for the Si rich phase.Adding more La upon La/Si?111?-3^1/2×3^1/2 reconstruction,the formation of high-quality LaSi₂ films occurs.X-ray diffraction and low electron energy diffraction verify the[112]orientation of the films.Based on the analysis of lattice epitaxial relationship between LaSi₂?112?and Si?111?,the films suffer 2%tensile strain at the interface,forming strain stripes.The stripe space increases with thinckness increasing,and vanishes when the film thick enough,reflecting the release of the strain.Electric transport measurement and STS evidence the superconductivity of the LaSi₂ thick films.The superconducting transition temperature is 3.2 K and the superconducting gap is 0.9meV at 2.26 K,lying in the strong coupling regime.In the thickness dependent study,when the thickness is beyond 21 ML?“ML”is the single layer of LaSi₂?112?here?,the size of the superconducting gap maintains with thickness increasing,representing the intrinsic superconductivity of LaSi₂.When the thinckness is between 10 and 21 ML,the superconducting gap is enhanced with thickness increasing,consistence with the increase of strain,indicating the appropriate strain can enhance the superconductivity.When the thickness is less than 10 ML,the superconductivity is suppressed due to the suppression of the density of states at the Fermi energy,caused by the bonding and charge rearrangement at the LaSi₂/Si?111?interface.Besides,the strain leads to the spatial modulation of the superconducting gap.Take the 7 ML thick film as an example,the superconducting gaps are suppressed at the positions corresponding to the maximum and minimum of the stripes,while they appear clearly in the medium regions.In addition,we analysis the electronic structure of LaSi₂ from charge transfer?in real space?and band structure?in reciprocal space?perspectives.X-ray photoelectron spectroscopy and differential charge calculation show that the charge transfer between La and Si is weak.Hall resistance measurement gives the hole carrier type of LaSi₂.The calculated electron density of states show that the f orbital contributes the density of states at E_F.The band structures of LaSi₂?112?in experiment and calculations are compared.In calculation,there are flat bands below 0.033 an 0.55 eV at the?point,which provides a possibility for the further superconducting adjustment of LaSi₂.On the Fermi level at the M point,there is a linear band crossing.Orbital projected band displays the band contribution by s,p,d,and f orbitals,showing a weak orbital hybridization between La and Si.The flat band at-0.033 eV is mainly contributed by s orbital.These results reveal the particular electronic characteristics of LaSi₂.In conclusion,based on the La/Si?111? interfaces,we studied the interaction between rare earth metal and Si?111?detailedly,and provide a method for the growth of high-quality rare earth metal silicides or even heavy fermion materials.Meanwhile,the superconductivity of the films influenced by the interfaces is clarified from both lattice and electronic aspects.