Hf Intercalation In Epitaxial Graphene And The Characterization Of Two-Dimensional Topological Insulator HfTe₅

Topological materials are a new class of materials emerging in recent years.Compared with ordinary materials,topological materials have peculiar physical properties due to their non-trivial energy band structures.Graphene,as a topological semimetal,has been widely investigated for the past decades.Energy bands in graphene are linear near the Dirac points and the carriers can be described as massless Dirac fermions,which results in the ultrahigh mobility of graphene and makes graphene promising in applications of microelectronics.In theory,graphene can be treated as a two-dimensional（2D）topological insulator when considering the spin orbit coupling（SOC）.2D topological insulator is a kind of topological material which is insulating in bulk and has spin polarized edge states at boundaries.The edge states consist of two channels with opposite spin and motion direction,which is the feature of quantum spin Hall effect.Since the spin polarrized edge states of topological insulator will play a significant role in future spin devices,looking for 2D topological insulators with large energy gaps is an attractive topic in recent years.From the perspective of graphene and topological insulator,the intercalation at the interface of epitaxial graphene and the supported substrate,and relative properties of 2D topological insulator were investigated in this thesis.The main contents are divided into three parts.In the first part,the Hf intercalation between graphene and Ir（111）substrate was studied using first principles calculations.Theoretical calculations can provide important guidance for experiments and help explain experimental phenomena.Currently,the theoretical models of heteroatoms intercalation are limited in Si atoms.Considering the differences between Hf and Si atoms,and the advantages of Hf O₂ than Si O₂ with respect to dielectric properties,theoretical models of Hf intercalation are also needed.Based on the cooperative intercalation mechanism,the processes of creation of defects in graphene,Hf penetration through defects and interfacial diffusion of Hf atoms were calculated.The results showed that the Hf intercalation processes can simultaneously happen in the fcc and hcp regions of graphene/Ir（111）,since all the energies or barriers are similar for these two regions.The defect formation energies and diffusion barriers are around 0.30-0.59 e V,which are comparable to that in Si intercalation,indicating a mild annealing temperature.However,the penetration barriers are significantly high（2.14 e V）,which are probably induced by the large Hf atomic size.The high penetration barriers make Hf intercalation quite distinct from Si intercalation.As a result,Hf intercalation should be carried out with low deposition amounts and high annealing temperatures,to eliminate the influences of large penetration barriers and avoid the aggregation of Hf clusters on graphene surface and the degradation of graphene quality.This work provides the theoretical guidance for the future integration of epitaxial graphene and the high-K Hf O₂ dielectrics.In the second part,the Hf intercalation and subsequent oxidation between graphene and Ir（111）substrate were investigated using molecular beam epitaxy（MBE）method.Although graphene has been exfoliated for several years,the issue that how to achieve free-standing graphene has not been settled.Epitaxy on metal substrates is an important method to grow large-area and high-quality graphene,but the strong interactions between epitaxial graphene and metal substrates destroy the intrinsic electronic properties of graphene and limit its applications.For this reason,the intercalation technique is developed to effectively decouple the epitaxial graphene and metal substrates.Graphene-based heterostructures can be constructed and in situ interfacial gate oxide can be formed via intercalation technique.Due to the excellent dielectric properties of Hf O₂,the achievements of Hf intercalation and interfacial oxidation between graphene and metal substrates will be of great practical significance.With the help theoretical instructions,the Hf intercalation between epitaxial graphene and Ir（111）substrate was performed in a mild way.Low energy electron diffraction（LEED）and scanning tunneling microscopy（STM）experiments showed that the Hf atoms were packed into 2×2 ordered structures and the moirépattern of graphene was changed after Hf intercalation.The Hf atoms were all underneath the atop region of moiré-changed graphene,which lowered the total energy of the systems.On the other hand,internal stress was increased due to the change of moirépattern of graphene,which restricted the continued intercalation of Hf atoms to some extent.With the help of the restriction,well patterned nano-structures can be constructued with proper intercalation amounts of Hf atoms.The Hf intercalation processes play a critical role in the subsequent interfacial oxidation.In the optimization experiments of parameters of Hf intercalation processes,it is found that low deposition amounts and high annealing temperatures facilitate the sufficient intercalation of Hf atoms and favour the maintaince of high-quality graphene,consequently the interfacial Hf atoms can be oxidized to a high degree.As the deposition amounts decrease to a extremely low value and the annealing temperature increases to 1000℃,the the interfacial Hf atoms were highly oxidized.Amorphous hafnium oxide was formed between graphene and Ir（111）and graphene remained intact,which offered the opportunity for integration of graphene-based functional devices.In the third part,the structures and properties of 2D topological insulator HfTe₅were investigated.Theoretical predictions have been made that the layered material HfTe₅ is a topological insulator which possesses large band gap.However,so far,studies on HfTe₅ have been limited in the bulk regime or few layers on metal substrates.In this part,the epitaxial growth of monolayer and few-layer HfTe₅ was realized on highly oriented pyrolytic graphite（HOPG）via MBE method.The crystal structures and surface morphologies of monolayer and few-layer HfTe₅ were characterized by LEED and low-temperature STM.The topological edge states of monolayer and few-layer HfTe₅ were exploited using scanning tunneling spectroscopy（STS）,which turned to be robust regardless of defects,impurities,and adjacent phases.In addition,the band gap of monolayer HfTe₅ was about 127 me V,and the fermi level shifted downward.Importantly,the monolayer HfTe₅ reconstructed in the perpendicular direction of prism chains,which was different from the bulk phase and showed an effect on the electronic states.Furthermore,epitaxial growth of HfTe₅ on Nb Se₂ substrate was also realized,which offered the opportunity for research on topological superconducting based on the heterostructure of 2D topological insulator/superconductor.Single-layer HfTe₅ on Nb Se₂ substrate also showed similar structures and robust edge states.The differences were that the band gap of monolayer HfTe₅ on Nb Se₂ substrate was about 107 me V and the fermi level was in the band gap.The successful growth and detailed characterization of HfTe₅ enriches the family of 2D topological insulators and lays a solid foundation for the future fabrication of spin devices.Furthermore,the construction of HfTe₅/Nb Se₂heterostructure paves an important way to the future research on topological superconducting.