Epitaxial Growth And Physical Properties Of Low-Dimensional Germanium

Monolayer silicon（Si）and germanium（Ge）have similar electronic properties to graphene referring to silicene and germanene.Both silicene and germanene are compatible with the modern silicon-based semiconductor industry.Compared with silicene,germanene has higher carrier mobility,which is expected to be used as an electronic device with better performance in the future microelectronics industry.The controllable preparation of germanene has been a hot research topic in recent years.At present,the preparation of germanene on different substrates has been reported.However,the growth mechanism of germanene is still far from be understood,and even the preparation of germanene on some substrates is still controversial.In particular,we need to pay attention to whether the germanene prepared on the substrate can preserve the Dirac electronic feature similar to that in graphene or not.Therefore,it is necessary to carefully study the growth of Ge on different substrates,understand the growth mechanism of Ge,and explore its structure and electrical properties.In this thesis,we employ ultrahigh vacuum molecular beam epitaxy（MBE）method to grow Ge on Ag（111）and InSe surfaces.The following contents are studied by combining scanning tunneling microscopy（STM）and scanning tunneling spectroscopy（STS）:（1）The growth morphology and electronic structure of germanium on Ag（111）surface were analyzed.The results show that at a low deposition flux,Ge will form two-dimensional islands with irregular shapes similar to fractal structures on Ag（111）substrates.Upon increasing the deposition flux,we found the irregular-shaped islands convert to compact 2D islands,and the differential conductivity spectrum of which shows a V-shaped character near the Fermi surface revealing a signature of 2D Dirac material.Furthermore,we found that Ge always grows on the Ag₂Ge surface alloy.Meanwhile,density functional theory（DFT）calculations show that only Ge on the double-layer Ag₂Ge surface alloy has the V-shaped density of state near the Fermi level.Thus,germanene was successfully synthesized on a bilayer of Ag₂Ge surface alloy supported by Ag（111）surface.（2）The growth mechanism and structural evolution of Ge on non-metallic InSe substrate were studied.The results of STM characterization showed that Ge grows in cluster mode after deposition.After proper annealing treatment,although the number of Ge clusters are reduced,the surface is not smooth,and there is a height difference between the substrate and the defect after deposition.Ge atoms tend to form cluster structure,rather than becoming thin films.Upon increasing the annealing temperature,all the Ge clusters disappeared and formed a bright and dark region on the surface,showing a uniform and ordered InSe periodic structure.According to the STS experiment results,Ge grows in clusters on the surface of InSe.After annealing,some Ge clusters intercalated from the defect,and someones gathered to form irregular aggregates.The results provide a scientific basis for the controllable preparation of related low-dimensional germanium structures.