Study On Structures And Electronic Properties Of Low Dimensional Silicon And Germanium Materials Based On DFTB Calculations

Low dimensional silicon and germanium materials are important candidates for the preparation of nano-electronic devices,and are the basis materials to develope novel generations of nano-electronic devices with high efficiency,low energy consumption and ultra-high speed.The excellent performances of low dimensional silicon and germanium materials have inspired interests in the researchers of the structures and properties of these materials.In order to understand physical and chemical properties of these low dimensional materials,this paper focus on atomic arrangements,stabilities,and electrical properties of these low dimensional materials including a zero-dimensional system(clusters),a one-dimensional system(nanowires(nanoribbons)),and a two-dimensional system(Si thin film)by using density functional tight binding(DFTB)approach,and the following studies are given.First,the geometric structures,stability and electronic properties of binary SixGey(x+y ?2?9)clusters are studied using the genetic algorithm(GA)and DFTB method.The results show that the size and composition of the clusters are main factors to determine the geometric structures.Starting from 5 atomic clusters,the structures are changed from flat polygon to bipyramid,and then forming accumulation structures of tetrahedron.In the clusters including 6 atoms,the structures between pure silicon and germanium clusters present differences.For the clusters including 7 atoms,the geometric structures of the binary clusters depend on the germanium composition.The different interaction strength of the Si-Si bond,Si-Ge bond and Ge-Ge bond is the main factor to determind the different positions of the Si atoms and Ge atoms of the binary clusters.The clusters including 9 atoms have lots of isomers.The size and composition of the clusters also affect their stability.The composition of the binary cluster plays an important role in energy gap changes.It can make energy gap value sudden rise or drop,or may also make its energy gap value be higher or lower than pure clusters.The charge on each atom in the binary clusters always transfer from the Ge atom being far from the center of the cluster to the Si atom being closer to the center of the cluster.Second,the structure,stability,and charge distribution of the silicon nanowires(nanoribbons)with their widths in the range of 0.272 nm?1.360 nm and the germanium nanowires(nanoribbons)with their widths in the range of 0.425 nm?0.851 nm and with the same thickness are studied by DFTB algorithm.Results show that the silicon nanowires(nanoribbons)with different widths present different patterns in the surface regions.One pattern is that the atoms have regular arrangement in dimers along the<110>direction,the other pattern is the interval appearance of dimers and trimers.Silicon nanowires(nanoribbons)present periodical arrangement along the extension direction;the changes of the nanowires(nanoribbons)widths make the length of the repetitive structure unit change.The germanium nanowires(nanoribbons)with different widths present regular arrangement in dimers along the<110>direction in the surface region.The repeating structural unit length is consistent with that in the germanium crystals.The cross sections of silicon and germanium nanowires(nanoribbons)appear similar spindle structures.The width changes of silicon and germanium nanowires(nanoribbons)have obvious influence on the bond length and bond angle.The changes of the surface structures are important to the surface charge distribution.The studies also find that the atomic charge transfer values in the nanowires(nanoribbons)are closely related to the distance of the atoms moving from the surface.The widths of the nanowires(nanoribbons)and atomic arrangements in the surface regions are the main factors to affect their stability.Finally,the structure,stability,and charge distribution of silicon thin films with different thicknesses in the range of 0.272 nm?1.494nm are studied by DFTB algorithm.Results show that the silicon thin films with different thicknesses present three patterns in the surface regions:the first is that the atoms have regular "Zigzag" arrangement of dimers along the<110>direction;the second pattern is the parallel trimers array along the<110>direction;the third pattern is that the appearance of dimers and trimers at the same time.The atomic rearrangement structures of the surfaces show some similarities as the thickness of the silicon thin film periodically increases.The thickness of silicon thin film is thinner,the range of the bond angle is larger.and the rearrangements of the surface greaterly influence the arrangement of the subsurface atoms.The dimers ratio on the arrangement surface of the silicon thin film is the main factors to affect its stability.The most stable structure of silicon thin film surface is the regular "Zigzag" dimer arrangement.The atomic charge transfer values of the silicon thin film surface are closely related to the distance of the atoms moving from the surface,and the charge transfer direction present regularity.