In Doping Of First-principles Study Of Low-dimensional Germanium Based Nanomaterials

Germanium (Ge), a tranditional semiconductor material, has been used for various applications, such as infrared detector, optical fiber, high-speed microelectronic devices, integrated circuits and thermoelectric material etc., due to its excellent physics and chemistry properties as well as good compatibility with silicon-based materials. With the rapid development of science and technology, the electronic devices need to be high-integrated, multi-functionalized and miniaturized. At the same time, nanoscale devices with low dimensions and small sizes have gradually become the new trend for the development in fields of microelectronics and optoelectronics. The exciton Bohr radius of Ge is far greater than that of silicon, indicating that obvious quantum size effect will appear in Ge. Therefore, Ge-based nanomaterials should posses much more unique properties.One-dimensional (1D) Ge-based nanomaterials have excellent photoelectric characteristics thus show a well prospect in applications of high-performance field-effect transistors (FETs), biochemical sensors, logic gates and efficient solar batteries. Recently, a lot of theoretical and experimental researches have been devoted to the synthesis, microstructure characterization and physicochemical characteristics analysis of1D Ge nanomaterials and finally achieved many breakthroughs. Nevertheless, the property-modification for1D Ge-based nanomaterials through hetero atom remains to be less exploited, thus needing to be systematically investigated and deeply understood. In this study, using first-principles projected-augmented wave (PAW) method, we pay special attention to the microstructure, stability, electronic and magnetic properties of the perfect, defected, adsorbed and doped Ge nanowires/nanoribbons. The principal conclusions are shown as follows:(1) Al and P adsorption on Ge nanowires. Using first-principles calculations, the electronic properties of Al and P adsorbed Ge nanowires with different configurations and concentrations have been extensively studied and the corresponding results been compared with those of Al and P doped nanowires. Al adatom prefers to bind on the hollow site surrounded by a surface octagon ring (MO) of Ge nanowire and does not break the adjacent Ge-Ge bond after adsorption, while P adatom prefers to bind on the bridge site shared by two surface pentagons (Bb) of Ge nanowire, and breaks the beneath Ge-Ge bond to form a new Ge-P-Ge bond. At higher adsorption concentration, an impurity-induced electronic band appears and crosses the Fermi level, thus resulting in a "semiconductor-metal" transition in Ge nanowires. With decreasing of Al (P) adsorption concentration, the impurity-induced electronic band becomes flat and eventually locates below (above) the Fermi level, thus leading a "metal-semiconductor" transition in Ge nanowires. In Al (P) adsorbed wire, the impurity-induced electronic band is located closer to conduction band (valence band), which does not follow the traditional acceptor (donor) mechanism in p-type (n-type) doped semiconductors. Such a reverse behavior is explained by the fact that the adatom is simply adsorbed on the surface of the wire, thus only interacts with the unoccupied electronic states from the dangling bonds of surface Ge atoms. In addition, the formed Al-Ge bonds mainly display a covalent bonding character, while the formed P-Ge bonds display both covalent bonding and ionic bonding characters.(2)3d transition-metal (TM) atoms adsorption on Ge nanowires. The microstructure, stability, electronic and magnetic properties of ten kinds of3d TM atoms adsorbed Ge nanowires have been investigated by spin-polarized first-principles PAW method. Except that Sc prefers to bind on the top site of the mutual Ge atom which belongs to the surface pentagons and hexagons of the wire, other TM atoms all prefer to bind on the hollow site of surface hexagon (HH) of the wrie. The variation trend of binding energies with'd'electron number agrees well with that of3d TM atoms adsorbed (8,0) carbon nanotubes, and by comparison, all TM atoms also form stronger bonding on Ge nanowires than on carbon nanotubes. Good conducting metals, such as Cu and Zn, can form weak bonding with the wire, whereas those such as Ti, V, Fe, Co and Ni have relative larger binding energies. Various types of wires can be obtained depending on the adatom species, including nonmagnetic (NM) metals (Sc or Cu adsorption) and semiconductors (Ni or Zn adsorption), weak ferromagnetic (FM) metals (Ti or V adsorption), FM semiconductors (Cr adsorption) and more interesting the FM half-metals (Mn, Fe or Co adsorption) which have potential application in spintronics. The magnetism of these wires originates mainly from spin-split of the TM-3d states, and the TM atom also induces some anti-parallel charge density around its adjacent Ge atoms. Furthermore, using DFT+U method, we also considered the effect of on-site Coulomb interaction on the stability of the three FM half-metallic wires and found the half-metallic ground state of Mn-or Co-adsorbed wire is more robust than that of Fe-adsorbed one.(3) The study of two-dimensional (2D) honeycomb Ge, perfect and defected Ge nanoribbons. The microstructure, stability, electronic and magnetic properties of2D honeycomb Ge, perfect and defected armchair Ge nanoribbons (AGeNRs) and zigzag Ge nanoribbons (ZGeNRs) have been studied in detail by using first-principles PAW calculations. The stable2D honeycomb Ge sheet is slightly buckled and shows semi-metallic character. Its electron and hole bands linearly across at the Fermi level thus the carriers behave like "massless" Dirac fermion near the K point in the Brillouin zone (BZ). The perfect AGeNRs are NM semiconductors with their band gaps exhibit a periodically oscillatory damping as the ribbon width increases, thus making AGeNRs to be classified into three types. The perfect ZGeNRs have stable antimagnetic (AFM) semiconducting ground state with their band gaps monotonously decrease as ribbon width increases. Their net spin charge densities are mainly localized at the edge Ge atoms and contributed by π/π*electronic states, and the spin states at opposite edges have different spin orientations. The band gaps of AGeNRs can be efficiently tuned by atomic defect (vacancy or di-vacancy) at different positions though no magnetism is introduced. The ZGeNRs can become AFM or FM metals by introducing atomic defect, thus can be well used in electronic conduction and spin storage.(4) TM (Cr, Mn, Fe and Co) atoms adsorption on2D honeycomb Ge and AGeNRs. The results indicate that, all TM atoms considered prefer to adsorb on the hollow site of hexagon of2D Ge whether in single-sided or double-sided adsorption cases, and NM semi-metallic2D Ge finally changes to be either FM or AFM metals depending on both TM species and coverage. For AGeNRs, the most preferential adsorption site is the hollow site of hexagon at the ribbon edge. Except for Co adsorption remaining NM state, Cr-, Mn-and Fe-adsorbed AGeNRs all possess FM state or AFM state according to ribbon width, TM species and coverage. Through Cr or Mn adsorption, some AGeNRs can also become FM or ferrimagnetic (FIM) half-metals. Moreover, considering the effect of on-site Coulomb interaction, we found the half-metallic ground state of Cr-adsorbed ones is more robust than that of Mn-adsorbed one thus can be suitable for spintronic devices.(5) The stability, electronic and magnetic properties of N, B doped and co-doped AGeNRs and ZGeNRs. Our first-principles calculation results show that, for both AGeNRs and ZGeNRs, edge Ge atoms are always easy to be substituted. Single N-doping and single B-doping can introduce a "semiconductor-metal" transition in AGeNRs, while N and B co-doped AGeNRs also remain its semiconducting character due to the effective charge compensation. Single N-doping or single B-doping usually makes AFM ZGeNRs to be FM semiconductors, and the "AFM-FM" transition originates from the perturbation of π/π*electronic states which localized at the ribbon edges. Some single impurity doped ZGeNRs also exhibit half-metallic properties. Double atom substitution (regardless of N-N, B-B, and N-B configurations) at the edges of ZGeNRs removes the spin-polarization at both edges and transforms them into NM semiconductors. Overall, N, B doped and co-doped AGeNRs and ZGeNRs have potential applications in Ge-based nanoelectronic devices, such as FETs, negative differential resistances (NDR) and spin filters (SF) etc.