From bare Ge nanowire to Ge/Si core/shell nanowires: A first-principles study (original) (raw)
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Properties of the bare, passivated and doped germanium nanowire: A density-functional theory study
Computational Materials Science, 2010
Using first-principles calculations based on density-functional theory, we systematically investigated the structural, energetic, and electronic properties of bare, hydrogen-passivated as well as doped germanium nanowires oriented along [0 0 1] direction. The bare nanowires with different diameters (from 5.65A˚to5.65 Å to 5.65A˚to19.96 Å) initially cut from the bulk Ge crystal in rod-like forms, are all metallic after relaxation. To better mimic the actual growth process, the hydrogen atoms were used to terminate the dangling bonds on the surface of bare wires which have been already relaxed, and subsequently the whole system containing H was relaxed again. Taking H-GeNW(57) as a prototype, we investigated both adsorption and substitution doping of wire by Al (group IIIA) and P (group VA) impurities. In the case of adsorption, the MO and the B b sites (see text for definitions) are the most preferential sites for Al and P, respectively, and P adatom also breaks its nearest Ge-Ge bond below to form a Ge-P-Ge bond. The impurity-induced band appears and crosses the Fermi level, making the wire become metallic. In the case of substitution, impurity atom substituted at the edge site was seen more favorable than that in the core site, and leads to usual p-type (Al) or n-type (P) behavior similar to the substitutional doped bulk Ge crystal. This behavior is reverse if impurity atom is adsorbed on the wire. Furthermore, for both adsorption and substitution of Ge nanowires, the charge density analysis indicates the formed Al-Ge or P-Ge bond mainly shows a covalent character, and the P-Ge bonding is much stronger than the Al-Ge bonding. The electronic properties of these doped germanium nanowires can be used in nanoscale device applications.
Electronic properties of strained Si/Ge core-shell nanowires
Applied Physics Letters, 2010
We investigated the electronic properties of strained Si/Ge core-shell nanowires along the [110] direction using first principles calculations based on density-functional theory. The diameter of the studied core-shell wire is up to 5 nm. We found the band gap of the core-shell wire is smaller than that of both pure Si and Ge wires with the same diameter. This reduced band gap is ascribed to the intrinsic strain between Ge and Si layers, which partially counters the quantum confinement effect. The external strain is further applied to the nanowires for tuning the band structure and band gap. By applying sufficient tensile strain, we found the band gap of Sicore/Ge-shell nanowire with diameter larger than ~3 nm experiences a transition from direct to indirect gap.
Strain-modulated electronic properties of Ge nanowires: A first-principles study
Physical Review B, 2009
We used density-functional theory based first principles simulations to study the effects of uniaxial strain and quantum confinement on the electronic properties of germanium nanowires along the [110] direction, such as the energy gap and the effective masses of the electron and hole. The diameters of the nanowires being studied are up to 50 Å. As shown in our calculations, the Ge [110] nanowires possess a direct band gap, in contrast to the nature of an indirect band gap in bulk. We discovered that the band gap and the effective masses of charge carries can be modulated by applying uniaxial strain to the nanowires.
Hole mobility in Ge/Si core/shell nanowires: What could be the optimum?
Applied Physics Letters, 2014
Recent experimental works have shown that Ge/Si core/shell nanowires (NWs) are very attractive for nanoelectronics and for low-temperature quantum devices, thanks to the confinement of holes in the Ge core. Reported hole mobilities of the order of 200 cm 2 /V/s are promising for high-performance field-effect transistors. However, we demonstrate that mobilities more than ten times higher, up to 8000 cm 2 /V/s, could be reached in Ge/Si NWs. Atomistic calculations reveal the considerable influence of the strains induced by the Si shell on the hole transport, whatever the NW orientation. The enhancement of electron-phonon interactions by confinement, which usually degrades the mobility in NWs, is therefore outbalanced by the effect of strains.
Si and Ge based metallic core/shell nanowires for nano-electronic device applications
Scientific Reports, 2018
One dimensional heterostructure nanowires (NWs) have attracted a large attention due to the possibility of easily tuning their energy gap, a useful property for application to next generation electronic devices. In this work, we propose new core/shell NW systems where Ge and Si shells are built around very thin As and Sb cores. The modification in the electronic properties arises due to the induced compressive strain experienced by the metal core region which is attributed to the lattice-mismatch with the shell region. As/Ge and As/Si nanowires undergo a semiconducting-to-metal transition on increasing the diameter of the shell. The current-voltage (I-V) characteristics of the nanowires show a negative differential conductance (NDC) effect for small diameters that could lead to their application in atomic scale device(s) for fast switching. In addition, an ohmic behavior and upto 300% increment of the current value is achieved on just doubling the shell region. The resistivity of na...
Journal of Physics: Condensed Matter, 2011
Strain modulated electronic properties of Si/Ge core-shell nanowires along [110] direction were reported based on first principles density-functional theory calculations. Particularly, the energy dispersion relationship of the conduction/valence band was explored in detail. At the point, the energy levels of both bands are significantly altered by applied uniaxial strain, which results in an evident change of band gap. In contrast, for the K vectors far away from , the variation of the conduction/valence band with strain is much reduced. In addition, with a sufficient tensile strain (~1%), the valence band edge (VBE) shifts away from , which indicates that the band gap of the Si/Ge core-shell nanowires experiences a transition from direct to indirect. Our studies further showed that effective masses of charge carriers can be also tuned by the external uniaxial strain. The effective mass of the hole increases dramatically with a tensile strain, while strain shows a minimal effect on tuning the effective mass of the electron. Finally, the relation between strain and the conduction/valence band edge is discussed thoroughly in terms of site-projected wave-function characters.
2010
Strain modulated electronic properties of Si/Ge core-shell nanowires along [110] direction were reported based on first principles density-functional theory calculations. Particularly, the energy dispersion relationship of the conduction/valence band was explored in detail. At the {\Gamma} point, the energy levels of both bands are significantly altered by applied uniaxial strain, which results in an evident change of band gap. In
Physica Status Solidi A-applications and Materials Science, 2015
Ge/Si core/shell nanowires are fabricated with narrow (20 nm) Ge core and thin (2 nm) Si shell. Ge nanowires are prepared by vapor-liquid-solid (VLS) chemical vapor deposition (CVD). The low-temperature (450 8C) process by using Si 2 H 6 gas as a Si CVD source is essential to form ultrathin layer of epitaxial Si film onto very narrow Ge nanowires. Accumulation of holes in Ge nanowires confined in the Si shell is confirmed by electrical measurement.
Doping of SiGe core-shell nanowires
Journal of Computational Electronics, 2012
Dopant deactivation in pure Si and pure Ge nanowires (NWs) can compromise the efficiency of the doping process at nanoscale. Quantum confinement, surface segregation and dielectric mismatch, in different ways, strongly reduce the carrier generation induced by intentional addition of dopants. This issue seems to be critical for the fabrication of high-quality electrical devices for various future applications, such as photovoltaics and nanoelectronics. By means of Density Functional Theory simulations, we show how this limit can be rode out in core-shell silicongermanium NWs (SiGe NWs), playing on the particular energy band alignment that comes out at the Si/Ge interface. We demonstrate how, by choosing the appropriate doping configurations, it is possible to obtain a 1-D electron or hole gas, which has not to be thermally activated and which can furnish carriers also at very low temperatures. Our findings suggest core-shell NWs as possible building blocks for highspeed electronic device and new generation solar cells.