Silicon Germanium Nanowires Research Articles

Fabrication of high-density Si and SixGe1−x nanowire arrays based on the single step plasma etching process

Dense arrays of silicon and silicon germanium nanowires are fabricated using a top–down approach, which exploits the excellent patterning capabilities of inductively coupled plasmas. Using standard deep UV lithography on a previously deposited silicon oxide hard mask, silicon nanowires with straight and smooth sidewalls and a high aspect ratio greater than 60:1 can be obtained with SF6/O2/HBr/SiF4 plasma chemistries. The best results are obtained using Cl2/N2 high-density plasmas to pattern Si0.5Ge0.5 nanowires with an aspect ratio of 10:1.

Thermal transport in crystalline Si/Ge nano-composites: Atomistic simulations and microscopic models

Thermal transport in Si/Ge nano-composites, consisting of crystalline silicon as matrix and aligned germanium nanowires as inclusions, is investigated here through non-equilibrium and equilibrium molecular dynamics (MD) simulations. Our results show increasing of temperature gradient at the interface between silicon and germanium, which is limited in an interfacial phase of few lattices. A thermal boundary phase is included explicitly in our three-phase model, in companion with the modified effective medium theory, to be compared with MD simulation results with various nanowire concentrations. The results suggest that the presence of nanowires leads to a dramatic decrease of κ for heat transfer across nanowires arising from interfacial phase, while along the interfaces, the reduction of phonon mean free path due to interfacial scattering lowers κ of silicon matrix and germanium nanowires.

Comparison Between the Performance of Silicon Nanowire, Germanium Nanowire and Carbon Nanotube Junctionless Transistors from First Principle Calculations

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Composition and Size Effects on the Optical Properties of Isolated Silicon-Germanium Nanowires

ABSTRACTSilicon and Germanium nanowires (NWs) have shown a strong ability to enhance both the absorption and scattering of light. Tailoring the optical properties of Si or Ge NWs can be obtained by adjusting the nanowire diameter. Another parameter that can be used is the chemical composition of silicon-germanium (Si1-xGex-NWs) alloys. In this work, we perform a numerical study on the optical properties of single Si1-xGex-NWs based on the Lorenz-Mie theory. The effects of Ge composition, light polarization and angle of incidence on the nanowire optical properties are investigated.

Spin dephasing in silicon germanium (Si1−xGex) nanowires

We study spin polarized transport in silicon germanium nanowires using a semiclassical monte carlo approach. Spin depolarization in the channel is caused due to D'yakonov-Perel (DP) relaxation associated with Rashba spin orbit coupling and due to Elliott- Yafet (EY) relaxation. We investigate the dependence of spin dephasing on germanium mole fraction in silicon germanium nanowires. The spin dephasing lengths decrease with an increase in the germanium mole fraction. We also find that the temperature has a strong influence on the dephasing rate and spin relaxation lengths increase with decrease in temperature. The ensemble averaged spin components and the steady state distribution of spin components vary with initial polarization.

Band structure analysis in SiGe nanowires

Abstract One of the main challenges for Silicon-Germanium nanowires (SiGe NWs) electronics is the possibility to modulate and engine their electronic properties in an easy way, in order to obtain a material with the desired electronic features. Diameter and composition constitute two crucial ways for the modification of the band gap and of the band structure of SiGe NWs. Within the framework of density functional theory we present results of ab initio calculations regarding the band structure dependence of SiGe NWs on diameter and composition. We point out the main differences with respect to the case of pure Si and Ge wires and we discuss the particular features of SiGe NWs that are useful for future technological applications.

Single-Electron Transport through Semiconducting Nanowires: A Comparison between Silicon and Germanium

Single-electron transistors (SETs) have been fabricated with n-type monocrystalline silicon nanowires (SiNWs) and germanium nanowires (GeNWs) separately. Comparisons of the single-electron transport characteristics between them have been studied at low temperatures. Coulomb oscillations of both devices were found with almost equidistant peak spacing and largely varied peak heights, while charge-stability diagrams showed almost identical diamond-shaped dimensions. Noticeably, for the GeNW-SET, the spacing between neighboring Coulomb-oscillation peaks alternately changed in some gate-voltage (Vg) regions, indicating the even–odd effect. On the contrary, the SiNW-SET did not show such effect in the entire measured Vg region. The comparison indicates that the quantum effect is much more prominent in the GeNW-SET than in the SiNW-SET.

Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results.

Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1) silicon and germanium nanoparticles embedded in wide band gap materials and (2) mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole.

Large thermoelectric figure of merit in Si1−xGex nanowires

By using first-principles electronic structure calculation and Boltzmann transport equation, we investigate composition effects on the thermoelectric properties of silicon-germanium (Si1−xGex) nanowires (NWs). The power factor and figure of merit in n-type Si1−xGex wires are much larger than those in their p-type counterparts with the same Ge content and doping concentration. Moreover, the maximal obtainable figure of merit can be increased by a factor of 4.3 in n-type Si0.5Ge0.5 NWs, compared with the corresponding values in pure silicon nanowires (SiNWs). Given the fact that the measured ZT of n-type SiNW is 0.6∼1.0, we expect ZT value of n-type Si1−xGex NWs to be 2.5∼4.0.

Study on Transport Characteristics of Silicon-Germanium Nanowire MOSFETs with Core–Shell Structure

Study on Transport Characteristics of Silicon-Germanium Nanowire MOSFETs with Core–Shell Structure

The impact of erbium incorporation on the structure and photophysics of silicon–germanium nanowires

In this paper, we report multi-step processes for the fabrication of Er3+-doped SiGe nanowires (NWs) and characterization of their emissive properties. Three different alloyed architectures are obtained by altering the deposition sequences of Si and Er3+ on a Ge core NW, each involving a fixed concentration of these three elements. The deposition of Si onto the Ge NW core, followed by an Er3+-rich layer on the outermost surface, permits facile formation of a SiGe alloy given the lack of an erbium diffusion barrier; yet clustering of the erbium centers on the NW surface produces the weakest emitter. For nanowires prepared by co-depositing Si and Er3+ on top of the Ge core, the presence of impurity Er3+ ions greatly reduces the alloying rate of Si and Ge such that less Si can diffuse into the Ge core. For this structure, the reduction of Er-Er interactions by a polycrystalline Si shell results in the strongest emission at 1540 nm. If an Er3+ layer is inserted between the Ge and Si layers (a sandwich structure), it is found that Er3+ ions diffuse preferentially into the SiGe core instead of the silicon-rich shell, with a correspondingly weaker luminescence intensity. A combination of high resolution transmission electron microscopy, energy dispersive X-ray mapping, micro-Raman spectroscopy, and photoluminescence spectroscopy are employed to derive these conclusions.

Formation of Compositionally Abrupt Axial Heterojunctions in Silicon-Germanium Nanowires

We have formed compositionally abrupt interfaces in silicon-germanium (Si-Ge) and Si-SiGe heterostructure nanowires by using solid aluminum-gold alloy catalyst particles rather than the conventional liquid semiconductor-metal eutectic droplets. We demonstrated single interfaces that are defect-free and close to atomically abrupt, as well as quantum dots (i.e., Ge layers tens of atomic planes thick) embedded within Si wires. Real-time imaging of growth kinetics reveals that a low solubility of Si and Ge in the solid particle accounts for the interfacial abruptness. Solid catalysts that can form functional group IV nanowire-based structures may yield an extended range of electronic applications.

Tunable thermal conductivity of Si1−xGex nanowires

By using molecular dynamics simulation, we demonstrate that the thermal conductivity of silicon-germanium nanowires (Si1−xGex NWs) depends on the composition remarkably. The thermal conductivity reaches the minimum, which is about 18% of that of pure Si NW, when Ge content is 50%. More interesting, with only 5% Ge atoms (Si0.95Ge0.05 NW), SiNW’s thermal conductivity is reduced to 50%. The reduction of thermal conductivity mainly comes from the localization of phonon modes due to random scattering. Our results demonstrate that Si1−xGex NW might have promising application in thermoelectrics.

The Higher Mobility Fabrication and Study for SiGe Nanowire

The thin-film-transistor (TFT) was utilized to confirm that oxidation enhanced the SiGe drive current. It shows the incensement of current by the condition of higher temperature and longer oxidation time. The study above employed to the SiGe nanowire. The SiGe nanowire was fabricated by spacer technique and oxidized at different temperatures and times. All the SiGe films with different Ge concentrations were deposited on SiO2 by cold-wall ultrahigh vacuum chemical vapor deposition (UHVCVD). We utilize scanning electron microscopy (SEM) to observe the silicon germanium nanowire's size. In addition, from I-V measurement, it also shows that the current is improved for SiGe nanowire after oxidation.

Quantum size effect in core-shell structured silicon-germanium nanowires

First-principles density functional theory has been employed to study the composition dependent quantum size effect in a series of silicon-germanium core-shell structured nanowires with the diameters ranging from $0.5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}3.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Analysis of the calculated band gap energies in Si-core/Ge-shell and Ge-core/Si-shell structured nanowires shows a nonlinear composition dependence for nanowires with fixed diameter (fixed total number $N={N}_{\mathit{\text{Core}}}+{N}_{\mathit{\text{Shell}}}$ of Si and Ge atoms in the unit cell). In contrast, for nanowires with fixed core size and varying shell thickness, our calculation results reveal a striking linear blueshift of the direct band gap with composition. The obtained linear composition effect implies an inverse square relation between the energy of the fundamental band gap and the size of nanowire, in agreement with experimental observations. Our results provide useful guidelines for experimental gap engineering in the core-shell structured nanowire heterostructures.

Vapor—Liquid—Solid Growth of Silicon—Germanium Nanowires.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Vapor–Liquid–Solid Growth of Silicon–Germanium Nanowires

SiGe alloy nanowires (see Figure) have been fabricated using vapor–liquid–solid (VLS) growth with silane (SiH4) and germane (GeH4) gas sources. Growth conditions have been identified that produce nanowires with homogeneous alloy composition with negligible Ge coating on the wire surface. The Ge composition in the nanowire can be controlled by varying the inlet GeH4/(GeH4 + SiH4) gas ratio.