Session J36: Si and Ge Nanowires: Electrical Transport and Simulation

Epitaxial growth of Ge-Si$_{x}$Ge$_{1-x}$ core-shell nanowire heterostructures with tunable shell content

Core-shell nanowire heterostructures are an interesting testbed for band engineering at the nanoscale. Here we present the growth of germanium (Ge) -- silicon-germanium (Si$_{x}$Ge$_{1-x})$ epitaxial core-shell nanowire (NW) heterostructures, with tunable Si and Ge shell content. The Ge NWs were grown using the Au-catalyzed vapor-liquid-solid (VLS) growth mechanism. Subsequently, the Si$_{x}$Ge$_{1-x}$ shells are grown \textit{in-situ}, conformal onto the Ge NW using ultra-high-vacuum chemical vapor deposition. We use transmission electron microscopy to confirm that both the core and shell are single crystal, and cross-sectional scanning transmission electron microscopy energy dispersive x-ray spectroscopy to determine the shell thickness and content. Our data show that the Si and Ge shell content can be tuned depending on the SiH$_{4}$ and GeH$_{4}$ partial pressures during the shell growth, effectively enabling band engineered core-shell nanowire heterostructures.   

The Effects of Strain and Quantum Confinement on the Electronic Properties of Germanium Nanowires

Germanium nanowires are expected to play an important role as both interconnects and functional components in future nanoscale electronic and optical devices, such as light-emitting diodes, field-effect transistors, chemical and biological sensors. The study of quantum confinement on the band gap of Ge nanowires have been addressed both using theoretical methods and experimental techniques. In the present work, using first principles density-functional theory we studied the uniaxial strain effects on the electronic properties in Ge wires along [110] direction with lateral diameter up to 5 nm. Ge [110] nanowires demonstrate a direct band gap, in contrast to the nature of indirect band gap in bulk. We discovered that the uniaxial strain modulates the band gap of Ge nanowires: compressive strain increases the gap while tensile strain reduces the gap. In addition, the strain also modifies the effective masses of the electron and the hole of Ge wires. Expansion increases the effective mass of the hole, while compression increases the effective mass of the electron. Our results suggest both strain and size can be used to tune the band structure of nanowires, which may help in design of future nanoelectronical devices.   

Atomic scale structure of Si nanowire

In this work, we have succeeded to observe the atomic structures of Au assisted Vapor-liquid-solid grown Si nanowire facetted sidewalls by scanning tunnelling microscopy (STM) at low temperature. By combining transmision microscopy observations with STM measurements, we were able to identify the differents facets along the growth direction of the nanowires. For nanowires with diameters larger than 150 nm, the facets orientation alternates between the [111] and [113] directions, whereas for smaller diameters, the {\{}113{\}} facets are replaced by facets with an orientations making a larger angle with the [111] direction. Imaging the facets at the atomic resolution clearly revealed that the facet reconstructions are induced by Au atoms. From the spectroscopic measurements, the facets are found to be metallic. In order to obtain the impurity distribution below the surface, 3D atom probe tomography analyses were performed. A uniform distribution of Boron impurities is observed in the core of the nanowire and the impurity concentration agrees well with the ratio of the flow rates between silane and diborane. Finally, such results are compared to the conductivities of single nanowires measured in field effect transistor devices.   

Structural and Electronic Properties of Boron Doped Multiply Twinned Silicon Nanowires

Previous studies of undoped multiply twinned (MT) silicon nanowires (SiNWs) have found these structures to be more stable than the bulk-cut single crystal SiNWs for diameters $<$ 6 nm [1]. The five segments that form the MT-SiNWs result in a strain field, causing the interior region of the MT-SiNW to compress while stretching its exterior. In fact, the distribution of the internal stress field in MT-SiNWs offers a unique opportunity for doping the MT-SiNW, including bi-polar doping, and thus opening doors to novel designs of photovoltaic elements. In this work, we will use highly efficient quantum mechanical simulations based on the semi-empirical Hamiltonian developed in Ref. [2] to investigate the electronic structure of boron doped MT-SiNWs of different diameters. We will first determine the most favorable locations for placing boron atoms by mapping out the stress fields of undoped MT-SiNWs. To understand the doping characteristics, we will compare the local site energies and local electronic density of states of MT-SiNWs of undoped and doped systems, and carry out the calculation for MT-SiNWs of different diameters. 1. Y. Zhao, Phys. Rev. Lett., 91, 035501 (2003). 2. C. Leahy \textit{et al.} Phys. Rev. B74, 155408 (2006).   

Nanometer-resolution studies of ``end-on'' metal contacts to vertical Si nanowires

There is great interest in semiconducting nanowires (NWs) and carbon nanotubes (NTs) for future electronic devices and fundamental studies of low-dimensional systems. However, the critical \textit{contacts} to NWs and NTs are still poorly understood. For example, it is predicted (but not yet demonstrated) that Fermi level pinning should be much weaker at small ``end-on'' NW or NT Schottky contacts [1]. We have previously used cross-sectional ballistic electron emission microscopy (BEEM) to quantify small-size effects in Schottky contacts to cleaved quantum wells [2]. Here we describe on-going work to study individual end-on contacts to Si NWs. Vertical Si NWs were grown on Si(111) substrates, embedded in spin-on-glass, and planarized with a chemical mechanical polish. A brief HF etch and thin Au film deposition were then used to make end-on NW contacts. Initial studies with AFM, SEM, internal photoemission spectroscopy, and BEEM demonstrate we can make and measure end-on Schottky contacts to 80nm diameter Si NWs. We will discuss on-going work to optimize sample processing (to reduce roughness near the NWs) and then to quantify the dependence of local contact properties on Si NW diameter. Work supported by NSF Grant No. DMR-0805237. [1] F. Leonard\textit{ et al}., Phys. Rev. Lett. \textbf{84}, 4693 (2000). [2] C. Tivarus\textit{ et al}., Phys. Rev. Lett. \textbf{94}, 206803 (2005).   

First-Principles Simulations of Silicon Nanowires with Different Surface Passivations

We report first-principles simulation results for the electronic band structure of silicon nanowires along $<$100$>$ and $<$110$>$ directions with different surface passivating groups such as hydrogen, hydroxyl, and methyl within an all-electron, Gaussian type orbital, local density functional approach. We discuss how these different groups affect the band gaps and electron distribution of silicon nanowires. And from the band structures we find that the carrier effective masses of $<$100$>$-oriented silicon nanowires exhibit much more dependence on the diameter and passivation compared to those of $<$110$>$-oriented nanowires.   

Study of Electronic Charge Distribution in Silicon Nanowire Transistors : An Atomistic Approach

Atomistic modeling has been performed to investigate the spatial electronic charge distribution in silicon nanowire cross-sections. The modeling approach involves solution of electronic bandstructure using the 20 band sp3d5s* -SO nearest neighbor Tight-binding (TB) method with spin orbit (SO) interaction (LCAO) solved self-consistently with a two dimensional Poisson equation. Nanowires with rectangular, circular and triangular cross-section shapes have been investigated, with cross-section size of 3.1 nm and 5.1 nm for three different crystal orientations namely [100], [110] and [111]. The observed charge distribution as observed in these wires, is a strong function of cross-section shape, size and crystal orientation. [100] and [110] wires show strong corner effects, however, [111] oriented wires have centralized charge distribution. Charge distribution is sensitive to the structural and crystal symmetry of the nanowire. Structural confinement breaks the symmetry that manifests in the 1D energy dispersion of these wires by lifting up the degeneracy at the gamma valley. Finally, we enable the understanding of atomistic treatment for charge distribution in the capacitance measurements in these ultra-scaled silicon nanowire transistors.   

Interface State Disorder Dominated Microwave Conductance in Silicon Nanowires

We have developed a technique to measure the microwave conductance spectra of nanomaterials at frequencies from 100 MHz to 50 GHz and at temperatures between 4 K and 300 K. We have used this technique to measure the microwave conductance spectra of doped silicon nanowires (SiNWs) which are found to increase sublinearly with frequency as $f^s$, with \textit{0.25 $<$ s $<$ 0.45}, indicative of disordered conduction. Additionally, the exponents are found to be nearly independent of temperature suggesting that structural disorder in nanomaterial morphology, rather than energetic trapping, dominates the AC transport. A model was developed that explains the SiNW conductance in terms of carrier confinement in a disordered electrostatic potential caused by charged Si/SiOx interface states. These results highlight the importance of topological effects in the microwave conductance of nanomaterials. Results from the measurement of other nanomaterials will also be briefly presented. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Effect of hydrogen passivation on the structure and energetics of silicon nanowires

In this work we explore systematically the structure, energetics and electronic properties of silicon nanowires (SiNWs) with different surface structures and growth directions, and the trend of such property variation with increasing nanowire diameters using first principles density functional theory with both local atomic basis and plane waves. Both passivated and unpassivated systems were studied. The unpassivated (100) and (111) wires are found to be metallic with the unpaired electrons on the surface of these wires acting as conducting channels. Hydrogen passivation of these surfaces introduces a direct band gap by confining the electrons to localized bonds. The nature of the electronic states is examined through local density of states and electron density distributions. The relative stability of SiNWs with different growth directions and surface structures are evaluated from the free energy of formation.   

Semiconducting nanowire devices in out-of-plane geometry.

Semiconducting nanowires are attractive components in the field of nanoelectronics, photonic and sensing applications. Experiments with nanowires have usually been performed in planar geometry. Here, we demonstrate the fabrication of nanowire devices in out-of-plane geometry by taking advantage of inherent growth direction of nanowire using the vapor-liquid-solid (VLS) method. Highly epitaxial semiconducting nanowires are grown on doped Si (111) substrate from Au nanoparticle seeds assembled in e-beam lithography patterns. The directed assembly of Au nanoparticles is achieved by molecular recognition through silanization process, or electrophoretic assembly. The versatility of the VLS method allows the growth of a wide range of semiconducting nanowires with controlled \textit{in-situ} doping. The post-growth processes include CVD of SiO$_{2}$ filler layer, chemical mechanical polishing and light etching of the SiO$_{2}$ layer to expose nanowire tips. Top metal contacts are then deposited for electrical characterization and sensing applications. We will present the results of the vertical nanowire device performance.   

Top-gate Ge-Si$_{x}$Ge$_{1-x}$ core-shell nanowire field effect transistors with highly doped source and drain.

Semiconductor nanowires (NWs) field effect transistors (FETs) have been considered as candidates for aggressively scaled complementary metal-oxide-semiconductor (CMOS) devices. In particular, germanium (Ge) NW have been of interest thanks to their higher carrier mobility, compared to silicon (Si). Most of the reported semiconductor NW FETs up to date are measured on devices with metal (Schottky) contacts, where the carrier injection efficiency into the channel is significantly limited by the Schottky barrier at the metal/NW interface. Using low (3keV) energy boron ion implantation, we demonstrate here top-gate Ge-Si$_{x}$Ge$_{1-x}$ core-shell NW p-type FETs, with highly doped source (S) and drain (D). The highly doped, up to $\sim $10$^{20}$ cm$^{3}$ levels, S/D areas of the NW FETs allow an efficient carrier injection into the NW and a low contact resistance. Compared to similar top gated NW FETs, but with undoped S/D and with metal-semiconductor contacts, the electrical characteristics of the top-gated NW FETs with doped S/D exhibit up to two orders of magnitude higher current, and an improved ON/OFF current ratio.   

Precision transport and assembling of nanowires in suspension by electric fields

We describe a method of precision transport of nanowires in suspension using a combination of dielectrophoretic force and electrophoretic force, which, respectively, aligns and transports the nanowires. We revealed the effect of electroosmosis flows on the nanowires and determined the ratio of viscous coefficients for nanowires moving parallel or perpendicular to the orientations. The transport of nanowires can be made to follow any prescribed trajectory with any orientation by the voltages applied to the patterned electrodes. As a demonstration of the high precision of manipulation, we have joined end-to-end two oppositely charged nanowires originally separated by 200 $\mu$m into a microelectromechanical device.   

Thermal properties measurements of silicon nanowires at low temperature

Phonons transport in nanowires and nanotubes is an effervescent field for theoretician as well as experimentalist. Especially at low temperature, where the dimensions of the sample approximate the dominant phonon wave length, the low dimensionality of these systems has strong impact on the thermal transport. Specific regimes have to be considered: transmission coefficient to the heat bath, quantum regime, transition between diffusive and specular regime etc{\ldots} Firstly, we have performed measurements with the 3$\omega $ method on various suspended silicon nanowires with a section of the order of 100nm$^{2}$ and a length of 10$\mu $m. Above 2 K, the thermal conductance varies like T$^{3}$(Casimir Regime); however at lower temperature, a quadratic regime in temperature appears: the signature of a change in the phonon transport regime. Secondly, we have measured nanowires with various geometries, to deduce the impact of geometrical factors at the mesoscopic scale on the thermal transport. All these results will be discussed in view of the different models describing the heat transfer at the nanoscale.   

Donor-pair defects and doping efficiency in silicon nanowires

We investigate the doping efficiency of dopants in Si nanowires through first-principles density-functional calculations. For hydrogen- passivated Si nanowires doped with group-V elements such as P, As, and Sb, we consider wire diameters in the range of 8 - 30 {\AA} and axis orientations along the [111] and [110] directions. A single substitutional donor prefers to locate on the wire center and acts as a shallow donor. When wire diameters are below a critical value, a donor-pair defect which consists of two dopants at the first-nearest distance can be stabilized, in contrast to bulk Si. The stability of the donor-pair defect is attributed to the confinement effect in nanostructures, which results in the increase of the band gap and thereby the destabilization of the shallow donor level. As the donor- pair defect with a deep level in the band gap is electrically inactive, the doping efficiency is expected to be low in small-diameter nanowires. The formation of the donor-pair defect is found to be more favorable for the P dopants, which have a smaller atomic radius than the As and Sb dopants.