AuxAg1-x alloy seeds: A way to control growth, morphology and defect formation in Ge nanowires

Germanium (Ge) nanowires are of current research interest for high speed nanoelectronic devices due to the lower band gap and high carrier mobility compatible with high K-dielectrics and larger excitonic Bohr radius ensuing a more pronounced quantum confinement effect [1-6]. A general way for the growth of Ge nanowires is to use liquid or a solid growth promoters in a bottom-up approach which allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, operating pressure, precursor flow rate etc [3, 7-11]. The Solid-phase seeding is preferred for more control processing of the nanomaterials and potential suppression of the unintentional incorporation of high dopant concentrations in semiconductor nanowires and unrequired compositional tailing of the seed-nanowire interface [2, 5, 9, 12]. There are therefore distinct features of the solid phase seeding mechanism that potentially offer opportunities for the controlled processing of nanomaterials with new physical properties. A superior control over the growth kinetics of nanowires could be achieved by controlling the inherent growth constraints instead of external parameters which always account for instrumental inaccuracy. The high dopant concentrations in semiconductor nanowires can result from unintentional incorporation of atoms from the metal seed material, as described for the Al catalyzed VLS growth of Si nanowires [13] which can in turn be depressed by solid-phase seeding. In addition, the creation of very sharp interfaces between group IV semiconductor segments has been achieved by solid seeds [14], whereas the traditionally used liquid Au particles often leads to compositional tailing of the interface [15] . Korgel et al. also described the superior size retention of metal seeds in a SFSS nanowire growth process, when compared to a SFLS process using Au colloids [12]. Here in this work we have used silver and alloy seed particle with different compositions to manipulate the growth of nanowires in sub-eutectic regime. The solid seeding approach also gives an opportunity to influence the crystallinity of the nanowires independent of the substrate. Taking advantage of the readily formation of stacking faults in metal nanoparticles, lamellar twins in nanowires could be formed

Hybrid InAs nanowire-vanadium proximity SQUID

We report the fabrication and characterization of superconducting quantum interference devices (SQUIDs) based on InAs nanowires and vanadium superconducting electrodes. These mesoscopic devices are found to be extremely robust against thermal cycling and to operate up to temperatures of $\sim2.5$~K with reduced power dissipation. We show that our geometry allows to obtain nearly-symmetric devices with very large magnetic-field modulation of the critical current. All these properties make these devices attractive for on-chip quantum-circuit implementation.Comment: 3 pages, 3 figure

Irreversible Thermodynamics of the Universe: Constraints from Planck

The present work deals with irreversible universal thermodynamics. The homogenous and isotropic flat model of the universe is chosen as open thermodynamical system and nonequilibrium thermodynamics comes into picture. For simplicity, entropy flow is considered only due to heat conduction. Further, due to Maxwell-Cattaneo modified Fourier law for nonequilibrium phenomenon, the temperature satisfies damped wave equation instead of heat conduction equation. Validity of generalized second law of thermodynamics (GSLT) has been investigated for universe bounded by apparent or event horizon with cosmic substratum as perfect fluid with constant or variable equation of state or interacting dark species. Finally, we have used three Planck data sets to constrain the thermal conductivity λ and the coupling parameter b2. These constraints must be satisfied in order for GSLT to hold for universe bounded by apparent or event horizons

Inducing imperfections in germanium nanowires

Nanowires with inhomogeneous heterostructures such as polytypes and periodic twin boundaries are interesting due to their potential use as components for optical, electrical, and thermophysical applications. Additionally, the incorporation of metal impurities in semiconductor nanowires could substantially alter their electronic and optical properties. In this highlight article, we review our recent progress and understanding in the deliberate induction of imperfections, in terms of both twin boundaries and additional impurities in germanium nanowires for new/enhanced functionalities. The role of catalysts and catalyst–nanowire interfaces for the growth of engineered nanowires via a three-phase paradigm is explored. Three-phase bottom-up growth is a feasible way to incorporate and engineer imperfections such as crystal defects and impurities in semiconductor nanowires via catalyst and/or interfacial manipulation. “Epitaxial defect transfer” process and catalyst–nanowire interfacial engineering are employed to induce twin defects parallel and perpendicular to the nanowire growth axis. By inducing and manipulating twin boundaries in the metal catalysts, twin formation and density are controlled in Ge nanowires. The formation of Ge polytypes is also observed in nanowires for the growth of highly dense lateral twin boundaries. Additionally, metal impurity in the form of Sn is injected and engineered via third-party metal catalysts resulting in above-equilibrium incorporation of Sn adatoms in Ge nanowires. Sn impurities are precipitated into Ge bi-layers during Ge nanowire growth, where the impurity Sn atoms become trapped with the deposition of successive layers, thus giving an extraordinary Sn content (>6 at.%) in Ge nanowires. A larger amount of Sn impingement (>9 at.%) is further encouraged by utilizing the eutectic solubility of Sn in Ge along with impurity trapping

Recent advances in the growth of germanium nanowires: synthesis, growth dynamics and morphology control

One-dimensional semiconductor nanostructures have been studied in great depth over the past number of decades as potential building blocks in electronic, thermoelectric, optoelectronic, photovoltaic and battery devices. Silicon has been the material of choice in several industries, in particular the semiconductor industry, for the last few decades due to its stable oxide and well documented properties. Recently however, Ge has been proposed as a candidate to replace Si in microelectronic devices due to its high charge carrier mobilities. A number of various ‘bottom-up’ synthetic methodologies have been employed to grow Ge nanowires, including chemical vapour deposition, thermal evaporation, template methods, supercritical fluid synthesis, molecular beam epitaxy and solution phase synthesis. These bottom-up methods afford the opportunity to produce commercial scale quantities of nanowires with controllable lengths, diameters and crystal structure. An understanding of the vapour-liquid-solid (VLS) and vapour-solid-solid (VSS) mechanism by which most Ge nanowires are produced, is key to controlling their growth rate, aspect ratio and morphology. This article highlights the various bottom-up growth methods that have been used to synthesise Ge nanowires over the past 5-6 years, with particular emphasis on the Au/Ge eutectic system and the VLS mechanism. Thermodynamic and kinetic models used to describe Ge nanowire growth and morphology control will also be discussed in detail

Chemical approaches for doping nanodevice architectures

Advanced doping technologies are key for the continued scaling of semiconductor devices and the maintenance of device performance beyond the 14 nm technology node. Due to limitations of conventional ion-beam implantation with thin body and 3D device geometries, techniques which allow precise control over dopant diffusion and concentration, in addition to excellent conformality on 3D device surfaces, are required. Spin-on doping has shown promise as a conventional technique for doping new materials, particularly through application with other dopant methods, but may not be suitable for conformal doping of nanostructures. Additionally, residues remain after most spin-on-doping processes which are often difficult to remove. In-situ doping of nanostructures is especially common for bottom-up grown nanostructures but problems associated with concentration gradients and morphology changes are commonly experienced. Monolayer doping (MLD) has been shown to satisfy the requirements for extended defect-free, conformal and controllable doping on many materials ranging from traditional silicon and germanium devices to emerging replacement materials such as III-V compounds but challenges still remain, especially with regard to metrology and surface chemistry at such small feature sizes. This article summarises and critically assesses developments over the last number of years regarding the application of gas and solution phase techniques to dope silicon-, germanium- and III-V-based materials and nanostructures to obtain shallow diffusion depths coupled with high carrier concentrations and abrupt junctions

Nanosize effect in germanium nanowire growth with binary metal alloys

This article describes feasible and improved ways towards enhanced nanowire growth kinetics by reducing the equilibrium solute concentration in the liquid collector phase in a vapor-liquid-solid (VLS) like growth model. Use of bi-metallic alloy seeds (AuxAg1-x) influences the germanium supersaturation for a faster nucleation and growth kinetics. Nanowire growth with ternary eutectic alloys shows Gibbs-Thompson effect with diameter dependent growth rate. In-situ transmission electron microscopy (TEM) annealing experiments directly confirms the role of equilibrium concentration in nanowire growth kinetics and was used to correlate the equilibrium content of metastable alloys with the growth kinetics of Ge nanowires. The shape and geometry of the heterogeneous interfaces between the liquid eutectic and solid Ge nanowires were found to vary as a function of nanowire diameter and eutectic alloy composition

Inherent control of growth, morphology and defect formation in germanium nanowires

The use of bimetallic alloy seeds for growing one-dimensional nanostructures has recently gained momentum among researchers. The compositional flexibility of alloys provides the opportunity to manipulate the chemical environment, reaction kinetics, and thermodynamic behavior of nanowire growth, in both the eutectic and the subeutectic regimes. This Letter describes for the first time the role of AuxAg1–x alloy nanoparticles in defining the growth characteristics and crystal quality of solid-seeded Ge nanowires via a supercritical fluid growth process. The enhanced diffusivity of Ge in the alloy seeds, compared to pure Ag seeds, and slow interparticle diffusion of the alloy nanoparticles allows the realization of high-aspect ratio nanowires with diameters below 10 nm, via a seeded bottom-up approach. Also detailed is the influence the alloyed seeds have on the crystalline features of nanowires synthesized from them, that is, planar defects. The distinctive stacking fault energies, formation enthalpies, and diffusion chemistries of the nanocrystals result in different magnitudes of {111} stacking faults in the seed particles and the subsequent growth of ⟨112⟩-oriented nanowires with radial twins through a defect transfer mechanism, with the highest number twinned Ge nanowires obtained using Ag0.75Au0.25 growth seeds. Employing alloy nanocrystals for intrinsically dictating the growth behavior and crystallinity of nanowires could open up the possibility of engineering nanowires with tunable structural and physical properties