Electron trapping in InP nanowire FETs with stacking faults

Semiconductor III-V nanowires are promising components of future electronic and optoelectronic devices, but they typically show a mixed wurtzite-zinc blende crystal structure. Here we show, theoretically and experimentally, that the crystal structure dominates the conductivity in such InP nanowires. Undoped devices show very low conductivities and mobilities. The zincblende segments are quantum wells orthogonal to the current path and our calculations indicate that an electron concentration of up to 4.6 × 10(18) cm(-3) can be trapped in these. The calculations also show that the room temperature conductivity is controlled by the longest zincblende segment, and that stochastic variations in this length lead to an order of magnitude variation in conductivity. The mobility shows an unexpected decrease for low doping levels, as well as an unusual temperature dependence that bear resemblance with polycrystalline semiconductors.     

Surface effects on stacking fault and twin formation in fcc nanofilms: A first-principles study

The free surface effects on stacking fault and twin formation in fcc metals (Al, Cu, and Ni) were examined by first-principles calculations based on density functional theory (DFT). It is found that the generalized planar fault (GPF) energies of Ni are much larger than bulk Ni with respect to Al and Cu. The discrepancy is attributed to the localized relaxation of Ni nanofilm to accommodate the large expansion of the interplanar separation induced at the fault plane. The localized relaxation can be coupled to the electronic structure of Ni nanofilms.     

Mechanisms of molecular beam epitaxy growth in InAs/InP nanowire heterostructures

InAs/InP axial nanowire heterostructures were grown by the Au-assisted vapour-liquid-solid method in a gas source molecular beam epitaxy system. The nanowire crystal structure and morphology were investigated by transmission electron microscopy for various growth conditions (temperature, growth rate, V/III flux ratio). Growth mechanisms were inferred from the InAs and InP segment lengths as a function of the nanowire diameter. Short InAs segment lengths were found to grow by depletion of In from the Au particle as well as by direct impingement, while longer segments of InAs and InP grew by diffusive transport of adatoms from the nanowire sidewalls. The present study offers a way to control the lengths of InAs quantum dots embedded in InP barriers.     

Kinetics of epitaxial growth and roughening

We review some recent results on epitaxial growth and surface roughening. Particular emphasis is placed on the concept of the critical island size in submonolayer growth and on the existence of scaling in both the submonolayer and multilayer growth regimes. The use of scaling ideas as well as Monte Carlo simulations and continuum equations is shown to be effective in understanding experimental results for submonolayer growth and surface roughening.     

III-V nanowire growth mechanism: V/III ratio and temperature effects

We have studied the dependence of Au-assisted InAs nanowire (NW) growth on InAs(111)B substrates as a function of substrate temperature and input V/III precursor ratio using organometallic vapor-phase epitaxy. Temperature-dependent growth was observed within certain temperature windows that are highly dependent on input V/III ratios. This dependence was found to be a direct consequence of the drop in NW nucleation and growth rate with increasing V/III ratio at a constant growth temperature due to depletion of indium at the NW growth sites. The growth rate was found to be determined by the local V/III ratio, which is dependent on the input precursor flow rates, growth temperature, and substrate decomposition. These studies advance understanding of the key processes involved in III-V NW growth, support the general validity of the vapor-liquid-solid growth mechanism for III-V NWs, and improve rational control over their growth morphology.     

Dislocations and stacking fault energy in silicon ditelluride

The stacking fault energy in silicon ditelluride single crystals has been determined for different dislocation configurations observed by electron microscopy. The configurations studied are dislocation ribbons and dislocation nodes, and the ratio of the stacking fault energy to the shear modulus is estimated as / = 1.6×10^–11 cm. Some common features observed in electron microscopy are also discussed in terms of dislocation networks of extended and contracted nodes and symmetrical or asymmetrical three-fold ribbons, as well as various dislocation interactions.     

The role of surface energies and chemical potential during nanowire growth

We present an approach to quantitatively determine the magnitudes and the variation of the chemical potential in the droplet (Δμ), the solid-liquid (γ(SL)) and the liquid-vapor (γ(LV)) interface energies upon variation of the group III partial pressure during vapor-liquid-solid-growth of nanowires. For this study, we use GaP twinning superlattice nanowires. We show that γ(LV) is the quantity that is most sensitive to the Ga partial pressure (p(Ga)), its dependence on p(Ga) being three to four times as strong as that of γ(SL) or Δμ, and that as a consequence the surface energies are as important in determining the twin density as the chemical potential. This unexpected result implies that surfactants could be used during nanowire growth to engineer the nanowire defect structure and crystal structure.     

Transients in the formation of nanowire heterostructures

We present results on the effect of seed particle reconfiguration on the growth of short InAs and InP nanowire segments. The reconfiguration originates in two different steady state alloy compositions of the Au/In seed particle during growth of InAs and InP. From compositional analysis of the seed particle, the In content in the seed particle is determined to be 34 and 44% during InAs and InP growth, respectively. When switching between growing InAs and InP, transient effects dominate during the time period of seed particle reconfiguration. We developed a model that quantitatively explains the effect and with the added understanding we are now able to grow short period (<10 nm) nanowire superlattices.     

Surface energy of monolayer formation during nanowire growth by vapor-liquid-solid mechanism

An exact expression for the effective surface energy of monolayer formation during the growth of a two-dimensional isotropic nanowire according to the vapor-liquid-solid (VLS) mechanism is obtained with allowance for faceting of the crystal side surface. Conditions necessary for the stability of a drop at the top of the growing nanowire and for the preferred nucleation at the triple phase line are theoretically analyzed in various geometries of the lateral L-S boundary. The obtained results correct the inaccuracies of the previous solution of an analogous problem and make it possible to calculate the nucleation barriers that determine the probabilities of formation of various crystalline phases in nanowires of III–V semiconductor compounds.     

Catalytic role of gold nanoparticle in GaAs nanowire growth: a density functional theory study

The energetics of Ga, As, and GaAs species on the Au(111) surface (employed as a model for Au nanoparticles) is investigated by means of density functional calculations. Apart from formation of the compound Au(7)Ga(2), Ga is found to form a surface alloy with gold with comparable ΔH ~ -0.5 eV for both processes. Dissociative adsorption of As(2) is found to be exothermic by more than 2 eV on both clean Au(111) and AuGa surface alloys. The As-Ga species formed by reaction of As with the surface alloy is sufficiently stable to cover the surface of an Au particle in vacuo in contact with a GaAs substrate. The results of the calculations are interpreted in the context of Au-catalyzed growth of GaAs nanowires. We argue that arsenic is supplied to the growth zone of the nanowire mainly by impingement of molecules on the gold particle and identify a regime of temperatures and As(2) partial pressures suitable for Au-catalyzed nanowire growth in molecular beam epitaxy.     

Critical assessment 19: stacking fault energies of austenitic steels

The stacking fault energy (SFE) can play a key role in the deformation mechanism (e.g. transformation-induced plasticity and twinning-induced plasticity) of austenitic steels. Therefore, tremendous efforts have been devoted to exploring the evaluation methods and controlling parameters (e.g. alloying elements and temperature) that determine the SFE and its relationship to mechanical twinning. We provide here a summary of recent progress in studies of the SFE of austenite and of unsolved issues that may stimulate further investigation.     

Electron microscope weak-beam imaging of stacking fault tetrahedra: observations and simulations

Weak-beam diffraction-contrast electron microscope images of stacking-fault tetrahedra (SFT) have been simulated by solving numerically the Howie–Basinski equations, which are well suited for studying the dependence of image contrast on experimental parameters. These simulated images are in good qualitative agreement with experimental transmission electron micrographs. The visibility of small SFT and the relationship between measured image sizes and real SFT sizes are discussed.

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First-step nucleation growth dependence of InAs/InGaAs/InP quantum dot formation in two-step growth

First-step nucleation growth has an important impact on the two-step growth of high-quality mid-infrared emissive InAs/InGaAs/InP quantum dots (QDs). It has been found that an optimized growth rate for first-step nucleation is critical for forming QDs with narrow size distribution, high dot density and high crystal quality. High growth temperature has an advantage in removing defects in the QDs formed, but the dot density will be reduced. Contrasting behavior in forming InAs QDs using metal-organic vapor phase epitaxy (MOVPE) by varying the input flux ratio of group-V versus group-III source (V/III ratio) in the first-step nucleation growth has been observed and investigated. High-density, 2.5 × 10(10)?cm(-2), InAs QDs emitting at>2.15?μm have been formed with narrow size distribution, ～1?nm standard deviation, by reducing the V/III ratio to zero in first-step nucleation growth.     

The role of interface concentration gradient in the formation of silver dendritic particles

Dendritic structures are widely present in nature, from river networks to snowflakes. There is a long-term interest in discovering their common formation mechanism, especially the driving force leading to the branching and the reason to keep symmetry. The inhibition of lithium dendrites in secondary batteries also calls for the deep understanding on the formation of dendritic structures. Here in this article, we report an investigation on the driving force of the formation of dendritic structures. Silver particles are synthesized by Galvanic replacement reaction (GRR) in which metal rods are immersed into the silver nitrate solution to reduce silver ions followed by silver particles formation on the surface of rods. The silver ions concentration profile near the rods is measured by Mach-Zehnder interferometer during the reaction. It is found that the formation of silver dendritic particles is accompanied by the interface concentration gradient. A regulation on the gradient leads to the change of silver morphology, experimentally confirming the dominant role of the interface concentration gradient in the formation of diverse structures.     

Manipulation of electron orbitals in hard-wall InAs/InP nanowire quantum dots

We present a novel technique for the manipulation of the energy spectrum of hard-wall InAs/InP nanowire quantum dots. By using two local gate electrodes, we induce a strong transverse electric field in the dot and demonstrate the controlled modification of its electronic orbitals. Our approach allows us to dramatically enhance the single-particle energy spacing between the first two quantum levels in the dot and thus to increment the working temperature of our InAs/InP single-electron transistors. Our devices display a very robust modulation of the conductance even at liquid nitrogen temperature, while allowing an ultimate control of the electron filling down to the last free carrier. Potential further applications of the technique to time-resolved spin manipulation are also discussed.     

The effects of Au surface diffusion to formation of Au droplets/clusters and nanowire growth on GaAs substrate using VLS method

Using VLS method with the separated 220 nm thick Au catalyst circles/stripes configurations sputtered onto GaAs substrate surface, this paper investigated the effects of the Au droplets/clusters formation as well as the nanowires growth process inside and outside the Au circles/stripes configurations. The Au surface outward diffusion from the Au layer edge up to several tens of micrometers has strongly dominated. The effects of Au surface diffusion to formation of Au droplets/cluster and to the nanowires growth on GaAs semiconductor substrate in the region outside the Au layers have been shown. The mechanism of the droplets/clusters formation outside the Au layer could explained by the surface cluster diffusion, meanwhile the nanowires have grown simultaneously during the Au outward diffusion. The growth could explain by the diffusion of Ga and As atoms into the diffusing Au droplets/clusters via dissociative mechanism to form nanowire seeds inside for nanowires growth. The Au droplets/clusters formation and nanowires growth on GaAs substrate outside Au layer could be applied for making nanodevices blocks outside the Au layer. Unfortunately if this Au surface diffusion phenomenon is occurring on the GaAs semiconductor containing the Au stripes interconnections in micro/nanocircuits this could also cause the short-circuits phenomenon, even at thin Au layer.