Effect of a ZnSe buffer layer on the surface, structural, and optical properties of the ZnTe/ZnSe/GaAs heterostructures

Lattice-mismatched ZnTe epilayers on GaAs (1 0 0) substrates with and without ZnSe buffer layers were grown by using molecular beam epitaxy. AFM, XRD, and TEM measurements were performed to investigate the surface and structural properties of the ZnTe thin films. Photoluminescence, Raman scattering, and TEM measurements showed that the crystallinity of a ZnTe epilayer grown on a GaAs substrate was remarkably improved by using a ZnSe buffer layer. Photoreflectance measurements showed that the strain of the ZnTe layer with the ZnSe buffer layer was smaller than that without the ZnSe buffer layer. These results indicate that ZnTe epitaxial films grown on GaAs substrates with ZnSe buffer layers hold promise for potential applications in optoelectronic devices operating in the blue-green spectral region.     

Synthesis and structural characterization of single-crystalline branched nanowire heterostructures

We report the synthesis of three-dimensional single-crystalline branched nanowire heterostructures, where the backbones and branches are assembled with ZnS and CdS, respectively. Growth of branch and backbones with control over the compositions was enabled via sequential seeding of gold nanocluster catalysts. Elemental mapping data confirmed that branched nanowire heterostructures were synthesized with the intended chemical modulation, CdS branches on ZnS backbones. Transmission electron microscopy studies showed that the growth of heterostructure branches occurs epitaxially from the backbone while maintaining single-crystalline structure. This unique class of heterostructures holds great potential in assembling electronics and photonics in three dimensions.     

Single crystalline PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices

We report the formation of PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices from such heterostructures. Scanning electron microscopy studies show that silicon nanowires can be converted into PtSi nanowires through controlled reactions between lithographically defined platinum pads and silicon nanowires. High-resolution transmission electron microscopy studies show that PtSi/Si/PtSi heterostructure has an atomically sharp interface with epitaxial relationships of Si[110]//PtSi[010] and Si(111)//PtSi(101). Electrical measurements show that the pure PtSi nanowires have low resistivities approximately 28.6 microOmega.cm and high breakdown current densities>1x10(8) A/cm2. Furthermore, using single crystal PtSi/Si/PtSi nanowire heterostructures with atomically sharp interfaces, we have fabricated high-performance nanoscale field-effect transistors from intrinsic silicon nanowires, in which the source and drain contacts are defined by the metallic PtSi nanowire regions, and the gate length is defined by the Si nanowire region. Electrical measurements show nearly perfect p-channel enhancement mode transistor behavior with a normalized transconductance of 0.3 mS/microm, field-effect hole mobility of 168 cm2/V.s, and on/off ratio>10(7), demonstrating the best performing device from intrinsic silicon nanowires.     

Self-organized growth of Si/Silica/Er2Si2O7 core-shell nanowire heterostructures and their luminescence

Self-organized Si-Er heterostructure nanowires showed promising 1.54 microm Er(3+) optical activity. Si nanowires of about 120-nm diameter were grown vertically on Si substrates by the vapor-liquid-solid mechanism in an Si-Er-Cl-H(2) system using an Au catalyst. Meanwhile, a single-crystalline Er(2)Si(2)O(7) shell sandwiched between nanometer-thin amorphous silica shells was self-organized on the surface of Si nanowires. The nanometer-thin heterostructure shells make it possible to observe a carrier-mediated 1.53 microm Er(3+) photoluminescence spectrum consisting of a series of very sharp peaks. The Er(3+) spectrum and intensity showed absolutely no change as the temperature was increased from 25 to 300 K. The luminescence lifetime at room temperature was found to be 70 micros. The self-organized Si nanowires show great potential as the material basis for developing an Si-based Er light source.     

Oxidative annealing of ZnSe/GaAs heterostructures

We annealed ZnSe/GaAs heterostructures in the oxygen atmosphere and investigated structural and optical properties of the forming films using X-ray diffraction and photoluminescence. While highly textured ZnO films were synthesized via low-temperature processing (~ 500 °C), high temperature processing (~ 800 °C) promoted reaction at the film/substrate interface and Zn loss from the film surface resulting in the polycrystalline ZnGa₂O₄ and ZnO₂.     

The morphology of axial and branched nanowire heterostructures

We present an extensive investigation of the epitaxial growth of Au-assisted axial heterostructure nanowires composed of group IV and III-V materials and derive a model to explain the overall morphology of such wires. By analogy with 2D epitaxial growth, this model relates the wire morphology (i.e., whether it is kinked or straight) to the relationship of the interface energies between the two materials and the particle. This model suggests that, for any pair of materials, it should be easier to form a straight wire with one interface direction than the other, and we demonstrate this for the material combinations presented here. However, such factors as kinetics and the use of surfactants may permit the growth of straight double heterostructure nanowires. Finally, we demonstrate that branched nanowire heterostructures, also known as nanotrees, can be successfully explained by the same model.     

Numerical simulation of the temperature dependence of photoluminescence in strained-Si1−xGex/Si heterostructures

In previous work it has been shown that the decay in photoluminescence from Si/strained-Si_1–xGe_x/Si quantum wells at temperatures over 100 K is controlled by surface recombination and that the photoluminescence intensity can be increased by over an order of magnitude by surface passivation. These results had been explained only by a simple phenomenological model, which could not explain why at high pump power density the observed luminescence was constant from 77 to 250 K. This paper uses a two-carrier heterojunction device simulator to determine the carrier profiles during optical pumping. The profiles are used to understand quantitatively luminescence as a function of temperature and pump power density without making the over-simplifying assumptions required for analytical modeling. Surface recombination velocities over 10³ cm/s drastically affect the results, and Auger recombination plays an important role at high pump power density.     

Microwave-assisted synthesis and highly photocatalytic activity of MWCNT/ZnSe heterostructures

The multi-walled carbon nanotubes (MWCNTs) coated with the face-center cubic ZnSe nanoparticles with a uniform and small diameter have been prepared to form MWCNT/ZnSe heterostructures by microwave irradiation. The morphology, loading quantity and size of the ZnSe nanoparticles in the range of 15–50 nm can be controlled easily by adjusting the microwave power, pH value of the initial solution, the molar ratios of the Zn(AC)₂/MWCNTs and the appropriate complexing agent. The photoluminescence measurement indicates that the MWCNT/ZnSe heterostructures are blue-shifted compared to reported bulk ZnSe. The UV–vis absorption spectra of the heterostructures appears two sharp absorption peaks at 336 and 344 nm, respectively. It was demonstrated that the heterostructures could photodegrade the fuchsine acid in the solution with highly photocatalytic activity.     

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.     

The dependence of the optoelectrical properties of silver nanowire networks on nanowire length and diameter

We have characterized the optoelectrical properties of networks of silver nanowires as a function of nanowire dimension by measuring transmittance (T) and sheet resistance (R(s)) for a large number of networks of different thicknesses fabricated from wires of different diameters (D) and lengths (L). We have analysed these data using both bulk-like and percolative models. We find the network DC conductivity to scale linearly with wire length while the optical conductivity is approximately invariant with nanowire length. The ratio of DC to optical conductivity, often taken as a figure of merit for transparent conductors, scales approximately as L/D. Interestingly, the percolative exponent, n, scales empirically as D2, while the percolative figure of merit, Π, displays large values at low D. As high T and low R(s) are associated with low n and high Π, these data are consistent with improved optoelectrical performance for networks of low-D wires. We predict that networks of wires with D = 25 nm could give sheet resistance as low as 25 Ω/□ for T = 90%.     

Molecular beam epitaxial growth and characterization of catalyst-free InN/InxGa1-xN core/shell nanowire heterostructures on Si(111) substrates

We report on the achievement of, for the first time, InN/InGaN core/shell nanowire heterostructures, which are grown directly on Si(111) substrates by plasma-assisted molecular beam epitaxy. The crystalline quality of the heterostructures is confirmed by transmission electron microscopy, and the elemental mapping through energy dispersive x-ray spectrometry further reveals the presence of an InGaN shell covering the sidewall and top regions of the InN core. The optical characterizations reveal two emission peaks centered at ～1685?nm and 1845?nm at 5?K, which are related to the emission from the InGaN shell and InN core, respectively. The InN/InGaN core/shell nanoscale heterostructures exhibit a very high internal quantum efficiency of ～62% at room temperature, which is attributed to the strong carrier confinement provided by the InGaN shell as well as the nearly intrinsic InN core.     

Observation of interface carrier states in no-common-atom heterostructures ZnSe/BeTe

The existence of intrinsic carrier interface states in heterostructures with no common atom at the interface (such as ZnSe/BeTe) is shown experimentally by ellipsometry and photoluminescence spectroscopy. These states are located on interfaces and lie inside the effective bandgap of the structure; they are characterized by a high density and a long lifetime. A tight binding model confirms theoretically the existence of these states in ZnSe/BeTe heterostructures for a ZnTe-type interface, in contrast to the case of the BeSe-type interface for which they do not exist.     

Initial stage and reconstruction of GaAs/Si heterostructures

GaAs epitaxial layers on Si(1 0 0) substrates having a single or a double domain 2×1 have been grown by molecular beam epitaxy using the two-step growth mode and thermal regrowth techniques. The initial stage and the reconstruction of the GaAs/Si heterostructures have been investigated in situ by Auger electron spectroscopy and reflection high-energy electron diffraction. GaAs layers grown by both methods show the reconstruction of a single domain, and models for the process of GaAs growth have been presented to explain the self-annihilation of the antiphase boundary.     

Three-dimensional GaN/AlN nanowire heterostructures by separating nucleation and growth processes

Bottom-up nanostructure assembly has been a central theme of materials synthesis over the past few decades. Semiconductor quantum dots and nanowires provide additional degrees of freedom for charge confinement, strain engineering, and surface sensitivity-properties that are useful to a wide range of solid state optical and electronic technologies. A central challenge is to understand and manipulate nanostructure assembly to reproducibly generate emergent structures with the desired properties. However, progress is hampered due to the interdependence of nucleation and growth phenomena. Here we show that by dynamically adjusting the growth kinetics, it is possible to separate the nucleation and growth processes in spontaneously formed GaN nanowires using a two-step molecular beam epitaxy technique. First, a growth phase diagram for these nanowires is systematically developed, which allows for control of nanowire density over three orders of magnitude. Next, we show that by first nucleating nanowires at a low temperature and then growing them at a higher temperature, height and density can be independently selected while maintaining the target density over long growth times. GaN nanowires prepared using this two-step procedure are overgrown with three-dimensionally layered and topologically complex heterostructures of (GaN/AlN). By adjusting the growth temperature in the second growth step either vertical or coaxial nanowire superlattices can be formed. These results indicate that a two-step method allows access to a variety of kinetics at which nanowire nucleation and adatom mobility are adjustable.     

Structural and optical properties of InGaN/GaN nanowire heterostructures grown by PA-MBE

The structural and optical properties of InGaN/GaN nanowire heterostructures grown by plasma-assisted molecular beam epitaxy have been studied using a combination of transmission electron microscopy, electron tomography and photoluminescence spectroscopy. It is found that, depending on In content, the strain relaxation of InGaN may be elastic or plastic. Elastic relaxation results in a pronounced radial In content gradient. Plastic relaxation is associated with the formation of misfit dislocations at the InGaN/GaN interface or with cracks in the InGaN nanowire section. In all cases, a GaN shell was formed around the InGaN core, which is assigned to differences in In and Ga diffusion mean free paths.     

The effect of postgrowth annealing on the structure and optical properties of multilayer Ge/Si heterostructures

The effect of the postgrowth laser and thermal annealing on the structure and optical properties of multilayer heterostructures comprising quantum dots of germanium in a silicon matrix has been studied by photoluminescence (PL) and transmission electron microscopy (TEM). The PL spectra of annealed samples reveal a decrease of emission from the quantum dots and display a new emission band as compared to the initial spectra. The TEM measurements show that this effect is related to smearing of the Ge-Si interface and to the appearance of a regular rectangular network of dislocations on the surface of the annealed structure.     

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.     

Semi-empirical tight binding modelling of CdSTe/CdTe, ZnSSe/ZnSe and ZnSSe/ CdSe heterostructures

We present a semi-empirical sp³s^? tight binding model to calculate the effects of alloy composition and strain on electronic band structure of Cd and Zn based group II-VI heterostructures for photovoltaic devices. The semi-empirical sp³s^? TB model Hamiltonian includes second nearest neighbor interactions and spin-orbit splitting of p-states. Bond lengths and atomic energies of cation and anion forming ternary semiconductors are taken as nonlinear function of composition. The 2NN sp³s^? tight binding model calculations are compared with those of the package program WIEN2k which uses the generalized gradient approximation (GGA) and local spin density approximation (LSDA) based scaling law for the scissor operator for the self energy corrections to the DFT energy band gaps of semiconductors. We found that both the GGA and LSDA corrected WIEN 2k simulations and 2NN sp³s^? TB model accurately reproduces the band gaps and both the valence band and conduction band dispersion curves at Γ, X and L high symmetry points of Brillouin zone, also in good agreement with experiment.     

The Faraday rotation angle of Ni nanowire arrays: its dependence on photon energy and nanowire size

The magneto-optical properties of Ni nanowire arrays embedded in anodic aluminum oxide templates are studied, for a selection of photon energies, as a function of their diameter and length for the first time. This was achieved by the determination of Stokes parameters of the transmitted light. The magneto-optical response is found to differ considerably from that of the bulk material. At all photon energies studied, a linear association of the Faraday rotation angle with nanowire length has been observed; moreover, a proportional relationship between rotation angle per unit length and nanowire diameter has also been also observed, consistent with our earlier work on Fe and Co nanowires. The relationship between the Faraday rotation angle per unit length with different nanowire diameters and photon energy has been found to exhibit clear spectroscopic structure.