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A study on the formation of InGaN ternary compounds in the three-dimensional growth mode is presented. For the first time, we demonstrate that the self-organization during InGaN growth is responsible for the formation of core–shell nanowires (NWs), nanotubes, zinc blende (ZB) phases, and nanoflowers. It is found that the core–shell InGaN NWs are formed at the very initial stage of growth. An increase in growth time results in the ascending indium diffusion from the cores of NWs and their accumulation at the NW tips, enabling three-dimensional lateral growth and formation of nanotubes. Further nanostructure growth leads to the formation of nanoflowers with empty stems and ZB/wurtzite (WZ) phase interface at the periphery. The observed structural transformations of NWs are supported by transmission electron microscopy and photoluminescence measurements, as well as theoretical estimates. Understanding the formation mechanisms of these complex three-dimensional nanostructures can facilitate the development of InGaN compounds for gas-sensing applications. [ABSTRACT FROM AUTHOR]

We present a study with a numerical model based on k → · p → , including electromechanical fields, to evaluate the electromechanical and optoelectronic properties of single GaAs quantum dots embedded in direct band gap AlGaAs nanowires. The geometry and the dimensions of the quantum dots, in particular the thickness, are obtained from experimental data measured by our group. We also present a comparison between the experimental and numerically calculated spectra to support the validity of our model. [ABSTRACT FROM AUTHOR]

Hybrid nanostructures based on InGaN nanowires with decorated plasmonic silver nanoparticles are investigated in the present study. It is shown that plasmonic nanoparticles induce the redistribution of room temperature photoluminescence between short-wavelength and long-wavelength peaks of InGaN nanowires. It is defined that short-wavelength maxima decreased by 20%, whereas the long-wavelength maxima increased by 19%. We attribute this phenomenon to the energy transfer and enhancement between the coalesced part of the NWs with 10–13% In content and the tips above with an In content of about 20–23%. A proposed Fröhlich resonance model for silver NPs surrounded by a medium with refractive index of 2.45 and spread 0.1 explains the enhancement effect, whereas the decreasing of the short-wavelength peak is associated with the diffusion of charge carriers between the coalesced part of the NWs and the tips above. [ABSTRACT FROM AUTHOR]

GaN nanowires were grown using selective area plasma-assisted molecular beam epitaxy on SiOx/Si(111) substrates patterned with microsphere lithography. For the first time, the temperature–Ga/N2 flux ratio map was established for selective area epitaxy of GaN nanowires. It is shown that the growth selectivity for GaN nanowires without any parasitic growth on a silica mask can be obtained in a relatively narrow range of substrate temperatures and Ga/N2 flux ratios. A model was developed that explains the selective growth range, which appeared to be highly sensitive to the growth temperature and Ga flux, as well as to the radius and pitch of the patterned pinholes. High crystal quality in the GaN nanowires was confirmed through low-temperature photoluminescence measurements. [ABSTRACT FROM AUTHOR]

Combinations of III–V nanowires (NWs) with quantum dots (QDs) are promising building blocks for quantum light sources. Herein, for the first time, the results of growing AlGaAs NWs with InGaAs QDs by molecular‐beam epitaxy on a silicon substrate are shown. The optimal growth temperature is determined and the physical properties of the grown nanostructures are studied. It is shown that the grown nanostructures exhibit photoluminescence (PL) signal up to room temperature in a wide wavelength range, including 1.3 μm emission which is important for the optical fiber transmission. It is found that in addition to InGaAs QDs radial InGaAs quantum wells are formed inside the NWs as a result of lateral/axial InGaAs growth competition. The proposed technology opens up new possibilities for the integration of direct‐band III–V materials with the silicon platform for various applications in the field of silicon photonics and quantum communication technology. [ABSTRACT FROM AUTHOR]

Experimental data on the rapid (≈4.4 nm s−1) axial growth rate of narrow (≈9 nm in radius) InAs nanowires (NWs) obtained by Au‐catalyzed molecular beam epitaxy on Si substrates at a low surface temperature of 270 °C are presented. These NWs exhibit pure wurtzite crystal structure and an unusually high ratio of the average NW length over the effective thickness of deposited InAs of about 60 despite the presence of parasitic InAs islands on the Si substrate. These trends are explained within a dedicated growth model. In the absence of In evaporation, In atoms either diffuse from the substrate to the NW tips or remain in the parasitic layer. This leads to a linear time dependence of the NW length and other unusual growth properties that are thought to have not previously been accessed. [ABSTRACT FROM AUTHOR]

Young's modulus of tapered mixed composition (zinc-blende with a high density of twins and wurtzite with a high density of stacking faults) gallium phosphide (GaP) nanowires (NWs) was investigated by atomic force microscopy. Experimental measurements were performed by obtaining bending profiles of as-grown inclined GaP NWs deformed by applying a constant force to a series of NW surface locations at various distances from the NW/substrate interface. Numerical modeling of experimental data on bending profiles was done by applying Euler–Bernoulli beam theory. Measurements of the nano-local stiffness at different distances from the NW/substrate interface revealed NWs with a non-ideal mechanical fixation at the NW/substrate interface. Analysis of the NWs with ideally fixed base resulted in experimentally measured Young's modulus of 155 ± 20 GPa for ZB NWs, and 157 ± 20 GPa for WZ NWs, respectively, which are in consistence with a theoretically predicted bulk value of 167 GPa. Thus, impacts of the crystal structure (WZ/ZB) and crystal defects on Young's modulus of GaP NWs were found to be negligible. [ABSTRACT FROM AUTHOR]

Rubber materials are the key components of flexible optoelectronic devices, especially for the light-emitting diodes based on arrays of inorganic nanowires (NWs). This paper reports on polydimethylsiloxane-graft-polystyrene (PDMS-St) as a new flexible substrate of GaP NW array structures. The NWs were encapsulated by the newly introduced G-coating method to substitute the inefficient mainstream spin-coating. To further exploit the flexibility and the stretchability of the NW/PDMS-St structures, the ferrocenyl-containing polymethylhydrosiloxane was synthesized and successfully used as an electrode for the NWs. In order to make an alternative highly efficient transparent electrode, a new application of conductive single-walled carbon nanotubes was demonstrated. The novel materials and methods demonstrated unsurpassed mechanical stability of the fabricated flexible electronic devices. [ABSTRACT FROM AUTHOR]

The influence of shadow effect originating from the neighboring nanowires on the nanowire growth is theoretically investigated. The nanowire axial and radial growth rates and the nanowire shape are shown to be strongly dependent on the nanowire surface density and the direction of incident flux. Theoretical predictions are compared with the experimental shapes of InAs nanowires grown by the Au-catalyzed molecular beam epitaxy. In particular, the barrel-like shape observed in dense arrays of InAs nanowires is well described by the model. Very importantly, we show that the shadow effect helps to avoid otherwise enabled radial growth and to preserve the cylindrical nanowire shape. [ABSTRACT FROM AUTHOR]

The Au-assisted molecular beam epitaxial growth of InAs nanowires is discussed. In situ reflection high-energy electron diffraction observations of phase transitions of the catalyst particles indicate that they can be liquid below the eutectic point of the Au-In alloy. The temperature range where the catalyst can be liquid covers the range where we observed nanowire formation (380–430 °C). The variation of nanowire growth rate with temperature is investigated. Pure axial nanowire growth is observed at high temperature while mixed axial/lateral growth occurs at low temperature. The change of the InAs nanowire shape with growth duration is studied. It is shown that significant lateral growth of the lower part of the nanowire starts when its length exceeds a critical value, so that their shape presents a steplike profile along their axis. A theoretical model is proposed to explain the nanowire morphology as a result of the axial and lateral contributions of the nanowire growth. [ABSTRACT FROM AUTHOR]

A semiquantitative theoretical model for nanowire growth, which allows obtaining simple theoretical expressions for the time evolution of nanowire length and radius, is described.

We propose a new insight into the mechanism of low-temperature Au-assisted growth of InAs nanowires during molecular beam epitaxy (MBE). The nanowire MBE growth was achieved at a temperature of 270 °C on both Si(111) and SiC/Si(111) substrates. A special procedure of substrate preparation was used to obtain a high yield of nanowires grown perpendicularly to the substrate. The morphology of the InAs nanowire array was studied by scanning electron microscopy (SEM) revealing a significantly higher percentage of vertical InAs nanowires compared with previous works. The structural properties of nanowires and the catalyst composition were investigated by analytical methods of transmission electron microscopy (TEM). A theoretical assessment of the growth of InAs nanowires in the frame of the classical nucleation theory has shown the possibility of vapor–solid–solid growth at extremely low temperature, e.g., at 270 °C. It was found that the presence of elastic stresses due to the lattice mismatch between the solid catalyst particle and the nanowire material influences the nanowire growth rate. This important feature of nucleation in solid in the case of vapor–solid–solid growth of III–V nanowires was investigated for the first time. Also, we have shown that the material transport of arsenic towards the interface between the catalyst particle and the nanowire top limits the nanowire growth rate. Further development of the low-temperature growth methods facilitates the integration of III–V semiconductors with silicon electronics. [ABSTRACT FROM AUTHOR]

Generation of electric current is observed when GaAs nanowires with wurtzite crystal structure are bent by the probe of an atomic force microscope. The current originates from a piezo active phase in the nanowires due to the piezoelectric effect. Increasing of the piezo‐potential in bent nanowires enhances tunneling through the probe–nanowire Schottky barrier due to the thermionic field emission. Laser illumination amplifies short‐circuit current pulses by two orders of magnitude from 9 pA to 1 nA due to the piezo‐phototronic effect. Utilization of such piezo‐phototronic effect in GaAs nanowires is a solution to accelerate the efficiency of hybrid energy sources “piezoelectric nanogenerator − solar cell” comprised of III–V nanowires. [ABSTRACT FROM AUTHOR]

The piezoelectric effect in wurtzite-type GaAs nanowires was theoretically analyzed using an electromechanical 3D model. The difference between the theoretical and measured piezoelectric strain coefficient is explained by an enhanced pressing force of the contact layer on the nanowire. Simulations show the potential decreasing when nanowires are partly inside in the top metal contact due to a decreasing effective length and pressing force enhancement. [ABSTRACT FROM AUTHOR]

We have designed GaAs/Al0.15Ga0.85As multilayer heterostructure (MH) with diagonal transitions and optimized oscillator strength - 0.425. We investigate the dependence of the MH energy band profile on the electric field and thermal properties of terahertz quantum cascade lasers (THz QCL) under different operation conditions. Furthermore, we develop a technique for the fabrication of THz QCL with double metal waveguide via low-temperature In-Au wafer bonding followed by substrate removal. Inductively coupled plasma reactive ion etching in BCl3/Ar at 15:15 sccm has been used to obtain ridge structure of various widths with vertical sidewalls. [ABSTRACT FROM AUTHOR]

The anomalous piezoelectric effect in GaAs nanowires was detected (the piezoelectric strain coefficient up to d33 ≈ 26 pC/N) that is 12 times larger than the theoretically estimated value. This result is explained by the dominant content of the phase with the wurtzite-type crystal structure in GaAs nanowires and an increased pressing force by the contact layer on the nanowire. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Ex post manipulation of ~1.1µm emitting InGaAs/GaAs-based quantum dot-quantum well tunnel injection light emitting devices is demonstrated experimentally. The devices were operated at elevated forward currents until irreversible alterations were observed. As a result, changes in the steady-state optical spectra (electroluminescence, photoluminescence, and photocurrent), in carrier kinetics, in transport properties, and real structure are found. Except for degradation effects, e.g., of larger quantum dots, also restoration/annealing effects such as increased tunnel barriers are observed. The results furnish evidence for a generic degradation mode of nanostructures. We qualitatively interpret the mechanisms involved on both the nanoscopic and the device scales. [ABSTRACT FROM AUTHOR]

We report on a numerical model of quasi-1-D and quasi-0-dimensional semiconductor heterostructures. This model is strictly based on experimental structures of cylindrical nanocolumns of AlGaAs grown by molecular-beam epitaxy in the (1 1 1) direction. The nanocolumns are of 20–50 nm in diameter and 0.5–1 \mum in length and contain a single GaAs quantum dot of 2 nm in thickness and 15–45 nm in diameter. Since the crystal phase of these nanowires spontaneously switches during the growth from zincblende (Zb) to wurzite (Wz) structures, we implement a continuum elastic model and an eight-band \veck \cdot \vecp  model for polymorph crystal structures. The model is used to compute electromechanical fields, wave-function energies of the confined states and optical transitions. The model compares a pure Zb structure with a polymorph in which the Zb disk of GaAs is surrounded by Wz barriers and results are compared to experimental photoluminescence excitation spectra. The good agreement found between theory and features in the spectra supports the polyphorm model. [ABSTRACT FROM PUBLISHER]

Tailorable synthesis of axially heterostructured epitaxial nanowires (NWs) with a proper choice of materials allows for the fabrication of novel photonic devices, such as a nanoemitter in the resonant cavity. An example of the structure is a GaP nanowire with ternary GaPAs insertions in the form of nano-sized discs studied in this work. With the use of the micro-photoluminescence technique and numerical calculations, we experimentally and theoretically study photoluminescence emission in individual heterostructured NWs. Due to the high refractive index and near-zero absorption through the emission band, the photoluminescence signal tends to couple into the nanowire cavity acting as a Fabry–Perot resonator, while weak radiation propagating perpendicular to the nanowire axis is registered in the vicinity of each nano-sized disc. Thus, within the heterostructured nanowire, both amplitude and spectrally anisotropic photoluminescent signals can be achieved. Numerical modeling of the nanowire with insertions emitting in infrared demonstrates a decay in the emission directivity and simultaneous rise of the emitters coupling with an increase in the wavelength. The emergence of modulated and non-modulated radiation is discussed, and possible nanophotonic applications are considered. [ABSTRACT FROM AUTHOR]

Control of directionality of emissions is an important task for the realization of novel nanophotonic devices based on nanowires. Most of the existing approaches providing high directionality of the light emitted from nanowires are based on the utilization of the tapered shape of nanowires, serving as nanoantenna coupling with the light waveguided in nanowire and the directional output beam. Here we report the beaming of the emitted light with wavelength near 800 nm by naturally formed core-shell AlGaAs NW with multiply GaAs quantum dots (QDs) diameter 30 nm and height 10 nm, while the diameter of NW 130 nm, what does not support efficient emission into waveguided modes, including the mode HE11. Experimental measurements show that intensity of emission for directions in the vicinity of the axis of NW is about two orders of magnitude higher than for perpendicular directions. The developed theoretical approach allowed us to calculate the probability of spontaneous emission for various directions and into waveguided modes and showed that highly directional radiation can be provided by the intrinsic emission properties of cylindrical NW. Our results suggest that for the small diameter of NW, directional emissions are associated with an TM0 leaky mode (when electric field oriented in axial direction) and therefore manifests in an existence of axial electric dipole transitions in quantum dots. [ABSTRACT FROM AUTHOR]

The passivation influence by ligands coverage with trioctylphosphine oxide (TOPO) and TOPO including colloidal CdSe/ZnS quantum dots (QDs) on optical properties of the semiconductor heterostructure, namely an array of InP nanowires (NWs) with InAsP nanoinsertion grown by Au-assisted molecular beam epitaxy on Si (111) substrates, was investigated. A significant dependence of the photoluminescence (PL) dynamics of the InAsP insertions on the ligand type was shown, which was associated with the changes in the excitation translation channels in the heterostructure. This change was caused by a different interaction of the ligand shells with the surface of InP NWs, which led to the formation of different interfacial low-energy states at the NW-ligand boundary, such as surface-localized antibonding orbitals and hybridized states that were energetically close to the radiating state and participate in the transfer of excitation. It was shown that the quenching of excited states associated with the capture of excitation to interfacial low-energy traps was compensated by the increasing role of the "reverse transfer" mechanism. As a result, the effectiveness of TOPO-CdSe/ZnS QDs as a novel surface passivation coating was demonstrated. [ABSTRACT FROM AUTHOR]

Controlled growth of heterostructured nanowires and mechanisms of their formation have been actively studied during the last decades due to perspectives of their implementation. Here, we report on the self-catalyzed growth of axially heterostructured GaPN/GaP nanowires on Si(111) by plasma-assisted molecular beam epitaxy. Nanowire composition and structural properties were examined by means of Raman microspectroscopy and transmission electron microscopy. To study the optical properties of the synthesized nanoheterostructures, the nanowire array was embedded into the silicone rubber membrane and further released from the growth substrate. The reported approach allows us to study the nanowire optical properties avoiding the response from the parasitically grown island layer. Photoluminescence and Raman studies reveal different nitrogen content in nanowires and parasitic island layer. The effect is discussed in terms of the difference in vapor solid and vapor liquid solid growth mechanisms. Photoluminescence studies at low temperature (5K) demonstrate the transition to the quasi-direct gap in the nanowires typical for diluted nitrides with low N-content. The bright room temperature photoluminescent response demonstrates the potential application of nanowire/polymer matrix in flexible optoelectronic devices. [ABSTRACT FROM AUTHOR]

Research regarding ways to increase solar cell efficiency is in high demand. Mechanical deformation of a nanowire (NW) solar cell can improve its efficiency. Here, the effect of uniaxial compression on GaAs nanowire solar cells was studied via conductive atomic force microscopy (C-AFM) supported by numerical simulation. C-AFM I–V curves were measured for wurtzite p-GaAs NW grown on p-Si substrate. Numerical simulations were performed considering piezoresistance and piezoelectric effects. Solar cell efficiency reduction of 50% under a −0.5% strain was observed. The analysis demonstrated the presence of an additional fixed electrical charge at the NW/substrate interface, which was induced due to mismatch between the crystal lattices, thereby affecting the efficiency. Additionally, numerical simulations regarding the p-n GaAs NW solar cell under uniaxial compression were performed, showing that solar efficiency could be controlled by mechanical deformation and configuration of the wurtzite and zinc blende p-n segments in the NW. The relative solar efficiency was shown to be increased by 6.3% under −0.75% uniaxial compression. These findings demonstrate a way to increase efficiency of GaAs NW-based solar cells via uniaxial mechanical compression. [ABSTRACT FROM AUTHOR]

Growth and properties of the self‐catalyzed heterostructured GaP nanowires (NWs) with GaP1 − xAsx insertions in the form of nanodiscs (NDs) grown by means of molecular‐beam epitaxy on Si (111) substrate are studied. To obtain the NDs with the different composition and optoelectronic properties, the ratio of As and P fluxes is varied. Structural properties of the synthesized heterostructures are characterized by means of transmission electron microscopy. Energy dispersive X‐ray spectroscopy is used to study chemical composition of the NDs. The maximum achieved fraction x in the NDs is nearly 60%. Sublinear dependence of As concentration in the ND on the As/P flux ratio is observed and theoretical description for the observed phenomenon is provided. The proposed model can be used to estimate the predicted As/P ratio for the synthesis of GaPAs NDs as well as NWs of the required composition. Microphotoluminescence (μPL) studies demonstrate the appearance of broadband PL signal in the spectral range between 600 and 700 nm, corresponding to the NDs with different compositions. Spectra intensity modulation is found due to longitudinal Fabry–Pérot‐type resonances in the individual NWs. [ABSTRACT FROM AUTHOR]

We report on the defect-free (i) nanostructures formation by sub-critical (less than 1 monolayer) inclusions of Ge in a Si matrix and appearance of the new photoluminescence lines from the multilayer structures correlated with the formation of these nanostructures, and (ii) multilayer structures containing close-to-critical Ge insertions in a Si matrix exhibiting strong photoluminescence at room temperature for the optimally grown samples. [Copyright &y& Elsevier]

Increasing the InN content in the In x Ga 1- x N compound is paramount for optoelectronic applications. It has been demonstrated in homogeneous nanowires or deliberately grown nanowire heterostructures. Here, we present spontaneous core-shell In x Ga 1- x N nanowires grown by molecular beam epitaxy on Si substrates at 625 °C. These heterostructures have a high InN fraction in the cores around 0.4 and sharp interfaces, and exhibit bright photoluminescence at 650 nm. The surprising effect of material separation is attributed to the periodically changing environment for instantaneous growth of single monolayers on top of nanowires. Due to a smaller collection length of N adatoms, each monolayer nucleates under a balanced V/III ratio, but then continues under highly group III rich conditions. As a result, the miscibility gap is suppressed in the cores but remains in the shells. These results provide a simple method for obtaining high-quality InGaN heterostructures emitting in the extended wavelength range.

The electronic properties of semiconductor AIIIBV nanowires (NWs) due to their high surface/volume ratio can be effectively controlled by NW strain and surface electronic states. We study the effect of applied tension on the conductivity of wurtzite In x Ga 1- x As ( x ∼ 0.8) NWs. Experimentally, conductive atomic force microscopy is used to measure the I - V curves of vertically standing NWs covered by native oxide. To apply tension, the microscope probe touching the NW side is shifted laterally to produce a tensile strain in the NW. The NW strain significantly increases the forward current in the measured I - V curves. When the strain reaches 4%, the I - V curve becomes almost linear, and the forward current increases by 3 orders of magnitude. In the latter case, the tensile strain is supposed to shift the conduction band minima below the Fermi level, whose position, in turn, is fixed by surface states. Consequently, the surface conductivity channel appears. The observed effects confirm that the excess surface arsenic is responsible for the Fermi level pinning at oxidized surfaces of III-As NWs.

Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we show that the Fermi level at oxidized cleavage surfaces of ternary Al x Ga 1-x As (0 ≤ x ≤ 0.45) and Ga x In 1-x As (0 ≤ x ≤ 1) alloys is pinned at the same position of 4.8 ± 0.1 eV with regard to the vacuum level. The finding implies a unified mechanism of the Fermi level pinning for such surfaces. Further investigation, performed by Raman scattering and photoluminescence spectroscopy, shows that photooxidation of the Al x Ga 1-x As and Ga x In 1-x As nanowires leads to the accumulation of an excess of arsenic on their crystal surfaces which is accompanied by a strong decrease of the band-edge photoluminescence intensity. We conclude that the surface excess arsenic in crystalline or amorphous forms is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires.

In this paper we study growth of quasi-one-dimensional GaN nanowires (NWs) and nanotube (NT)-like nanostructures on Si(111) substrates covered with a thin AlN layer grown by means of plasma-assisted molecular beam epitaxy. In the first part of our study we investigate the influence of the growth parameters on the geometrical properties of the GaN NW arrays. First, we find that the annealing procedure carried out prior to deposition of the AlN buffer affects the elongation rate and the surface density of the wires. It has been experimentally demonstrated that the NW elongation rate and the surface density drastically depend on the substrate growth temperature, where 800 °C corresponds to the maximum elongation rate of the NWs. In the second part of the study, we introduce a new dopant-stimulated method for GaN nanotube-like nanostructure synthesis using a high-intensity Si flux. Transmission electron microscopy was used to investigate the morphological features of the GaN nanostructures. The synthesized structures have a hexagonal cross-section and possess high crystal quality. We propose a theoretical model of the novel nanostructure formation which includes the role of the dopant Si. Some of the Si-doped samples were studied with the photoluminescence (PL) technique. The analysis of the PL spectra shows that the highest value of donor concentration in the nanostructures exceeds 5∙10 19 cm -3 .

We bury vertical free-standing core-shell GaAs/AlGaAs nanowires by a planar GaAs overgrowth. As the nanowires get buried, their crystalline structure progressively transforms: whereas the upper emerging part retains its initial wurtzite structure, the buried part adopts the zinc blende structure of the burying layer. The burying process also suppresses all the stacking faults that existed in the wurtzite nanowires. We consider two possible mechanisms for the structural transition upon burying, examine how they can be discriminated from each other, and explain why the transition is favorable.

We report on the fabrication by Au-assisted molecular beam epitaxy of InP nanowires with embedded InAsP insertions. The growth temperature affects the nucleation on the nanowire lateral surface. It is therefore possible to grow the wires in two steps: to fabricate an axial heterostructure (at 420 degrees C), and then cover it by a shell (at 390 degrees C). The InAsP alloy composition could be varied between InAs0.35P0.65 and InAs0.5P0.5 by changing the As to P flux ratio. When a shell is present, the InAsP segments show strong room-temperature photoluminescence with a peak wavelength tunable from 1.2 to 1.55 mum by adjusting the As content. If the axial heterostructure has no shell, luminescence intensity is drastically reduced. Low-temperature microphotoluminescence performed on isolated single wires shows narrow peaks with a line width as small as 120 microeV.