Remote p-doping of InAs nanowires

We report on remote p-type doping of InAs nanowires by a p-doped InP shell grown epitaxially on the core nanowire. This approach addresses the challenge of obtaining quantitative control of doping levels in nanowires grown by the vapor-liquid-solid (VLS) mechanism. Remote doping of III-V nanowires is demonstrated here with the InAs/InP system. It is especially challenging to make p-type InAs wires because of Fermi level pinning around 0.1 eV above the conduction band. We demonstrate that shielding with a p-doped InP shell compensates for the built-in potential and donates free holes to the InAs core. Moreover, the off-current in field-effect devices can be reduced up to 6 orders of magnitude. The effect of shielding critically depends on the thickness of the InP capping layer and the dopant concentration in the shell.     

First-principles study of the electronic properties of wurtzite, zinc-blende, and twinned InP nanowires

The electronic properties of zinc-blende, wurtzite, and rotationally twinned InP nanowires were studied using first-principles calculations. The results show that all the simulated nanowires exhibit a semiconducting character, and the band gap decreases with increasing the nanowire size. The band gap difference between the zinc-blende, wurtzite, and twinned InP nanowires and bulk InP can be described by ΔE(g)(wire) = 0.88/D(1.23), ΔE(g)(wire) = 0.79/D(1.22) and ΔE(g)(twin) = 1.3/D(1.19), respectively, where D is the diameter of the nanowires. The valence band maximum (VBM) and conduction band minimum (CBM) originate mainly from the p-orbitals of the P atoms and s-orbitals of the In atoms at the core regions of the nanowires, respectively. The hexagonal (2H) stacking inside the cubic (3C) stacking has no effect on the electronic properties of thin InP nanowires.     

Dynamics of strongly degenerate electron-hole plasmas and excitons in single InP nanowires

Low-temperature time-resolved photoluminescence spectroscopy is used to probe the dynamics of photoexcited carriers in single InP nanowires. At early times after pulsed excitation, the photoluminescence line shape displays a characteristic broadening, consistent with emission from a degenerate, high-density electron-hole plasma. As the electron-hole plasma cools and the carrier density decreases, the emission rapidly converges toward a relatively narrow band consistent with free exciton emission from the InP nanowire. The free excitons in these single InP nanowires exhibit recombination lifetimes closely approaching that measured in a high-quality epilayer, suggesting that in these InP nanowires, electrons and holes are relatively insensitive to surface states. This results in higher quantum efficiencies than other single-nanowire systems as well as significant state-filling and band gap renormalization, which is observed at high electron-hole carrier densities.     

Bidirectional growth of indium phosphide nanowires

We present a bidirectional growth mode of InP nanowires grown by selective-area metalorganic vapor-phase epitaxy (SA-MOVPE). We studied the effect of the supply ratio of DEZn ([DEZn]) on InP grown structure morphology and crystal structures during the SA-MOVPE. Two growth regimes were observed in the investigated range of the [DEZn] on an InP(111)B substrate. At low [DEZn], grown structures formed tripod structures featuring three nanowires branched toward the [111]A directions. At high [DEZn], we obtained hexagonal pillar-type structures vertically grown on the (111)B substrate. These results show that the growth direction changes from [111]A to [111]B as [DEZn] is increased. We propose a growth mechanism based on the correlation between the incident facet of rotational twins and the shapes of the grown structures. Our results bring us one step closer to controlling the direction of nanowires on a Si substrate that has a nonpolar nature. They can also be applied to the development of InP nanowire devices.     

Impact of Pb doping on the optical and electrical properties of ZnO nanowires

In this letter, effect of Pb-doping on the electrical and optical properties of the as grown ZnO nanowires (NWs) have been investigated. The microstructural investigations show that the Pb-dopant substituted into wurtzite ZnO nanowires without forming any secondary phase. The amount of contents and valence state of Pb ions has been investigated through energy dispersive spectroscopy and X-ray photospectroscopy. The doped nanowires show a remarkable reduction of 15.3 nm (127.4 meV) in the optical band gap, while an increase amount of deep-level defects transition in visible luminescence. Furthermore, the reduction in the band gap and the presence of deep-level defects induces strong effect in the electrical resistivity of doped NWs, which makes their potential for the fabrication of nanodevices. The possible growth mechanism is also briefly discussed.     

Growth of GaInAs/AlInAs heterostructure nanowires for long-wavelength photon emission

We investigated the growth of GaInAs/AlInAs heterostructure nanowires on InP(111)B and Si(111) substrates in a metalorganic vapor phase epitaxy reactor. Au colloids were used to deposit Au catalysts 20 and 40 nm in diameter on the substrate surfaces. We obtained vertical GaInAs and AlInAs nanowires on InP(111)B surfaces. The GaInAs nanowires capped with GaAs/AlInAs layers show room-temperature photoluminescence. The peak exhibits a blue-shift when the Ga content in the core GaInAs nanowire is increased. For the GaInAs/AlInAs heterostructure growth, it is possible to change the Ga content sharply but Al also exists in the GaInAs layer regions. We also found that the ratios of Ga and Al contents to In content tend to increase and the axial growth rate to decrease along the nanowire toward the top. We were also able to make vertical GaInAs nanowires on Si(111) surfaces after a short growth of GaP and InP.     

Fabrication of luminescent silver doped PbS nanowires in polymer

We report here fabrication of silver (0 to 1.76 mol%) doped PbS nanowires (radius r approximately 1.75 nm) in polymer by a simple wet chemical process. An X-ray photoelectron spectroscopy study clearly confirms the possibility of silver (Ag) doping in PbS nanowires. Both absorption and photoluminescence spectra reveal very strong quantum confinement effect in PbS nanowires as expected for a r/Bohr radius ratio approximately 0.0972 nm. Visible excitonic emission is observed at room temperature in the photoluminescence spectra of undoped and silver doped PbS nanowires in polymer. The excitonic emission is appreciably blue-shifted when doped by silver (1.76 mol%) indicating strong modification of the electronic states by magnetic silver ions. While Ag2+ centers at the substitutional lattice site show an emission band around 525 nm, Ag0 at the interstitial site act as nonradiative recombination centers. Effect of silver doping on the luminescence intensity is also discussed.     

VLS growth of alternating InAsP/InP heterostructure nanowires for multiple-quantum-dot structures

We investigated the Au-assisted growth of alternating InAsP/InP heterostructures in wurtzite InP nanowires on InP(111)B substrates for constructing multiple-quantum-dot structures. Vertical InP nanowires without stacking faults were obtained at a high PH(3)/TMIn mole flow ratio of 300-1000. We found that the growth rate changed largely when approximately 40 min passed. Ten InAsP layers were inserted in the InP nanowire, and it was found that both the InP growth rate and the background As level increased after the As supply. We also grew the same structure using TBAs/TBP and could reduce the As level in the InP segments. A simulation using a finite-difference time-domain method suggests that the nanowire growth was dominated by the diffusion of the reaction species with long residence time on the surface. For TBAs/TBP, when the source gases were changed, the formed surface species showed a short diffusion length so as to reduce the As background after the InAsP growth.     

Photomodulated rayleigh scattering of single semiconductor nanowires: probing electronic band structure

The internal electronic structures of single semiconductor nanowires can be resolved using photomodulated Rayleigh scattering spectroscopy. The Rayleigh scattering from semiconductor nanowires is strongly polarization sensitive which allows a nearly background-free method for detecting only the light that is scattered from a single nanowire. While the Rayleigh scattering efficiency from a semiconductor nanowire depends on the dielectric contrast, it is relatively featureless as a function of energy. However, if the nanowire is photomodulated using a second pump laser beam, the internal electronic structure can be resolved with extremely high signal-to-noise and spectral resolution. The photomodulated Rayleigh scattering spectra can be understood theoretically as a first derivative of the scattering efficiency that results from a modulation of the band gap and depends sensitively on the nanowire diameter. Fits to spectral lineshapes provide both the band structure and the diameter of individual GaAs and InP nanowires under investigation.     

Gallium nitride nanowires doped with magnesium

GaN nanowires doped with Mg have been synthesized on Si (111) substrate through ammoniating Ga₂O₃ films doped with Mg under flowing ammonia atmosphere. The Mg-doped GaN nanowires were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL). The results demonstrate that the nanowires were single crystalline with hexagonal wurzite structure. The diameters of the nanowires ranged 20-30 nm and the lengths were about hundreds of micrometers. The intense PL peak at 359 nm showed a blueshift from the bulk band gap emission, attributed to Burstein-Moss effect. The growth mechanism of the crystalline GaN nanowires is discussed briefly.     

Growth of InAs/InP core-shell nanowires with various pure crystal structures

We have studied the epitaxial growth of an InP shell on various pure InAs core nanowire crystal structures by metal-organic vapor phase epitaxy. The InP shell is grown on wurtzite (WZ), zinc-blende (ZB), and {111}- and {110}-type faceted ZB twin-plane superlattice (TSL) structures by tuning the InP shell growth parameters and controlling the shell thickness. The growth results, particularly on the WZ nanowires, show that homogeneous InP shell growth is promoted at relatively high temperatures (～500?°C), but that the InAs nanowires decompose under the applied conditions. In order to protect the InAs core nanowires from decomposition, a short protective InP segment is first grown axially at lower temperatures (420-460?°C), before commencing the radial growth at a higher temperature. Further studies revealed that the InP radial growth rate is significantly higher on the ZB and TSL nanowires compared to WZ counterparts, and shows a strong anisotropy in polar directions. As a result, thin shells were obtained during low temperature InP growth on ZB structures, while a higher temperature was used to obtain uniform thick shells. In addition, a schematic growth model is suggested to explain the basic processes occurring during the shell growth on the TSL crystal structures.     

Zinc blende and wurtzite crystal phase mixing and transition in indium phosphide nanowires

Indium phosphide (InP) nanowires, which have crystal phase mixing and transition from zinc blende (ZB) to wurtzite (WZ), are grown in intermediate growth conditions between ZB and WZ by using selective-area metalorganic vapor phase epitaxy (SA-MOVPE). The shape of InP nanowires is tapered unlike ZB or WZ nanowires. A growth model has been developed for the tapered nanowires, which is simply described as the relationship between tapered angle and the ratio of ZB and WZ segments. In addition, the peak energy shift in photoluminescence measurement was attributed to the quantum confinement effect of the quantum well of the ZB region located in the polytypic structure of ZB and WZ in nanowires.     

Formation of wurtzite InP nanowires explained by liquid-ordering

We report an in situ surface X-ray diffraction study of liquid AuIn metal alloys in contact with zinc-blende InP (111)(B) substrates at elevated temperatures. We observe strong layering of the liquid metal alloy in the first three atomic layers in contact with the substrate. The first atomic layer of the alloy has a higher indium concentration than in bulk. In addition, in this first layer we find evidence for in-plane ordering at hollow sites, which could sterically hinder nucleation of zinc-blende InP. This can explain the typical formation of the wurtzite crystal structure in InP nanowires grown from AuIn metal particles.     

Structural and optical characterization of strained free-standing InP nanowires

The structural and optical properties of high-quality crystalline strained InP nanowires are reported in this article. The nanowires were produced by the vapor-liquid-solid growth method in a chemical-beam epitaxy reactor, using 20 nm gold nanoparticles as catalysts. Polarization-resolved photoluminescence experiments were carried out to study the optical properties of the InP nanowires. These experiments revealed a large blue shift of 74 meV of the first electron-to-heavy hole optical transition in the nanowires, which cannot be solely explained by quantum size effects. The blue shift is mainly attributed to the presence of biaxial compressive strain in the inward radial direction of the InP nanowires. High-resolution transmission electron microscopy Electron and selected area electron diffraction experiments show that the nanowires have high crystal quality and grow along a [001] axes. These experiments also confirmed the presence of 1.8% compressive radial strain and 2% tensile longitudinal strain in the nanowires. A simple theoretical model including both quantum confinement and strain effects consistently describes the actual energy position of the InP nanowires optical emission.     

Structural and optical properties of Al-doped SnO2 nanowires

We report a facile thermal evaporation method for the syntheses of Al-doped SnO₂ nanowires using Al-doped SnO₂ nanoparticles as precursors. High-density, single-crystalline Al-doped SnO₂ nanowires were directly grown on the 6H-SiC substrates without any catalyst. X-ray diffraction patterns show that the Al dopants are incorporated into the rutile SnO₂ nanowires. The X-ray photoelectron spectra confirm the SnO₂ nanowires doped with 5 at.% Al. The photoluminescence spectra of the Al-doped SnO₂ nanowires exhibit that the large blue shift of the emission band can be observed in the Al-doped SnO₂ nanowires compared with undoped nanowires. The distortion of the crystal lattices caused by incorporation of Al atoms at the interstitials should be responsible for the large blue shift of the emission band.     

The effects of oxygen on the surface passivation of InP nanowires

The effects of surface passivation on the electronic and structural properties of InP nanowires have been investigated by first-principles calculations. We compare the properties of nanowires whose surfaces have been passivated in several ways, always by H atoms and OH radicals. Taking as the initial reference nanowires that are fully passivated by H atoms, we find that the exchange of these atoms at the surface by OH radicals is always energetically favorable. A nanowire fully passivated by OH radicals is about 2.5?eV per passivated dangling bond more stable than a nanowire fully passivated by H atoms. However, the energetically most stable passivated surface is predicted to have all In atoms bonded to OH radicals and all P atoms bonded to H atoms. This mixed passivation is 2.66?eV per passivated dangling bond more stable than a nanowire fully passivated by H atoms. Our results show that, in comparison with the fully H-saturated nanowire, the fully OH-saturated nanowire has a smaller energy band gap and localized states near the energy band edges. Also, more interestingly, concerning optical applications, the most stable H+OH passivated nanowire has a well-defined energy band gap, only 10% smaller than the H-saturated nanowire energy gap, and few localized states always close to the valence band maximum.     

On the p-type character of Cd-and Zn-doped InAs nanowires

InAs nanowires are potential materials for high speed nanoelectronic devices due to their high electron mobility among the semiconductor nanostructures. One of the main challenges, however, is to obtain a p-type InAs material, since the Fermi level is usually pinned at the conduction band, leading to an intrinsic n-type behaviour. Here we show through first principles calculations that InAs nanowires, doped with Cd or Zn substitutional impurities, can behave as p-type materials. Differently from other III-V nanowires, these impurities introduce shallow acceptor levels. We show that the Zn impurity can be equally distributed along the nanowire radius, naturally compensating the surface levels. On the other hand, the Cd impurity is preferentially found in the core region, requiring a surface treatment to eliminate the surface pinning levels. These results explain the available experiments and show how and why p-type InAs nanowires can be obtained.     

Network-bridge structure of CdSxSe₁-x nanowire-based optical sensors

We report on the synthesis of CdS(x)Se(1-x) nanowires by pulsed-laser deposition and their application to optical sensors. We developed a suspended structure for a nanowire-based optical sensor. This structure comprised separated nanowires that were suspended in the desired position between two pre-patterned electrodes. We found from measuring photoluminescence that the direct bandgap energy of the nanowires changes linearly with the composition of sulfur in the nanowires. These findings show that the bandgap energy of the nanowires can be systematically modulated in the range of 1.7-2.4 eV. The cutoff wavelength of the fabricated optical sensors shifted toward the longer wavelength with increasing sulfur composition. We found that the CdS(x)Se(1-x) nanowires have sufficient potential for a broad band optoelectronic device involving photosensors.     

Optical properties of rotationally twinned InP nanowire heterostructures

We have developed a technique so that both transmission electron microscopy and microphotoluminescence can be performed on the same semiconductor nanowire over a large range of optical power, thus allowing us to directly correlate structural and optical properties of rotationally twinned zinc blende InP nanowires. We have constructed the energy band diagram of the resulting multiquantum well heterostructure and have performed detailed quantum mechanical calculations of the electron and hole wave functions. The excitation power dependent blue-shift of the photoluminescence can be explained in terms of the predicted staggered band alignment of the rotationally twinned zinc blende/wurzite InP heterostructure and of the concomitant diagonal transitions between localized electron and hole states responsible for radiative recombination. The ability of rotational twinning to introduce a heterostructure in a chemically homogeneous nanowire material and alter in a major way its optical properties opens new possibilities for band-structure engineering.     

Unit cell parameters of wurtzite InP nanowires determined by x-ray diffraction

High resolution x-ray diffraction is used to study the structural properties of the wurtzite polytype of InP nanowires. Wurtzite InP nanowires are grown by metal-organic vapor phase epitaxy using S-doping. From the evaluation of the Bragg peak position we determine the lattice parameters of the wurtzite InP nanowires. The unit cell dimensions are found to differ from the ones expected from geometric conversion of the cubic bulk InP lattice constant. The atomic distances along the c direction are increased whereas the atomic spacing in the a direction is reduced in comparison to the corresponding distances in the zinc-blende phase. Using core/shell nanowires with a thin core and thick nominally intrinsic shells we are able to determine the lattice parameters of wurtzite InP with a negligible influence of the S-doping due to the much larger volume in the shell. The determined material properties will enable the ab initio calculation of electronic and optical properties of wurtzite InP nanowires.