Optical properties of heavily doped GaAs nanowires and electroluminescent nanowire structures

We present GaAs electroluminescent nanowire structures fabricated by metal organic vapor phase epitaxy. Electroluminescent structures were realized in both axial pn-junctions in single GaAs nanowires and free-standing nanowire arrays with a pn-junction formed between nanowires and substrate, respectively. The electroluminescence emission peak from single nanowire pn-junctions at 10 K was registered at an energy of around 1.32 eV and shifted to 1.4 eV with an increasing current. The line is attributed to the recombination in the compensated region present in the nanowire due to the memory effect of the vapor-liquid-solid growth mechanism. Arrayed nanowire electroluminescent structures with a pn-junction formed between nanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eV and two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed to the recombination in the p-doped GaAs. The other two lines correspond to the tunneling-assisted photon emission and band-edge recombination in the abrupt junction, respectively. Electroluminescence spectra are compared with the micro-photoluminescence spectra taken along the single p-, n- and single nanowire pn-junctions to find the origin of the electroluminescence peaks, the distribution of doping species and the sharpness of the junctions.     

Band-offset driven efficiency of the doping of SiGe core-shell nanowires

Impurity doping of semiconducting nanowires has been predicted to become increasingly inefficient as the wire diameter is reduced, because impurity states get deeper due to quantum and dielectric confinement. We show that efficient n- and p-type doping can be achieved in SiGe core-shell nanowires as thin as 2 nm, taking advantage of the band offset at the Si/Ge interface. A one-dimensional electron (hole) gas is created at the band-edge and the carrier density is uniquely controlled by the impurity concentration with no need of thermal activation. Additionally, SiGe core-shell nanowires provide naturally the separation between the different types of carriers, electron and holes, and are ideally suited for photovoltaic applications.     

Doping GaP Core–Shell Nanowire pn‐Junctions: A Study by Off‐Axis Electron Holography

The doping process in GaP core–shell nanowire pn‐junctions using different precursors is evaluated by mapping the nanowires' electrostatic potential distribution by means of off‐axis electron holography. Three precursors, triethyltin (TESn), ditertiarybutylselenide, and silane are investigated for n‐type doping of nanowire shells; among them, TESn is shown to be the most efficient precursor. Off‐axis electron holography reveals higher electrostatic potentials in the regions of nanowire cores grown by the vapor–liquid–solid (VLS) mechanism (axial growth) than the regions grown parasitically by the vapor–solid (VS) mechanism (radial growth), attributed to different incorporation efficiency between VLS and VS of unintentional p‐type carbon doping originating from the trimethylgallium precursor. This study shows that off‐axis electron holography of doped nanowires is unique in terms of the ability to map the electrostatic potential and thereby the active dopant distribution with high spatial resolution.     

Realization of a linear germanium nanowire p-n junction

Germanium nanowires grown by chemical vapor deposition exhibit a peculiar dopant incorporation mechanism. The dopant atoms, such as boron and phosphorus, get incorporated through the wire surface, a mechanism which limits the doping modulation along the wire length, and therefore the fabrication of more elaborate structures that combine both n- and p-type doping. Using a novel device design that circumvents these constraints, we demonstrate here a linear Ge nanowire p-n junction.     

Growth of p-type Si nanotubes by catalytic plasma treatments

We have demonstrated a new technique to transform bulk materials into one-dimensional nanostructures. We have shown that p-type Si nanotubes (SiNTs) can be grown by a simple dual RF plasma treatment of p-type Si substrates at 500?°C. These SiNTs have diameters of ～50-80?nm with tubular wall thickness of ～10-15?nm. The use of Cu vapor and reactive plasma has enabled the growth of these SiNTs instead of Si nanowires.     

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.     

p-Type modulation doped InGaN/GaN dot-in-a-wire white-light-emitting diodes monolithically grown on Si(111)

Full-color, catalyst-free InGaN/GaN dot-in-a-wire light-emitting diodes (LEDs) were monolithically grown on Si(111) by molecular beam epitaxy, with the emission characteristics controlled by the dot properties in a single epitaxial growth step. With the use of p-type modulation doping in the dot-in-a-wire heterostructures, we have demonstrated the most efficient phosphor-free white LEDs ever reported, which exhibit an internal quantum efficiency of ～56.8%, nearly unaltered CIE chromaticity coordinates with increasing injection current, and virtually zero efficiency droop at current densities up to ～640 A/cm(2). The remarkable performance is attributed to the superior three-dimensional carrier confinement provided by the electronically coupled dot-in-a-wire heterostructures, the nearly defect- and strain-free GaN nanowires, and the significantly enhanced hole transport due to the p-type modulation doping.     

Zn(1-x)Mg(x)Te nanowires grown by solid source molecular beam epitaxy

This paper reports on the epitaxial growth of single-crystalline ternary Zn(1-x)Mg(x)Te nanowires covering a broad compositional range of molar fraction 0≤x≤0.75. The nanowires were grown on (100), (110), and (111) GaAs substrates using a vapor-liquid-solid mechanism. Solid source molecular beam epitaxy and an Au-based nanocatalyst were used for these purposes. The composition of nanowires can be adjusted by changing the ratio of Mg to Zn molecular beam fluxes. Electron microscopy images show that the nanowires are smooth and slightly tapered. The diameters of the obtained nanowires are from?30 to 70?nm and their length is around 1?μm. X-ray diffraction analysis and transmission electron microscopy reveal that the nanowires have a zinc-blende structure throughout the whole range of obtained compositions, and have a [Formula: see text] growth axis. The Raman measurements reveal both the expected splitting and shift of phonon lines with increasing Mg content, thus proving the substitutional incorporation of Mg into metallic sites of the ZnTe lattice.     

Solid-state diffusion as an efficient doping method for silicon nanowires and nanowire field effect transistors

In this work we investigate doping by solid-state diffusion from a doped oxide layer, obtained by plasma-enhanced chemical vapor deposition (PECVD), as a means for selectively doping silicon nanowires (NWs). We demonstrate both n-type (phosphorous) and p-type (boron) doping up to concentrations of 10(20) cm(-3), and find that this doping mechanism is more efficient for NWs as opposed to planar substrates. We observe no diameter dependence in the range of 25 to 80 nm, which signifies that the NWs are uniformly doped. The drive-in temperature (800-950?°C) can be used to adjust the actual doping concentration in the range 2 × 10(18) to 10(20) cm(-3). Furthermore, we have fabricated NMOS and PMOS devices to show the versatility of this approach and the possibility of achieving segmented doping of NWs. The devices show high I(on)/I(off) ratios of around 10(7) and, especially for the PMOS, good saturation behavior and low hysteresis.     

Device fabrication with solid-liquid-solid grown silicon nanowires

High quality, single-crystal silicon nanowires were successfully grown from silicon wafers with a nickel catalyst by utilizing a solid-liquid-solid (SLS) mechanism. The nanowires were composed of a crystalline silicon core with an average diameter of 10?nm and a thick outer oxide layer of between 20 and 30?nm at a growth temperature of 1000?°C. When utilizing the SLS growth mechanism, the diameter of the silicon nanowire is dependent solely upon the growth temperature, and has no relation to either the size or the shape of the catalyst. The characteristics of the silicon nanowires are highly dependent upon the properties of the silicon substrate, such as the crystal phase of silicon itself, as well as the doping type. The possibility of doping of silicon nanowires grown via the SLS mechanism without any external dopant source was demonstrated by measuring the electrical properties of a silicon nanowire field effect transistor.     

Ion beam doping of silicon nanowires

We demonstrate n- and p-type field-effect transistors based on Si nanowires (SiNWs) implanted with P and B at fluences as high as 10(15) cm (-2). Contrary to what would happen in bulk Si for similar fluences, in SiNWs this only induces a limited amount of amorphization and structural disorder, as shown by electrical transport and Raman measurements. We demonstrate that a fully crystalline structure can be recovered by thermal annealing at 800 degrees C. For not-annealed, as-implanted NWs, we correlate the onset of amorphization with an increase of phonon confinement in the NW core. This is ion-dependent and detectable for P-implantation only. Hysteresis is observed following both P and B implantation.     

Fluorescence spectroscopy of electrochemically self-assembled ZnSe and Mn:ZnSe nanowires

We report room temperature fluorescence spectroscopy (FL) studies of ZnSe and Mn-doped ZnSe nanowires of different diameters (10, 25, 50?nm) produced by an electrochemical self-assembly technique. All samples exhibit increasing blue-shift in the band edge fluorescence with decreasing wire diameter because of quantum confinement. The 10?nm ZnSe nanowires show four distinct emission peaks due to band-to-band recombination, exciton recombination, recombination via surface states and via band gap (trap) states. The exciton binding energy in these nanowires exhibits a giant increase (～10-fold) over the bulk value due to quantum confinement, since the effective wire radius (taking into account side depletion) is smaller than the exciton Bohr radius in bulk ZnSe. The 25 and 50?nm diameter wires show only a single FL peak due to band-to-band electron-hole recombination. In the case of Mn-doped ZnSe nanowires, the band edge luminescence in 10?nm samples is significantly quenched by Mn doping but not the exciton luminescence, which remains relatively unaffected. We observe additional features due to Mn(2+) ions. The spectra also reveal that the emission from Mn(2+) states increases in intensity and is progressively red-shifted with increasing Mn concentration.     

Confinement and surface effects in B and P doping of silicon nanowires

Several experimental groups have achieved effective n- and p-type doping of silicon nanowires (SiNWs). However, theoretical analyses on ultrathin SiNWs suggest that dopants tend to segregate to their surfaces, where they would combine with defects such as dangling bonds (DB), becoming electronically inactive. Using fully ab initio calculations, we show that the differences in formation energies among surface and core substitutional sites decrease rapidly as the diameters of the wires increase, indicating that the dopants will be uniformly distributed. Moreover, occurrence of the electronically inactive impurity/DB complex rapidly becomes less frequent for NWs of larger diameters. We also show that the high confinement in the ultrathin SiNWs causes the impurity levels to be deeper than in the silicon bulk, but our results indicate that for NWs of diameters larger than approximately 3 nm the impurity levels recover bulk characteristics. Finally, we show that different surfaces will lead to different dopant properties in the gap.     

Distribution of active impurities in single silicon nanowires

The distribution of electrically active B concentration in single SiNWs (nanowires) grown by a vapor-liquid-solid (VLS) process was studied by analyzing Fano resonance in Raman spectra. We found a gradient of active B concentration along the growth direction; the B concentration was the largest at the substrate side and the smallest at the catalyst side. The observed concentration gradient suggests the conformal growth of a high B concentration layer during a VLS process. To confirm this effect, we grew SiNWs with controlled impurity profiles, that is, p-type/intrinsic ( p-i) and intrinsic/ p-type ( i-p) SiNWs, by controlling the supply of B source during SiNWs growth. We found that p-i SiNWs can be grown by just stopping the supply of B source in the middle of the growth, while i-p SiNWs were not realized; that is, the whole region of nominal " i-p" SiNWs was B-doped even if we started the supply of B source in the middle of the growth. These results confirm the above doping model. We also found that the distribution of active B concentration was significantly modified by high temperature annealing. By annealing at 1100 degrees C for 1 min, B concentration became almost uniform along 10 mum long SiNWs irrespective of initial B profiles. This suggests very efficient diffusion of B atoms in a defective high B concentration surface layer of SiNWs.     

Preparation and electrical properties of electrospun tin-doped indium oxide nanowires

Well-aligned tin-doped indium (ITO) nanowires have been prepared using the electrospinning process. The Sn doping mechanism and microstructure have been characterized by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Devices for I-V measurement and field-effect transistors (FETs) were assembled using ITO nanowires with top contact configurations. The effect of Sn doping on the electrical conductivity was significant in that it enhanced the conductance by over 10(7) times, up to ～1?S?cm(-1) for ITO nanowires with an Sn content of 17.5 at.%. The nanowire FETs were operated in the depletion mode with an electron mobility of up to 0.45?cm(2)?V(-1)?s(-1) and an on/off ratio of 10(3).     

Manganese-induced growth of GaAs nanowires

GaAs nanowires have been grown on SiO2 and GaAs by molecular beam epitaxy using manganese as growth catalyst. Transmission electron microscopy shows that the wires have a wurtzite-type lattice and that alpha-Mn particles are found at the free end of the wires. X-ray absorption fine structure measurements reveal the presence of a significant fraction of Mn-As bonds, suggesting Mn diffusion and incorporation during wire growth. Transport measurements indicate that the wires are p-type, as expected from doping of GaAs with Mn.     

Fabrication of selective-emitter silicon heterojunction solar cells using hot-wire chemical vapor deposition and laser doping

The Si heterojunction (HJ) solar cells were fabricated on the textured p-type mono-crystalline Si (c-Si) substrates using hot-wire chemical vapor deposition (HWCVD). In view of the potential for the bottom cell in a hybrid junction structure, the microcrystalline Si (μc-Si) film was used as the emitter with various PH₃ dilution ratios. Prior to the n-μc-Si emitter deposition, a 5 nm-thick intrinsic amorphous Si layer (i-a-Si) was grown to passivate the c-Si surface. In order to improve the indium-tin oxide (ITO)/emitter front contact without using the higher PH₃ doping concentration, a laser doping technique was employed to improve the ITO/n-μc-Si contact via the formation of the selective emitter structure. For a cell structure of Ag grid/ITO/n-μc-Si emitter/i-a-Si/textured p-c-Si/Al-electrode, the conversion efficiency (AM1.5) can be improved from 13.25% to 14.31% (cell area: 2 cm × 2 cm) via a suitable selective laser doping process.     

p and n-type germanium layers grown using iso-butyl germane in a III-V metal-organic vapor phase epitaxy reactor

We report on the growth of n- and p-doped Germanium (Ge) on Ge substrates by Metal-Organic Vapor Phase Epitaxy (MOVPE). Iso-butyl germane, a liquid metal-organic source less toxic than Germane, is used as Ge precursor. We demonstrate the p-doping of Germanium by MOVPE using Trimethylgallium. The influence of the growth parameters for n and p-type doping is studied in order to optimize the morphology, the structural and the electrical properties of the Ge layers. The controlled growth of p and n doped Ge layers opens the possibility to realize totally epitaxially grown Ge diodes with improved performances, for example, for solar cell applications.     

Electron Tomography Reveals the Droplet Covered Surface Structure of Nanowires Grown by Aerotaxy

For the purpose of functionalizing III‐V semiconductor nanowires using n‐doping, Sn‐doped GaAs zincblende nanowires are produced, using the growth method of Aerotaxy. The growth conditions used are such that Ga droplets, formed on the nanowire surface, increase in number and concentrations when the Sn‐precursor concentration is increased. Droplet‐covered wires grown with varying Sn concentrations are analyzed by transmission electron microscopy and electron tomography, which together establish the positioning of the droplets to be preferentially on {?111}B facets. These facets have the same polarity as the main wire growth direction, [?1?1?1]B. This means that the generated Ga particles can form nucleation sites for possible nanowire branch growth. The concept of azimuthal mapping is introduced as a useful tool for nanowire surface visualization and evaluation. It is demonstrated here that electron tomography is useful in revealing both the surface and internal morphologies of the nanowires, opening up for applications in the analysis of more structurally complicated systems like radially asymmetrical nanowires. The analysis also gives a further understanding of the limits of the dopants which can be used for Aerotaxy nanowires.     

The growth of ultralong and highly blue luminescent gallium oxide nanowires and nanobelts, and direct horizontal nanowire growth on substrates

We report the growth of ultralong β-Ga(2)O(3) nanowires and nanobelts on silicon substrates using a vapor phase transport method. The growth was carried out in a tube furnace, with gallium metal serving as the gallium source. The nanowires and nanobelts can grow to lengths of hundreds of nanometers and even millimeters. Their full lengths have been captured by both scanning electron microscope (SEM) and optical images. X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) images have been used to study the crystal structures of these nanowires and nanobelts. Strong blue emission from these ultralong nanostructures can be readily observed by irradiation with an ultraviolet (UV) lamp. Diffuse reflectance spectroscopy measurements gave a band gap of 4.56?eV for these nanostructures. The blue emission shows a band maximum at 470?nm. Interestingly, by annealing the silicon substrates in an oxygen atmosphere to form a thick SiO(2) film, and growing Ga(2)O(3) nanowires over the sputtered gold patterned regions, horizontal Ga(2)O(3) nanowire growth in the non-gold-coated regions can be observed. These horizontal nanowires can grow to as long as over 10?μm in length. Their composition has been confirmed by TEM characterization. This represents one of the first examples of direct horizontal growth of oxide nanowires on substrates.