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(In, Ga) alloy droplets are used to catalyse the growth of (In, Ga)As nanowires by molecular beam epitaxy on Si(111) substrates. The composition, morphology and optical properties of these nanowires can be tuned by the employed elemental fluxes. To incorporate more than 10% of In, a high In/(In+Ga) flux ratio above 0.7 is required. We report a maximum In content of almost 30% in bulk (In, Ga)As nanowires for an In/(In+Ga) flux ratio of 0.8. However, with increasing In/(In+Ga) flux ratio, the nanowire length and diameter are notably reduced. Using photoluminescence and cathodoluminescence spectroscopy on nanowires covered by a passivating (In, Al)As shell, two luminescence bands are observed. A significant segment of the nanowires shows homogeneous emission, with a wavelength corresponding to the In content in this segment, while the consumption of the catalyst droplet leads to a spectrally-shifted emission band at the top of the nanowires. The (In,Ga)As nanowires studied in this work provide a new approach for the integration of infrared emitters on Si platforms. [ABSTRACT FROM AUTHOR]

The self-assembly of heteroepitaxial GaN nanowires using either molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results in wafer-scale ensembles with ultrahigh (>10 μ m−2) or ultralow (<1 μ m−2) densities, respectively. A simple means to tune the density of well-developed nanowire ensembles between these two extremes is generally lacking. Here, we examine the self-assembly of SiN x patches on TiN(111) substrates which are eventually acting as seeds for the growth of GaN nanowires. We first found that if prepared by reactive sputtering, the TiN surface is characterized by {100} facets for which the GaN incubation time is extremely long. Fast GaN nucleation is only obtained after deposition of a sub-monolayer of SiN x atoms prior to the GaN growth. By varying the amount of pre-deposited SiN x , the GaN nanowire density could be tuned by three orders of magnitude with excellent uniformity over the entire wafer, bridging the density regimes conventionally attainable by direct self-assembly with MBE or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of the GaN nanowires on nanometric SiN x patches. The photoluminescence analysis of single freestanding GaN nanowires reveals a band edge luminescence dominated by excitonic transitions that are broad and blue shifted compared to bulk GaN, an effect that is related to the small nanowire diameter and to the presence of a thick native oxide. The approach developed here can be principally used for tuning the density of most III–V semiconductors nucleus grown on inert surfaces like 2D materials. [ABSTRACT FROM AUTHOR]

We introduce a facile route for the top-down fabrication of ordered arrays of GaN nanowires with aspect ratios exceeding 10 and diameters below 20 nm. Highly uniform thin GaN nanowires are first obtained by lithographic patterning a bilayer Ni/SiN x hard mask, followed by a combination of dry and wet etching in KOH. The SiN x is found to work as an etch stop during wet etching, which eases reproducibility. Arrays with nanowire diameters down to (33 ± 5) nm can be achieved with a uniformity suitable for photonic applications. Next, a scheme for digital etching is demonstrated to further reduce the nanowire diameter down to 5 nm. However, nanowire breaking or bundling is observed for diameters below ≈20 nm, an effect that is associated to capillary forces acting on the nanowires during sample drying in air. Explicit calculations of the nanowire buckling states under capillary forces indicate that nanowire breaking is favored by the incomplete wetting of water on the substrate surface during drying. The observation of intense nanowire photoluminescence at room-temperature indicates good compatibility of the fabrication route with optoelectronic applications. The process can be principally applied to any GaN/SiN x nanostructures and allows regrowth after removal of the SiN x mask. [ABSTRACT FROM AUTHOR]

A systematic study of layered structures containing an InGaNAs/GaAs single quantum well (SQW) sandwiched between type-II AlAs/GaAs superlattices (SL) was carried out. The predominant optically detected cyclotron resonance (ODCR) peak was shown to be related to carriers with a 2D character and a cyclotron resonance effective mass.

We investigated the origin of the high reverse leakage current in light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. To this end, capacitance deep level transient spectroscopy(DLTS) and temperature-dependent current-voltage (I-V) measurements were performed on a fully processed NW-LED. The DLTS measurements reveal the presence of two distinct electron traps with high concentrations in the depletion region of the p-i-n junction. These band gap states are located at energies of 570 ± 20 and 840 ± 30 meV below the conduction band minimum. The physical origin of these deep level states is discussed. The temperature-dependent I-V characteristics, acquired between 83 and 403 K, show that different conduction mechanisms cause the observed leakage current. On the basis of all these results, we developed a quantitative physical model for charge transport in the reverse bias regime. By taking into account the mutual interaction of variable range hopping and electron emission from Coulombic trap states, with the latter being described by phonon-assisted tunnelling and the Poole-Frenkel effect, we can model the experimental I-V curves in the entire range of temperatures with a consistent set of parameters. Our model should be applicable to planar GaN-based LEDs as well. Furthermore, possible approaches to decrease the leakage current in NW-LEDs are proposed. [ABSTRACT FROM AUTHOR]

Si doped (In,Ga)N nanowires (In content up to 0.4) are grown on Si(111) substrates by plasmaassisted molecular beam epitaxy. By increasing the Si doping level, coalescence between nanowires is reduced and a more uniform morphology is obtained. The Raman spectra from highly doped samples show a characteristic broad band in the optical phonon frequency range, which became more prominent at higher doping levels. This Raman band can be explained by plasmonphonon scattering from a free electron gas with strong wave-vector nonconservation, providing evidence for successful n-type doping. The measured plasmon-phonon modes are explained by lineshape simulations taking into account the simultaneous contribution of both the charge-density fluctuation and the impurity induced Fröhlich scattering mechanisms. The according lineshape analysis allows for an estimate of the carrier concentration. [ABSTRACT FROM AUTHOR]

Piezoelectric semiconductor III-nitride nanostructures have received increasing interest as an alternative material for energy harvesters, sensors, and self-sustainable electronics, demanding further clarification of their piezoelectric behavior. Despite the feasibility of piezoresponse force microscopy (PFM) to resolve piezoresponses at the nanoscale, several difficulties arise when the measurements are performed on low piezocoefficient materials due to various artifacts. This work shows that semi-quantitative PFM on low piezocoefficient III-nitrides can be achieved in high-aspect-ratio nanostructures, such as nanowires or nanorods. For conventional bulk and thin films, accurate determination of their piezoresponses is limited because of clamping and bending effects which can occur simultaneously during PFM measurements. While the clamping effect only reduces the piezoresponse amplitude, the bending motion either increases or decreases this amplitude and can also rotate the phase by 180°. Improved electric field distribution in nanowires minimizes both artifacts, allowing correct determination of crystal polarities and piezocoefficients. In contrast to the reports in the literature, we do not observe giant piezoelectricity in III-N nanowires with a diameter in the range of 30–80 nm. This work provides an access to fundamental parameters for developing III-N-based piezoelectric nanodevices. [ABSTRACT FROM AUTHOR]

We report on our results from a systematic study of layered structures containing an InGaNAs/GaAs single quantum well (SQW) enclosed between staggered type II AlAs/GaAs superlattices (SL), by the photoluminescence (PL) and optically detected cyclotron resonance (ODCR) techniques. Besides the ODCR signal known to originate from electrons in GaAs, the predominant ODCR peak is shown to be related to carriers with a two-dimensional character and a cyclotron resonance effective mass of m*≈(0.51–0.56)m0. The responsible carriers are ascribed to electrons on the ellipsoidal equienergy surface at the AlAs X point of the Brillouin zone within the SL, based on results from angular and spectral dependences of the ODCR signal. No ODCR signal related to the InGaNAs SQW was detected, presumably due to low carrier mobility despite the high optical quality. Multiple absorption of photons with energy below the band gap energy of the SL and the GaAs barriers was observed, which bears implication on the efficiency of light-emitting devices based on these structures. [ABSTRACT FROM AUTHOR]

Photoluminescence in (GaIn)(NAs) quantum wells designed for laser emission was studied experimentally and theoretically. The observed temperature dependences of the luminescence Stokes shift and of the spectral linewidth evidence the essential role of disorder in the dynamics of the recombining excitations. The spatial and energy disorders can cause a localization of photocreated excitations supposedly in the form of excitons. Theoretical study of the exciton dynamics is performed via kinetic Monte Carlo simulations of exciton hopping and recombination in the manifold of localized states. Direct comparison between experimental spectra and theoretical calculations provides quantitative information on the energy scale of the potential fluctuations in (GaIn)(NAs) quantum wells. The results enable one to quantify the impact of annealing on the concentration of localized states and/or on the localization length of excitons in (GaIn)(NAs) quantum wells. [ABSTRACT FROM AUTHOR]

InAs(N) quantum dots (QDs) were grown on GaAs(001) by solid source molecular beam epitaxy. Nitrogen was supplied by a radio-frequency plasma cell. The samples were characterized by atomic force microscopy, photoluminescence (PL) spectroscopy, and transmission electron microscopy. With an increasing amount of nitrogen supplied to the surface, large QDs form and the small QDs typical for nitrogen free growth gradually disappear. For intermediate amounts of nitrogen, small and large QDs coexist in a bimodal distribution. The large QDs contain misfit dislocations and are thus plastically relaxed. These dislocations are already induced in the wetting layer by exposure to the nitrogen plasma and act as nucleation centers for the large QDs. The incorporation of material into relaxed QDs is preferred, which leads to an elimination of the small strained QDs. When increasing the amount of nitrogen, the expected strong redshift in the PL wavelength does not occur. This leads to the conclusion that only a little nitrogen is incorporated into the small coherent QDs. [ABSTRACT FROM AUTHOR]

A growth diagram for molecular beam epitaxy of AlN on sapphire and 6H-SiC was established using reflection high energy electron diffraction, atomic force microscopy, and Rutherford backscattering spectrometry. In varying the Al/N ratio and growth temperature, distinctive surface morphologies emerge, which are assigned to three regimes of growth, one N-rich (Al/N < 1) and two Al-rich (Al/N > 1) regimes. Under N-rich conditions, AlN films exhibit rough surface morphologies. In contrast, Al-rich conditions produce excellent smooth surface morphologies, but with the constraint of Al droplet formation at very high Al/N ratios and low temperatures. The differentiation between N-rich and Al-rich regimes is given only by the Al/N ratio, while the two Al-rich regimes (intermediate self-regulated and droplet regime) are separated by the boundary line of Al droplet formation. For this boundary an Arrhenius dependence of growth temperature was found, yielding an activation energy of 3.4 ± 0.1 eV. The observed morphology transitions are attributed to varying surface adatom mobilities present under the different Al/N ratios. [ABSTRACT FROM AUTHOR]

The strong light-matter interaction in transition metal dichalcogenide (TMD) monolayers (MLs) is governed by robust excitons. Important progress has been made to control the dielectric environment surrounding the MLs, especially through hexagonal boron nitride (hBN) encapsulation which drastically reduces the inhomogeneous contribution to the exciton linewidth. Most studies use exfoliated hBN from high quality flakes grown under high pressure. In this work, we show that hBN grown by molecular beam epitaxy (MBE) over a large surface area substrate has a similarly positive impact on the optical emission from TMD MLs. We deposit MoS2 and MoSe2 MLs on ultrathin hBN films (few MLs thick) grown on Ni/MgO(111) by MBE. Then, we cover them with exfoliated hBN to finally obtain an encapsulated sample: exfoliated hBN/TMD ML/MBE hBN. We observe improved optical quality of our samples compared to TMD MLs exfoliated directly on SiO2 substrates. Our results suggest that hBN grown by MBE could be used as a flat and charge-free substrate for fabricating TMD-based heterostructures on a larger scale. [ABSTRACT FROM AUTHOR]

We study the optical properties of coaxial GaAs/InxGa1-xAs core/multishell nanowires with x between 0.2 and 0.4 at 10K. The evolution of the photoluminescence energy of the InxGa1-xAs quantum well shell with x and shell thickness agrees with the result of 8-band k · p calculations, demonstrating that the shell growth is pseudomorphic. At low excitation power, the photoluminescence from the shell is dominated by the recombination of exciton states deeply localized within the shell. We show that these states are associated with crystal-phase quantum rings that form at polytype segments of the InxGa1-xAs quantum well shell. [ABSTRACT FROM AUTHOR]

We investigate GaN nanowire ensembles spontaneously formed in plasma-assisted molecular beam epitaxy by non-destructive low-energy electron diffraction (LEED) and x-ray photoelectron diffraction (XPD). We show that GaN nanowire ensembles prepared on AlN-buffered 6H-SiC(000Ï) substrates with well-defined N polarity exhibit similar LEED intensity-voltage curves and angular distribution of photo-emitted electrons as N-polar free-standing GaN layers. Therefore, as in the case of GaN layers, LEED and XPD are found to be suitable techniques to assess the polarity of GaN nanowire ensembles on a macroscopic scale. The analysis of GaN nanowire ensembles prepared on bare Si(111) allows us to conclude that, on this non-polar substrate, the majority of nano-wires is also N-polar. [ABSTRACT FROM AUTHOR]

We investigate the effect of the p-type top contact on the optoelectronic characteristics of light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. We compare devices fabricated with either Ni/Au or indium tin oxide (ITO) top contact. The NW-LEDs with ITO exhibit a number density of NWs emitting electroluminescence about ten times higher, significantly lower turn-on voltage and series resistance, and a relative external quantum efficiency more than one order of magnitude higher than the sample with Ni/Au. These results show that limitations in the performance of such devices reported so far can be overcome by improving the p-type top-contact. [ABSTRACT FROM AUTHOR]

The oscillating piezoelectric fields accompanying surface acoustic waves are able to transport charge carriers in semiconductor heterostructures. Here, we demonstrate high-frequency (above 1 GHz) acoustic charge transport in GaAs-based nanowires deposited on a piezoelectric substrate. The short wavelength of the acoustic modulation, smaller than the length of the nanowire, allows the trapping of photo-generated electrons and holes at the spatially separated energy minima and maxima of conduction and valence bands, respectively, and their transport along the nanowire with a well defined acoustic velocity towards indium-doped recombination centers. [ABSTRACT FROM AUTHOR]

We use Raman scattering to investigate how light penetrates into and escapes out of GaN nanowires with diameters and interspacings smaller than the effective wavelength of the light. Quantitatively, the efficiencies for light coupling into and out of the nanowires can be explained by considering the nanowire ensemble as a homogeneous effective medium. The observed phonon modes in the Raman spectra, however, suggest that light enters the nanowires to a considerable extent through their side facets even for normal incidence. This finding is supported by rigorous calculations of the spatially distributed light intensity within a periodic nanowire structure. [ABSTRACT FROM AUTHOR]

The article reports that relatively small changes in growth parameters, such as the active layer growth temperature, have been shown to have dramatic effects on the threshold current density for GaInNAs-based lasers. Lasers based upon GaInNAs/GaAs have become of major interest as an alternative to GaInAsP/InP. Better electron confinement is expected because of the much larger conduction band offset and because wide-bandgap AlGaAs can be used to form the cladding regions. A is the monomolecular recombination coefficient, describing recombination through defects and is proportional to the defect density ND.

Employs the ion beam mass spectrometer to study chemical vapor deposition plasmas. Measurement of the ion composition and the ion energy distribution of the plasma in the immediate vicinity of the substrate surface; Construction and technical performance of the system; Components of the spectrometer.

In previous studies, bacterial cultures were isolated that had the ability to degrade the nitrate ester glyceryl trinitrate (i.e. nitroglycerin). The goal of the present study was to examine the ability of resting cells and cell-free extracts of the isolate Bacillus sp. ATCC 51912 to degrade the more recalcitrant nitrate ester propylene glycol dinitrate (PGDN). It was observed that the PGDN-denitrating activity was expressed during growth even when cells were cultured in the absence of nitrate esters. This indicates that nitrate esters are not required for expression of denitration activity. Using cell-free extracts, PGDN was observed to be sequentially denitrated to propylene glycol mononitrate (PGMN) and propylene glycol with the second denitration step proceeding more slowly than the first. Also it was observed that dialysis of the cell-free extracts did not affect denitration activity indicating that regenerable cofactors [e.g. NAD(P)H or ATP] are not required for denitration. [ABSTRACT FROM AUTHOR]

The nucleation process of self-induced GaN nanowires grown by molecular beam epitaxy has been investigated by reflection high-energy electron diffraction measurements. It is found that stable nuclei in the form of spherical cap-shaped islands develop only after an incubation time that is strongly dependent upon the growth conditions. Its evolution with the growth temperature and gallium rate has been described within standard island nucleation theory, revealing a nucleation energy of 4.9 ± 0.1 eV and a very small nucleus critical size. The consideration of the incubation time is critical for the control of the nanowire morphology. [ABSTRACT FROM AUTHOR]

Self-induced GaN nanowires are grown by plasma-assisted molecular beam epitaxy, with InxGa1-xN quantum wells inserted to form an axial superlattice. From the ω-2θ scans of a laboratory x-ray diffraction experiment, we obtain the superlattice period, the thickness of the quantum wells, and the In content in this layer. The axial growth rate of the InxGa1-xN quantum wells is significantly enhanced, which we attribute to increased Ga diffusion along the nanowire sidewalls in the presence of In. [ABSTRACT FROM AUTHOR]

The evolution of the density of self-induced GaN nanowires as a function of the growth time, gallium rate, and growth temperature has been investigated by scanning and transmission electron microscopy. Nucleation and coalescence effects have been disentangled and quantified by distinguishing between single nanowires and nanowire clusters. Owing to the very specific nanowire nucleation mechanism involving a shape transition from spherical-cap-shaped islands, the nanowire density does not follow the standard island nucleation theory. Furthermore, the detrimental nanowire coalescence process can be significantly reduced by raising the growth temperature. [ABSTRACT FROM AUTHOR]

Recently discovered effect of spin-filtering and spin amplification in GaNAs enables us to reliably obtain detailed information on the degree of spin loss during optical spin injection across a semiconductor heterointerface at room temperature. Spin polarization of electrons injected from GaAs into GaNAs is found to be less than half of what is generated in GaNAs by optical orientation. We show that the observed reduced spin injection efficiency is not only due to spin relaxation in GaAs, but more importantly due to spin loss across the interface due to structural inversion asymmetry and probably also interfacial point defects. [ABSTRACT FROM AUTHOR]