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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 spatial distribution of the leakage current through ZrAlxOy thin films with different degrees of crystallinity was investigated at the nanometer-scale. Conductive atomic-force microscopy shows leakage currents at low electric fields in the polycrystalline but not in the amorphous films. Leakage occurs at large crystallites that protrude from the surface. Ring-shaped current distributions around some of the crystallites suggest a conduction at the boundary of crystalline grain and amorphous matrix, and not through the grain. In contrast, the leakage spots that are observed in amorphous films at high electric fields are not correlated to the morphology. [ABSTRACT FROM AUTHOR]

A correction to the article "Analyzing the growth of InxGa1-xN/GaN superlattices in self-induced GaN nanowires by x-ray diffraction" that was published in the 2011 issue is presented.

We investigate the Raman intensity of E2H phonons in wurtzite GaAs nanowire ensembles as well as single nanowires as a function of excitation wavelength. For nanowires with radii in the range of 25 nm, an almost complete quenching of the E2H phonon line is observed for excitation wavelengths larger than 600 nm. The observed behavior is quantitatively explained by the dielectric polarization contrast for the coupling of light into the GaAs nanowires. Our results define the limits of Raman spectroscopy for the detection of the wurtzite phase in semiconductor nanowires. [ABSTRACT FROM AUTHOR]

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]

We investigate the strain state of ensembles of thin and nearly coalescence-free self-assembled GaN nanowires prepared by plasma-assisted molecular beam epitaxy on Ti/Al 2 O 3 (0001) substrates. The shifts of Bragg peaks in high-resolution X-ray diffraction profiles reveal the presence of a homogeneous tensile strain in the out-of-plane direction. This strain is inversely proportional to the average nanowire radius and results from the surface stress acting on the nanowire sidewalls. The superposition of strain from nanowires with different radii in the same ensemble results in a broadening of the Bragg peaks that mimics an inhomogeneous strain on a macroscopic scale. The nanowire ensembles show a small blueshift of the bound-exciton transitions in photoluminescence spectra, reflecting the existence of a compensating in-plane compressive strain, as further supported by grazing incidence X-ray diffraction measurements carried out at a synchrotron. By combining X-ray diffraction and photoluminescence spectroscopy, the surface stress components f x and f z of the air-exposed GaN { 1 1 ¯ 00 } planes that constitute the nanowire sidewalls are determined experimentally to be 2.25 and − 0.7 N/m, respectively. [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]

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 examine how substrate selection impacts the resulting film properties in graphene growth by molecular beam epitaxy (MBE). Graphene growth on metallic as well as dielectric templates was investigated. We find that MBE offers control over the number of atomic graphene layers regardless of the substrate used. High structural quality could be achieved for graphene prepared on Ni (111) films which were epitaxially grown on MgO (111). For growth either on Al 2 O 3 (0001) or on (6√3×6√3)R30°-reconstructed SiC (0001) surfaces, graphene with a higher density of defects is obtained. Interestingly, despite their defective nature, the layers possess a well defined epitaxial relation to the underlying substrate. These results demonstrate the feasibility of MBE as a technique for realizing the scalable synthesis of this two-dimensional crystal on a variety of substrates. [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 nature of excitons bound to I, basal-plane stacking faults [(I1.Y)] in GaN nanowire ensembles by continuous-wave and time-resolved photoluminescence spectroscopy. Based on the linear increase of the radiative lifetime of these excitons with temperature, they are demonstrated to exhibit a two-dimensional density of states, i.e., a basal-plane stacking fault acts as a quantum well. From the slope of the linear increase, we determine the oscillator strength of the (I1, X) and show that the value obtained reflects the presence of large internal electrostatic fields across the stacking fault. While the recombination of donor-bound and free excitons in the GaN nanowire ensemble is dominated by nonradiative phenonema already at 10 K, we observe that the (I1,X) recombines purely radiatively up to 60 K. This finding provides important insight into the nonradiative recombination processes in GaN nanowires. First, the radiative lifetime of about 6 ns measured at 60 K sets an upper limit for the surface recombination velocity of 210 cms-1 considering the nanowires mean diameter of 50 nm. Second, the density of nonradiative centers responsible for the fast decay of donor-bound and free excitons cannot be higher than 6 × 1016 cm-3. As a consequence, the nonradiative decay of donor-bound excitons in these GaN nanowire ensembles has to occur indirectly via the free exciton state. [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]

Abstract: A study of GaN–AlGaN nanowire heterostructures grown by molecular beam epitaxy on sapphire substrates is presented. Nanowire growth was promoted using nickel seeds formed by in-situ annealing of a Ni thin film. Deposition sequences were designed to form two nanowire samples with embedded alternating multilayers of GaN and Al0.2Ga0.8N of variable thicknesses. Subsequent analyses showed the formation of two types of nanostructures in both samples, namely long and tapered nanowires and short columnar structures with uniform diameters along the whole length. Ni seeds were found at the end of the long nanowires only, indicating a catalyst-induced growth mechanism. No Ni seeds were found at the top of columnar structures indicating a catalyst-free growth mechanism. Analytical electron microscopy techniques were applied to the study of the Al distribution along both types of nanostructures. The layered heterostructure was found only for the columnar structures, whereas in nanowires the mediation of the Ni seed led to an incorporation of Al varying at monolayer level unrelated to the deposition sequence. The Al to Ga concentration ratio was found to decrease from the central segment of the nanowires towards the tip. Residual radial growth was also found to interfere with the effective deposition of the multilayer NW heterostructures. [Copyright &y& Elsevier]

Abstract: We investigate the strain in core–shell nanowires consisting of a highly mismatched materials system of a GaAs core and a MnAs shell. The strain in the GaAs core is directly traced using Raman spectroscopy, whereas that in the MnAs shell is assessed by magnetization measurements. The Raman peak positions in the MnAs-capped nanowires are shifted in comparison to those in bare GaAs nanowires in a nonuniform manner. We theoretically explore the influence of the anisotropic strain in the core–shell nanowires on the peak shift. When the shell thickness considerably exceeds the core diameter, the shell is only weakly strained. This can be interpreted as the shell taking over the role of the rigid substrate. In addition, we evaluate the diffusion length in the MnAs growth. [Copyright &y& Elsevier]

Abstract: Scanning transmission electron microscopy has been used to investigate the growth mechanism of gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy. The composition of nickel seeds used to promote nanowire-type growth has been studied. These seeds remain at the end of the nanowires after growth as Ni-based metallic crystallites. The concentration of gallium, nickel, oxygen and nitrogen in the seeds were quantified by analysing electron energy-loss spectra. The seed crystallites at the ends of the GaN nanowires are found to comprise of a metallic core of mainly nickel and gallium, with a surrounding oxide shell consisting of Ni, Ga, and O, attributed to the post-growth atmospheric exposure of the nanowires. Only a background concentration of nitrogen was found in the metallic nickel seed cores. [Copyright &y& Elsevier]

We show that the growth kinetics of dense arrays of self-induced GaN nanowires involves the exchange of Ga atoms between nanowires: Ga atoms desorbed from the side surfaces of nanowires readsorb on neighboring nanowires. This process favors the growth of shorter nanowires and gives rise to a narrow nanowire height distribution during the late stages of growth. We propose a stochastic differential equation model which describes the growth of dense nanowire ensembles. The model calculations are in good agreement with the experiments. [ABSTRACT FROM AUTHOR]

The dewetting of thin Pt films on different surfaces is investigated as a means to provide the patterning for the top-down fabrication of GaN nanowire ensembles. The transformation from a thin film to an ensemble of nanoislands upon annealing proceeds in good agreement with the void growth model. With increasing annealing duration, the size and shape uniformity of the nanoislands improves. This improvement speeds up for higher annealing temperature. After an optimum annealing duration, the size uniformity deteriorates due to the coalescence of neighboring islands. By changing the Pt film thickness, the nanoisland diameter and density can be quantitatively controlled in a way predicted by a simple thermodynamic model. We demonstrate the uniformity of the nanoisland ensembles for an area larger than 1 cm 2 . GaN nanowires are fabricated by a sequence of dry and wet etching steps, and these nanowires inherit the diameters and density of the Pt nanoisland ensemble used as a mask. Our study achieves advancements in size uniformity and range of obtainable diameters compared to previous works. This simple, economical, and scalable approach to the top-down fabrication of nanowires is useful for applications requiring large and uniform nanowire ensembles with controllable dimensions., (Creative Commons Attribution license.)

We study the molecular beam epitaxy of rock-salt ScN on the wurtzite GaN(11̅00) surface. To this end, ScN is grown on freestanding GaN(11̅00) substrates and self-assembled GaN nanowires exhibiting (11̅00) sidewalls. On both substrates, ScN crystallizes twin-free thanks to a specific epitaxial relationship, namely ScN(110)[001]∥GaN(11̅00)[0001], providing a congruent, low-symmetry interface. The 13.1% uniaxial lattice mismatch occurring in this orientation mostly relaxes within the first few monolayers of growth by forming a near-coincidence site lattice, where 7 GaN planes coincide with 8 ScN planes, leaving the ScN surface nearly free of extended defects. Overgrowth of the ScN with GaN leads to a kinetic stabilization of the zinc blende phase, that rapidly develops wurtzite inclusions nucleating on {111} nanofacets, commonly observed during zinc blende GaN growth. Our ScN/GaN(11̅00) platform opens a new route for the epitaxy of twin-free metal-semiconductor heterostructures including closely lattice-matched GaN, ScN, HfN, and ZrN compounds.

(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., (Creative Commons Attribution license.)