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We investigate the formation of the ternary alloy InxGa1–xN in the growth of self-induced nanowires by plasma-assisted molecular beam epitaxy. A series of samples grown at different temperatures were analyzed in situ by line-of-sight quadrupole mass spectrometry, and the predicted composition was verified by X-ray diffraction on as-grown nanowire ensembles. The InxGa1–xN composition is determined by the thermally activated loss of In due to InN decomposition and In desorption, similar to the composition of planar layers. The convolution of decomposition, reincorporation, and desorption is described by an apparent activation energy of about 2.5 eV. We show the feasibility of in situ control of the InxGa1–xN nanowire composition by line-of-sight quadrupole mass spectrometry.

The electrical properties of InN nanowires were investigated in four-point probe measurements. The dependence of the conductance on the wire diameter allows distinguishing between "core" bulk (quadratic) and "shell" sheet (linear) contributions. Evidence of the formation of a thin In(2)O(3) layer at the surface of the nanowires is provided by X-ray core level photoemission spectroscopy. The shell conductivity is therefore ascribed to an electron accumulation layer forming at the radial InN/In(2)O(3) interface. Although conductance through the accumulation layer dominates for nanowires below a critical diameter of about 55 nm, the core channel cannot be neglected, even for small nanowires.

InN-nanocolumns are an attractive system for light harvesting applications. To understand the mechanism of self organized growth of nanocolumns in plasma assisted MBE, InN samples were produced under various conditions on p-Si(111). Depending on the growth parameters different growth regimes for nanocolumns were identified according to their final shape. High-resolution TEM pictures show a very good crystal quality. This is also confirmed by Raman and PL measurements. Nanocolumns with diameters of 20-200 nm and lengths of up to 2 mm were produced. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Light emitting diodes (LEDs) were fabricated using ensembles of free-standing (In,Ga)N/GaN nanowires (NWs) grown on Si substrates in the self-induced growth mode by molecular beam epitaxy. Electron beam induced current analysis, cathodoluminescence as well as biased $\mu$-photoluminescence spectroscopy, transmission electron microscopy, and electrical measurements indicate that the electroluminescence of such LEDs is governed by the differences in the individual current densities of the single-NW LEDs operated in parallel, i.e. by the inhomogeneity of the current path in the ensemble LED. In addition, the optoelectronic characterization leads to the conclusion that these NWs exhibit N-polarity and that the (In,Ga)N quantum well states in the NWs are subject to a non-vanishing quantum confined Stark effect., Comment: This is an author-created, un-copyedited version of an article accepted for publication in Nanotechnology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at doi:10.1088/0957-4484/23/46/465301

We present comparative studies of optical properties of GaN nanowires (NWs) obtained by two different self-formation techniques: Plasma-Assisted Molecular Beam Epitaxy (PAMBE) growth; and plasma etching of GaN layers deposited by Metal-Organic Vapor Phase Epitaxy (MOVPE). The effects of the coalescence process on grown NW and plasma-induced defects in etched NWs have been studied by photoluminescence (PL) and Raman scattering. In MBE grown NWs, the coalescence-associated defects are extended toward the NW top for intermediate Ga flux. Using High Resolution Electron Microscopy of reactive plasma etching (RIE) NWs, it was found that NWs obtained with an optimal combination of inductive (ICP) and capacitive (RF) plasma are free of extended structural defects. The PL efficiency is strongly increased in plasma etched NWs. However, plasma-induced point defects have to be taken into account for explaining the changes of the PL spectra. Less plasma-induced degradation is observed for high ICP/RF power ratios.

GaN nanowires (NWs) were grown selectively on Si(111) substrate without catalyst by plasma-assisted molecular-beam epitaxy under N-rich conditions. The selective growth was obtained using regular arrays of holes patterned in a silicon oxide layer on top of a thin AlN buffer. The optical properties of the selectively grown GaN NWs have been studied using cathodoluminescence (CL) spectroscopy. Both, CL spectra measured on NW ensembles and spatially resolved monochromatic images were investigated. From a comparison of morphology and CL studies it emerges that NW coalescence is responsible for the appearance of the defect related emission.

This paper discusses some of the advantages of nanowire structures for use in LEDs as well as the challenges that need to be overcome towards the realisation of real-world devices. Our experimental results pertain to group-III nitride nanowire structures grown by MBE. We present clear evidence that the catalyst-free growth approach on Si yields best results with respect to structural and optical material properties. We elucidate the mechanism of nanowire nucleation and the factors determining the initial nanowire diameter, discuss the issue of InGaN growth in small-diameter nitride nanowires and review the results reported for nanowire-based group-III nitride LEDs reported so far.

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.

Molecular beam epitaxy (MBE) on patterned Si/AlN/Si(111) substrates was used to obtain regular arrays of uniform-size GaN nanowires (NWs). The silicon top layer has been patterned with e-beam lithography, resulting in uniform arrays of holes with different diameters (d(h)) and periods (P). While the NW length is almost insensitive to the array parameters, the diameter increases significantly with d(h) and P till it saturates at P values higher than 800 nm. A diffusion induced model was used to explain the experimental results with an effective diffusion length of the adatoms on the Si, estimated to be about 400 nm. (C) 2011 American Institute of Physics. [doi:10.1063/1.3559618]

Single GaN nanowires and larger GaN ensembles are investigated by Raman spectroscopy. Spectra of nanowire ensembles prove the high crystal quality and are in agreement with selection rules for the wurtzite structure. Single nanowires are studied with a spatial resolution of the order of 400 nm for different polarization directions of the incident laser beam relative to the nanowire axis. In the single wire spectrum, only the A1(TO) was observed and the Raman intensity was suppressed for perpendicular polarization. These results confirm that Raman scattering in isolated GaN nanowires is governed by size effects.

In Europe, Borrelia burgdorferi sensu stricto, Borrelia afzelii and Borrelia garinii are known as pathogens among the Borrelia burgdorferi sensu lato group. Since it is not yet known which Borrelia are responsible for Lyme borreliosis in France, the objective of this study was to identify the species of Borrelia responsible for erythema migrans in the region of Alsace, France.Eighteen patients with erythema migrans (EM) of more than 5 cm of diameter were included in this prospective study. All patients were investigated at the Strasbourg University Hospital. Patients were biopsied on the active border of their lesion. Cutaneous biopsies of the active border of the lesion were cultivated in BSK-H medium (Sigma) and analysed in vitro by PCR after 8 weeks of culture, using flagellin consensus sequences which are present in all species of Borrelia burgdorferi sensu lato group as primers. Species-specific oligotyping was used for species identification.Among the 18 patients biopsied, 7 had evidence of borrelia infection revealed by culture and/or PCR. Borrelia afzelii was detected in 4 patients and Borrelia garinii in three.These preliminary results appear to confirm that Borrelia afzelii and Borrelia garinii are the predominant borrelial species in EM lesions in our geographic area, as in other European countries.

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.

High quality, well-separated, homogeneous sizes and high aspect ratio Si-doped InN nanowires (NWs) were grown by catalyst-free molecular beam epitaxy (MBE) after optimization of the growth conditions. To this end, statistical analysis of NW density and size distribution was performed. The high crystal quality and smooth NW surfaces were observed by high resolution transmission electron microscopy. Spectral photoluminescence has shown the increase of the band filling effect with Si flux, indicating successful n-type doping. A Raman LO scattering mode appears with a pronounced low energy tail, also reported for highly doped InN films.

InGaN/GaN nanowire (NW) heterostructures grown by plasma assisted molecular beam epitaxy were studied in comparison to their GaN and InGaN counterparts. The InGaN/GaN heterostructure NWs are composed of a GaN NW, a thin InGaN shell, and a multifaceted InGaN cap wrapping the top part of the GaN NW. High-resolution transmission electron microscopy (HRTEM) images taken from different parts of a InGaN/GaN NW show a wurtzite structure of the GaN core and the epitaxial InGaN shell around it, while additional crystallographic domains are observed whithin the InGaN cap region. Large changes in the lattice parameter along the wire, from pure GaN to higher In concentration demonstrate the successful growth of a complex InGaN/GaN NW heterostructure. Photoluminescence (PL) spectra of these heterostructure NW ensembles show rather broad and intense emission peak at 2.1 eV. However, {mu}-PL spectra measured on single NWs reveal a reduced broadening of the visible luminescence. The analysis of the longitudinal optical phonon Raman peak position and its shape reveal a variation in the In content between 20% and 30%, in agreement with the values estimated by PL and HRTEM investigations. The reported studies are important for understanding of the growth and properties of NW heterostructures suitable for applicationsmore » in optoelectronics and photovoltaics.« less

In the literature, there are controversies on the interpretation of the appearance in InN Raman spectra of a strong scattering peak in the energy region of the unscreened longitudinal optical (LO) phonons, although a shift caused by the phonon-plasmon interaction is expected for the high conductance observed in this material. Most measurements on light scattering are performed on ensembles of InN nanowires (NWs). However, it is important to investigate the behavior of individual nanowires and here we report on micro-Raman measurements on single nanowires. When changing the polarization direction of the incident light from parallel to perpendicular to the wire, the expected reduction of the Raman scattering was observed for transversal optical (TO) and E(2) phonon scattering modes, while a strong symmetry-forbidden LO mode was observed independently on the laser polarization direction. Single Mg- and Si-doped crystalline InN nanowires were also investigated. Magnesium doping results in a sharpening of the Raman peaks, while silicon doping leads to an asymmetric broadening of the LO peak. The results can be explained based on the influence of the high electron concentration with a strong contribution of the surface accumulation layer and the associated internal electric field.