The growth and optoelectronic properties of core–shell nanostructures are influenced by the strain induced by the lattice mismatch between core and shell. In contrast with planar films, nanostructures contain multiple facets that act as independent substrates for shell growth, which enables different relaxation mechanisms. In this study, X-ray diffraction data are presented that show that In α Ga 1−α N shells grown on GaN cores are strained along each of the facets independently. Reciprocal space maps reveal multiple Bragg peaks, corresponding to different parts of the shell being strained along the individual facet planes. The strained lattice constants were found from the positions of the Bragg peaks. Vegard's law and Hooke's law for an anisotropic medium were applied in order to find the composition and strain in the InGaN shells. A range of nanowire samples with different InGaN shell thicknesses were measured and it is concluded that, with an In concentration of around 30%, major strain relaxation takes place when the thickness reaches 23 nm. InGaN shells of 6 and 9 nm thickness remain nearly fully strained biaxially along each of the \{10{\overline 1}0\} facets of the nanowires and the \{10{\overline 1}1\} facets of the nanopyramids.

We report on the direct observation of the diffusion of carriers in graded InGaN/GaN quantum wells in a nanowire. By probing the local dynamics at the nanoscale, along the wire for different temperatures between 4 and 250 K, we conclude that this diffusion process is thermally activated. In addition, the analysis of the cathodoluminescence lifetime for different temperatures shows that the carrier motion is isotropic and does not follow the indium gradient. Our observations are interpreted in terms of a hopping process between localized states. We find that the random alloy fluctuations prevent any directional drift of excitons along the In gradient and therefore any carrier accumulation. Our results therefore confirm the potential of core-shell nanowires for lighting devices. Indeed, the short lifetime of m-plane quantum wells together with their large active area and the homogeneous distribution of carrier along the quantum well will decrease influence of any high carrier density effect on the efficiency of these light-emitting diodes.

X-ray nanobeams are unique nondestructive probes that allow direct measurements of the nanoscale strain distribution and composition inside the micrometer thick layered structures that are found in most electronic device architectures. However, the method is usually extremely time-consuming, and as a result, data sets are often constrained to a few or even single objects. Here we demonstrate that by special design of a nanofocused X-ray beam diffraction experiment we can (in a single 2D scan with no sample rotation) measure the individual strain and composition profiles of many structures in an array of upright standing nanowires. We make use of the observation that in the generic nanowire device configuration, which is found in high-speed transistors, solar cells, and light-emitting diodes, each wire exhibits very small degrees of random tilts and twists toward the substrate. Although the tilt and twist are very small, they give a new contrast mechanism between different wires. In the present case, we image complex nanowires for nanoLED fabrication and compare to theoretical simulations, demonstrating that this fast method is suitable for real nanostructured devices.

We report on the electron beam induced current (EBIC) microscopy and cathodoluminescence (CL) characterization correlated with compositional analysis of light emitting diodes based on core/shell InGaN/GaN nanowire arrays. The EBIC mapping of cleaved fully operational devices allows to probe the electrical properties of the active region with a nanoscale resolution. In particular, the electrical activity of the p-n junction on the m-planes and on the semi-polar planes of individual nanowires is assessed in top view and cross-sectional geometries. The EBIC maps combined with CL characterization demonstrate the impact of the compositional gradients along the wire axis on the electrical and optical signals: the reduction of the EBIC signal toward the nanowire top is accompanied by an increase of the CL intensity. This effect is interpreted as a consequence of the In and Al gradients in the quantum well and in the electron blocking layer, which influence the carrier extraction efficiency. The interface between the nanowire core and the radially grown layer is shown to produce in some cases a transitory EBIC signal. This observation is explained by the presence of charged traps at this interface, which can be saturated by electron irradiation.

Core/shell InGaN/GaN nanowire light emitting diodes (LEDs) based on vertically standing single nanowires and nanowire arrays were fabricated and extensively characterized. The emission of single wire LEDs with the same conformal contact geometry as the array device exhibits the same broadening as the array LED electroluminescence, which proves an excellent wire-to-wire homogeneity. The electroluminescence spectra present two peaks corresponding to the m-plane InGaN quantum well (blue emission) and to an In-rich region at the m-plane-semipolar plane junction (green emission), in agreement with structural characterizations. Modification of the contact layout and a post-growth plasma treatment enable strongly suppressing the unwanted green electroluminescence while increasing the intensity in the blue spectral range for the same injected electrical power. Electron beam induced current mapping proves the inhibition of the electrical activity of the top part of the nanowire after plasma treatment. Inductively coupled plasma etching of the In-rich region permits one to completely remove the green emission for all injection currents, but loss of intensity in the blue spectral range is observed. Selectively contacting the m-plane and plasma treatment of the top part of the nanowire appear as a viable solution for controlling the color of core/shell nanowire LEDs with an inhomogeneous indium composition.

We report on the demonstration of MOVPE-grown single nanowire InGaN/GaN core-shell light emitting diodes (LEDs) with a transparent graphene contact for hole injection. The electrical homogeneity of the graphene-contacted LED has been assessed by electron beam induced current microscopy. By comparing graphene-contacted and metal-contacted nanowire LEDs, we show that the contact layout determines the electroluminescence spectrum. The electroluminescence changes color from green to blue with increasing injection current. High-resolution cathodoluminescence on cleaved nanowires allows the location with high precision of the origin of different emitted wavelengths and demonstrates that the blue peak originates from the emission of the radial quantum well on the m-planes, whereas the green peak arises from the In-rich region at the junction between the m-planes and the semipolar planes. The spectral behavior of the electroluminescence is understood by modeling the current distribution within the nanowire.

Core-shell GaN nanowires are expected to be building blocks of future light emitting devices. Here we apply off-axis electron holography to map the electrostatic potential distributions in such nanowires. To access the cross-section of selected individual nanowires, focused ion beam (FIB) milling is used. Furthermore, to assess the influence of FIB damage, the dopant potential measured from an intact NW is compared with a FIB prepared one. It is shown that in addition to the built-in potential between the p-type shell and unintentionally n-type under-layer there is a potential barrier between the core and under-layer which are both unintentionally n-type doped.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

III-nitride nanowire LEDs grown by MOCVD are demonstrated with external quantum efficiency exceeding 10% at 20 A/cm2 with a peak wavelength of 520 nm. Advantages of this device structure over planar LEDs will be described. Article not available.