130 research outputs found
Epitaxial lateral overgrowth of AlN on self-assembled patterned nanorods
We report an inexpensive nanoscale patterning process for epitaxial lateral overgrowth (ELOG) in AlN layers grown by metal organic vapour phase epitaxy (MOVPE) on sapphire. The pattern was produced by an inductively coupled plasma etch using a self-assembled monolayer of silica spheres on AlN as the lithographic mask. The resulting uniform 1 [small mu ]m length rod structure across a wafer showed a massive reduction in threading dislocations (TDs) when annealed at 1100 [degree]C. Overgrowing homoepitaxial AlN on top of the nanorods, at a temperature of 1100 [degree]C, produced a crack free coalesced film with approximately 4 [small mu ]m of growth, which is formed at a much lower temperature compared to that typically required for microscale ELOG. The improved crystal quality, in terms of TD reduction, of the AlN above the rods was determined by detailed weak beam (WB) electron microscopy studies and showed that the threading dislocation density (TDD) was greatly reduced, by approximately two orders of magnitude in the case for edge-type dislocations. In situ reflectance measurements during the overgrowth allowed for thickness coalescence to be estimated along with wafer curvature changes. The in situ measurements also confirmed that tensile strain built up at a much slower rate in the ELOG AlN layer compared to that of AlN prepared directly on sapphire
Ultra-high-density arrays of defect-free AlN nanorods: a "space-filling" approach
Nanostructured semiconductors have a clear potential for improved optoelectronic devices, such as high-efficiency light-emitting diodes (LEDs). However, most arrays of semiconductor nanorods suffer from having relatively low densities (or “fill factors”) and a high degree of nonuniformity, especially when produced by self-organized growth. Ideally an array of nanorods for an optoelectronic emitter should have a fill factor close to 100%, with uniform rod diameter and height. In this article we present a “space-filling” approach for forming defect-free arrays of AlN nanorods, whereby the separation between each rod can be controlled to 5 nm due to a self-limiting process. These arrays of pyramidal-topped AlN nanorods formed over wafer-scale areas by metal organic chemical vapor deposition provide a defect-free semipolar top surface, for potential optoelectronic device applications with the highest reported fill factor at 98%
Direct imaging of spatial heterogeneities in type II superconductors
Understanding the exotic properties of quantum materials, including
high-temperature superconductors, remains a formidable challenge that demands
direct insights into electronic conductivity. Current methodologies either
capture a bulk average or near-atomically-resolved information, missing direct
measurements at the critical intermediate length scales. Here, using the
superconductor Fe(Se,Te) as a model system, we use low-temperature conductive
atomic force microscopy (cAFM) to bridge this gap. Contrary to the uniform
superconductivity anticipated from bulk assessments, cAFM uncovers micron-scale
conductive intrusions within a relatively insulating matrix. Subsequent
compositional mapping through atom probe tomography, shows that differences in
conductivity correlated with local changes in composition. cAFM, supported by
advanced microscopy and microanalysis, represents a methodological breakthrough
that can be used to navigate the intricate landscape of high-temperature
superconductors and the broader realm of quantum materials. Such fundamental
information is critical for theoretical understanding and future guided design
Diameter controlled germanium nanowires with lamellar twinning and polytypes
One-dimensional nanostructures with controllable morphologies and defects are appealing for use in nanowire devices. This paper details the influence of colloidal magnetite iron oxide nanoparticle seeds to regulate the radial dimension and twin boundary formation in Ge nanowires grown through a liquid-injection chemical vapor deposition process. Control over the mean nanowire diameter, even in the sub-10 nm regime, was achieved due to the minimal expansion and aggregation of iron oxide nanoparticles during the growth process. The uncommon occurrence of heterogeneously distributed multiple layer {111} twins, directed perpendicular to the nanowire growth axis, were also observed in 〈111〉-directed Ge nanowires, especially those synthesized from patterned hemispherical Fe3O4 nanodot catalysts. Consecutive twin planes along 〈111〉-oriented nanowires resulted in a local phase transformation from 3C diamond cubic to hexagonal 4H allotrope. Localized polytypic crystal phase heretostructures were formed along 〈111〉-oriented Ge nanowire using magnetite nanodot catalysts
Charge carriers in dynamic ferroelectric domain walls
Ferroelectric domain walls (DWs) are the subject of intense research at present in the search for high dielectric, gigahertz responsive materials with novel functionalities[1]. Crucial to the integration of DWs into nanoelectronics is a proper understanding of the local electronic landscape around the wall and the influence this has on the behaviour of the DW under variable electric fields. A high degree of mobility under small electric fields is especially desirable for low power applications which escape from the critical current thresholds required to move magnetic domain walls[2]. Perovskite oxides are prime candidates for tuning the thermodynamic variables affecting the energy landscape of DWs and thus controlling their orientation/charge state[3]. Here we present an investigation into the behaviour of ferroelectric DWs under dynamic fields and the specific charge carriers present at DWs
An enhanced surface passivation effect in InGaN/GaN disk-in-nanowire light emitting diodes for mitigating Shockley-Read-Hall recombination
We present a detailed study of the effects of dangling bond passivation and the comparison of different sulfide passivation processes on the properties of InGaN/GaN quantum-disk (Qdisk)-in-nanowire based light emitting diodes (NW-LEDs). Our results demonstrated the first organic sulfide passivation process for nitride nanowires (NWs). The results from Raman spectroscopy, photoluminescence (PL) measurements, and X-ray photoelectron spectroscopy (XPS) showed that octadecylthiol (ODT) effectively passivated the surface states, and altered the surface dynamic charge, and thereby recovered the band-edge emission. The effectiveness of the process with passivation duration was also studied. Moreover, we also compared the electro-optical performance of NW-LEDs emitting at green wavelength before and after ODT passivation. We have shown that the Shockley–Read–Hall (SRH) non-radiative recombination of NW-LEDs can be greatly reduced after passivation by ODT, which led to a much faster increasing trend of quantum efficiency and higher peak efficiency. Our results highlighted the possibility of employing this technique to further design and produce high performance NW-LEDs and NW-lasers
Observation of Antiferroelectric Domain Walls in a Uniaxial Hyperferroelectric
Ferroelectric domain walls are a rich source of emergent electronic
properties and unusual polar order. Recent studies showed that the
configuration of ferroelectric walls can go well beyond the conventional
Ising-type structure. N\'eel-, Bloch-, and vortex-like polar patterns have been
observed, displaying strong similarities with the spin textures at magnetic
domain walls. Here, we report the discovery of antiferroelectric domain walls
in the uniaxial ferroelectric PbGeO. We resolve highly
mobile domain walls with an alternating displacement of Pb atoms, resulting in
a cyclic 180 flip of dipole direction within the wall. Density
functional theory calculations reveal that PbGeO is
hyperferroelectric, allowing the system to overcome the depolarization fields
that usually suppress antiparallel ordering of dipoles along the longitudinal
direction. Interestingly, the antiferroelectric walls observed under the
electron beam are energetically more costly than basic head-to-head or
tail-to-tail walls. The results suggest a new type of excited domain-wall
state, expanding previous studies on ferroelectric domain walls into the realm
of antiferroic phenomena
Ultra-High Carrier Mobilities in Ferroelectric Domain Wall Corbino Cones at Room Temperature
Recently, electrically conducting heterointerfaces between dissimilar band-insulators (such as lanthanum aluminate and strontium titanate) have attracted considerable research interest. Charge transport has been thoroughly explored and fundamental aspects of conduction firmly established. Perhaps surprisingly, similar insights into conceptually much simpler conducting homointerfaces, such as the domain walls that separate regions of different orientations of electrical polarisation within the same ferroelectric band-insulator, are not nearly so well-developed. Addressing this disparity, we herein report magnetoresistance in approximately conical 180° charged domain walls, which occur in partially switched ferroelectric thin film single crystal lithium niobate. This system is ideal for such measurements: firstly, the conductivity difference between domains and domain walls is extremely and unusually large (a factor of at least 1013) and hence currents driven through the thin film, between planar top and bottom electrodes, are overwhelmingly channelled along the walls; secondly, when electrical contact is made to the top and bottom of the domain walls and a magnetic field is applied along their cone axes (perpendicular to the thin film surface), then the test geometry mirrors that of a Corbino disc, which is a textbook arrangement for geometric magnetoresistance measurement. Our data imply carriers at the domain walls with extremely high room temperature Hall mobilities of up to ~ 3,700cm2V-1s-1. This is an unparalleled value for oxide interfaces (and for bulk oxides too) and is most comparable to mobilities in other systems typically seen at cryogenic, rather than at room, temperature
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