Critical assessment 23: Gallium nitride-based visible light-emitting diodes

Solid-state lighting based on light-emitting diodes (LEDs) is a technology with the potential to drastically reduce energy usage, made possible by the development of gallium nitride and its alloys. However, the nitride materials family exhibits high defect densities and, in the equilibrium wurtzite crystal phase, large piezo-electric and polarisation fields arising at polar interfaces. These unusual physical properties, coupled with a high degree of carrier localisation in devices emitting visible light, result in ongoing challenges in device development, such as efficiency ‘droop’ (the reduction in efficiency of nitride LEDs with increasing drive current density), the ‘green gap’ (the relatively low efficiency of green emitters in comparison to blue) and the challenge of driving down the cost of LED epitaxy.This is the author accepted manuscript. The final version is available from Taylor & Francis via http://dx.doi.org/10.1080/02670836.2015.111622

Growth and optical characterisation of multilayers of InGaN quantum dots

We report on the growth (using metal-organic vapour phase epitaxy) and optical characterization of single and multiple layers of InGaN quantum dots (QDs), which were formed by annealing InGaN epilayers at the growth temperature in nitrogen. The size and density of the nanostructures have been found to be fairly similar for uncapped single and three layer QD samples if the GaN barriers between the dot layers are grown at the same temperature as the InGaN epilayer. The distribution of nanostructure heights of the final QD layer of three is wider and is centred around a larger size if the GaN barriers are grown at two temperatures (first a thin layer at the dot growth temperature, then a thicker layer at a higher temperature). Micro-photoluminescence studies at 4.2 K of capped samples have confirmed the QD nature of the capped nanostructures by the observation of sharp emission peaks with full width at half maximum limited by the resolution of the spectrometer. We have also observed much more QD emission per unit area in a sample with three QD layers, than in a sample with a single QD layer, as expected

Porous nitride semiconductors reviewed

Porous nitride semiconductors are a fast-developing area of study, which open up a wide range of new properties and applications, including strain free optical reflectors, chemical sensors and as a pathway to device lift-off. This article reviews the current progress in porous nitrides formed through electrochemical and photoelectrochemical methods. Using a simple electrochemical cell, pores are formed by injecting holes into the surface layer in order to oxidise the material into a soluble form and releasing nitrogen gas. The process is controlled principally by the electric field that drives the injection of holes and hence the applied potential and doping density are the key parameters for controlling pore morphology, along with how and whether illumination is used. We describe the mechanisms responsible for this process in detail and outline the trends for changing pore size and pore shape. For example, larger applied potential creates a larger electric field and hence larger pores. These methods have been used to produce a wide variety of different structures. We present a layered porous structure created by the modulation of the applied potential. Alternatively, layered structures can be produced by growing alternate doped and non-intentionally doped layers. Electrochemical etching can then create pores only in the doped layers, as they are conductive. This process can be performed by etching laterally through access trenches that expose the doped material or through the etching of dislocations to create nanopipes that allow subsurface porosity to form. This process requires no prior processing steps. We combine this method with patterning of surface protective layers to influence where the resulting pores grow. Based on these various fabrication processes, significant progress has been made towards applications of porous GaN across optoelectronics, sensing and for improving material quality.The authors gratefully acknowledge funding from the EPSRC under grant numbers: EP/M010589/1, EP/L015455/1, and EP/R511675/1

Evaluation of Transbronchial Lung Cryobiopsy Size and Freezing Time: A Prognostic Animal Study

© 2016 S. Karger AG, Basel. Copyright: All rights reserved. Background: Transbronchial lung biopsy using a cryoprobe is a novel way of sampling lung parenchyma. Correlation of freezing time with biopsy size and complications has not been evaluated in vivo. Objectives: The primary aim of the study is to evaluate the correlation between transbronchial cryobiopsy freezing time and size. The secondary aims are to evaluate histological quality of the biopsy and evaluate procedure-associated complications. Methods: Transbronchial lung cryobiopsies were obtained from two anaesthetised sheep using a 1.9-mm cryoprobe inserted into a flexible bronchoscope under fluoroscopic guidance. Freezing times ranged from 1 to 6 s (n = 49). The cryobiopsies were evaluated histologically with respect to their size and quality. Complications of bleeding and pneumothorax were recorded. Results: The mean cross-sectional area of the cryobiopsy ranged from 4.7 ± 2.1 to 15.7 ± 15.3 mm2. There was a significant positive correlation between increasing freezing time and cryobiopsy cross-sectional area (p = 0.028). All biopsies contained lung tissue with preserved parenchyma. Crush and freeze artefacts were not observed and tissue architecture was intact in all specimens. Small blood vessels and terminal bronchioles were observed in 88% of specimens. All cryobiopsies caused nil or mild haemorrhage with the exception of only 1 episode of severe haemorrhage at 6 s freezing time. Pneumothoraces occurred at 2, 5 and 6 s freezing time and required chest tube insertion. The most significant haemorrhage and pneumothoraces occurred at 5 and 6 s. Our results suggest an initial freezing time of 3 s can provide the maximal biopsy size while minimising major complications. Conclusion: The optimal transbronchial cryobiopsy freezing time is initially 3 s. This time is associated with minimal complications and large artefact-free biopsies

Improved Quantification of Arterial Spin Labelling Images using Partial Volume Correction Techniques

Arterial Spin Labelling (ASL) MRI suffers from a phenomenon known as the partial volume effect (PVE), which causes a degradation in accuracy of quantitative perfusion estimates. The effect is caused by inadequate spatial resolution of the imaging system. Resolution of the system is determined by point spread function (PSF) of the imaging process and the voxel grid on which the image is sampled. ASL voxels are comparatively large, which leads to tissue signal mixing within an individual voxel that results in an underestimation of grey matter (GM) and overestimation of white matter (WM) perfusion. PV correction of ASL images is not routinely applied. When PVC is applied, it usually takes the form of correcting for tissue fraction only, often by masking voxels with a partial volume fraction below a certain threshold. There are recent efforts to correct for tissue fraction effect through the use of linear regression or Bayesian inferencing using high resolution tissue posterior probability maps to estimate tissue concentration. This thesis reports an investigation into techniques for PVC of ASL images. An extension to the linear regression method is described, using a 3D kernel to reduce the inherent blurring of this method and preserve spatial detail. An investigation into the application of a Bayesian inferencing toolkit (BASIL) to single timepoint ASL data to estimate GM and WM perfusion in the absence of kinetic information is described. BASIL is found to rely heavily on the spatial prior for perfusion when the number of signal averages is less than three, and is outperformed by linear regression in terms of spatial smoothing until five or more averages are used. An existing method of creating partial volumes estimates from low resolution data is modified to use a voxelwise estimation for the longitudinal relaxation of GM, which improves segmentation estimates in the deep GM structures and improves GM perfusion estimates. An estimate for the width of the PSF for the 3D GRASE imaging sequence used in these studies is made and incorporated into a complete solution for PVC of ASL data, which deblurs the data through the process of deconvolution of the PSF, prior to a correction for the tissue fraction effect. This is found to elevate GM and reduce WM perfusion to a greater extent than correcting for tissue fraction alone, even in the case of a segmented acquisition. The new method for PVC is applied to two clinical cohorts; a Frontal Temporal Dementia and Posterior Cortical Atrophy groups. These two populations exhibit differential patterns of cortical atrophy and reduced tissue metabolism, which remains after PV correction

Non-polar InGaN quantum dot emission with crystal-axis oriented linear polarization

Polarization sensitive photoluminescence is performed on single non-polar InGaN quantum dots. The studied InGaN quantum dots are found to have linearly polarized emission with a common polarization direction defined by the [0001] crystal axis. Around half of ∼40 studied dots have a polarization degree of 1. For those lines with a polarization degree less than 1, we can resolve fine structure splittings between −800 μeV and +800 μeV, with no clear correlation between fine structure splitting and emission energy.This research was supported by the Engineering and Physical Sciences Research Council (EPSRC) UK (Grant No. EP/H047816/1).This is the accepted manuscript of a paper published in Applied Physics Letters (Reid BPL, Kocher C, Zhu T, Oehler F, Chan CCS, Oliver RA, Taylor RA, Applied Physics Letters, 2015, 106, 171108, doi:10.1063/1.4919656). The final version is available at http://dx.doi.org/10.1063/1.491965

Polarisation-controlled single photon emission at high temperatures from InGaN quantum dots

Solid-state single photon sources with polarisation control operating beyond the Peltier cooling barrier of 200 K are desirable for a variety of applications in quantum technology. Using a non-polar InGaN system, we report the successful realisation of single photon emission with a g((2))(0) of 0.21, a high polarisation degree of 0.80, a fixed polarisation axis determined by the underlying crystallography, and a GHz repetition rate with a radiative lifetime of 357 ps at 220 K in semiconductor quantum dots. The temperature insensitivity of these properties, together with the simple planar epitaxial growth method and absence of complex device geometries, demonstrates that fast single photon emission with polarisation control can be achieved in solid-state quantum dots above the Peltier temperature threshold, making this system a potential candidate for future on-chip applications in integrated systems

Observations of Rabi oscillations in a non-polar InGaN quantum dot

Experimental observation of Rabi rotations between an exciton excited state and the crystal ground state in a single non-polar InGaN quantum dot is presented. The exciton excited state energy is determined by photoluminescence excitation spectroscopy using two-photon excitation from a pulsed laser. The population of the exciton excited state is seen to undergo power dependent damped Rabi oscillations.This research was supported by the Engineering and Physical Sciences Research Council (EPSRC) U.K. (Grant No. EP/H047816/1).This version is the author accepted manuscript. The published version can also be found on the publisher's website at: http://scitation.aip.org/content/aip/journal/apl/104/26/10.1063/1.4886961 © 2014 AIP Publishing LL

Highly polarized electrically driven single-photon emission from a non-polar InGaN quantum dot

© 2017 Author(s). Nitride quantum dots are well suited for the deterministic generation of single photons at high temperatures. However, this material system faces the challenge of large in-built fields, decreasing the oscillator strength and possible emission rates considerably. One solution is to grow quantum dots on a non-polar plane; this gives the additional advantage of strongly polarized emission along one crystal direction. This is highly desirable for future device applications, as is electrical excitation. Here, we report on electroluminescence from non-polar InGaN quantum dots. The emission from one of these quantum dots is studied in detail and found to be highly polarized with a degree of polarization of 0.94. Single-photon emission is achieved under excitation with a constant current giving a g(2)(0) correlation value of 0.18. The quantum dot electroluminescence persists up to temperatures as high as 130 K