Cathodoluminescence hyperspectral imaging of trench-like defects in InGaN/GaN quantum well structures

Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (90 meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (10 meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes

Effects of quantum well growth temperature on the recombination efficiency of InGaN/GaN multiple quantum wells that emit in the green and blue spectral regions

InGaN-based light emitting diodes and multiple quantum wells designed to emit in the green spectral region exhibit, in general, lower internal quantum efficiencies than their blue-emitting counter parts, a phenomenon referred to as the “green gap.” One of the main differences between green-emitting and blue-emitting samples is that the quantum well growth temperature is lower for structures designed to emit at longer wavelengths, in order to reduce the effects of In desorption. In this paper, we report on the impact of the quantum well growth temperature on the optical properties of InGaN/GaN multiple quantum wells designed to emit at 460 nm and 530 nm. It was found that for both sets of samples increasing the temperature at which the InGaN quantum well was grown, while maintaining the same indium composition, led to an increase in the internal quantum efficiency measured at 300 K. These increases in internal quantum efficiency are shown to be due reductions in the non-radiative recombination rate which we attribute to reductions in point defect incorporation.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Numbers EP/I012591/1 and EP/H011676/1.This is the final version of the article. It first appeared from AIP via http://dx.doi.org/10.1063/1.4932200 All data created during this research are openly available from the University of Manchester eScholar archive at http://dx.doi.org/10.15127/1.26974

Effect of QW growth temperature on the optical properties of blue and green InGaN/GaN QW structures

In this paper we report on the impact that the quantum well growth temperature has on the internal quantum efficiency and carrier recombination dynamics of two sets of InGaN/GaN multiple quantum well samples, designed to emit at 460 and 530 nm, in which the indium content of the quantum wells within each sample set was maintained. Measurements of the internal quantum efficiency of each sample set showed a systematic variation, with quantum wells grown at a higher temperature exhibiting higher internal quantum efficiency and this variation was preserved at all excitation power densities. By investigating the carrier dynamics at both 10 K and 300 K we were able to attribute this change in internal quantum efficiency to a decrease in the non-radiative recombination rate as the QW growth temperature was increased which we attribute to a decrease in incorporation of the point defects

Superconductor-ferromagnet nanocomposites created by co-deposition of niobium and dysprosium

We have created superconductor-ferromagnet composite films in order to test the enhancement of critical current density, Jc, due to magnetic pinning. We co-sputter the type-II superconductor niobium (Nb) and the low-temperature ferromagnet dysprosium (Dy) onto a heated substrate; the immiscibility of the two materials leads to a phase-separated composite of magnetic regions within a superconducting matrix. Over a range of compositions and substrate temperatures, we achieve phase separation on scales from 5 nm to 1 micron. The composite films exhibit simultaneous superconductivity and ferromagnetism. Transport measurements show that while the self-field Jc is reduced in the composites, the in-field Jc is greatly enhanced up to the 3 T saturation field of Dy. In one instance, the phase separation orders into stripes, leading to in-plane anisotropy in Jc.Comment: 7 pages, 7 figures. Matches the version published in SUST: Added one reference and some discussion in Section

Investigation of InGaN facet-dependent non-polar growth rates and composition for core-shell LEDs

Core–shell indium gallium nitride (InGaN)/gallium nitride (GaN) structures are attractive as light emitters due to the large nonpolar surface of rod-like cores with their longitudinal axis aligned along the c-direction. These facets do not suffer from the quantum-confined Stark effect that limits the thickness of quantum wells and efficiency in conventional light-emitting devices. Understanding InGaN growth on these submicron three-dimensional structures is important to optimize optoelectronic device performance. In this work, the influence of reactor parameters was determined and compared. GaN nanorods (NRs) with both {11-20} a-plane and {10-10} m-plane nonpolar facets were prepared to investigate the impact of metalorganic vapor phase epitaxy reactor parameters on the characteristics of a thick (38 to 85 nm) overgrown InGaN shell. The morphology and optical emission properties of the InGaN layers were investigated by scanning electron microscopy, transmission electron microscopy, and cathodoluminescence hyperspectral imaging. The study reveals that reactor pressure has an important impact on the InN mole fraction on the {10-10} m-plane facets, even at a reduced growth rate. The sample grown at 750°C and 100 mbar had an InN mole fraction of 25% on the {10-10} facets of the NRs

Room Temperature Ferrimagnetism and Ferroelectricity in Strained, Thin Films of BiFe0.5Mn0.5O3.

Highly strained films of BiFe0.5Mn0.5O3 (BFMO) grown at very low rates by pulsed laser deposition were demonstrated to exhibit both ferrimagnetism and ferroelectricity at room temperature and above. Magnetisation measurements demonstrated ferrimagnetism (TC ∼ 600K), with a room temperature saturation moment (MS ) of up to 90 emu/cc (∼ 0.58 μB /f.u) on high quality (001) SrTiO3. X-ray magnetic circular dichroism showed that the ferrimagnetism arose from antiferromagnetically coupled Fe3+ and Mn3+. While scanning transmission electron microscope studies showed there was no long range ordering of Fe and Mn, the magnetic properties were found to be strongly dependent on the strain state in the films. The magnetism is explained to arise from one of three possible mechanisms with Bi polarization playing a key role. A signature of room temperature ferroelectricity in the films was measured by piezoresponse force microscopy and was confirmed using angular dark field scanning transmission electron microscopy. The demonstration of strain induced, high temperature multiferroism is a promising development for future spintronic and memory applications at room temperature and above.This is the final published version. It's also available from Advanced Functional Materials: http://onlinelibrary.wiley.com/doi/10.1002/adfm.201401464/full

Segregation of In to dislocations in InGaN.

Dislocations are one-dimensional topological defects that occur frequently in functional thin film materials and that are known to degrade the performance of InxGa1-xN-based optoelectronic devices. Here, we show that large local deviations in alloy composition and atomic structure are expected to occur in and around dislocation cores in InxGa(1-x)N alloy thin films. We present energy-dispersive X-ray spectroscopy data supporting this result. The methods presented here are also widely applicable for predicting composition fluctuations associated with strain fields in other inorganic functional material thin films.This work was funded in part by the Cambridge Commonwealth trust, St. John’s College and the EPSRC. SKR is funded through the Cambridge-India Partnership Fund and Indian Institute of Technology Bombay via a scholarship. MAM acknowledges support from the Royal Society through a University Research Fellowship. Additional support was provided by the EPSRC through the UK National Facility for Aberration-Corrected STEM (SuperSTEM). The Titan 80- 200kV ChemiSTEMTM was funded through HM Government (UK) and is associated with the capabilities of the University of Manchester Nuclear Manufacturing (NUMAN) capabilities. SJH acknowledges funding from the Defence Treat Reduction Agency (DTRA) USA (grant number HDTRA1-12-1-0013).This is the accepted manuscript. The final version is available at http://pubs.acs.org/doi/abs/10.1021/nl5036513

Electron microscopy of defects in ultra-wide band gap nitrides

AlGaN is currently being studied for the development of UV applications, owing to its ultra-wide band gap which varies with aluminium concentration. Key challenges are to reduce the density of threading dislocations in AlGaN films which arise due to lattice inbrnatch with the substrate, and to p- and n-dope nitride films to obtain sufficient carrier ncentrations. In this study, transmission electron microscopy (TEM) was used to examine the microstructure of AlGaN/GaN heterostructures for use in UV light-emitters.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Dataset for Investigation of InGaN facet-dependent non-polar growth rates and composition for core-shell LEDs

This dataset contains the results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements carried out on core-shell nanostructures. The samples are highly regular arrays of GaN plasma etched cores onto which thick InGaN layers were grown using different metal organic vapour phase epitaxy (MOVPE) growth parameters. Three different InGaN growth conditions were considered with the following parameters: 750°C at 300 mbar, 700°C at 300 mbar and 750°C at 100 mbar. Statistical growth rates were determined on the non-polar crystal planes from measurements of increase in diameter using SEM images. TEM analysis was carried out on a single nanorod for greater detail.Secondary electron images were captured using a Hitachi S-4300 scanning electron microscope (SEM). An accelerating voltage of 5 kV was used to collect the images at constant magnification of 6000x from 5 samples, corresponding to a plasma etched GaN template, a GaN regrowth and three InGaN growths. Approximately 80 individual nanorods and their diameters are identified in each image. Each image was processed by binary threshold and low pass filter in Vision Assistant 2011 to obtain individual nanorod dimensions as detailed in the *.xls files. Each xls file contains separate columns for object number, calibrated area, equivalent disk diameter and image area. Statistical distributions of diameters were determined. A planar cross section was prepared for TEM measurements using a focused ion beam to thin down the cross section to 100-200 nm thickness. A Tecnai Osiris scanning-TEM (STEM) operating at 200 kV was used to obtain a high-angle annular dark-field (HAADF) Z-contrast image. The same image processing as in the SEM images was used to determine diameters and InGaN shell thickness.Hitachi S-4300 scanning electron microscope (SEM) Focused Ion Beam (FIB) etching and platinum deposition Tecnai Osiris scanning transmission electron microscope (STEM