Christopher X. Ren

Carrier localization in the vicinity of dislocations in InGaN

This project is funded in part by the European Research Council under the European Communitys Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 279361 (MACONS). The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement 312483-ESTEEM2 (Integrated Infrastructure InitiativeI3). F.M. would also like to acknowledge the financial support from EPSRC Doctoral Prize Awards and Cambridge Philosophical Society. M.H. would like to acknowledge support from the Lindemann Fellowship.

Ultrafast, Polarized, Single-Photon Emission from m-Plane InGaN Quantum Dots on GaN Nanowires

We demonstrate single-photon emission from self-assembled m-plane InGaN quantum dots (QDs) embedded on the side-walls of GaN nanowires. A combination of electron microscopy, cathodoluminescence, time-resolved microphotoluminescence (μPL), and photon autocorrelation experiments give a thorough evaluation of the QD structural and optical properties. The QD exhibits antibunched emission up to 100 K, with a measured autocorrelation function of g(2)(0) = 0.28(0.03) at 5 K. Studies on a statistically significant number of QDs show that these m-plane QDs exhibit very fast radiative lifetimes (260 ± 55 ps) suggesting smaller internal fields than any of the previously reported c-plane and a-plane QDs. Moreover, the observed single photons are almost completely linearly polarized aligned perpendicular to the crystallographic c-axis with a degree of linear polarization of 0.84 ± 0.12. Such InGaN QDs incorporated in a nanowire system meet many of the requirements for implementation into quantum information systems and could potentially open the door to wholly new device concepts.

Estimating Fault Friction From Seismic Signals in the Laboratory

Nearly all aspects of earthquake rupture are controlled by the friction along the fault that progressively increases with tectonic forcing, but in general cannot be directly measured. We show that fault friction can be determined at any time, from the continuous seismic signal. In a classic laboratory experiment of repeating earthquakes, we find that the seismic signal follows a specific pattern with respect to fault friction, allowing us to determine the faults position within its failure cycle. Using machine learning, we show that instantaneous statistical characteristics of the seismic signal are a fingerprint of the fault zone shear stress and frictional state. Further analysis of this fingerprint leads to a simple equation of state quantitatively relating the seismic signal power and the friction on the fault. These results show that fault zone frictional characteristics and the state of stress in the surroundings of the fault can be inferred from seismic waves, at least in the laboratory.

Wafer-scale Fabrication of Non-Polar Mesoporous GaN Distributed Bragg Reflectors via Electrochemical Porosification

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices. For many device applications, it is highly desirable to achieve not only high reflectivity and low absorption, but also good conductivity to allow effective electrical injection of charges. Here, we demonstrate the wafer-scale fabrication of highly reflective and conductive non-polar gallium nitride (GaN) DBRs, consisting of perfectly lattice-matched non-polar (11–20) GaN and mesoporous GaN layers that are obtained by a facile one-step electrochemical etching method without any extra processing steps. The GaN/mesoporous GaN DBRs exhibit high peak reflectivities (>96%) across the entire visible spectrum and wide spectral stop-band widths (full-width at half-maximum >80 nm), while preserving the material quality and showing good electrical conductivity. Such mesoporous GaN DBRs thus provide a promising and scalable platform for high performance GaN-based optoelectronic, photonic, and quantum photonic devices.

Structural impact on the nanoscale optical properties of InGaN core-shell nanorods

The authors would like to thank OSRAM Opto Semiconductors for the provision of the GaN/Silicon templates and acknowledge the financial support from the European Union FP7 under Contract Nos. 228999 (SMASH) and 279361 (MACONS) and the EPSRC, UK (EP/M015181/1 “Manufacturing of nanoengineered III Nitride semiconductors”).

Nanoscopic insights into the effect of silicon on core-shell InGaN/GaN nanorods: Luminescence, composition, and structure

GaN-based nanorods and nanowires have recently shown great potential as a platform for future energy-efficient photonic and optoelectronic applications, such as light emitting diodes and nanolasers. Currently, the most industrially scalable method of growing III-nitride nanorods remains metal-organic vapour phase epitaxy: whilst this growth method is often used in conjunction with extrinsic metallic catalyst particles, these particles can introduce unwanted artifacts in the nanorods such as stacking faults. In this paper, we examine the catalyst-free growth of GaN/InGaN core-shell nanorods by metal-organic vapor phase epitaxy for optoelectronic applications using silane to enhance the vertical growth of the nanorods. We find that both the silane concentration and exposure time can greatly affect the nanorod properties, and that larger concentrations and longer exposure times can severely degrade the nanorod structure and thus result in reduced emission from the InGaN QW shell. Finally, we report that the ...

Defects in III-Nitride Microdisk Cavities

The original research shown in this article has been funded by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ ERC grant agreement no. 279361 (MACONS). RAO acknowledges the Royal Academy of Engineering Leverhulme Trust Senior Research Fellowship scheme.

Vertical leakage mechanism in GaN on Si high electron mobility transistor buffer layers

Control of leakage currents in the buffer layers of GaN based transistors on Si substrates is vital for the demonstration of high performance devices. Here, we show that the growth conditions during the metal organic chemical vapour deposition growth of the graded AlGaN strain relief layers (SRLs) can significantly influence the vertical leakage. Using scanning capacitance microscopy, secondary ion mass spectrometry, and transmission electron microscopy, we investigate the origins of leakage paths and show that they result from the preferential incorporation of oxygen impurities on the side wall facets of the inverted hexagonal pyramidal pits which can occur during the growth of the graded AlGaN SRL. We also show that when 2D growth of the AlGaN SRL is maintained a significant increase in the breakdown voltage can be achieved even in much thinner buffer layer structures. These results demonstrate the importance of controlling the morphology of the high electron mobility transistor buffer layer as even at a very low density the leakage paths identified would provide leakage paths in large area devices.

Research data supporting "Carrier Localization in the Vicinity of Dislocations in InGaN"

FIG. 1. AFM (a), SEM (b), panchromatic CL (c), and ADF-STEM (d) performed on the same micrometre-scale area. To guide the eye, a few dislocations are indicated by arrows in each picture. (e) High-resolution (HR) STEM image of the dislocation indicated by a square in (a)-(d), enabling the identification of the core structure (here dissociated 7/4/8/5-atom ring), and (f) geometric phase analysis (GPA) showing the e_xx strain component of the dislocation core region. FIG. 2. Schematic showing the electron probe in the SEM-CL scanning across a V-pit. The scale of the schematic, although indicative, is representative of the experimental conditions in which the experiment was conducted. Distance to nearest neighbor dependence of the intensity ratio (a)(c) and energy shift (b)(d) measured at the center (a)(b) and facet (c)(d) of the V-pits. FIG. 3. (a) Histogram of the number of In-N chains as a function of the number of indium atoms in the chains, located within a 10 A radius centered on the dislocation, in the case of a random distribution of indium (i.e. initial configuration of the simulation) or segregation of indium (i.e. equilibrium configuration of the simulation). Abstract representation of the data in (a), in the case of a random distribution (b) or segregation (c) of indium atoms. FIG. 4. ADF-STEM image of the clustered dislocations 26 (a) and 87 (b). The white strain-related contrast between the neighboring dislocations is indicated by an arrow. Aberration-corrected HAADF-STEM image of the core of dislocation 26 (dissociated 7/4/8/4/9-atom ring)(c) and 87 (undissociated double 5/6-atom ring)(d). An ABSF-filter (Average Background Subtraction Filter) has been applied to (c) and (d) in order to remove noise from the images. FIG. 5. 16K CL integrated intensity (a)(c) and peak emission energy (b)(d) maps of isolated (a)(b) and clustered (c)(d) dislocations. To guide the eye, the position of the bright spots, directly observable in (a) and (c), is indicated by circles in all the maps. To emphasize the relative variations in intensity and energy between isolated and clustered configurations, a common color scale is used in (a) and (c) and in (b) and (d).