We are extremely pleased to announce the appointment of Dr. Michael E. Coltrin as the new Director of Sandia’s Solid-State Lighting Science (SSLS) Energy Frontier Research Center (EFRC). Mike has had a long and distinguished career advancing the science underlying chemically reacting fluid flow, and in applying that science to solve practical problems in the chemical vapor deposition of virtually all of the important semiconducting materials. These materials include Si, C, GaAs, and, most recently, the GaN-based materials that are the foundation for solid-state lighting devices. Mike has served as Co-Director of our SSLS EFRC since its inception in August, 2009. He was instrumental in creating the original SSLS EFRC proposal; has helped guide the continued evolution of our SSLS EFRC these past two years in technical direction, personnel, and budget; and has served for the past half year as its Acting Director.
Jerry Simmons , Founding Director of the SSLS EFRC, will continue to be an active supporter of our SSLS EFRC, and will chair its Senior Leadership Council.
EFRC researchers from Sandia have recently published an article titled “Optical performance of top-down fabricated InGaN/GaN nanorod light emitting diode arrays” in Optics Express. This paper details the development of a two-step top-down process for creating nanorod LED arrays.
Abstract: Vertically aligned InGaN/GaN nanorod light emitting diode (LED) arrays were created from planar LED structures using a new top-down fabrication technique consisting of a plasma etch followed by an anisotropic wet etch. The wet etch results in straight, smooth, well-faceted nanorods with controllable diameters and removes the plasma etch damage. 94% of the nanorod LEDs are dislocation-free and a reduced quantum confined Stark effect is observed due to reduced piezoelectric fields. Despite these advantages, the IQE of the nanorod LEDs measured by photoluminescence is comparable to the planar LED, perhaps due to inefficient thermal transport and enhanced nonradiative surface recombination.
In early November, both Mary Crawford (EFRC Thrust Leader) and Fred Schubert (EFRC external partner and Professor at Rensselaer Polytechnic Institute) gave plenary talks at the Optical Society of America conference in Austin, Texas. This event is part of the Renewable Energy and the Environment Congress, allowing attendees to access to all meetings within the Congress and to collaborate on topics of mutual interest. Crawford spoke on the “Pathways to Ultra-Efficient Sold-State Lighting”, while Schubert addressed the “Promises and Challenges in Light-Emitting Diodes for High-Power Lighting Applications”.
The full agenda of the conference is available here.
EFRC Scientist Weng Chow was recently featured in an article in the November 2011 edition of the IEEE Spectrum. He was highlighted for his calculations involving the behavior of LEDs in relations to the debate over the droop theory.
The debate over the origin of droop has consumed the LED community. As current is increased, droop occurs when a decline in the effectiveness of blue and white emitters occurs. If this problem can be resolved, solid-state lighting will become brighter and cheaper. Chow has found a way to bring all models of light emission together.
Chow’s work is a new breakthrough in the heated droop debate as does his model combines the defects and carrier recombination in quantum wells, also in addition to integrating Auger recombination.
For the full article, see the IEEE website.
The Journal of Solid State Chemistry recently released a new EFRC-supported publication written by Grant C. Bleier, May Nyman, Lauren E.S. Rohwer, and Mark A. Rodriguez. The article, titled “Seeking the optimal LaTaO4:Eu phosphor”, investigates the relationship of Eu-concentration in LaTaO4:Eu materials with the Eu quantum-yield.
Abstract: Lanthanum orthotantalate, LaTaO4, is an excellent host lattice for rare-earth luminescent ions such as Eu3þ for red emission. However, there are multiple RETaO4 (RE¼rare earth) polymorphs, and the stability of these is controlled predominantly by the RE-radius. Thus it is difficult to obtain a pure phase of LaTaO4:Eu as Eu concentration and consequently the RE radius is varied. We recently reported a ‘soft- chemical’ route that allows crystallization of pure-phaseLaTaO4:Eu at temperatures as low as 800 1C. In the current report, we investigate polymorph evolution and Eu emission as a function of Eu concentration and annealing temperature. We obtain a maximum quantum yield (QY) of 83% at the highest Eu substitution (25%) for which the low temperature orthorhombic (Pbca) polymorph is stable. Therefore, QY is not limited necessarily by concentration quenching; rather it is limited by polymorph stability as the RE-radius decreases with increasing Eu substitution.
Weng Chow, EFRC scientist, recently had his paper, “Modeling excitation-dependent bandstructure effects on InGaN light-emitting diode efficiency” published in Optics Express. It details a procedure for modeling InGaN LEDs that takes into account of bandstructure properties that are affected by carrier population.
Abstract: Bandstructure properties in wurtzite quantum wells can change appreciably with changing carrier density because of screening of quantum confined Stark effect. An approach for incorporating these changes in an InGaN light-emitting-diode model is described. Bandstructure is computed for different carrier densities by solving Poisson and k・p equations in the envelop approximation. The information is used as input in a dynamical model for populations in momentum-resolved electron and hole states. Application of the approach is illustrated by modeling device internal quantum efficiency as a function of excitation.
On October 5-6, 2011 Jerry Simmons (Director of the SSLS EFRC) and Jeff Tsao (EFRC Scientist) participated in two roundtables on Solid-State Lighting sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) and the Office of Basic Energy Sciences (BES).
The intent of the roundtables was to enable these Offices to maintain an ongoing and up to date understanding of the critical technical and scientific challenges facing solid-state lighting (SSL), and to foster collaboration between the EERE SSL and BES programs and researchers. The first day was devoted to SSL based on inorganic light-emitting diodes; the second day was devoted to SSL based on organic light-emitting diodes. Both days had lively discussions of underlying scientific challenges facing SSL with participants drawn from industry, academia and national laboratories.
Jerry Simmons and Jeff Tsao participated in the first day, presenting short talks on “Foundational InGaN materials physics: microscopic processes associated with macroscopic light emission” and “Novel emitter architectures: aiming for near-100% efficiency.” Both of these topics are being pursued at Sandia’s Energy Frontier Research Center for Solid-State Lighting Science. Jerry Simmons also was an observer for the second day.
In a new paper titled “Gallium Nitride based Logpile Photonic Crystals”, published in Nanoletters, EFRC scientists Ganapathi Subramania, Qiming Li, George T. Wang, and Arthur J. Fischer present their work on nine layer logpile 3DPC structures. The paper established that nine layer logpile 3DPC structures with and without a cavity are composed entirely from epitaxial single crystalline GaN fabricated using a template-directed selective MOCVD growth approach.
Abstract: We demonstrate a nine-layer logpile three-dimensional photonic crystal (3DPC) composed of single crystalline gallium nitride (GaN) nanorods, 100 nm in size with lattice constants of 260, 280, and 300 nm with photonic band gap in the visible region. This unique GaN structure is created through a combined approach of a layer-by-layer template fabrication technique and selective metal organic chemical vapor deposition (MOCVD). These GaN 3DPC exhibit a stacking direction band gap characterized by strong optical reflectance between 380 and 500 nm. By introducing a “line-defect” cavity in the fifth (middle) layer of the 3DPC, a localized transmission mode with a quality factor of 25–30 is also observed within the photonic band gap. The realization of a group III nitride 3DPC with uniform features and a band gap at wavelengths in the visible region is an important step toward realizing complete control of the electromagnetic environment for group III nitride based optoelectronic devices.
The full article is available at the Nanoletters website.
The Solid-State Lighting Science Energy Frontier Research Center (SSLS EFRC) and the Center for Integrated Nanotechnology (CINT) hosted a joint workshop in conjunction with CINT’s Annual User Conference. The workshop was held at the Marriott Pyramid Hotel in Albuquerque on September 14, 2011, with 90 scientists registering for the all-day event.
The workshop was co-organized by Dr. Igal Brener and Dr. Willie Luk, scientists associated both with the SSLS EFRC and with CINT. Dr. Jerry Simmons, Director of the SSLS EFRC, gave an opening welcome, and Dr. Jeff Tsao, Chief Scientist of the SSLS EFRC, gave an introduction to solid-state lighting and also discussed how the fundamental research of the workshop addresses issues of potential long-term importance to solid-state lighting. The workshop featured invited talks in two theme areas which intersect nanoscience and solid-state lighting: novel emitters; and light-matter interactions in nanostructured materials.
The workshop provided an opportunity for the nanoscience community to learn about issues associated with solid-state lighting, and for the solid-state lighting community to be exposed to further-out research that could impact their technology. Nanowires and nanodots, for example, are emerging as exciting platforms not just for exploring fundamental materials and physics issues at the nanoscale, but for practical light emission. Strong coupling between exciton and photon cavity resonances to form polaritons is emerging as an exciting way to manipulate light-matter interactions.
In the theme area of novel emitters, our plenary speaker, Professor Lars Samuelson of Lund University (Sweden), discussed recent progress in nanowire-based nano-optical devices, including recent breakthroughs in nanowire LEDs. Other invited speakers in this theme area were: Dr. John Schlager (NIST), who discussed optical characterization of nanowires, pointing out the possible error of converting from EQE to IQE by the customary normalization to a low temperature curve; Professor Silvija Gradecak (MIT), who discussed fundamental growth mechanisms of nanowires; Dr. George Wang (Sandia National Laboratories), who reviewed progress in GaN nanowires at Sandia; and Dr. Jennifer Hollingsworth (Los Alamos National Laboratory), who discussed “giant” nanocrystal quantum dots. Taken together, the work discussed in these talks indicate that real-world application of nanowires and nanodots is not a fantasy anymore.
In the theme area of light-matter interactions in nanostructured materials, our invited speakers were: Professor Leonid Butov (UC San Diego); Dr. Weng Chow (Sandia National Laboratories); Professor Vladimir Bulovic (MIT); and Dr. Stephane Kena-Cohen (Imperial College). In the strong coupling sessions, Professor Vladimir Bulovic (MIT) and Dr. Stephane Kena-Cohen (Imperial College) presented evidence that strong coupling can occur at room temperature in organic systems. Although not as photostable as their inorganic counterparts, the large binding energy leads to large Rabi splittings which alleviates the need for low temperature. Professor Leonid Butov spoke about his work on polariton condensates in GaAs heterostructures and at very low temperatures (<1K). Dr. Kena-Cohen also mentioned the recent work from Professor Bhattacharya’s group at the University of Michigan in creating a polariton condensate in a GaN nanowire with carrier densities several orders of magnitude lower than the Mott density. Dr. Weng Chow (Sandia National Laboratories) showed that phonon interaction and strong coupling can co-exist at room temperatures, and might facilitate rapid cooling of excitons in GaN.
SSLS EFRC Workshop Agenda and Abstracts
Many of the talks were videotaped and are available for viewing on our video page.
EFRC researchers recently demonstrated that the kinetic Wulff plot (v-plot) is a powerful tool for understanding and controlling GaN heteroepitaxy on foreign substrates over polar, nonpolar, and semipolar orientations. The findings were included in a paper published by Journal of Applied Physics in September 2011. The scientists behind this proposal included Micheal E. Coltrin (EFRC scientist), Qian Sun, Christopher D. Yerino, Benjamin Leung, and Jung Han.
Abstract: This work represents a comprehensive attempt to correlate the heteroepitaxial dynamics in experiments with fundamental principles in crystal growth using the kinetic Wulff plot (or v-plot). Selective area growth is employed to monitor the advances of convex and concave facets toward the construction of a comprehensive v-plot as a guidepost for GaN heteroepitaxy. A procedure is developed to apply the experimentally determined kinetic Wulff plots to the interpretation and the design of evolution dynamics in nucleation and island coalescence. This procedure offers a cohesive and rational model for GaN heteroepitaxy on polar, nonpolar, and semipolar orientations and is broadly extensible to other heteroepitaxial material systems. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and geometrical blocking of stacking faults, paving a way to device-quality heteroepitaxial nonpolar and semipolar GaN materials.
The full paper can be read online at the Journal of Applied Physics website.
The EFRC has recently finished its second round of reviews by its External Advisory Board. Unlike our first year review of entire EFRC, this year we held separate reviews of individual thrusts or challenges in order to examine them more deeply. Our reviewers helped us assess how our research is addressing important long-term SSL technology challenges and fundamental and exciting science issues. Our review schedule and review team was as follows:
June 13: Beyond Free-Space Spontaneous Emission
Reviewers:
July 26: Nanowires and Non-Planar Growth
Reviewers:
July 27: Competing Radiative & Non-Radiative Processes
Reviewers:
July 28: Point Defects in InGaN
Reviewers:
August 8: Atoms & Dots
Reviewers:
EFRC scientist Ganapathi Subramania and Stavroula Foteinpoulou from the University of Exeter (UK) chaired the Active Photonic Materials IV conference at the recent SPIE Optics+Photonics conference in San Diego, California. The conference included sessions in:
EFRC scientist Igal Brener was invited to speak twice at the SPIE Nanophotonics conference in San Diego in August.
His first talk involved “Active Infrared Metamaterials”. In it, he reviewed the current status of electrically tunable metamaterials, both at Terahertz and shorter infrared frequencies. Scaling these active devices to mid and near infrared optical frequencies poses considerable challenges and requires new tuning mechanisms. Some examples include controlling coupling to other dipolar resonances such as phonons and engineered transitions in semiconductor heterostructures.
Brener’s second talk was titled “Interactions in Planar Metamatials: From Strong Coupling to Active Tuning”. The issue of interactions between different types of engineered metamaterial resonances has received considerable attention since it was shown that electromagnetically induced transparency behavior can be mimicked using coupling between metamaterials and/or plasmonic resonators. He presented some of our recent results on strong coupling between metamaterial resonators and other excitations such as phonons, free carriers and intersubband transitions in semiconductor heterostructures, and ways to exploit these for active tuning of metamaterial properties.
The full agenda is available in PDF form here.
In addition to his participation in the EAB review of the SSLS EFRC, Professor Sir Colin Humphreys, Director of Research at the Dept. of Materials Science and Metallurgy at Cambridge University, gave a seminar at Sandia National Laboratories. His seminar titled “Carrier localization mechanisms in InGaN quantum wells and growth on large area silicon substrates” discussed mechanisms of carrier localization in InGaN alloys that have a profound impact on the efficiency of InGaN semiconductor alloys , as well as the challenges and recent advances of growth of InGaN LED heterostructures on low-cost, large-area silicon substrates.
An EFRC publication titled “On the temperature dependence of electron leakage from the active region of GaInN/GaN light-emitting diodes”, by EFRC scientists Fred Schubert and his research group at RPI has recently appeared in Applied Physics Letters.
Abstract: Reduction in the light-output power in GaN-based light-emitting diodes (LEDs) with increasing temperature is a well-known phenomenon. In this work, temperature dependent external-quantum-efficiency versus current curves are measured, and the mechanisms of recombination are discussed. Shockley-Read-Hall recombination increases with temperature and is found to greatly reduce the light output at low current densities. However, this fails to explain the drop in light-output power at high current densities. At typical current density (35 A/cm2), as temperature increases, our results are consistent with increased Shockley-Read-Hall recombination and increased electron leakage from the active region. Both of these effects contribute to the reduction in light-output power in GaInN/GaN LEDs at high temperatures.
In a paper recently published in SPIE’s nanotechnology newsroom, EFRC scientists George Wang and Qiming Li’s discuss how advances in the fabrication and characterization of gallium nitride-based nanowires and nanowire devices may lead to enhanced performance solid-state lighting. The paper can be read online at the SPIE Newsroom.
Figure 3. (a) Schematic for vertically integrated, electrically injected nanorod-based GaN/InGaN LED shown in (b), in which a p-doped GaN layer has been coalesced into a planar layer over the nanorods. MQW: Multiquantum well. p, n: Doping. Al2O3: Sapphire.
EFRC partner Fred Schubert and members of his research group recently published an article titled “Effects of polarization-field tuning in GaInN light-emitting diodes” in Applied Physics Letters.
Abstract: III-V nitrides form the backbone of light-emitting diode (LED) technology. However, the relevance of the very strong polarization fields in III-V nitride LEDs remains unclear. Here, we demonstrate the tuning of polarization fields by mechanical force. For compressive strain in a GaInN LED epitaxial layer, we find: (i) redistribution of intensity within the electroluminescence spectrum; (ii) a decrease in the peak efficiency at low current densities; and (iii) an increase in light-output power at high current densities. These findings show the relevance of transport effects in the efficiency droop.
EFRC Scientists Mary Crawford and Jeff Tsao participated at the International Conference on Nitride Semiconductors in Glasgow, Scotland held July 10-15. ICNS-9 is a leading international forum for reporting advances in group III – nitride semiconductors such as gallium nitride and indium nitride. Both fundamental research and applications are fully covered, with sessions focusing on topics such as Epitaxial Growth, Bulk Crystal Growth, Theory and Simulation, Optical and Electronic Devices (LEDs, lasers, transistors, etc), Material Characterization and Development and Nanostructures involving Nitride Semiconductors.
Mary Crawford’s talk was titled “Application of a Microscopic Model to Efficiency Droop of InGaN LEDs”
Abstract: (Excerpt) Understanding the efficiency loss of InGaN LEDs with increasing current is critical for realizing high efficiency solid-state lighting. Efforts to reveal the non-radiative mechanisms behind efficiency droop typically involve application of the “ABC model” to LED efficiency-versus-current data. This rate equation model describes carrier losses as a function of total carrier density “n” and employs empirical fitting parameters A, B, C, ascribed to Shockley-Read-Hall, radiative and Auger recombination, respectively. Despite its wide-spread use, this model has important limitations.
We describe an improved model for the description of LED radiative efficiency into the high-carrier-density regime. While still based on rate equations, our approach replaces total carrier density with momentum-resolved carrier distributions, allowing direct implementation of bandstructure properties into the rate equations. Our approach further provides a more accurate description of carrier-carrier and carrier-phonon interactions, includes a treatment of carrier leakage and capture, and derives radiative recombination via bandstructure and carrier distributions.
Jeff Tsao’s talk centered on the energy economics of solid-state lighting at a rump session on “The Next Big Thing in Nitride Optoelectronics.”
Solid-state lighting is currently based on light-emitting diodes (LEDs) and phosphors. Solid-state lighting based on lasers would offer significant advantages including high potential efficiencies at high current densities. Light emitted from lasers, however, has a much narrower spectral linewidth than light emitted from LEDs or phosphors. Therefore it is a common belief that white light produced by a set of lasers of different colors would not be of high enough quality for general illumination.June 22
In a paper just published in a special “Optics in LEDs for Lighting” issue of Energy Express (a supplement to Optics Express), Sasha Neumann (graduate student at the University of New Mexico), Jon Wierer (EFRC scientist), Wendy Davis (scientist at NIST Gaithersburg), Yoshi Ohno (scientist at NIST Gaithersburg), Steve Brueck (Professor at the University of New Mexico and EFRC external partner), and Jeff Tsao (EFRC scientist), tested this belief experimentally. They found the opposite to be true: in terms of color rendering quality, white light from a four-color (RYGB) laser setup is virtually indistinguishable from white reference light, including that from an incandescent lamp and three phosphor-converted LEDs (warm white, neutral white and cool white). This result paves the way for the use of lasers in solid-state lighting.
The rebound (or take-back) effect) is the term in energy economics used to describe the effectin which increases in energy efficiency do not necessarily lead to simple 1:1 decreases in energy consumption, but instead are “taken back” in the form of higher consumption of the goods and services that the energy is used to create. Although taken very seriously in Europe, both in academia and in policy circles, the rebound effect has not been taken very seriously in the U.S. There are pockets of activity, but no serious government programs aimed at understanding it.
Nonetheless, there is a small community in the U.S. is working in this area, particularly passionate around its importance to U.S. and world policy aimed at mitigating global climate change. As a result of their efforts, the first U.S. workshop on this effect just took place: the “Energy Efficiency and the Rebound Effect” workshop, held at the AAAS Building, Washington DC June 27-28 2011, and organized by Ines Azevedo (Co-Director) and Granger Morgan (Director) of Carnegie Mellon University’s NSF-supported Center for Climate and Energy Decision Making. The workshop was relatively small, about 35 participants from universities (both U.S. and international) and national laboratories (LBNL, NREL, ORNL, Sandia) and about 5 observers (AAAS, DOE-EERE, EPA). The participants all gave short presentations (including a presentation by Jeff Tsao on historical trends in the consumption of light) followed by vigorous discussion.
In a paper titled “Nanoscale Effects on Heterojunction Electron Gases in GaN/AlGaN Core/Shell Nanowires” published in Nano Letters, Sandia scientist Bryan M. Wong, and EFRC scientists François Léonard, Qiming Li, and George T. Wang, present a theoretical and computational study of the electronic properties of core/shell nanowires. They find that the nanometer size scales combined with the highly anisotropic cross-sections of these nanowires strongly influences the behavior of the electron distribution, leading to confinement at corners and polar faces, and transitions between core-centered and interface-confined electron gases. More generally, their results indicate that electron gases in closed nanoscale systems are qualitatively different from their bulk counterparts.
Bulk semiconductor-semiconductor heterojunctions have been instrumental in enabling technological breakthroughs in electronics and optoelectronics, including for solid-state lighting. Such heterojunctions in core/shell nanowires have recently been proposed as a novel route for solid-state lighting technology. In order to assess the potential of such systems, it is important to understand the fundamental electronic and optical properties of these core/shell nanowires.
EFRC scientists John Reno, Joel R. Wendt, Aaron Gin, Michael C. Wanke, Michael B. Sinclair, Eric Shaner, and Igal Brener recently published a paper titled “Interaction between metamaterial resonators and intersubband transitions in semiconductor quantum wells” in Applied Physics Letters.
Abstract: We report on the coupling and interaction between the fundamental resonances of planar metamaterials split ring resonators and intersubband transitions in GaAs/AlGaAs quantum wells structures in the mid-infrared. An incident field polarized parallel to the sample surface is converted by the metamaterial resonators into a field with a finite component polarized normal to the surface and interacts strongly with the large dipole moment associated with quantum well intersubband transitions.
The full paper is available at the Applied Physics Letters website.
EFRC scientists Willie Luk, Weng Chow, Ganesh Subramania, and Art Fischer and colleagues from the University of New Mexico have published a new article titled “Anomalous enhanced emission from PbS quantum dots on a photonic-crystal microcavity” in Optics Express.
Abstract: We report up to 75 times enhancement in emission from lithographically produced photonic crystals with postprocessing close-packed colloidal quantum-dot incorporation. In our analysis, we use the emission from a close-packed free-standing film as a reference. After discounting the angular redistribution effect, our analysis shows that the observed enhancement is larger than the combined effects of Purcell enhancement and dielectric enhancement with the microscopic local field. The additional enhancement mechanisms, which are consistent with all our observations, are thought to be spectral diffusion mediated by phonons and local polarization fluctuations that allow off-resonant excitons to emit at the cavity wavelengths.
The Solid-State Lighting Science Energy Frontier Research Center (SSLS EFRC) and Sandia’s Physical, Chemical, & Nano Sciences Center participated in the 2011 Take our Daughters and Sons to Work Day on April 28th. During the event, our booth and display areas were filled with students and parents who actively engaged in a series of “Coloring Your World” exhibits. One example was a hands-on exhibit on solid-state lighting that illustrated different ways of creating white light from single-colored light-emitting diodes and the resulting differences in how the colors of typical objects in our lives look under these lights. In addition, we challenged our visitors to participate in an electronic “LED Lighting 101” Jeopardy game with an emphasis on understanding LEDs. The game explored the evolution of LED technologies and potential technologies.
That evening the EFRC enjoyed the end of the day with a fun, informal social after work gathering and celebration at O’Neill’s Irish Pub. SSLS EFRC members toasted Zhengshan Yang, a post doc with EFRC Scientist Weng Chow, who has accepted a full professor position in the College of Physical Science, Information Technology & Engineering at Liaocheng University. Liaocheng University has 28,000 full-time students and a teaching a research staff of 1,700. Zhengshan will leave Sandia at the end of April. The food was great and a good time was had by all.
Recently, the EFRC put together an entry for the Life at the Frontiers of Energy Research video contest, associated with the upcoming EFRC Summit & Forum. It features George Wang and Tania Henry explaining some of the ways that research at the EFRC helps enable greater energy efficiency. The Energy Frontier Research Center‘s video with the most votes by 5 pm EST on May 24, 2011 will win the People‘s Choice Award, to be presented at the EFRC Summit on May 25, 2011 in Washington DC.
Please take a few minutes to view our video. (It is about halfway down the page, titled: Enabling Energy Efficiency.) Also, please share the link with anyone you know that might be interested, so we can spread the word about some of the great things we do at the SSLS EFRC.
Jeff Tsao, Distinguished Member of Technical Staff at Sandia National Laboratories and Chief Scientist of the EFRC, gave a Physical, Chemical, and Nano Sciences Colloquium at Sandia on April 13, 2011. The colloquium was an opportunity to give an overview of Sandia’s Energy Frontier Research Center for Solid-State Lighting Science, and to review: (1) its funding and organization; (2) SSL technology; and (3) the foundational science being done in the EFRC aimed at making long-term impact on SSL technology.
Abstract: Solid-state lighting is on the verge of replacing every lamp on earth, from the smallest signal lamp to the largest stadium-scale flood lamp. But many challenges remain, if solid-state lighting is to fulfill its ultimate promise. In this talk, I will give an overview of Sandia’s Energy Frontier Research Center (EFRC) for Solid-State Lighting Science, one of 46 EFRCs supported by DOE’s Office of Science to “establish the scientific foundation for a fundamentally new U.S. energy economy.” Our EFRC aims to deepen the foundational science of energy conversion and manipulation in tailored photonic structures, while informing, and being informed by, solid-state lighting technology. Its overarching scientific thrusts are: (1) Competing Radiative and Non-radiative Processes (aimed at developing a microscopic understanding of the competition between radiative and non-radiative e-h recombination); (2) Beyond Free-Space Spontaneous Emission (aimed at exploring energy conversion routes that short-circuit conventional free-space spontaneous emission but end in free-space photons); and (3) Beyond-2D (aimed at exploring the use of non-planar nanoscale structures to modify energy conversion routes).
An audio recording of this presentation is available here.
Strong coupling between material excitations and optical resonators is an intriguing means for creating composite polariton excitations with new properties. For example, strong coupling between semiconductor excitons and high-Q optical cavities can create exciton polaritons which, at modest densities, may form a Bose-Einstein condensate with coherent, laser-like emission.
In principle, strong coupling may occur between a wide range of material excitations and a wide range of optical resonators. In a paper (“Strong Coupling between Nanoscale Metamaterials and Phonons”) published in Nano Letters, researchers from CREOL (D.J. Shelton and G. D. Boreman), AMPAC (K. R. Coffey), Plasmonics Inc (D.J. Shelton and J. C. Ginn), Sandia National Labs (M. B. Sinclair and D. W. Peters) and from our EFRC (I. Brener), have demonstrated strong coupling between a metamaterial resonator and infrared active phonons.
The metamaterial was based on split ring resonators (SRRs) and the infrared active phonons were those of SiO2 at 130 meV (31 THz). Strong anti-crossing of these resonances was observed, indicative of strong coupling between the metamaterial and phonon excitations. The observed anti-crossing behavior results in the opening of a transparency window within the absorption band of the uncoupled metamaterial. The amount of coupling can be altered through the design of the metamaterial resonators, the proximity of the dielectric film containing the vibrational species to the resonator, the dielectric film thickness, and the amount of field overlap with the dielectric layer. This new approach could have important implications for the design of future active and nonlinear metamaterials, passive and active plasmonic filters, and novel light emitters through active manipulation of phonons and carrier-phonon dynamics.
Figure: (a) Schematic cross section showing thin film interface between metallic SRR elements and Si wafer including a thin SiO2 layer. (b) An SEM micrograph of the SRR arrays. (c) Dimensional parameters.
March 30, 2011
Our EFRC is exploring the use of nanowires based on gallium nitride (GaN) in solid-state lighting applications. Ultimately one anticipates that charge carriers (electrons and holes) would recombine to create light. Enroute to this recombination, however, carriers must be transported. Up until now, however, it has been difficult to measure the transport behavior of carriers, particularly minority carriers, in these extremely small nanostructures. Determination of minority carrier diffusion lengths (Ld) is generally performed using electron beam induced current (EBIC), light induced transient grating, or a combination of measurements of lifetime with estimates of minority carrier mobility. Strengths and limitations of these approaches vary but most either require contacts or lack the spatial resolution for nanostructure characterization.
In a paper published in Applied Physics Letters, Naval Postgraduate School researchers (Lee Baird, C. P. Ong, R. Adam Cole, and N. M. Haegel) and EFRC scientists (A. Alec Talin, Qiming Li, and George T. Wang) devised a combination technique to make direct measure of Ld in GaN, GaN/AlGaN, and GaN/InGaN core-shell nanowires. The technique combined (a) near-field scanning optical microscopy (NSOM) to image the luminescence associated with carrier diffusion and recombination with (b) charge generation by a scanning electron microscope (SEM).
The results depended strongly on the shell material. An AlGaN shell increases Ld, due to confinement of carriers in the GaN away from the surface where fast recombination can take place. In contrast, an InGaN shell reduces Ld, due to band bending which enhances diffusion of minority carrier holes to the surface.
Figure: Topography (left) and near-field optical intensity (right) images during e-beam excitation of a GaN nanowire.
GaN nanowires are a promising vehicle for fundamental studies of electron and hole carrier dynamics in, and for optoelectronic devices that could benefit from, the absence of the dislocations normally present in planar GaN structures. Up until now, the electrical, optoelectronic, and thermal properties of GaN nanowires have been studied extensively, but less so the mechanical properties of interest to devices that might be stressed during fabrication or operation.
In a paper just published in NanoLetters, “In Situ Nanomechanics of GaN Nanowires,” we report a novel, in situ, real-time dynamical study of the mechanics of GaN nanowires. The authors of the paper are: Jian Yu Huang (EFRC scientist, Sandia Labs), He Zheng (University of Pittsburgh and Wuhan University), S.K. Mao (University of Pittsburgh), Qiming Li (EFRC scientist, Sandia Labs) and George Wang (EFRC scientist, Sandia Labs).
The study involved real-time transmission-electron microscopy measurements of nanowires as they are mechanically contacted, stressed, and ultimately fractured, by a scanning probe “punch.” Real-time movies of the process provide unique insight into the dynamics of GaN nanowire mechanics. In all cases, fracture is brittle, but preceded by local (but not global) plastic deformation via dislocation motion. That the fracture is brittle is consistent with the known difficulty with which dislocations in GaN move; that some pre-fracture local plastic deformation occurs near the contact surface is an indication that dislocation dynamics (including nucleation) are strongly modified by surfaces.
Figure Caption. In-situ compression of a GaN nanowire. Left and middle are transmission electron microscopy images (TEM) of a GaN nanowire before and after compression, respectively, showing the top of the nanowire was plastically deformed to a mushroom shape after compression. Right is a high resolution TEM lattice image showing the fracture surface was kinked at (10-10) and (10-11) planes.
The fundamental interaction governing light emission from semiconductor materials is the coupling between electronic states in the semiconductor and a local electromagnetic field. The current generation of solid-state lighting devices operates in the so-called “weak-coupling” regime, in which the rate of energy transfer between the electronic states and the local electromagnetic field is lower than the rates at which these states or fields relax. For a possible future generation of solid-state lighting devices, the Solid-State Lighting Science Energy Frontier Research Center is exploring the so-called “strong-coupling” regime, in which the rate of energy transfer between the electronic states and the local electromagnetic field is higher than the rates at which these states fields relax. In this regime, energy oscillates between the electronic states and the local electromagnetic field—so-called Rabi oscillations which manifest themselves as Rabi splittings in the electronic states and in measured optical emission spectra.
In a paper just published in NanoLetters, “Observation of Rabi Splitting from Surface Plasmon Coupled Conduction State Transitions in Electrically Excited InAs Quantum Dots,” we observe Rabi splitting between electronic states in InAs quantum dots and a surface-plasmon polariton field. The electronic states are populated by electrical excitation, and are conduction-band states connected by quantum-cascade-like intra-conduction-band transitions. The surface plasmon polariton field is supported by a metallic (Au) film in close proximity to the quantum dots, and the film perforated with holes arranged to facilitate coupling of the surface-plasmon polariton to free-space light.
The electroluminescence spectra for devices emitting in the 9-11 µm wavelength range show distinct spectral splittings that we believe, through comparisons with model calculations, arise from Rabi oscillations. This result indicates that electrically excited intra-conduction-band transitions in self-assembled quantum dot samples can be strongly coupled to surface-plasmon polaritons. A striking result of the research is that Rabi oscillations can be observed from a largely inhomogenously broadened medium of narrow linewidth oscillators. Beyond this, these results indicate that quantum dots and surface-plasmon polaritons show promise for quantum-optics experiments and for non-classical long-wavelength plasmonic optoelectronic devices.
The authors of the paper are: Brandon Passmore (EFRC student, Sandia Labs), David Adams (EFRC student, University of Massachusetts), Troy Ribaudo (EFRC student, University of Massachusetts), Dan Wasserman (EFRC external partner, University of Massachusetts), Stephen Lyon (Princeton University), Paul Davids (EFRC student, Sandia Labs), Weng Chow (EFRC scientist, Sandia Labs), and Eric Shaner (EFRC scientist, Sandia Labs).
The EFRC enjoyed its first annual “Ski Day” at Ski Santa Fe on February 26, 2011. Jeff Tsao, Weng Chow, and Chris Monroe coordinated the outing. 10 people took part in this fun team-building event. The day started with an early breakfast at the French Pastry Shop at La Fonda Hotel in Santa Fe, followed by a day of skiing, with additional attendees joining the group for dinner at the Zia diner. The Santa Fe resort had excellent snow depth and nearly 100% of the runs were open. Weng Chow, who was most familiar with the mountain, was the group’s guide on the slopes. Over the course of the day, the group had the opportunity to both ski and have fun together. Overall, it was a great ski day with no injured skiers and a good time was had by all.
On February 17, students from the University of New Mexico visited the EFRC to learn about solid-state lighting technology. The visitors started the afternoon with a presentation by EFRC director Jerry Simmons, which covered an overview of SSL technology and a discussion of future potential for SSL in a variety of applications. Later, Dan Koleske showed students what it is like to work in the Fab, and EFRC thrust leaders George Wang and Mary Crawford gave tours of their labs. The day ended with some hands-on demonstrations of SSL technology. The students came as an activity for their class in technical writing at UNM, and will be using SSL and the EFRC as topics in upcoming assignments.
EFRC management and staff (Jerry Simmons, Mike Coltrin, Jeff Tsao, Bob Biefeld, Dan Koleske and Andy Armstrong) and external partner Professor Jung Han (Yale University) participated in this week’s “Transformations in Lighting,” the 2011 DOE Solid-State Lighting R&D Workshop sponsored by the Office of Energy Efficiency and Renewable Energy (EERE).
The three-day workshop, held in San Diego, was an opportunity for the U.S. SSL community to hear talks covering a wide range of SSL topics ranging from research to development to commercialization, and to provide input into the DOE-EERE SSL R&D program and priority research directions.
One session was devoted to R&D (Core Technology) projects funded through the DOE-EERE SSL program. EFRC scientists Dan Koleske and Andy Armstrong, and external partner Professor Jung Han presented posters at this session. They all have projects in the DOE-EERE SSL program which build on EFRC and Office of Basic Energy Sciences’ fundamental research investments. Dan’s poster was entitled Semi-polar GaN materials technology for high IQE green LEDs“. Andy’s poster was entitled “Novel defect spectroscopy of InGaN materials for improved green LEDs.” Professor Jung Han’s poster was entitled “Multicolor, high efficiency, nanotextured LEDs.”
Another session was devoted to discussing hot topics that emerged from the SSL Roundtable discussions held in Washington DC in November, 2010. EFRC Chief Scientist Jeff Tsao was one of four panelists in the LED portion of this session. Their panel presented forward-looking ideas that could impact the R&D directions of the LED part of the DOE-EERE SSL R&D program.
The ABC model is often used to describe carrier recombination in the active region of a LED. In a recent paper published in Applied Physics Letters titled “On the symmetry of efficiency-versus-carrier concentration curves in GaInN/GaN light-emitting diodes and relation to droop-causing mechanisms,” a careful examination of thecarrier-density dependence of LED internal quantum efficiency (IQE) showed marked differences between experiment and this simple model. Fourth-order (and higher) terms must be added to the ABC model to match the symmetry properties of measured IQE data. This work was jointly performed by members of Fred Schubert’s research group at RPI and at Sandia.
Abstract: The internal quantum efficiency (IQE)-versus-carrier-concentration (n) curves of GaN-based light-emitting diodes have been frequently described by the ABC model: IQE = Bn2/(An+Bn2+Cn3). We show that this model predicts IQE-versus-n curves that have even symmetry. Phase-space filling makes the B and C coefficients concentration-dependent. We also show that IQE-versus-n curves that take into account phase-space filling possess even symmetry. In contrast, experimental IQE-versus-n curves exhibit asymmetry. The asymmetry requires a fourth-power or higher-power contribution to the recombination rate and provides insight into the mathematical form of the droop-causing mechanisms.
EFRC scientists from Rensselaer Polytechnic Institute and Sandia recently published a paper titled “Temperature-dependent light-output characteristics of GaInN light-emitting diodes with different dislocation densities” in Physica Status Solidi A. Their publication examines the impact of threading dislocation density on the temperature stability of LEDs.
Abstract: We have experimentally investigated the temperature dependence of optical-output power of light-emitting diodes (LEDs) with different threading dislocation densities (TDDs) to assess the influence of the TDD on the temperature stability of LEDs. Whereas the LED with high TDD shows a 64% decrease in optical-output power when the ambient temperature increases from 20 to 150 °C, the LED with low TDD shows only a 54% decrease. The temperature dependence of the optical-output power and current dependence of the characteristic temperature Tch of LEDs shows that short radiative recombination lifetime and low TDDs are essential to obtain LED characteristics that are tolerant of high temperatures.
ERFC scientist Weng W. Chow and collaborators from the Technical University of Berlin have recently published an article titled “Inductive equation of motion approach for a semiconductor QD-QED: Coherence induced control of photon statistics” in Physica Status Solidi B.
Abstract: This paper presents an inductive method for the microscopic description of quantum dot (QD) QED. Our description reproduces known effects up to an arbitrary accuracy, and is extendable to typical semiconductor effects, like many electron- and phonon-interactions. As an application, this method is used to theoretically examine quantum coherence phenomena and their impact on photon statistics for a Λ-type semiconductor QD strongly coupled to a single mode cavity and simultaneously excited with an external laser.
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