Energy and Climate

2014

Toward Smart and Ultra-efficient Solid-State Lighting

June 27, 2014

The article “Solid-State Lighting: Toward Smart and Ultra-Efficient” was recently published by Advanced Optical Materials. The article was written by Jeffrey Tsao, Mary Crawford, Michael Coltrin, Arthur Fischer, Daniel Koleske, Ganesh Subramania, George Wang, Jonathan Wierer, and Robert Karlicek. Work was done at both Sandia National Laboratories and Rensselaer Polytechnic Institute.

Advanced Optical Materials

 

 

Abstract: Solid-state lighting has made tremendous progress this past decade, with the potential to make much more progress over the coming decade. In this article, the current status of solid-state lighting relative to its ultimate potential to be “smart” and ultra-efficient is reviewed. Smart, ultra-efficient solid-state lighting would enable both very high “effective” efficiencies and potentially large increases in human performance. To achieve ultra-efficiency, phosphors must give way to multi-color semiconductor electroluminescence: some of the technological challenges associated with such electroluminescence at the semiconductor level are reviewed. To achieve smartness, additional characteristics such as control of light flux and spectra in time and space will be important: some of the technological challenges associated with achieving these characteristics at the lamp level are also reviewed. It is important to emphasize that smart and ultra-efficient are not either/or, and few compromises need to be made between them. The ultimate route to ultra-efficiency brings with it the potential for smartness, the ultimate route to smartness brings with it the potential for ultra-efficiency, and the long-term ultimate route to both might well be color-mixed RYGB lasers.

 

Energy Frontier Research Center for Solid-State Lighting Science: Exploring New Materials Architectures and Light Emission Phenomena

June 26, 2014

Journal of Physical Chemistry C

 

 

 

 

 

 

 

 

 

 

The SSLS review article entitled “Energy Frontier Research Center for Solid-State Lighting Science: Exploring New Materials Architectures and Light Emission Phenomena” made the cover of The Journal of Physical Chemistry C for the week of June 26, 2014. The article covers the research done at the SSLS Energy Frontier Research Center.

Abstract: The Energy Frontier Research Center (EFRC) for Solid-State Lighting Science (SSLS) is one of 46 EFRCs initiated in 2009 to conduct basic and use-inspired research relevant to energy technologies. The overarching theme of the SSLS EFRC is the exploration of energy conversion in tailored photonic structures. In this article we review highlights from the research of the SSLS EFRC. Major research themes include: studies of the materials properties and emission characteristics of III-nitride semiconductor nanowires; development of new phosphors and II–VI quantum dots for use as wavelength downconverters; fundamental understanding of competing radiative and nonradiative processes in current-generation, planar light-emitting diode architectures; understanding of the electrical, optical, and structural properties of defects in InGaN materials and heterostructures; exploring ways to enhance spontaneous emission through modification of the environment in which the emission takes place; and investigating routes such as stimulated emission that might outcompete nonradiative processes.

 

Radiative Emission Enhancement Using Nano-antennas Made of Hyperbolic Metamaterial Resonators

June 23, 2014

A team of our SSLS EFRC participants recently had a new paper accepted for publication in Applied Physics Letters. The paper is titled “Radiative Emission Enhancement Using Nano-antennas Made of Hyperbolic Metamaterial  Resonators” and is written by EFRC authors Willie Luk and George Wang, as well as Caner Guclu and Filippo Capolino from UC Irvine.

Radiative Emission 1Radiative Emission 2

Abstract: A hyperbolic metamaterial (HM) resonator was analyzed as a nano-antenna for enhancing the radiative emission of quantum emitters in its vicinity. It has been  shown that the spontaneous emission rate by an emitter near a hyperbolic metamaterial substrate is enhanced dramatically due to very large density of states. However, enhanced coupling to the free-space, which is central to applications such as solid-state lighting, had not been investigated significantly. Here, we numerically demonstrated approximately 100 times enhancement of the free-space radiative emission at 660 nm wavelength by utilizing a cylindrical HM resonator with a radius of 54 nm and a height of 80 nm on top of an opaque silver-cladded substrate. We also showed how the free-space radiation enhancement factor depends on the dipole orientation and the location of the emitter near the subwavelength resonator. Furthermore, we calculate that an array of HM resonators with subwavelength  spacings can maintain most of the enhancement effect of a single resonator.

 

Emission properties of nanolasers during transition to lasing

May 7, 2014

Weng Chow and co-authors Frank Jahnke and Christopher Gies (Institute fur Theoretische Physik, Universitat Bremen, Germany) just had a paper accepted into the nature journal Light: Science & Applications entitled “Emission properties of nanolasers during transition to lasing.” The paper is motivated by on-going discussions of recent nanolaser experiments, relating to the criteria for lasing and determination of laser threshold, where much ambiguity exists in cases of apparent thresholdless lasing.

Abstract: The emission characteristics of nanolasers are investigated, using a fully quantized laser theory. The active medium is treated as consisting of inhomogeneously-broadened semiconductor quantum dots embedded in a quantum well. Threshold behaviors, in terms of intracavity intensity, coherence time and second order correlation function are computed during nonlasing to lasing transition. Comparisons are made between a conventional nanolaser configuration and when approaching unity-spontaneous-emission-factor or few-quantum-dot situations. A motivation for the study is to contribute to discussions involving recent nanolaser experiments, in particular, those relating to threshold lasing.

 

Review: Down Conversion Materials for Solid-State Lighting

May 6, 2014

Down conversion materials

Fellow SSLS scientists Lauren Rohwer and co-authoer Joanna McKittrick from UC San Diego published a paper titled, “Review: Down Conversion Materials for Solid-State Lighting”. This comprehensive, 26-journal-page-work reviews wavelength down conversion from near-UV or blue emission from InGaN LEDs in order to produce visible light. They discuss the many criteria for practical application to SSL, and the advantages and disadvantages of different classes of phosphor materials. They provide an extensive analysis of core/shell quantum dot architectures and strategies to improve their photostability and reduce the thermal quenching. The article also discusses different methods to integrate the phosphors and QDs with the LED for SSL use.

Abstract: The wavelength down conversion approach to solid-state lighting (SSL) uses down conversion materials to produce visible light when excited by near-UV or blue emission from InGaN LEDs. This review discusses two classes of down conversion materials: phosphors and semiconductor quantum dots (QDs). Strong absorption of the excitation wavelength; high luminous efficacy of radiation, which enables white light with a high color rendering index and a low correlated color temperature; high quantum efficiency; and thermal and chemical stability are some of the criteria for down converters used in SSL. This review addresses the challenges in the development of down converters that satisfy all these criteria. We will discuss the advantages and disadvantages of several phosphor compositions for blue and near-UV LEDs. The use of core/shell architectures to improve the photoluminescence and moisture resistance of phosphors is presented. QDs are another class of down conversion materials for near-UV and blue LEDs. Strategies to improve the photostability and reduce the thermal quenching of QDs include strain-graded core/shell interfaces and alloying. We discuss Cd-containing II–VI QDs, and Cd-free III–V and I–III–VI QDs and their potential for SSL applications. Finally, a description of different methods to integrate the phosphors and QDs with the LED is given.

 

On the reliable analysis of indium mole fraction within InxGa1−xN quantum wells using atom probe tomography

April 22, 2014

Indium mole fraction

In the paper titled, “On the reliable analysis of indium mole fraction within InxGa1−xN quantum wells using atom probe tomography,” SSLS scientists Jim Riley and Lincoln Lauhon from Northwestern University and Theeradeth Detchprohm and Christian Wetzel from Rensselaer Polytechnic Institute examine the effect of surface crystallography and polarity on the detection probability of In, Ga and N ions in the atom probe tomography (APT) of InGaN quantum wells. It was known that  group-III and group-V  species in compound semiconductors exhibit distinct evaporation characteristics and APT detection probabilities. This work showed that while Ga and N are generally not detected with equal probability when evaporating from facets of different polarity, In and Ga are seen to evaporate similarly. This enables reliable measurement of group-III mole fraction via APT, as was demonstrated here for InGaN QWs grown on m-plane, c-plane, and (20-21) surfaces of GaN.

Abstract: Surface crystallography and polarity are shown to influence the detection probability of In, Ga, and N ions during atom probe tomography analysis of InxGa1−xN m-plane, c-plane, and (202¯1¯) quantum wells. A N deficit is observed in regions of the reconstruction generated from Ga-polar surfaces, and the probability of detecting group-III atoms is lower in InxGa1−xN quantum wells than in GaN barrier layers. Despite these artifacts, the detected In mole fraction is consistent throughout a given quantum well regardless of the crystal orientation of the quantum well or the evaporation surface from which the reconstruction was generated.

 

Connections between GaN and InGaN Growth Mechanisms and Surface Morphology

March 25, 2014

GaN_InGaN-Growth-Mechanisms

SSLS EFRC scientists Dan Koleske, Steve Lee, Mary Crawford, Karen Cross, Mike Coltrin, and Jeff Kempisty collaborated on a paper titled, “Connection between GaN and InGaN growth Mechanisms and Surface Morphology” for the Journal of Crystal Growth. They describe how smoothing and roughening mechanisms result in the growth morphologies that are observed during GaN and InGaN growth.

Abstract: Power spectral density (PSD) analysis of atomic force microscopy (AFM) images is used to determine the roughening and smoothing mechanisms that contribute to InGaN and GaN morphology during metalorganic chemical vapor deposition epitaxy (MOCVD). The analysis finds that non-stochastic surface roughening  occurs from deposition onto the pre-existing GaN surface roughening becoming more evident at lower growth temperatures. Counteracting the surface roughening are two smoothing mechanisms that operate over different temperature ranges and length scales. The first an evaporation g and recondensation mechanism, which dominates at higher temperatures (>900°C) and longer length-scales ranging from one to tens of microns. The second is a surface diffusion mechanism, which dominates at lower temperatures (

 

Excitons in a quasi-one-dimensional quantum nanorod under a strong electric field

Excitons in a quasi-one-dimensional quantum nanorod

SSLS EFRC colleague Ken Lyo published a paper titled, “Excitons in a quasi-one-dimensional nanorod under a strong electric field”. Lyo examines the response of an exciton in the ground and first excited states to a strong DC electric field in a quasi-one-dimensional nano quantum well (i.e., nanorod). Lyo looked at the interplay between barrier confinement, e-h attraction, and the field-induced e-h separation for exciton binding. For a long nanorod , the exciton energy, as well as the oscillator strength, drops abruptly as a function of the field near the exciton-dissociation field, while the average e-h separation rises rapidly. For shorter rods, the transition was found to be more gradual due to the combined effect of the confinement and the long-range e-h interaction. The dependence of a number of physical properties was studied as a function of rod length for varying electric fields.

 

Controlling indium incorporation in InGaN barriers with dilute hydrogen flows

March 15, 2013

Controlling Indium Incorporation

SSLS EFRC scientists Dan Koleske, Steve Lee, Mary Crawford, Karen Cross, Mike Coltrin, and Jeff Kempisty collaborated on a paper titled, “Controlling indium incorporation in InGaN barriers with dilute hydrogen flows.” The purpose of the paper is to show that the extent of In incorporation into InGaN barriers could be controlled by changing the accompanying hydrogen carrier gas flow. They were successful in developing a chemistry-based model that suggests that hydrogen enhances indium surface desorption through the formation of more volatile indium-hydride species, thereby decreasing the surface indium concentration available for incorporation into the barriers.

Abstract: InxGa1−xN multiple quantum wells (MQWs) with InyGa1−yN barriers were grown by adding dilute hydrogen flows to the QW growth conditions in order to modify the barrier indium composition. With the H2 flow off, the indium concentration in the InxGa1−xN QWs were x=0.183, x=0.163, and x=0.096 for growth temperatures of 730, 750 and 770 °C respective. Using these same QW growth conditions, the H2 flow was increased up to 3 SLM resulting in a gradual decrease in the indium concentration in the InyGa1−yN barriers. Kinetic analysis suggests that hydrogen enhances indium surface desorption through the formation of more volatile indium-hydride species, thereby decreasing the surface indium concentration available for incorporation into the InyGa1−yN barriers. For the MQW structures grown at 750 °C, the photoluminescence (PL) wavelength blue-shifts from 477 to ~450 nm as the indium in the InyGa1−yN barriers increases as expected from the reduced influence of the piezoelectric fields. While a corresponding increase of spontaneous emission from the increasing overlap of electron states within the QW is also expected, the PL intensity of the QW instead decreases. This conflicting expectation of increased spontaneous emission and the observation of decreased PL intensity suggest either decreases in the QW-barrier height or increases in non-radiative defects offset any efficiency gains obtained when InGaN barriers to reduce polarization fields within the QWs.

 

Contribution of deep-level defects to decreasing radiative efficiency of InGaN/GaN quantum wells with increasing emission wavelength

Sandia SSLS scientists Andrew Armstrong, Mary Crawford and Daniel Koleske recently published a paper titled, “Contribution of deep-level defects to decreasing radiative efficiency of InGaN/GaN quantum wells with increasing emission wavelength”. They review how deep-level optical spectroscopy and photoluminescence were used to understand the role of defects in reducing the IQE of InxGa1-xN/GaN multiple quantum wells as the emission wavelength increased from approximately 450 to 530 nm. DLOS studies of LEDs identified QW defects whose concentrations increased significantly with increasing In fraction. The effect of increased QW defect density on IQE was determined by examining the PL of MQW samples. Green-emitting MQWs had lower IQE and required higher pump power to reach peak IQE, confirming the important impact of enhanced non-radiative recombination at defects as wavelength (and In fraction) increases.

Abstract: Deep-level optical spectroscopy (DLOS) and photoluminescence (PL) were used to understand the role of defects in reducing the internal quantum efficiency (IQE) of InxGa1−xN/GaN multiple quantum wells (MQWs) as the emission wavelength increased from approximately 450 to 530 nm, i.e., the “green gap”. DLOS studies of light emitting diodes (LEDs) identified QW defects whose concentration increased significantly with increasing x. The effect of increased QW defect density on IQE was assessed by examining the PL of MQW samples. Green-emitting MQWs had lower IQE and required higher pump power to reach peak IQE, corroborating the important impact of enhanced non-radiative recombination at defects.

 

Distributed Feedback Gallium Nitride Nanowire Lasers

January 6, 2014

Distributed Feedback Gallium Nitride Nanowire Lasers

In the paper, “Distributed Feedback Gallium Nitride Lasers”, SSLS scientists Jeremy Wright, Salvatore Campione, Sheng Liu, Julio Martinez, Hiuwen Xu, Willie Luk, Qiming Li, George Wang, Brian Swartzentruber, Luke Lester, and Igal Brenner discuss the topic of single-mode lasting using distributed feeback by externally coupling GaN nanowires to a dielectric grating to achieve mode-control. The effective periodicity of the grating experience by the nanowire was altered using nanomanipulation to change the angular alignment between the nanowire and the grating. Single-mode emission was achieved at an alignment where the designed periodicity of the grating was experienced by the nanowire.

Abstract: Achieving single-mode laser operation in nanowire lasers remains a challenge due to a lack of mode selection approaches. We have implemented single-mode lasing using distributed feedback by externally coupling gallium nitride nanowires to a dielectric grating to achieve mode-control. The effective periodicity of the grating experienced by the nanowire was altered using nanomanipulation to change the angular alignment between the nanowire and the grating. The effective periodicity controls the spectral location of the distributed feedback stop-band. Single-mode emission was achieved at an alignment, where the designed periodicity of the grating was experienced by the nanowire.

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