Energy and Climate
Spatial Mapping of Efficiency of GaN/InGaN Nanowire Array Solar Cells Using Scanning Photocurrent Microscopy

November 10,2013

spatial mapping

SSLS scientists Sarah Howell, Sonal Padalkar, KunHo Yoon, Qiming Li, Dan Koleske, Jon Wierer, George Wang and Lincoln Lauhon published an article titled, “Spatial Mapping of Efficiency of GaN/InGaN Nanowire Array Solar Cells Using Scanning Photocurrent Microscopy” in the Journal of Nano Letters. This research article takes a detailed look into GaN-InGaN core-shell nanowire array devices using the process of scanning photocurrent microscopy.

Abstract: GaN-InGaN core-shell nanowire array devices are characterized by spectrally resolved scanning photocurrent microscopy (SPCM). The spatially resolved external quantum efficiency is correlated with structure and composition inferred from atomic force microscope (AFM) topography, scanning transmission electron microscope (STEM) imaging, Raman microspectroscopy, and scanning photocurrent microscopy (SPCM) maps of the effective adsorption edge. The experimental analyses are coupled with finite difference time domain simulations to provide mechanistic understanding of spatial variations in carrier generation and collection, which is essential to the development of heterogeneous novel architecture solar cell devices.


Group-velocity slowdown in a double quantum dot molecule

November 8, 2013

group velocity

Research article, “Group-velocity slowdown in a double quantum dot molecule” was recently published in the Journal Physical Review B.. SSLS EFRC scientist, Weng Chow, in conjunction with Stephan Michael and Hans Christian Schneider, discuss the effects in slowdown of optical pulses in double quantum dot molecules.

Abstract: The slowdown of optical pulses due to quantum-coherence effects is investigated theoretically for an “active material” consisting of InGaAs-based double quantum dot molecules. These are designed to exhibit a long-lived coherence between two electronic levels, which is an essential part of the quantum-coherence scheme that makes use of electromagnetically induced transparency effect to achieve group-velocity slowdown. We apply a many-particle approach based on realistic semiconductor parameters that allows us to calculate the quantum dot material dynamics including microscopic carrier scattering and polarization dephasing dynamics. The group-velocity reduction is characterized in the frequency domain by the quasiequilibrium slowdown factor and in the time domain by the probe-pulse slowdown obtained from a calculation of the spatiotemporal material dynamics coupled to the propagating optical field. The group-velocity slowdown in the quantum dot molecule is shown to be substantially higher than what is achievable from similar transitions in typical InGaAs-based single quantum dots. The dependencies of slowdown and shape of the propagating probe pulses on lattice temperature and drive intensities are investigated.


Stark effect of optical properties of excitons in a quantum nanorod with parabolic confinement

October 21, 2013

Lyo_Figs_Tiff.pptxRecently, the journal article titled, “Stark effect of optical properties of excitons in a quantum nanorod with parabolic confinement” was published in the Journal of Luminescence. In this publication, SSLS EFRC scientist Ken Lyo examined the effects of confinement, electron-hole attraction and an electric field on binding energy, oscillator strength and average e-h separation distance. As a result, major controlling parameters were found to be the length of the quantum rod and the accompanying change in confinement.

Abstract: We study the quantum Stark effect of optical properties of a quasi-one-dimensional quantum rod with parabolic confinement. Interplays between the competing/cooperative forces from confinement, electron–hole (e–h) attraction, and an external field are examined by studying the binding energy, the oscillator strength, and the root-mean-square (RMS) average of the e–h separation in a nonlinear electric field. In a long rod with weak confinement, the e–h interaction dominates over the confinement effect, yielding an abrupt drop of the exciton binding energy, oscillator strength, and a sudden increase of the RMS average e–h separation as the excitons are dissociated at the threshold field as the field increases. The exciton-dissociation transition is gradual in a short rod, where the confinement force dominates over the e–h attraction. We show that a DC field can induce an optically active excited exciton state in a narrow field range, causing a sharp peak in the oscillator strength and a dip in the RMS average of the e–h separation as the field increases. The Stark effects are also investigated as a function of the linear confinement length (i.e., rod length) at fixed fields.

2013 SSL Symposium

October 7, 2013

Rainbow OLEDs

Last month, SSLS EFRC Director Mike Coltrin, in conjunction with Mark Thompson from the University of Southern California, organized a solid-state lighting symposium. The symposium was sponsored by the Division of Physical Chemistry at last month’s American Chemical Society national meeting in Indianapolis. The goal of the symposium revolved around the discussion of efforts to improve the efficiency of light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) for lighting application. Possible methods that were featured involved nanowires or manipulating light-emitting features of organic materials. To see a comprehensive overview of the symposium and future solid-state lighting applications, please click here.


Lasers for Solid-State Lighting

October 4, 2013

ssl laserThe discussion surrounding the use of lasers in solid-state lighting is gaining momentum thanks to SSLS EFRC scientist Jon Wierer! Recently, Jon Weirer and Jeff Tsao were interviewed by Richard Stevenson, a writer for the “Compound Semiconductor Magazine” on the topic of lasers for solid-state lighting. Jon spearheaded a review article on this topic, and was invited to give a talk about lasers for SSL at the International Conference on Nitride Semiconductors.  Keep an eye out for Jon and Jeff’s interview in the Compound Semiconductor Magazine, scheduled for released around November/December.


Multi-Colour Nanowire Photonic Crystal Laser Pixels

September 25, 2013

multi-colour nanowireAn SSLS EFRC team consisting of Jeremy Wright, Sheng Liu, George Wang, Qiming Li, Alexander Benz, Dan Koleske, Ping Lu, Huiwen Xu, Luke Lester, Willie Luk, Igal Brener and Ganesh Subramania collaborated in a publication titled, “Multi-Colour Nanowire Photonic Crystal Laser Pixels”, an SSLS EFRC team consisting of Jeremy Wright, Sheng Liu, George Wang, Qiming Li, Alexander Benz, Dan Koleske, Ping Lu, Huiwen Xu, Luke Lester, Willie Luk, Igal Brener and Ganesh Subramania, recently released in Scientific Reports. In this publication, the EFRC team achieved 60nm of spectral tuning across an array of vertically emitting photonic crystal nanowire lasers on a single chip. The emission wavelength of each individual photonic crystal laser was precisely controlled using e-beam lithography by tuning the spacing and diameter of the nanowires.

Abstract: Emerging applications such as solid-state lighting and display technologies require micro-scale vertically emitting lasers with controllable distinct lasing wavelengths and broad wavelength tunability arranged in desired geometrical patterns to form “super-pixels”. Conventional edge-emitting lasers and current surface-emitting lasers that require abrupt changes in semiconductor bandgaps or cavity length are not a viable solution. Here, we successfully address these challenges by introducing a new paradigm that extends the laser tuning range additively by employing multiple monolithically grown gain sections each with a different emission centre wavelength. We demonstrate this using broad gain-bandwidth III-nitride multiple quantum well (MQW) heterostructures and a novel top-down nanowire photonic crystal nanofabrication. We obtain single-mode lasing in the blue-violet spectral region with a remarkable 60 nm of tuning (or 16% of the nominal centre wavelength) that is determined purely by the photonic crystal geometry. This approach can be extended to cover the entire visible spectrum.


Highly nonlinear defect-induced carrier recombination rates in semiconductors

September 20, 2013

nonlinear defect-induced carrierSSLS EFRC scientists Normand Modine, Andy Armstrong, Mary Crawford and Weng Chow recently released a publication titled, “Highly nonlinear defect-induced carrier recombination rates in semiconductor” published in the Journal of Applied Physics. The article features research that highlights the defects with multiple defect charge levels that can induce non-radiative recombination with a highly nonlinear dependence on carrier concentration.

Abstract: Defects in semiconductors can induce recombination of carriers and thus can strongly influence the efficiency and performance of solid-state devices. In the analysis of device performance, defect-induced recombination is often assumed to depend linearly on the carrier concentration or to be given by a sum of Shockley-Read-Hall expressions taken independently for each known defect level. Under these assumptions, defect-induced recombination increases with carrier concentration more slowly than both band-to-band radiative recombination and Auger recombination and becomes relatively less important at higher carrier concentrations. However, we show that defects with multiple defect levels can induce recombination with a highly nonlinear dependence on carrier concentration. For such defects, the usual assumptions about the relative importance of different recombination mechanisms at different carrier concentrations may fail. In order to demonstrate the potential impact of this phenomenon on realistic devices, we incorporate the defect-induced recombination rates obtained from our analysis into a microscopic InGaN/GaN light-emitting diode (LED) model. Our results indicate that a particular class of defects with plausible properties can induce a loss of optical efficiency at carrier concentrations relevant for high-power LED operation.


Senator Heinrich Touts Energy Savings & Job Creation through Shaheen-Portman Bill

Heinrich Banner

September 17. 2013

“New Mexico is already capitalizing on a diversified but rapidly transforming energy sector and stands to benefit from leveraging investments in efficiency projects and innovative technologies.” -Senator Martin Heinrich

Senator Martin Heinrich of New Mexico, a member of the Committee on Energy and Natural Resources, delivered a speech on the Senate floor in support of the Energy Savings and Industrial Competitiveness Act.

Within the speech, Senator Heinrich addressed the importance of energy-efficient technologies, including a highlight on Sandia’s Solid-State Lighting Science Energy Frontier Research Center (SSLS EFRC) and its research. Heinrich emphasized the importance and need of basic research and science, which are, “the kind of investments that have made our nation great.”

To view the whole speech, click here.


SSLS Scientist Andy Armstrong Receives 2013 Employee Recognition Award

SSLS EFRC Scientist, Andy Armstrong, recipient of Sandia's 2013 Employee Recognition Award
SSLS EFRC Scientist, Andy Armstrong, recipient of Sandia’s 2013 Employee Recognition Award

September 9, 2013

Congratulations to SSLS EFRC Sr. Investigator Andy Armstrong, who was honored with Sandia’s 2013 Employee Recognition Award. Andy was awarded in recognition of his development and application of novel defect characterization for III-Nitride materials.

Sandia’s prestigious Employee Recognition Awards (ERA) are presented to individual employees and teams nominated by their peers and chosen by a Laboratory selection committee for their accomplishments during the past year.

The ERA winners are honored for their exceptional contributions to Sandia mission success to underscore the importance placed on individual and team contributions. ERA winners are recognized for their exceptional service, leadership, technical accomplishments, or teamwork.

Congratulations to Andy!


EFRC Scientist Weng Chow Awarded the Quantum Electronics Award

July 24, 2013

EFRC scientist, Weng Chow, recipient of the 2013 Quantum Electronics Award.

EFRC Sr. Investigator Weng Chow is the recipient of the 2013 Quantum Electronics Award. This highly prestigious award presented by the IEEE Photonics Society is given to a select group of scientists who are considered pioneers in the field of quantum electronics. Chow is receiving the award on behalf of his, “contributions to semiconductor-laser theory, in particular the implementation and verification of many body effects.” The awards ceremony will be hosted by the 2013 IEEE Photonics Conference on September 9, 2013, held at the Hyatt Regency Bellevue, in Bellevue, Washington, USA. The full press release of Chow’s award can be found here: . Congratulations to our very own Weng Chow!


SSLS Exhibit at Explora Museum

April 14, 2013

EFRC scientist Jeff Tsao, in conjunction with Sandian Erik Spoerke and UNM scientist Patrick Johnson, volunteered at Explora Musem over the weekend. The three scientists participated in a program event hosted by Explora called Portal to the Public, whose goal is to, “Engage in experiential activities with scientists that are your neighbors in the community.”  Each scientist brought their own mobile exhibit showcasing different areas of science; Patrick’s exhibit highlighted the difference between fluorescence and phosphorescence, Erik’s exhibit explored the composite nature of bones, and Jeff’s exhibit demonstrated how wavelengths of colored lights combine together. Due to public response, Explora turned our SSL exhibit into a permanent exhibit which can now be enjoyed by the public all year round.


George Wang's Invited Talk at 2013 tSSL

March 26, 2013

SSLS scientist George Wang presented at tSSL on March 26th-27th.

George Wang, a senior investigator in solid-state lighting science, was invited to speak at the 7th Taiwan Solid State Lighting (2013 tSSL). The conference was held on March 26th-27th in Taipei, Taiwan. The purpose of tSSL is “to bring together scientists and engineers actively engaged in research, development, and manufacturing on all aspects of Solid State Lighting….” George presented before an audience of around 200 attendees, on the emergence of nanowires as a future architecture for solid state lighting devices.


SSLS Scientist Jeff Tsao Wins the Asian American Engineer of the Year Award!

March 2, 2013

Congratulations to our very own Jeff Tsao, who was awarded the 2013 Asian American Engineer of the Year Award! Jeff is the 13th Sandian to be honored with this prestigious award. In recognition of this exceptional accomplishment, Jeff is featured in an article by Sandia titled, “Sandia’s Dr. Jeffrey Tsao Is Recognized as an Asian-American Engineer of the Year “. Congratulations once again to Jeff!


BES Web Highlight: Single-mode gallium nitride nanowire lasers

January 28, 2013

A new top-down method for fabricating gallium nitride (GaN) nanowires with precisely controlled geometries enables single-mode, rather than typically-observed multi-mode, lasing behavior. Single-mode lasing arises from strong mode competition induced by careful restriction of the nanowire dimensions. This research was published by Sandia National Laboratories and is featured in a BES EFRC newsletter that highlights the research done by each of the 46 EFRC centers to demonstrate the collective impact of the EFRC program. The article can be found here: Overcoming the First Grand Challenge 


SPIE Interviews George Wang: Developing next-generation lighting using nanowires

January 14, 2013 

spielogod_4c_ntGeorge Wang, a senior investigator in the SSLS, was interviewed about his work in the EFRC on nanowire growth and the possibility of achieving ultra-efficient solid-state lighting. You may view his interview at:


Solid-state Lighting: 'The case' 10 Years After and Future Prospects paper will be translated in Chinese

January 14, 2013

In 2011, EFRC scientist Jeffrey Tsao, in conjunction with Roland Haitz and Taiming Zhou, published a SSLS paper titled, “Solid state Lighting: ‘The Case’ 10 Years After and Future Prospects”. The paper critiques a previously released white paper titled, “The Case for a National Research Program on Semiconductor Lighting” while providing new predictions for the next coming 10-20 years in the field of solid-state lighting. The paper will be published in Chinese, making it accessible to a larger scientific audience. 

Abstract: Ten years ago, a white paper titled “The Case for a National Research Program on Semiconductor Lighting” outlined the promise of semiconductor light-emitting diodes (LEDs) for general illumination. Since then, investments in the now-renamed field of solid-state lighting (SSL) have accelerated and considerable progress has been made, not always in the directions envisioned at the time. In this paper, two of the original four authors comment on the white paper’s hits and misses, while making the original white paper available archivally as supplemental online materials. Finally, we make predictions for the coming 10-20 years. 


NASA Earth at Night Video

January 9, 2013

Have you ever wondered what the Earth looks like at night? NASA provides a clear, cloud-free view of the Earth at night using the Suomi National Polar-orbiting Partnership Satellite. The satellite utilizes an instrument known as the Visible Infrared Radiometer Suite (VIIRS), which allows the satellite to capture images of a “remarkably detailed view of the Earth of night.” VIIRS combines the Earth at night view created by NASA’s Earth Observatory with data processes by the NOAA’s National Geophysical Data Center. The video can be viewed here: NASA Earth at Night 


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