Publications

Abstract not provided.

Dipicolinic acid (DPA, 2,6-pyridinedicarboxylic acid) is a substance uniquely present in bacterial spores such as that from anthrax (B. anthracis). It is known that DPA can be detected by the long-lived fluorescence of its terbium chelate; the best limit of detection (LOD) reported thus far using a large benchtop gated fluorescence instrument using a pulsed Xe lamp is 2 nM. We use a novel AlGaN light-emitting diode (LED) fabricated on a sapphire substrate that has peak emission at 291 nm. Although the overlap of the emission band of this LED with the absorption band of Tb-DPA ({lambda}{sub max} doublet: 273, 279 nm) is not ideal, we demonstrate that a compact detector based on this LED and an off-the-shelf gated photodetection module can provide an LOD of 0.4 nM, thus providing a basis for convenient early warning detectors.

The goal of this one year LDRD was to improve the overall efficiency of InGaN LEDs by improving the extraction of light from the semiconductor chip. InGaN LEDs are currently the most promising technology for producing high efficiency blue and green semiconductor light emitters. Improving the efficiency of InGaN LEDs will enable a more rapid adoption of semiconductor based lighting. In this LDRD, we proposed to develop photonic structures to improve light extraction from nitride-based light emitting diodes (LEDs). While many advanced device geometries were considered for this work, we focused on the use of a photonic crystal for improved light extraction. Although resonant cavity LEDs and other advanced structures certainly have the potential to improve light extraction, the photonic crystal approach showed the most promise in the early stages of this short program. The photonic crystal (PX)-LED developed here incorporates a two dimensional photonic crystal, or photonic lattice, into a nitride-based LED. The dimensions of the photonic crystal are selected such that there are very few or no optical modes in the plane of the LED ('lateral' modes). This will reduce or eliminate any radiation in the lateral direction so that the majority of the LED radiation will be in vertical modes that escape the semiconductor, which will improve the light-extraction efficiency. PX-LEDs were fabricated using a range of hole diameters and lattice constants and compared to control LEDs without a photonic crystal. The far field patterns from the PX-LEDs were dramatically modified by the presence of the photonic crystal. An increase in LED brightness of 1.75X was observed for light measured into a 40 degree emission cone with a total increase in power of 1.5X for an unencapsulated LED.

Materials studies of high Al-content (> 30%) AlGaN epilayers and the performance of AlGaN-based LEDs with emission wavelengths shorter than 300 nm are reported. N-type AlGaN films with Al compositions greater than 30% reveal a reduction in conductivity with increasing Al composition. The reduction of threading dislocation density from the 1-5 x10{sup 10} cm{sup -2} range to the 6-9 x 10{sup 9}cm{sup -2} range results in an improvement of electrical conductivity and Al{sub 0.90}Ga{sub 0.10}N films with n= 1.6e17 cm-3 and f{acute Y}=20 cm2/Vs have been achieved. The design, fabrication and packaging of flip-chip bonded deep UV LEDs is described. Large area (1 mm x 1 mm) LED structures with interdigitated contacts demonstrate output powers of 2.25 mW at 297 nm and 1.3 mW at 276 nm when operated under DC current. 300 f{acute Y}m x 300 f{acute Y}m LEDs emitting at 295 nm and operated at 20 mA DC have demonstrated less than 50% drop in output power after more than 2400 hours of operation. Optimization of the electron block layer in 274 nm LED structures has enabled a significant reduction in deep level emission bands, and a peak quantum well to deep level ratio of 700:1 has been achieved for 300 f{acute Y}m x 300 f{acute Y}m LEDs operated at 100 mA DC. Shorter wavelength LED designs are described, and LEDs emitting at 260 nm, 254nm and 237 nm are reported.

Abstract not provided.

Forward-to-reverse bias step-recovery measurements were performed on In.07Ga.93N/GaN and Al.36Ga.64N/Al.46Ga.54N quantum-well (QW) light-emitting diodes grown on sapphire. With the QW sampling the minority-carrier hole density at a single position, distinctive two-phase optical decay curves were observed. Using diffusion equation solutions to self-consistently model both the electrical and optical responses, hole transport parameters tp = 758 {+-} 44 ns, Lp = 588 {+-} 45 nm, and up = 0.18 {+-} 0.02 cm2/Vs were obtained for GaN. The mobility was thermally activated with an activation energy of 52 meV, suggesting trap-modulated transport. Optical measurements of sub-bandgap peaks exhibited slow responses approaching the bulk lifetime. For Al.46Ga.54N, a longer lifetime of tp = 3.0 us was observed, and the diffusion length was shorter, Lp = 280 nm. Mobility was an order of magnitude smaller than in GaN, up = 10-2 cm2/Vs, and was insensitive to temperature, suggesting hole transport through a network of defects.

The junction temperature of AlGaN/GaN ultraviolet (UV) Light-Emitting Diodes (LEDs) emitting at 295 nm is measured by using the temperature coefficients of the diode forward volt-age and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate method (,,b 3 ,,aC). A theoretical model for the dependence of the diode junction voltage (Vj) on junction temperature (T) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6 K/W is obtained with the AlGaN/GaN LED sample mounted with thermal paste on a heat sink.

An AlGaN Light-emitting diode (LED) emitting with a peak wavelength at 291 nm and a radiant power of 0.5 mW @ 100 mA was fabricated on a sapphire substrate. A compact gated fluorescence detection system was built using this LED as the excitation light source. We demonstrate that it provides sufficient power using Terbium enhanced fluorescence to detect subnanomolar concentrations of dipicolinic acid (DPA, 2, 6-pyridinedicarboxylic acid), a substance uniquely present in bacterial spores such as that from B. anthracis, providing a basis for convenient early warning detectors. We also describe initial results from a novel approach for biological aerosol detection using long lived fluorescence from a Europium tagged dye that binds to proteins.

The junction temperature of AlGaN ultraviolet light-emitting diodes emitting at 295 nm is measured by using the temperature coefficients of the diode forward voltage and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate ({+-}3 C). A theoretical model for the dependence of the diode forward voltage (V{sub f}) on junction temperature (T{sub j}) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6 K/W is obtained with the device mounted with thermal paste on a heat sink.

Abstract not provided.

Abstract not provided.

Solid-state light sources emitting at wavelengths less than 300 nm would enable technological advances in many areas such as fluorescence-based biological agent detection, non-line-of-sight communications, water purification, and industrial processing including ink drying and epoxy curing. In this paper, we present our recent progress in the development of LEDs with emission between 237 and 297 nm. We will discuss growth and design issues of deep-UV LEDs, including transport in Si-doped AlGaN layers. The LEDs are designed for bottom emission so that improved heat sinking and light extraction can be achieved by flip chipping. To date, we have demonstrated 2.25 mW of output power at 295 nm from 1 mm x 1 mm LEDs operated at 500 mA. Shorter wavelength LEDs emitting at 276 nm have achieved an output power of 1.3 mW at 400 mA. The heterostructure designs that we have employed have suppressed deep level emission to intensities that are up to 330 x lower than the primary quantum well emission.

Femtosecond differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found only for positive time delay in both DRS and FWM experiments. The rise time at negative time delay for the DRS was much slower than the FWM signal or differential transmission spectroscopy at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing, that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

This SAND report is the final report on Sandia's Grand Challenge LDRD Project 27328, 'A Revolution in Lighting -- Building the Science and Technology Base for Ultra-Efficient Solid-state Lighting.' This project, which for brevity we refer to as the SSL GCLDRD, is considered one of Sandia's most successful GCLDRDs. As a result, this report reviews not only technical highlights, but also the genesis of the idea for Solid-state Lighting (SSL), the initiation of the SSL GCLDRD, and the goals, scope, success metrics, and evolution of the SSL GCLDRD over the course of its life. One way in which the SSL GCLDRD was different from other GCLDRDs was that it coincided with a larger effort by the SSL community - primarily industrial companies investing in SSL, but also universities, trade organizations, and other Department of Energy (DOE) national laboratories - to support a national initiative in SSL R&D. Sandia was a major player in publicizing the tremendous energy savings potential of SSL, and in helping to develop, unify and support community consensus for such an initiative. Hence, our activities in this area, discussed in Chapter 6, were substantial: white papers; SSL technology workshops and roadmaps; support for the Optoelectronics Industry Development Association (OIDA), DOE and Senator Bingaman's office; extensive public relations and media activities; and a worldwide SSL community website. Many science and technology advances and breakthroughs were also enabled under this GCLDRD, resulting in: 55 publications; 124 presentations; 10 book chapters and reports; 5 U.S. patent applications including 1 already issued; and 14 patent disclosures not yet applied for. Twenty-six invited talks were given, at prestigious venues such as the American Physical Society Meeting, the Materials Research Society Meeting, the AVS International Symposium, and the Electrochemical Society Meeting. This report contains a summary of these science and technology advances and breakthroughs, with Chapters 1-5 devoted to the five technical task areas: 1 Fundamental Materials Physics; 2 111-Nitride Growth Chemistry and Substrate Physics; 3 111-Nitride MOCVD Reactor Design and In-Situ Monitoring; 4 Advanced Light-Emitting Devices; and 5 Phosphors and Encapsulants. Chapter 7 (Appendix A) contains a listing of publications, presentations, and patents. Finally, the SSL GCLDRD resulted in numerous actual and pending follow-on programs for Sandia, including multiple grants from DOE and the Defense Advanced Research Projects Agency (DARPA), and Cooperative Research and Development Agreements (CRADAs) with SSL companies. Many of these follow-on programs arose out of contacts developed through our External Advisory Committee (EAC). In h s and other ways, the EAC played a very important role. Chapter 8 (Appendix B) contains the full (unedited) text of the EAC reviews that were held periodically during the course of the project.

To observe the effects of polarization fields and screening, we have performed contacted electroreflectance (CER) measurements on In{sub 0.07}Ga{sub 0.93}N/GaN single quantum well light emitting diodes for different reverse bias voltages. Room-temperature CER spectra exhibited three features which are at lower energy than the GaN band gap and are associated with the quantum well. The position of the lowest-energy experimental peak, attributed to the ground-state quantum well transition, exhibited a limited Stark shift except at large reverse bias when a redshift in the peak energy was observed. Realistic band models of the quantum well samples were constructed using self-consistent Schroedinger-Poisson solutions, taking polarization and screening effects in the quantum well fully into account. The model predicts an initial blueshift in transition energy as reverse bias voltage is increased, due to the cancellation of the polarization electric field by the depletion region field and the associated shift due to the quantum-confined Stark effect. A redshift is predicted to occur as the applied field is further increased past the flatband voltage. While the data and the model are in reasonable agreement for voltages past the flatband voltage, they disagree for smaller values of reverse bias, when charge is stored in the quantum well, and no blueshift is observed experimentally. To eliminate the blueshift and screen the electric field, we speculate that electrons in the quantum well are trapped in localized states.

Abstract not provided.

Abstract not provided.

The goal of this project was to increase the power of vertical cavity surface emitting lasers and to convert their wavelength into the blue/ultraviolet and the infrared for sensing applications. We have increased the power to the multi-watt level and have generated several milliwatts of blue light using optical pumping. Electrical pump has been less successful, but we have identified the problems and begun work to overcome them using a bottom emitting design.

Abstract not provided.

Selectively oxidized vertical cavity surface emitting lasers (VCSEL) typically operate in multiple transverse optical modes. High power VCSEL operation is desirable for many applications such as optical storage and printing, modulation spectroscopy, bar code scanning, and data communication over single mode optical fiber. The modal discrimination can be augmented by creating a central region of gain surrounded by a region of optical loss. A VCSEL fabricated via hybrid ion implantation and selective oxidation device structure is designed to demonstrate a single mode output of more than 5 mW for 850 nm.

A record high fundamental-mode power of 5.1 mW was achieved from coupled-resonator vertical-cavity lasers (CRVCLs). In conventional VCSELs, the extent to which the gain volume may be increased is limited by the onset of multi-mode operation. Results indicate that this limitation is circumvented in a coupled-resonator device allowing high power fundamental-mode operation.

The continuous wave operation of 1.3 μm vertical cavity surface emitting laser (VCSEL) grown on GaAs substrates is achieved up to 55 °C, as motivated by demands of emerging VCSEL network applications. These VCSELs employ the mature AlGaAs/GaAs distributed Bragg reflector mirror technology, including selective oxidation for efficient cavity designs. By incorporating a tunnel junction near the optical cavity, both mirrors are doped n-type, which provides the benefits of low optical loss.

The authors report a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity which exhibits bistable behavior in the light output versus injection current. Large bistability regions over current ranges as wide as 18 mA have been observed with on/off contrast ratios of greater than 20 dB. The position and width of the bistability region can be varied by changing the bias to the top cavity. Switching between on and off states can be accomplished with changes as small as 250 {micro}W to the electrical power applied to the top cavity. Theoretical analysis suggests that the bistable behavior is the response of the nonlinear susceptibility in the top cavity to the changes in the bottom intracavity laser intensity as the bottom cavity reaches the thermal rollover point.

Selectively oxidized vertical cavity lasers emitting at 1294 nm using InGaAsN quantum wells are reported for the first time which operate continuous wave at and above room temperature. The lasers employ two n-type Al{sub 0.94}Ga{sub 0.06}As/GaAs distributed Bragg reflectors each with a selectively oxidized current aperture adjacent to the optical cavity, and the top output mirror contains a tunnel junction to inject holes into the active region. Continuous wave single mode lasing is observed up to 55 C. These lasers exhibit the longest wavelength reported to date for vertical cavity surface emitting lasers grown on GaAs substrates.

The authors have developed electrically-injected coupled-resonator vertical-cavity lasers and have studied their novel properties. These monolithically grown coupled-cavity structures have been fabricated with either one active and one passive cavity or with two active cavities. All devices use a selectively oxidized current aperture in the lower cavity, while a proton implant was used in the active-active structures to confine current in the top active cavity. They have demonstrated optical modulation from active-passive devices where the modulation arises from dynamic changes in the coupling between the active and passive cavities. The laser intensity can be modulated by either forward or reverse biasing the passive cavity. They have also observed Q-switched pulses from active-passive devices with pulses as short as 150 ps. A rate equation approach is used to model the Q-switched operation yielding good agreement between the experimental and theoretical pulseshape. They have designed and demonstrated the operation of active-active devices which la.se simultaneously at both longitudinal cavity resonances. Extremely large bistable regions have also been observed in the light-current curves for active-active coupled resonator devices. This bistability can be used for high contrast switching with contrast ratios as high as 100:1. Coupled-resonator vertical-cavity lasers have shown enhanced mode selectivity which has allowed devices to lase with fundamental-mode output powers as high as 5.2 mW.