Publications

Abstract not provided.

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.

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.

We report micro-Raman studies of self-heating in an AlGaN/GaN heterostructure field-effect transistor using below (visible 488.0 nm) and near (UV 363.8 nm) GaN band-gap excitation. The shallow penetration depth of the UV light allows us to measure temperature rise ({Delta}T) in the two-dimensional electron gas (2DEG) region of the device between drain and source. Visible light gives the average {Delta}T in the GaN layer, and that of the SiC substrate, at the same lateral position. Combined, we depth profile the self-heating. Measured {Delta}T in the 2DEG is consistently over twice the average GaN-layer value. Electrical and thermal transport properties are simulated. We identify a hotspot, located at the gate edge in the 2DEG, as the prevailing factor in the self-heating.

Abstract not provided.

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.

The goal of this LDRD was to investigate III-antimonide/nitride based materials for unique semiconductor properties and applications. Previous to this study, lack of basic information concerning these alloys restricted their use in semiconductor devices. Long wavelength emission on GaAs substrates is of critical importance to telecommunication applications for cost reduction and integration into microsystems. Currently InGaAsN, on a GaAs substrate, is being commercially pursued for the important 1.3 micrometer dispersion minima of silica-glass optical fiber; due, in large part, to previous research at Sandia National Laboratories. However, InGaAsN has not shown great promise for 1.55 micrometer emission which is the low-loss window of single mode optical fiber used in transatlantic fiber. Other important applications for the antimonide/nitride based materials include the base junction of an HBT to reduce the operating voltage which is important for wireless communication links, and for improving the efficiency of a multijunction solar cell. We have undertaken the first comprehensive theoretical, experimental and device study of this material with promising results. Theoretical modeling has identified GaAsSbN to be a similar or potentially superior candidate to InGaAsN for long wavelength emission on GaAs. We have confirmed these predictions by producing emission out to 1.66 micrometers and have achieved edge emitting and VCSEL electroluminescence at 1.3 micrometers. We have also done the first study of the transport properties of this material including mobility, electron/hole mass, and exciton reduced mass. This study has increased the understanding of the III-antimonide/nitride materials enough to warrant consideration for all of the target device applications.

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.

InxGa1-xAs1-yNy quaternary alloys offer the promise of longer wavelength, ≥ 1.3 μm optical transceivers grown on GaAs substrates. To achieve acceptable radiative efficiencies at 1.3 μm, highly-strained InGaAsN quantum wells (x ≈ 0.4, y ≈ 0.005) are being developed as laser active regions. By introducing GaAsP layers into the active region for strain-compensation, gain can be increased using multiple InGaAsN quantum wells. In this work, we report the first strain-compensated, 1.3 μm InGaAsN MQW lasers. Our devices were grown by metal-organic chemical vapor deposition. Lasers with InGaAsN quantum well active regions are proving superior to lasers constructed with competing active region materials. Under pulsed operation, our 1.3 μm InGaAsN lasers displayed negligible blue-shift from the low-injection LED emission, and state-of-the-art characteristic temperature (159 K) was obtained for a 1.3 μm laser.

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.

Abstract not provided.

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 optical gain spectra for GaInNAs/GaAs quantum wells are computed using a microscopic laser theory. From these spectra, the peak gain and carrier radiative decay rate as functions of carrier density are determined. These dependences allow the study of the lasing threshold current density of GaInNAs/GaAs quantum well structures.

Electron and hole transport in compensated, InGaAsN ({approx} 2% N) are examined through Hall mobility, photoconductivity, and solar cell photoresponse measurements. Short minority carrier diffusion lengths, photoconductive-response spectra, and doping dependent, thermally activated Hall mobilities reveal a broad distribution of localized states. At this stage of development, lateral carrier transport appears to be limited by large scale (>> mean free path) material inhomogeneities, not a random alloy-induced mobility edge.

The authors have investigated the properties of GaAsSb/InGaAs type-II bilayer quantum well structures grown by molecule beam epitaxy for use in long-wavelength lasers on GaAs substrates. Structures with layer, strains and thicknesses designed to be thermodynamically stable against dislocation formation exhibit room-temperature photoluminescence at wavelengths as long as 1.43 {mu}m. The photoluminescence emission wavelength is significantly affected by growth temperature and the sequence of layer growth (InGaAs/GaAsSb vs GaAsSb/InGaAs), suggesting that Sb and/or In segregation results in non-ideal interfaces under certain growth conditions. At low injection currents, double heterostructure lasers with GaAsSb/InGaAs bilayer quantum well active regions display electroluminescence at wavelengths comparable to those obtained in photoluminescence, but at higher currents the electroluminescence shifts to shorter wavelengths. Lasers have been obtained with threshold current densities as low as 120 A/cm{sup 2} at 1.17 {mu}m, and 2.1 kA/cm{sup 2} at 1.21 {mu}m.

The authors report a measurement of the variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1 and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy. The samples were grown by metal-organic chemical vapor deposition and the photoluminescence measurements were performed a 4K. The authors find that the value of the excitonic linewidth increases as a function of pressure until about 100 kbars after which it tends to saturate. This change in the excitonic linewidth is used to derive the pressure variation of the reduced mass of the exciton using a theoretical formalism which is based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The variation of the excitonic reduced mass thus derived is compared with that recently determined using a first-principles band structure calculation based on local density approximation.

The variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1% and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy is studied at 4 K. The excitonic linewidth increases as a function of pressure until about 100 kbar after which it tends to saturate. This pressure dependent excitonic linewidth is used to derive the pressure variation of the exciton reduced mass using a theoretical formalism based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The linewidth derived ambient pressure masses are compared and found to be in agreement with other mass measurements. The variation of this derived mass is compared with the results from a nearly first-principles approach in which calculations based on the local density approximation to the Kohn-Sham density functional theory are corrected using a small amount of experimental input.