This report describes an investigation of the piezoelectric field in strained bulk GaAs. The bound charge distribution is calculated and suitable electrode configurations are proposed for (1) uniaxial and (2) biaxial strain. The screening of the piezoelectric field is studied for different impurity concentrations and sample lengths. Electric current due to the piezoelectric field is calculated for the cases of (1) fixed strain and (2) strain varying in time at a constant rate.
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.
This paper investigates theoretically the modification of dynamical properties in a semiconductor laser by a strong injected signal. It is found that enhanced relaxation oscillations are governed by the pulsations of the intracavity field and population at frequencies determined by the injected field and cavity resonances. Furthermore, the bandwidth enhancement is associated with the undamping of the injection-induced relaxation oscillation and strong population pulsation effects. There are two limitations to the modulation-bandwidth enhancement: Overdamping of relaxation oscillation and degradation of flat response at low frequencies. The injected-laser rate-equations used in the investigation reproduce the relevant aspects of modulation-bandwidth enhancement found in the experiment on injection-locked vertical-cavity surface-emitting lasers.
A theory is presented which couples a dynamical laser model to a fully microscopic calculation of scattering effects. Calculations for two optically pumped GaInNAs laser structures show how this approach can be used to analyze nonequilibrium and dynamical laser properties over a wide range of system parameters.
This paper investigates nonlinear behavior of coupled lasers. Composite-cavity-mode approach and a class-B description of the active medium are used to describe nonlinearities associated with population dynamics and optical coupling. The multimode equations are studied using bifurcation analysis to identify regions of stable locking, periodic oscillations, and complicated dynamics in the parameter space of coupling-mirror transmission T and normalized cavity-length mismatch dL/{lambda}. We further investigate the evolution of the key bifurcations with the linewidth enhancement factor {alpha}. In particular, our analysis reveals the formation of a gap in the lockband that is gradually occupied by instabilities. We also investigate effects of the cavity-length on chaotic dynamics.
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 paper presents a theoretical study of synchronization between two coupled lasers. A theory valid for arbitrary coupling between lasers is used. Its key feature is that the laser field is decomposed in terms of the composite-cavity modes reflecting the spatial field dependence over the entire coupled-laser system. The ensuing multimode equations are reduced to class-B, and further to class-A equations which resemble competing species equations. Bifurcation analysis, supported by insight provided by analytical solutions, is used to investigate influences of pump, carrier decay rate, polarization decay rate, and coupling mirror losses on synchronization between lasers. Population pulsation is found to be an essential mode competition mechanism responsible for bistability in the synchronized solutions. Finally, we discovered that the mechanism leading to laser synchronization changes from strong composite-cavity mode competition in class-A regime to frequency locking of composite-cavity modes in class-B regime.
This paper explores quantum-coherence phenomena in a semiconductor quantum-dot structure. The calculations predict the occurrence of inversionless gain, electromagnetically induced transparency, and refractive-index enhancement in the transient regime for dephasing rates typical under room temperature and high excitation conditions. They also indicate deviations from atomic systems because of strong many-body effects. Specifically, Coulomb interaction involving states of the quantum dots and the continuum belonging to the surrounding quantum well leads to collision-induced population redistribution and many-body energy and field renormalizations that modify the magnitude, spectral shape, and time dependence of quantum-coherence effects.
Quantum dot nanostructures were investigated experimentally and theoretically for potential applications for optoelectronic devices. We have developed the foundation to produce state-of-the-art compound semiconductor nanostructures in a variety of materials: In(AsSb) on GaAs, GaSb on GaAs, and In(AsSb) on GaSb. These materials cover a range of energies from 1.2 to 0.7 eV. We have observed a surfactant effect in InAsSb nanostructure growth. Our theoretical efforts have developed techniques to look at the optical effects induced by many-body Coulombic interactions of carriers in active regions composed of quantum dot nanostructures. Significant deviations of the optical properties from those predicted by the ''atom-like'' quantum dot picture were discovered. Some of these deviations, in particular, those relating to the real part of the optical susceptibility, have since been observed in experiments.
The capabilities of a fully microscopic approach for the calculation of optical material properties of semiconductor lasers are reviewed. Several comparisons between the results of these calculations and measured data are used to demonstrate that the approach yields excellent quantitative agreement with the experiment. It is outlined how this approach allows one to predict the optical properties of devices under high-power operating conditions based only on low-intensity photo luminescence (PL) spectra. Examples for the gain-, absorption-, PL- and linewidth enhancement factor-spectra in single and multiple quantum-well structures, superlattices, Type II quantum wells and quantum dots, and for various material systems are discussed.
We have used selective AlGaAs oxidation, dry-etching, and high-gain semiconductor laser simulation to create new in-plane lasers with interconnecting passive waveguides for use in high-density photonic circuits and future integration of photonics with electronics. Selective oxidation and doping of semiconductor heterostructures have made vertical cavity surface emitting lasers (VCSELs) into the world's most efficient low-power lasers. We apply oxidation technology to improve edge-emitting lasers and photonic-crystal waveguides, making them suitable for monolithic integrated microsystems. Two types of lasers are investigated: (1) a ridge laser with resonant coupling to an output waveguide; (2) a selectively-oxidized laser with a low active volume and potentially sub-milliAmp threshold current. Emphasis is on development of high-performance lasers suited for monolithic integration with photonic circuit elements.
A wave-optical model that is coupled to a microscopic gain theory is used to investigate lateral mode behavior in group-III nitride quantum-well lasers. Beam filamentation due to self-focusing in the gain medium is found to limit fundamental-mode output to narrow stripe lasers or to operation close to lasing threshold. Differences between nitride and conventional near-infrared semiconductor lasers arise because of band structure differences, in particular, the presence of a strong quantum-confined Stark effect in the former. Increasing mirror reflectivities in plane-plane resonators to reduce lasing threshold current tends to exacerbate the filamentation problem. On the other hand, a negative-branch unstable resonator is found to mitigate filament effects, enabling fundamental-mode operation far above threshold in broad-area lasers.
A new class of semiconductor lasers that can potentially produce much more short pulse energy is presented. This new laser is not limited in volume or aspect ratio by the depth of a p-n junction and are created from current filaments in semi-insulating GaAs. A current filament semiconductor lasers (CFSL) that have produced 75 nJ of 890 nm radiation in 1.5 ns were tested. A filaments as long as 3.4 cm and several hundred microns in diameter in high gain GaAs photoconductive switches were observed. Their smallest dimension can be more than 100 times the carrier diffusion length in GaAs. The spectral narrowing, lasing thresholds, beam divergence, temporal narrowing and energies which imply lasing for several configurations of CFSL are reported.