Publications

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This report summarizes a brief and unsuccessful attempt to grow indium nitride via the electrochemical solution growth method and a modification thereof. Described in this report is a brief effort using a $50,000 LDRD award to explore the possibilities of applying the Electrochemical Solution Growth (ESG) technique to the growth of indium nitride (InN). The ability to grow bulk InN would be exciting from a scientific perspective, and a commercial incentive lies in the potential of extending the ESG technique to grow homogeneous, bulk alloys of In{sub x}Ga{sub 1-x}N for light emitting diodes (LEDs) operating in the green region of the spectrum. Indium nitride is the most difficult of the III-nitrides to grow due to its very high equilibrium vapor pressure of nitrogen1. It is several orders of magnitude higher than for gallium nitride or aluminum nitride. InN has a bandgap energy of 0.7eV, and achieving its growth in bulk for large area, high quality substrates would permit the fabrication of LEDs operating in the infrared. By alloying with GaN and AlN, the bulk material used as substrates would enable high efficiency emission wavelengths that could be tailored all the way through the deep ultraviolet. In addition, InN has been shown to have very high electronic mobilities (2700 cm{sup 2}/V s), making it a promising material for transistors and even terahertz emitters. Several attempts at synthesizing InN have been made by several groups. It was shown that metallic indium does not interact with unactivated nitrogen even at very high temperatures. Thus sets up an incompatibility between the precursors in all growth methods: a tradeoff between thermally activating the nitrogen-containing precursor and the low decomposition temperature of solid InN. We have been working to develop a novel growth technique that circumvents the difficulties of other bulk growth techniques by precipitating the column III nitrides from a solvent, such as a molten chloride salt, that provides an excellent host environment for the gallium nitride and indium nitride precursors. In particular, we have found that molten halide salts can solubilize both gallium (Ga{sup 3+}) and nitride (N{sup 3-}) ions without reacting with them to the extent that they are no longer available for reaction with each other. Literature reports indicate measured nitride ion concentrations in LiCl at 650 C as high as 10 mol% - a sufficient concentration to yield growth rates on the order of 0.1 to {approx}1 mm/hr under diffusion-limited growth conditions. Also, molten salts are compatible with the 400-1200 C temperatures likely to be necessary for growth of high-quality single-crystal III-nitrides. Since they can be worked with at (or close to) atmospheric pressure, scalability is not a problem and manufacturability issues are thus minimized, including capital equipment costs. Although the III-nitrides cannot be float-zone refined to remove impurities due to their high melting temperatures and vapor pressures, the salts can be, thus reducing sources of impurities before growth begins. Finally, the molten salts offer a number of pathways to improve the solubility and control the growth of the III-nitrides by functioning as an electrolyte in electrochemical processes. We have already demonstrated growth of wurtzite GaN particles ranging from 0.2 to 0.9 mm in two hours in our laboratory using these techniques. It was the goal of this work to extend this ESG approach to the growth of indium nitride. The hope was that the abundance of the activated form of nitrogen, namely the triply-charged nitride ion (N{sup -3}) would enable the facile growth of InN in solution at low temperatures.

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An atmospheric pressure approach to growth of bulk group III-nitrides is outlined. Native III-nitride substrates for optoelectronic and high power, high frequency electronics are desirable to enhance performance and reliability of these devices; currently, these materials are available in research quantities only for GaN, and are unavailable in the case of InN. The thermodynamics and kinetics of the reactions associated with traditional crystal growth techniques place these activities on the extreme edges of experimental physics. The novel techniques described herein rely on the production of the nitride precursor (N{sup 3-}) by chemical and/or electrochemical methods in a molten halide salt. This nitride ion is then reacted with group III metals in such a manner as to form the bulk nitride material. The work performed during the period of funding (February 2006-September 2006) focused on establishing that mass transport of GaN occurs in molten LiCl, the construction of a larger diameter electrochemical cell, the design, modification, and installation of a made-to-order glove box (required for handling very hygroscopic LiCl), and the feasibility of using room temperature molten salts to perform nitride chemistry experiments.

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Gallium nitride is a promising material for numerous optoelectronic and high power, high frequency, and high temperature applications (e.g., synthetic aperture radar, chem.-bio sensing, utility power switching, down-hole sensors, water purification, solar-blind detectors, etc.). While many GaNbased opto/electronic devices have been demonstrated, all applications that require high power operation suffer in performance and reliability from the lack of native substrates. The advent of a scalable, manufacturable, and affordable process for growth of bulk GaN for substrates would greatly advance these technologies. Melt growth is impractical as it requires temperatures >1600°C and pressures >45,000 atmospheres, and is kinetically slow. A novel materials synthesis technique is described that takes place in a molten salt at atmospheric pressure. The salt is a relatively good solvent for molecular GaN and an excellent solvent for its ionic precursors, which can be continuously created electrochemically, thus enabling boule growth. This flexible, unique, and interdisciplinary approach solves the kinetic, thermodynamic, scalability, cost, and manufacturability issues inherent in alternative methods. Electrochemical growth of gallium nitride crystals at atmospheric pressure at 450°C has been demonstrated, and options are discussed for improving and extending the technique to production of large area boules. Copyright © 2006 International Microelectronics And Packaging Society.

An atmospheric pressure approach to growth of bulk group III-nitrides is outlined. Native III-nitride substrates for optoelectronic and high power, high frequency electronics are desirable to enhance performance and reliability of these devices; currently, these materials are available in research quantities only for GaN, and are unavailable in the case of InN. The thermodynamics and kinetics of the reactions associated with traditional crystal growth techniques place these activities on the extreme edges of experimental physics. The technique described herein relies on the production of the nitride precursor (N^3-) by chemical and/or electrochemical methods in a molten halide salt. This nitride ion is then reacted with group III metals in such a manner as to form the bulk nitride material. The work performed during the period of funding (July 2004-September 2005) focused on the initial measurement of the solubility of GaN in molten LiCl as a function of temperature, the construction of electrochemical cells, the modification of a commercial glove box (required for handling very hygroscopic LiCl), and on securing intellectual property for the technique.

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We develop a reciprocal-space model that describes the (hkl) dependence of the broadened Bragg peakwidths produced by x-ray diffraction from a dislocated epilayer. We compare the model to experiments and find that it accurately describes the peakwidths of 16 different Bragg reflections in the [010] zone of both GaN and AlN heterolayers. Using lattice-distortion parameters determined by fitting the model to selected reflections, we estimate threading-dislocation densities for seven different GaN and AlGaN samples and find improved agreement with transmission electron microscopy measurements.

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Using in situ wafer-curvature measurements of thin-film stress, we determine the critical thickness for strain relaxation in Al{sub x}Ga{sub 1-x}N/GaN heterostructures with 0.14 {le} x {le} 1. The surface morphology of selected films is examined by atomic force microscopy. Comparison of these measurements with critical-thickness models for brittle fracture and dislocation glide suggests that the onset of strain relaxation occurs by surface fracture for all compositions. Misfit-dislocations follow initial fracture, with slip-system selection occurring under the influence of composition-dependent changes in surface morphology.

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.

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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.

GaAsSbN was grown by organometallic vapor phase epitaxy (OMVPE) as an alternative material to InGaAsN for long wavelength emission on GaAs substrates. OMVPE of GaAsSbN using trimethylgallium, 100% arsine, trimethylantimony, and 1,1-dimethylhydrazine was found to be kinetically limited at growth temperatures ranging from 520 C to 600 C, with an activation energy of 10.4 kcal/mol. The growth rate was linearly dependent on the group III flow and has a complex dependence on the group V constituents. A room temperature photoluminescence wavelength of >1.3 {micro}m was observed for unannealed GaAs{sub 0.69}Sb{sub 0.3}N{sub 0.01}. Low temperature (4 K) photoluminescence of GaAs{sub 0.69}Sb{sub 0.3}N{sub 0.01} shows an increase in FWHM of 2.4-3.4 times the FWHM of GaAs{sub 0.7}Sb{sub 0.3}, a red shift of 55-77 meV, and a decrease in intensity of one to two orders of magnitude. Hall measurements indicate a behavior similar to that of InGaAsN, a 300 K hole mobility of 350 cm{sup 2}/V-s with a 1.0 x 10{sup 17}/cm{sup 3} background hole concentration, and a 77 K mobility of 1220 cm{sup 2}/V-s with a background hole concentration of 4.8 x 10{sup 16}/cm{sup 3}. The hole mass of GaAs{sub 0.7}Sb{sub 0.3}/GaAs heterostructures was estimated at 0.37-0.40m{sub o}, and we estimate an electron mass of 0.2-0.3m{sub o} for the GaAs{sub 0.69}Sb{sub 0.3}N{sub 0.01}/GaAs system. The reduced exciton mass for GaAsSbN was estimated at about twice that found for GaAsSb by a comparison of diamagnetic shift vs. magnetic field.

We present low-temperature (T = 4K) photoluminescence studies of the effect of adding nitrogen to 6-nm-wide single-strained GaAsSb quantum wells on GaAs. The samples were grown by both MBE and MOCVD techniques. The nominal Sb concentration is about 30%. Adding about 1 to 2% N drastically reduced the bandgap energies from 1 to 0.75 eV, or 1.20 to 1.64 μm. Upon performing ex situ rapid thermal anneals, 825°C for 10s, the band gap energies as well as the photoluminescence intensities increased. The intensities increased by an order of magnitude for the annealed samples and the band gap energies increased by about 50 - 100 meV, depending on growth temperatures. The photoluminescence linewidths tended to decrease upon annealing. Preliminary results of a first-principles band structure calculation for the GaAsSbN system are also presented.

This LDRD is aimed to place Sandia at the forefront of GaN-based technologies. Two important themes of this LDRD are: (1) The demonstration of novel GaN-based devices which have not yet been much explored and yet are coherent with Sandia's and DOE's mission objectives. UV optoelectronic and piezoelectric devices are just two examples. (2) To demonstrate front-end monolithic integration of GaN with Si-based microelectronics. Key issues pertinent to the successful completion of this LDRD have been identified to be (1) The growth and defect control of AlGaN and GaN, and (2) strain relief during/after the heteroepitaxy of GaN on Si and the separation/transfer of GaN layers to different wafer templates.

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Optically pumped near ultraviolet vertical cavity laser operation (VCSEL) has been obtained under quasi-continuous wave conditions at room temperature near 383 nm from shallow InGaN/GaN multiple quantum wells (MQWs). Low loss optical resonators were fabricated by using in-situ grown (Al,Ga)N distributed Bragg reflectors that featured strain engineering design for high optical morphology, in combination with low-loss dielectric multilayer mirrors.

We demonstrate that the insertion of low-temperature (LT) AlGaN interlayers is effective in reducing mismatch-induced tensile stress and suppressing the formation of cracks during growth of AlGaN directly upon GaN epilayers., Stress evolution and relaxation is monitored using an in-situ optical stress sensor. The combination of in-situ and ex-situ. characterization techniques enables us to determine the degree of pseudomorphism in the interlayers. It is observed that the elastic tensile mismatch between AlGaN and GaN is mediated by the relaxation of interlayers; the use of interlayers offers tunability in the in-plane lattice parameters.