Publications

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Solid-state lighting (SSL) technologies, based on semiconductor light emitting devices, have the potential to reduce worldwide electricity consumption by more than 10%, which could significantly reduce U.S. dependence on imported energy and improve energy security. The III-nitride (AlGaInN) materials system forms the foundation for white SSL and could cover a wide spectral range from the deep UV to the infrared. For this LDRD program, we have investigated the synthesis of single-crystalline III-nitride nanowires and heterostructure nanowires, which may possess unique optoelectronic properties. These novel structures could ultimately lead to the development of novel and highly efficient SSL nanodevice applications. GaN and III-nitride core-shell heterostructure nanowires were successfully synthesized by metal organic chemical vapor deposition (MOCVD) on two-inch wafer substrates. The effect of process conditions on nanowire growth was investigated, and characterization of the structural, optical, and electrical properties of the nanowires was also performed.

This Report summarizes the first year progress (October 1, 2004 to September 30, 2005) made under a NETL funded project entitled ''Improved InGaN Epitaxy Yield by Precise Temperature Measurement''. This Project addresses the production of efficient green LEDs, which are currently the least efficient of the primary colors. The Project Goals are to advance IR and UV-violet pyrometry to include real time corrections for surface emissivity on multiwafer MOCVD reactors. Increasing wafer yield would dramatically reduce high brightness LED costs and accelerate the commercial manufacture of inexpensive white light LEDs with very high color quality. This work draws upon and extends our previous research (funded by DOE) that developed emissivity correcting pyrometers (ECP) based on the high-temperature GaN opacity near 400 nm (the ultraviolet-violet range, or UVV), and the sapphire opacity in the mid-IR (MIR) near 7.5 microns.

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We developed a pyrometer that operates near the high-temperature bandgap of GaN, thus solving the transparency problem once a {approx} 1 {micro}m thick GaN epilayer has been established. The system collects radiation in the near-UV (380-415 nm) and has an effective detection wavelength of {approx}405 nm. By simultaneously measuring reflectance we also correct for emissivity changes when films of differing optical properties (e.g. AlGaN) are deposited on the GaN template. We recently modified the pyrometer hardware and software to enable measurements in a multiwafer Veeco D-125 OMVPE system. A method of synchronizing and indexing the detection system with the wafer platen was developed; so signals only from the desired wafer(s) could be measured, while rejecting thermal emission signals from the platen. Despite losses in optical throughput and duty cycle we are able to maintain adequate performance from 700 to 1100 C.

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The AlGaInN material system is used for virtually all advanced solid state lighting and short wavelength optoelectronic devices. Although metal-organic chemical vapor deposition (MOCVD) has proven to be the workhorse deposition technique, several outstanding scientific and technical challenges remain, which hinder progress and keep RD&A costs high. The three most significant MOCVD challenges are: (1) Accurate temperature measurement; (2) Reliable and reproducible p-doping (Mg); and (3) Low dislocation density GaN material. To address challenge (1) we designed and tested (on reactor mockup) a multiwafer, dual wavelength, emissivity-correcting pyrometer (ECP) for AlGaInN MOCVD. This system simultaneously measures the reflectance (at 405 and 550 nm) and emissivity-corrected temperature for each individual wafer, with the platen signal entirely rejected. To address challenge (2) we measured the MgCp{sub 2} + NH{sub 3} adduct condensation phase diagram from 65-115 C, at typical MOCVD concentrations. Results indicate that it requires temperatures of 80-100 C in order to prevent MgCp{sub 2} + NH{sub 3} adduct condensation. Modification and testing of our research reactor will not be complete until FY2005. A new commercial Veeco reactor was installed in early FY2004, and after qualification growth experiments were conducted to improve the GaN quality using a delayed recovery technique, which addresses challenge (3). Using a delayed recovery technique, the dislocation densities determined from x-ray diffraction were reduced from 2 x 10{sup 9} cm{sup -2} to 4 x 10{sup 8} cm{sup -2}. We have also developed a model to simulate reflectance waveforms for GaN growth on sapphire.

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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.

Magnesocene (biscyclopentadienylmagnesium) is a common precursor used for the p-type doping of GaN and other group III nitride materials. Unfortunately, difficulties remain with predictably controlling the incorporation of Mg during metal organic chemical vapor deposition (MOCVD) film growth, which often exhibits poorly understood 'memory effects.' Although the formation of a reaction product between magnesocene and ammonia has been previously speculated, one has never been experimentally isolated or identified. We have spectroscopically observed and identified, for the first time, the adducts formed between magnesocene and ammonia. Density functional theory (DFT) quantum chemistry calculations have also been performed on the system to determine the structures and energetics of the reaction products. It was found that ammonia can form condensable Lewis acid-base complexes with magnesocene in both 1:1 and 2:1 ratios (i.e., NH{sub 3}-MgCp{sub 2} and (NH{sub 3}){sub 2}-MgCp{sub 2}) via nucleophilic attack of NH{sub 3} at the positively charged Mg center of MgCp{sub 2}. Adduct formation is reversible, and the 1:1 and 2:1 products can be converted to one another by controlling the NH{sub 3} partial pressure. The formation and condensation of both adducts at room temperature is the probable parasitic source that leads to many of the observed Mg incorporation difficulties during the p-type doping of group III nitride materials.

Using in situ laser light scattering, we have observed gas-phase nanoparticles formed during AlN, GaN and InN OMVPE. The response of the scattering intensity to a wide range of conditions indicates that the AlN parasitic chemistry is considerably different from the corresponding GaN and InN chemistry. A simple CVD particle-growth mechanism is introduced that can qualitatively explain the observed particle size and yields a strong residence time dependence. We also used FTIR to directly examine the reactivity of the metalorganic precursors with NH{sub 3} in the 25-300 C range. For trimethylaluminum/NH{sub 3} mixtures a facile CH{sub 4} elimination reaction is observed, which also produces gas-phase aminodimethylalane, i.e. Al(CH{sub 3}){sub 2}NH{sub 2}. For trimethylgallium and trimethylindium the dominant reaction is reversible adduct formation. All of the results indicate that the AlN particle-nucleation mechanism is predominately of a concerted nature, while the GaN and InN particle-nucleation mechanisms involve homogeneous pyrolysis and radical chemistry.