Fundamental Materials Physics

Conventional GaN-based materials are grown on highly-lattice-mismatched sapphire substrates and hence have very high defect densities.  Their typical p-type dopant is Mg, which is not always active but can be compensated by H and other common impurities.  And, optimal device designs often require incorporation of Al, which requires high temperatures, and of In, which requires low temperatures.  These and other complications result in materials properties that depend sensitively on growth conditions and are poorly understood.

Sandia is exploring some of the fundamental aspects of the physics of GaN materials, including

·   calculations of defect and impurity energetics and kinetics,

·   measurements and models of H trapping and release (pdf file - 140kb)

·   exploring MOCVD growth conditions and methods for optimized AlGaInN properties

·   MOCVD growth methods, such as stress-engineered buffers epitaxial lateral overgrowth (pdf file - 172kb) and cantilever epitaxy (pdf file - 162kb)

·   the connection between fundamental materials properties of GaN -based materials and the performance of optoelectronic devices

MOCVD Science and Technology

MOCVD is used to grow the GaN and GaAs based materials used in efficient semiconductor visible light emitters. There is a consensus in the community, though, that MOCVD tools and processes are at an early stage of development, particularly for the demanding conditions required for GaN epitaxy. Some of the fundamental challenges include:

·   an understanding of reactor fluid flow, particularly at the high operating temperatures and pressures where buoyancy effects tend to destabilize flow

·   an understanding of parasitic chemical reactions (pdf file - 159kb), particularly when both Al-bearing organometallic and N-bearing hydride precursors are simultaneously present

·   in situ monitoring technologies for real-time measurement of temperature (pdf file - 150kb) and stress (pdf file - 176kb) during growth

·   an overall understanding of the complex chemically reacting fluid flow associated with chemical vapor deposition

Advanced Light-Emitting Devices

GaN -based materials are also challenging to process, as they are relatively inert, refractory materials.  Because they are typically grown on sapphire, which lacks a cleavage plane for creating laser mirror facets, anisotropic dry etching is a key technology Sandia is developing.  Also, for some approaches to epitaxial lateral overgrowth, Sandia is studying methods for deep etching of sapphire substrates.  A combination of these device fabrication technologies with advanced materials growth recently enabled the first UV VCSEL in a collaboration between Sandia and Brown University as well as other ongoing VCSEL studies (pdf file - 152kb).  The combination is also enabling other heterostructure devices, such as GaN -based power HEMTs and other 2D electron gas devices (pdf file - 133kb).  Sandia is also investing in an emerging area that may someday enable very high external extraction efficiencies: photonic crystals.

Phosphors and Encapsulants

Sandia is also studying a range of packaging-level issues associated with high-brightness white LED-based lighting.  In one approach, single-color or multi-color phosphors are combined with blue or UV LEDs to create white light.  Most existing commercial phosphors, however, have been optimized for absorption at the shorter wavelengths present in fluorescent lamps, or for excitation by electron beams.  Sandia is studying alternative phosphors, including nanoclusters, which may be better optimized for blue or soft UV excitation.  Issues associated with polymer encapsulation as part of a broader encapsulant research activity at Sandia, including degradation by UV light, are also being studied.  In order to achieve the highest brightness, semiconductor LEDs are usually driven quite hard.  Hence, Sandia is also studying some of their reliability issues.

Review Papers and Presentations

Sandia has authored or co-authored a number of review papers and given a number of overview presentations which explore the technical challenges associated with solid-state lighting, and the potential impact of solid-state lighting.  A few of these are:

·   2004 May: " Solid-State Lighting: Lamps, Chips and Materials for Tomorrow" (pdf - 0.6mb), by Jeff Tsao, invited review published in IEEE Circuits and Devices, May/June, 2004.

·   2003 November: " Solid-State Lighting: The Promise and the Potential" (MS Powerpoint file - 1.1mb) invited talk presented at the American Vacuum Society Meeting, November, 2003, by Jeff Tsao.

·   2003 May: " Roadmap projects significant LED penetration of lighting markets by 2010" (pdf - 0.2mb) invited review published in Laser Focus World, May 2003.

·   2000 April: The Case for a National Research Program on Semiconductor Lighting (pdf - 303kb), by Roland Haitz and Fred Kish, Hewlett-Packard Company, and Jeff Tsao and Jeff Nelson, Sandia National Labs.

·   2000 March: Another Semiconductor Revolution: This Time It's Lighting"  (pdf - 142kb), by Roland Haitz and Fred Kish, Hewlett-Packard Company, and Jeff Tsao and Jeff Nelson, Sandia National Labs.