Optical Materials Metrology

21st century optoelectronics emphasizes energy technologies, and modern security issues have heightened needs for advanced sensors. Energy-inspired thrusts for high-efficiency solid state lighting, photovoltaics, and thermoelectrics present new metrology challenges since many materials may be combined into dense nano-engineered structures for increased performance. Modern sensors rely on similar structures and materials. Measurement science must keep pace with emerging technologies since fundamental properties established for bulk and thin film materials do not generalize to the required nanoscale dimensions and morphologies. NIST scientists have made rapid advances in measurement sciences aimed at nanoengineered semiconductors which will accelerate all of these applications.

It is internationally recognized that LED based solid state lighting using III-nitride semiconductors (GaN, AlGaN, InGaN) is poised to significantly reduce energy demand for 21st century. Illumination consumes approximately 30 % of the U.S. energy budget. Due to various complications, conventional planar LEDs made from these materials are inherently inefficient. Moreover, the planar morphology also results in low light extraction efficiency. There is much room for improving LEDs in order to meet the looming worldwide demands on reduced lighting costs in the face of mounting energy costs.

NIST scientists found that the GaN is fundamentally superior when grown as nanowires instead of planar films. They also quickly realized that dense nanowire arrays would enable LEDs with greatly improved light extraction efficiency. These findings, conclusions, and motivations aimed at developing nanowires for LED illumination technology have been independently arrived at by competing research groups around the world. NIST is in a strong position since NIST-grown nanowires are proven to be some of the best in the world, and NIST has developed new metrology methods to analyze these nanostructures. NIST can also grow nanowires on silicon substrates, thus enabling numerous possibilities in new device integration. NIST has teamed with a DARPA sponsored "iMINT" University Focus Center on nanotechnology. Of eleven such Centers now sponsored by the DARPA MTO office, DARPA has publicly recognized NIST’s progress in GaN nanowires as one of the 3 most significant outcomes of all 11 Centers. The dense nanowire morphology also enables improved light collection efficiency. NIST is starting work to apply nanowires for improving photovoltaic devices as well.

Nanowire LEDs and lasers are also developed for new active-tip NSOM tools for metrology. Furthermore, NIST is working on the application of nanowires for cancer research and advanced sensors for biological and chemical agents.

• Demonstrated first core-sleeve nanowire heterostructures employing MBE-grown Si-doped cores and HVPE-grown Mg-doped sleeves
• Demonstrated best measurement to date for the surface recombination velocity of GaN, enabled by the use of nanowire test structures
• Demonstrated best GaN nanowire MESFET devices to date
• Demonstrated use of photoconductivity to estimate carrier concentration and mobility in nanowires.
• Demonstrated nanowire UV detectors sensitive to only a few hundred photons