Semiconductor Metrology for Energy Conversion

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.

Semiconductor Growth

The goal of this project is to advance the state-of-the-art in semiconductor crystal growth for optoelectronic device applications, with an emphasis on nanostructures of compound semiconductors. Nanowire and quantum dot devices have unique properties including high luminescent output and broad wavelength tunability, but their application is limited by the inadequacy of conventional methods to quantify basic properties such as carrier concentration and composition. We collaborate with teams inside and outside of NIST to develop new nanoscale metrologies. We also provide data, materials and devices to support the efficient manufacture of compound semiconductor optoelectronic devices and to support research in NIST, other government laboratories, and industry.

Optical Materials

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.

Semiconductor Growth

Compound semiconductor materials form the basis for the diode lasers, LEDs, photodetectors, and high-efficiency solar cells critical to optical communication, display, data storage, and energy conservation and generation. Many of these semiconductor devices now incorporate structures with a high degree of strain and nanostructures so small that the properties of the devices depend on their physical dimensions as well as the bulk materials properties.

A major focus in this project is the growth and processing of GaN nanowires for nanometrology and device applications. We have demonstrated that the GaN nanowires grown in our laboratory with catalyst-free molecular beam epitaxy (MBE) are strain-free, very low in chemical impurities and usually entirely free of structural defects.

These properties lead to high optical emission intensity, consistent doping behavior, and high mechanical resonance quality factors, all of which are needed for future technical applications that will surpass GaN thin films in performance. The nanowire architecture is also a framework for inventing new characterization tools and devices. A simple example is that the optical emission can be used to measure externally applied strain. Future devices will extend our work on nanowires-as-transistors and light emitters to include nanowire lasers, biosensor applications, near-field optical probes, oscillators for communications equipment, single-photon emitters and detectors, and integration of photonics with silicon circuitry.

The scientific understanding needed to engineer these devices will make use of our basic research in quantum dot and nanowire nucleation and growth mechanisms, including the role of strain in nanostructures. This project supports manufacturing of compound semiconductor devices with the world's only composition standards for these materials. We also lead a cavity ringdown spectroscopy program that focuses on direct measurement of impurities in the source gases used in compound semiconductor manufacturing. We have achieved sensitivity below 50 nmol mol-1 for water in arsine and below 10 nmol mol-1 for water in phosphine.

Optical Materials

• 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
  

Semiconductor Growth

• Defect-free GaN nanowires grown and characterized, leading to an R&D Micro Nano 25 Award for NIST in 2006
• Comprehensive model developed for estimating carrier concentration and mobility based on nanowire resistance
• Role of growth parameters in the uniformity of InGaAs quantum dot size and spacing explained
• First application of cavity ring down spectroscopy to measure water vapor as a contaminant in arsine and phosphine
• AlGaAs composition Standard Reference Materials released in 2006