III-Nitride Semiconductor Nanowires - Novel Materials for Optoelectronic and Energy Applications

Research on semiconductor nanowires has grown exponentially over the last decade, with much attention focusing on their synthesis, fundamental properties, and potential applications. Nanowires are high aspect ratio, wire-like structures with diameters typically ranging from a few nanometers to a few hundred nanometers. Nanowires comprised of virtually every semiconductor system, including Si/Ge, II-VIs, and III-Vs, have been synthesized to date and exhibit a variety of interesting morphologies including hexagonal, rectangular, triangular, cylindrical, and even branched.

Interest in semiconductor nanowires is due in large part to their unique thermal, mechanical, optical, chemical, and electrical properties, a result of their high surface-to-volume ratio and their size, which intersects a number of physical characteristic length scales, such as the exciton diffusion length and Bohr radius, UV-visible wavelengths, phonon mean free path, and critical size of magnetic domains. These novel properties have led to a number of intriguing demonstrations of semiconductor nanowires as individual or integrated nanoscale elements in a variety of applications ranging from thermoelectrics, nanophotonics, sensing, piezoelectrics, energy harvesting and storage, and nanoelectronics.¹

The III-nitrides (AlGaInN) are technologically important direct band-gap semiconductors which absorb and emit over a very broad and attractive energy range from the UV to visible to infrared wavelengths, and are the basis for commercial products like visible light-emitting diodes (LEDs) and blue laser diodes (e.g. BluRay). As such, nanowires based on III-nitride semiconductors are being explored for potential use in LEDs, lasers, photovoltaics, water splitting, high speed/power electronics, and other applications.

However, before such nanowire-based applications can be practically realized, several challenges exist in the areas of controlled and ordered nanowire synthesis, fabrication of advanced nanowire heterostructures, and understanding and controlling the nanowire thermal, electrical, mechanical, and optical properties. At Sandia National Laboratories, under the Solid-State Lighting Science Energy Frontier Research Center and other programs, Dr. George T. Wang and colleagues are investigating the synthesis and properties of III-nitride based nanowires with the goal of addressing these many challenges.

Semiconductor nanowires can be fabricated by a variety of techniques, including bottom-up approaches often involving a nanoscale metal catalyst particle to direct 1D growth via the vapor-liquid-solid (VLS) mechanism, to top-down lithographic approaches. While both of these synthetic approaches are being explored at Sandia, the primary focus has been on VLS-based growth of GaN and III-nitride core-shell nanowires using metal-organic chemical vapor deposition (MOCVD). Figure 1 shows the template-free, aligned growth of GaN nanowires on a sapphire substrate via this method.

The high nanowire density and degree of vertical alignment, which is desirable for vertical device integration, is achieved by the proper selection of the substrate crystal orientation and careful control of the metal catalyst as well as the growth conditions.^2-4 The nanowires are single crystals, with triangular cross-sections (Figure 1b), and are free of the device-detrimental defects known as dislocations which are common in III-nitride films. This high crystalline quality of the nanowires, along with the ability to make doped and alloy heterostructures over a broad, tunable bandgap range, makes them attractive candidates for energy efficient devices.

Figure 1. (a) Scanning electron microscope (SEM) image of aligned GaN nanowire growth on sapphire; (b) transmission electron microscope (TEM) image of a GaN nanowire with AlGaN shell layer showing its triangular cross-section.

variety of nanocharacterization techniques are also being employed by Dr. Wang and his colleagues in order to understand and ultimately improve the nanowire properties. For example, spatially-resolved cathodoluminescence experiments are being used to map the frequencies and intensities of light emission from these nanowires with nanoscale resolution,⁵ as shown in Figure 2.

This and other optical techniques that have been adapted to studying these nanostructures, including near-field scanning microscopy⁶ and ultrafast⁷ and deep-level optical spectroscopies⁸, have revealed details such as the origin and concentration of impurities and other point defects in the nanowires, with the goal of reducing them and their impact on nanowire-based devices. Powerful 3D⁹ and in-situ electron microscopy techniques¹⁰ have enabled, for example, observing the physical breakdown of a nanowire device under high electrical power in real-time at atomic-scale resolutions.¹¹

Figure 2. (a) Cathodoluminescence (CL) image showing blue light emission from GaN/InGaN core-shell nanowires; (b) CL image showing defect-related yellow luminescence from the surface region of a GaN nanowire.

In addition to single nanowire devices, ensembles of nanowires can also be leveraged in interesting and advantageous ways. At Sandia, Dr. Wang and colleagues have developed a technique that uses vertically aligned GaN nanowire arrays as a high quality template for the growth of high quality GaN films on inexpensive, lattice-mismatched substrates, as shown in Figure 3.¹² The nanowires serve as strain compliant "bridges" between the coalesced GaN film and the underlying, lattice-mismatched sapphire substrate, which helps to minimize defect formation in the GaN film and hence improve device performance.

Figure 3. (a) Artist's rendering of nanowire-templated growth of a GaN film; (b) cross-section SEM image showing demonstration of nanowire-templated GaN growth.

In summary, III-nitride semiconductor nanowires are intriguing new structures that show great promise as efficient, nanoscale building blocks for applications ranging from solid-state lighting and displays to photovoltaics. Many efforts around the world are currently underway to better understand their synthesis and properties in order to realize their full potential.

Acknowledgments

Funding from DOE Basic Energy Sciences (BES) DMSE, DOE EERE National Energy Technology Laboratory, Sandia's LDRD program, and Sandia's Solid-State Lighting Science Energy Frontier Research Center (DOE BES). Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

References

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