The gallium nitride nanowires grown by PML scientists may only be a few tenths of a micrometer in diameter, but they promise a very wide range of applications, from new light-emitting diodes and diode lasers to ultra-small resonators, chemical sensors, and highly sensitive atomic probe tips.

Structure of an n-type GaN nanowire grown by MBE and coated in a thin shell of p-type GaN grown by halide vapor phase epitaxy. (Credit: Aric Sanders and Albert Davydov/MML)
In the two decades since GaN was first employed in a commercially viable LED, the III-V semiconductor has been produced and investigated numerous ways, in both thin-film and nanowire form. At PML's Quantum Electronics and Photonics Division in Boulder, CO, much of the recent effort has been devoted to growing and characterizing extremely high-quality GaN nanowires. GaN emits light when holes and electrons recombine at a junction created by doping the crystal to create p-type and n-type regions. These layers are formed by a variety of deposition methods, typically on a sapphire or silicon carbide substrate. Conventional methods produce crystals with relatively high defect densities. Unfortunately, defects in the lattice limit light emission, introduce signal noise, and lead to early device failure.

The Boulder team, by contrast, grows virtually defect-free hexagonal GaN nanowires very slowly from a silicon base. Their deposition method is molecular beam epitaxy (MBE), which allows the nanowires to form spontaneously without the use of catalyst particles. Although catalyst particles are widely used for nanowire growth, they leave behind trace impurities that can degrade GaN. It takes two to three days for the structures to reach a length around 10 micrometers (about one-tenth the thickness of a human hair), but the wait pays off because the crystal structure is very nearly perfect. Among other advantages, flawless crystals produce more light.

GaN and its related alloy system (including semiconductors containing indium and aluminum) form the basis of the rapidly expanding solidstate lighting industry. It could move faster, experts believe, if industry could develop an economical method to grow low-defect-density material. GaN nanowire LED technology offers significant improvements since the wires grow essentially free of strain and defects and should thus enable fundamentally more efficient devices. Furthermore, the morphology provided by a “forest” of densely arrayed nanowire LEDs offers improvements in the lightextraction efficiency of these structures compared with their planar counterparts.

Researchers are currently looking at ways to grow nanowires in regular arrays, with careful control of the spacing and dimensions of each individual wire. Recently they found that by creating a grid-like pattern of openings on the order of 200 nanometers wide in a silicon nitride “mask layer” placed over the substrate, they could achieve selective growth of highly regular wires. For more information, visit http://info.hotims.com/40435-307