news digest ♦ LEDs
resonators, chemical sensors, and highly sensitive atomic probe tips.
In the two decades since GaN was first employed in a commercially viable LED, ushering in a dazzling future for low-power lighting and high- power transistors, 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, Colorado, much of the recent effort has been devoted to growing and characterising extremely high-quality GaN nanowires – “some of the best, if not the best, in the world,” says Norman Sanford, co-leader of the Semiconductor Metrology for Energy Conversion project.
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 SiC 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 MBE deposition method 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 microns (about one- tenth the thickness of a human hair), but the wait pays off because the crystal structure is very nearly perfect.
(≈370 nm) and invisible to the unaided eye. The length of the lasing nanowire is roughly 10 microns and the diameter is roughly 200 nm. The metal probe tip at the top of the image is used to examine proximity effects on the lasing properties of the nanowire. Other (non-lasing) nanowires are also seen in the image.
Among other advantages, flawless crystals produce more light. «Now, for the first time, the electroluminescence from a single GaN nanowire LED is sufficiently bright that we can measure its spectrum and track the spectrum with drive current to see evidence of heating,» says project co-leader Kris Bertness. «There are no other examples of electroluminescence spectra from a single MBE- grown GaN nanowire in the literature.
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)
GaN and its related alloy system (including semiconductors containing indium and aluminium) form the basis of the rapidly expanding solid state lighting industry. It could move faster, experts believe, if industry could develop an economical method to grow low-defect-density material.
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Optically pumped GaN nanowire laser shown glowing orange. The actual laser output is UV
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www.compoundsemiconductor.net November/December 2011
“Conventional GaN-based LEDs grown on cost- effective but non-lattice-matched substrates (such as sapphire) suffer from unavoidable strain and defects which compromise efficiency,” Sanford says. “Additionally, light extraction from conventional planar (flat) LED structures is impeded by total internal reflection resulting in wasted
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