Lasers ♦ news digest
glowing orange. The actual laser output is UV (≈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.”
photons which are trapped in the device rather than radiating outward as useful light.”
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. What’s more, the morphology provided by a “forest” of densely arrayed nanowire LEDs offers improvements in the light-extraction efficiency of these structures compared with their planar counterparts.
A “forest” of nanowires
Testing and measuring those and other properties, however, poses significant challenges. “P-type GaN is difficult to grow by any common growth method,” Bertness says. “And what turns out to be very hard is making good electrical contacts to the nanowire, because it is not flat, and its thickness is larger than most of the metal films used to contact planar films.
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.
“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
“This 3D geometry encourages void formation and trapping of chemical impurities near the contacts, both of which degrade the contact, sometimes to the point of being unusable. This is an area we are actively investigating.”
The team is 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 nanometres wide in a SiN “mask layer” placed over the substrate, they could achieve selective growth of highly regular wires. The ability to produce ordered patterns of uniform GaN devices, Bertness says, “is essential for reliable manufacturing.”
GaN is not only a light source. It also has multiple uses in different fields. “Another nice thing about GaN is that it’s insensitive to high temperatures,” says Robert Hickernell, leader of the Optoelectronic
November/December 2011
www.compoundsemiconductor.net 161
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