AlGaN nanowires photonic crystal makes low-threshold UV laser
First the researchers used simulation to design and study the properties of a photonic crystal based on AlGaN nanowires. Described in a paper titled “An electrically injected AlGaN nanowire defect-free photonic crystal ultraviolet laser” published in Optics Express, the AlGaN nanowire photonic crystal laser was grown on a sapphire substrate using a site-controlled selective area growth (SAG) process.
In selective area growth, the nanowire formation is directly controlled by the nanoscale apertures created on a substrate using a well-defined top-down process. AlGaN nanowire arrays with controlled size and spacing were selectively grown on GaN-on-sapphire template using plasma-assisted molecular beam epitaxy. They were fabricated into photonic crystal lasers using standard photolithography, e-beam lithography, dry etching and contact-metallization techniques. The researchers deposited Ni/Au and Ti/Au metal layers on the nanowire top surfaces and n-GaN template to serve as p-and n-metal contacts, respectively. Because the nanowire structures exhibit nearly identical size distribution, they limit optical scattering losses and do not require any particular mirroring structure.
Thanks to the topological high-Q resonance (Q factor of 11,460 according to initial simulations) of a defect-free nanowire photonic crystal, the authors were able to create electrically pumped lasers capable of operating at 369.5nm with a relatively low threshold current density of about 2.1 kA/cm2 under continuous wave operation at room-temperature.
What’s more, the 500 µm2 device emitted its UV laser with a very narrow spectral linewidth of about 0.2nm.
Further characterization showed that the fabricated nanowire photonic crystal lasers exhibit good current-voltage characteristics, with a relatively low turn on voltage of about 4V and a low leakage current of about 69µA at −9.5V, significantly smaller than that of previously reported nanowire devices, the authors claim.
The researchers anticipate that device performance could be further improved by optimizing the design of the nanowire photonic crystal optical cavity to achieve predominantly surface or edge emission.
McGill University – www.mcgill.ca
University of Michigan – www.umich.edu