Laser diodes stretch further into the UV

07 Jan 2009

Hamamatsu extends the emission of UV diodes to 336 nm.

Researchers from Hamamatsu Photonics' Central Research Laboratories have broken the record for the shortest wavelength UV laser diode.

The team surpassed its previous best of 342 nm with a 336 nm emitter that features higher aluminium content in and around the active region, and produced 3 mW output power when driven in pulsed mode at room temperature.

Harumasa Yoshida and co-workers, who report their work in the 15 December issue of Applied Physics Letters, say that their source could be used for material processing and high-density data storage. The 336 nm, 500 µm long diode had a threshold current of 17.6 kA cm^–2, which is more than twice as high as its longer wavelength equivalent.

Short wavelength laser diode fabrication demands a suitable technique for dealing with tensile strain arising from lattice mismatch between the AlGaN device and its sapphire substrate. If the strain is too high, it leads to cracking, destroying the device. One promising option is to use AlN substrates. However, these are not widely available, so the Japanese team has developed a technique called hetero-facet-controlled epitaxial layer overgrowth.

This approach, which the team has already used to fabricate its 342 nm lasers, begins by growing pyramid-shaped GaN seed crystals. These 2.5 µm high structures are grown on a GaN underlayer with periodically spaced SiO₂ stripes.

Yoshida's team deposits Al_0.3Ga_0.7N on top of these pyramids, which grows laterally to create a flat platform for growth of the laser structure. Although dislocations are formed in the AlGaN layer, they generally propagate in the horizontal direction, and dislocation density falls as more material is deposited. A 2% increase in the AlN mole fraction of the guiding layers and the multiple quantum well is the only difference between the epitaxial structure of the 342 nm diode and that of its 336 nm cousin.

The lasers feature 5 µm wide ridge stripes produced by dry etching, plus Ni/Au and Ti/Al contact pads affixed to the p-type GaN and the n-type Al_0.3Ga_0.7N contact layer, respectively. The laser's dry-etched facets were left uncoated.