A low-cost surface emitting green laser

Technology News | January 17, 2020

By eeNews Europe

The results puplished under the title “An electrically pumped surface-emitting semiconductor green laser” in Science Advances detail an all-epitaxially grown device that exploits the photonic band edge modes formed in dislocation-free gallium nitride nanocrystal arrays, instead of using conventional distributed Bragg reflectors (DBRs).

Fig. 1: Schematics of the nanocrystal
surface-emitting laser (NCSEL) in
operation. Credit: Science Advances.

So far, room temperature surface-emitting green laser diodes relied on dual dielectric distributed Bragg reflectors (DBRs) and water bonding to a copper plate for low thermal resistance, but such devices exhibited a very large threshold current density at room temperature, and their light emission was limited to 400 and 460nm (violet blue).

Fig. 2: Schematic illustration of the full NCSEL fabrication, including passivation, planarization,
photolithography, and contact metallization techniques. Credit: Science Advances.

In contrast, the 10μm-diameter device operates at around 523nm and exhibits a threshold current of about 400A/cm2, over one order of magnitude lower compared to previously reported blue laser diodes. First explored through simulation, the so-called nanocrystal surface-emitting laser (NCSEL) was carefully engineered from InGaN/AlGaN (indium gallium nitride/aluminum gallium nitride) nanocrystal arrays of precisely controlled size, spacing and surface morphology.

The efficient strain relaxation in the conical core-shell InGaN/AlGaN multiple quantum disks within the nanowires ensured the nanostructures were free of dislocations.

Further design refinements allowed the researchers to reduce the quantum confined stark effect (QCSE) and suppress surface recombination (using an AlGaN shell structure around the active region).

Fig. 3: (A) STEM-HAADF image of the core-shell
InGaN/AlGaN multiple quantum disk (MQD)
heterostructure nanocrystal. (B) High-magnification
image taken from the region marked in (A) and (C)
schematic illustration for the quasi-3D structure of
the semipolar active region.

What’s more, the authors conclude that their approach could be applied across the entire visible as well as mid- and deep UV wavelengths to create lasers on low-cost and large-area Si wafers, opening the field to many applications including projection displays such as pico projectors, plastic optical fiber communication, wireless communication, smart lighting, optical storage and biosensors.

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