Laser Diode Emits Deep UV Light

Far field pattern of UV-C laser projected onto a fluorescent screen. (Source: Asahi Kasei / Nagoya U.)

Nagoya University scientists, in coopera­tion with Asahi Kasei Cor­poration, have succeeded in designing a laser diode that emits deep-ultra­violet light. “Our laser diode emits the world’s shortest lasing wavelength, at 271.8 nanometers, under pulsed current injection at room tempera­ture,” says Chiaki Sasaoka of Nagoya University’s Center for Integrated Research of Future Electronics. Previous efforts in the development of ultra­violet laser diodes had only managed to achieve emissions down to 336 nm, Sasaoka explains.

Laser diodes that emit short-wavelength ultra­violet light in the wavelength region of 200 to 280 nm could be used for disinfection in healthcare, for treating skin conditions such as psoriasis, and for analyzing gases and DNA. The new deep-ultra­violet laser diode overcomes several issues encountered by scientists in their work towards the develop­ment of these semi­conducting devices. The team used a high quality aluminum nitride (AlN) substrate as their base for building up the layers of the laser diode. This, they say, is necessary, since lower quality AlN contains a large amount of defects, which ulti­mately impact the effi­ciency of a laser diode’s active layer in converting electrical into light energy.

In laser diodes, a p-type and n-type layer are separated by a quantum well. When an electric current is passed through a laser diode, posi­tively charged holes in the p-type layer and negatively charged electrons in the n-type layer flow towards the center to combine, releasing energy in the form of photons. The researchers designed the quantum well so that it would emit deep UV light. The p- and n-type layers were made from aluminum gallium nitride (AlGaN). Cladding layers, also made from AlGaN, were placed on either side of the p- and n-type layers.

The cladding below the n-type layer included silicon impu­rities. This doping is used as a technique to modify a material’s properties. The cladding above the p-type layer underwent distributed polari­zation doping, which dopes the layer without adding impu­rities. The aluminum content in the p-side cladding was designed so that it was highest at the bottom, decreasing towards the top. The researchers believe this aluminum gradient enhances the flow of posi­tively charged holes. A top contact layer was finally added that was made from p-type AlGaN doped with mag­nesium.

The researchers found that the polari­zation doping of the p-side cladding layer meant that a pulsed electric current of remarkably low operating voltage of 13.8V was needed for the emission of the shortest wavelength reported so far. The team is now conducting advanced joint research with Asahi Kasei to achieve continuous room tempera­ture deep-UV lasing for the development of UV-C semi­conductor laser products. (Source: Nagoya U.)

Reference: Z. Zhang et al.: A 271.8 nm deep-ultraviolet laser diode for room temperature operation, Appl. Phys. Exp. 12, 124003 (2019); DOI: 10.7567/1882-0786/ab50e0

Link: Center for Integrated Research of Future Electronics, Institute of Materials Research and System for Sustainability, Nagoya University, Aichi, Japan

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