Nagoya University researchers, in cooperation with Asahi Kasei Corp., have succeeded in designing a laser diode that emits deep-ultraviolet light, according to research published in the journal Applied Physics Express. “Our laser diode emits the world’s shortest lasing wavelength, at 271.8 nm, under pulsed current injection at room temperature,” said Professor Chiaki Sasaoka of Nagoya University’s Center for Integrated Research of Future Electronics. Sasaoka explained that previous efforts in the development of ultraviolet laser diodes had managed to achieve emissions only down to 336 nm. Laser diodes that emit short-wavelength UV light could be used for disinfection in health care, for treating skin conditions such as psoriasis, and for analyzing gases and DNA. The base of the laser diode layers is constructed of a high-quality aluminum nitride (AlN) substrate, which the researchers opted for to avoid the defects associated with lower quality AlN. The quality of the AlN is said to affect 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 toward 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, a process called doping. 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 and decreasing toward 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 13.8 V was needed for the emission of the UV wavelength. The team is now conducting advanced joint research with Asahi Kasei Corp. to achieve continuous room temperature deep-UV lasing for the development of UV-C semiconductor laser products.
