Nagoya University scientists have succeeded in designing a laser diode that emits deep-ultraviolet light. It emits the world's shortest lasing wavelength, at 271.8 nanometers, under pulsed electric current injection at room temperature. They could be used for disinfection in healthcare, for treating skin conditions such as psoriasis, and for analyzing gases and DNA.
The Nagoya University 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 number 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; a process called doping. 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.