Over the years, the short-wavelength limit of laser diodes has moved from the red end of the visible spectrum to the near-ultraviolet range. However, many applications, such as Raman spectroscopy for chemical and biochemical sensing, surface analysis, and medical applications, can benefit from the development of deep UV-emitting laser diodes that can be battery-powered for use in portable instruments. The laser diode can be doubled and quadrupled to produce narrowband output deep UV light with a wavelength shorter than 193 nm. Although these lasers are more compact and more user friendly than other types of deep UV light sources, such as excimer lasers, they are still not as compact, simple and as low power as conventional laser diodes. Recently, researchers at McGill University in Canada created an AlGaN laser diode capable of outputting deep UV light with a wavelength of 239 nm, operating at room temperature, and being electrically pumped. In addition, the prototype has a very low threshold current of about 0.35 mA. Reverse tapered nanowires Simulation studies show that randomly distributed AlGaN nanowires can strongly constrain deep UV photons in the 240 nm spectral region. The researchers determined the reverse tapered nanowire structure to minimize the loss through the underlying silicon (Si) substrate. In the manufacturing process, nanowires are spontaneously formed on a Si substrate and each have a structure composed of an n-GaN contact layer, an n-AlGaN cladding layer, an AlGaN active region, a p-AlGaN cladding layer and a p-GaN contact layer Structure. The researchers said that the repeated scattering of photons caused by the random arrangement of nanowires leads to interference and therefore to strong light localization. The average fill factor of AlGaN nanowires is 0.55. Due to the inhomogeneity of nanowires and imperfections in manufacturing, actual current injection and laser operation occur in only about 50% of them. The calculated cavity volume and carrier recombination volume of single nanowire are 0.627μm3 and 0.165μm3, respectively. First, a room temperature photoluminescence (PL) study was performed using a 193 nm excitation source. The resulting PL spectrum has an emission peak at 20 nm bandwidth at 246 nm, indicating up to 70% Al composition and good Al homogeneity. The second peak, located at about 210 nm, indicates that AlN shells are formed on the sidewalls of AlGaN nanowires, which helps to suppress non-radiative surface recombination. Next, an electrically injected laser diode is fabricated by lithography and metallization methods. The researchers measured room temperature electroluminescence spectra below and above the laser threshold. Working at a threshold produces a broad emission spectrum. When the current reaches the threshold of 0.35mA, the 239nm laser line begins to appear. At the threshold, the line width is about 0.9 nm, but as the current gradually increases to about 1.4 mA, the line width gradually increases to about 1.4 nm. Researchers have previously demonstrated electrically injected AlGaN nanowire deep UV lasers at 262 nm and 289 nm. These lasers have a much higher compositional regulation, forming a quantum dot-like structure, producing a very low threshold current of just a few tens of microamperes. But because of the high compositional modulation that inhibits laser emission at shorter wavelengths, researchers have had to improve the compositional uniformity to produce laser emission at 239nm. The higher uniformity results in the disappearance of quantum dot properties and the increase of the threshold current to 0.35 mA. However, the threshold current of 239nm is still quite low, which will help to achieve a laser-based, battery-powered deep UV instrument.
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