Over the years, the short-wavelength limit of laser diodes has moved from the red end of the visible spectrum to the near-UV. However, there are many applications, such as Raman spectroscopy for chemical and biochemical sensing, surface analysis, and medical uses, that could benefit from the development of a deep-UV-emitting laser diode, which could be battery-powered for portable instrumentation.
Laser diodes can be frequency-doubled and quadrupled to produce deep-UV light with a narrowband output at wavelengths down to just below 193 nm. While these lasers are more compact and user-friendly than other deep-UV sources such as excimer lasers, they still are not nearly as small, simple, and low in power usage as a conventional laser diode.
Now, researchers at McGill University (Montreal, QC, Canada) have fabricated aluminum gallium nitride (AlGaN) laser diodes that produce deep-UV light at a 239 nm wavelength, operate at room temperature, and are electrically pumped.1 As a bonus, the prototype has a very low threshold current of about 0.35 mA.
Inversely tapered nanowires
Simulation studies showed that randomly distributed AlGaN nanowires can strongly confine deep-UV photons in the 240 nm spectral region. The researchers settled on an inversely tapered nanowire configuration to minimize loss through the underlying silicon (Si) substrate.
In the fabrication process, the nanowires are spontaneously formed on the Si substrate and each have a structure consisting of a n-GaN contact layer, n-AlGaN cladding layer, AlGaN active region, p-AlGaN cladding layer, and p-GaN contact layer (see figure). The researchers say that the repeated scattering of photons caused by the random arrangement of the nanowires results in interference and, thus, strong light localization.
The average fill factor for the AlGaN nanowires was 0.55. Because of inhomogeneity of nanowires and imperfect fabrication, actual current injection and laser operation takes place in only about 50% of them. The calculated cavity volume and carrier-recombination volume for the individual nanowires was 0.627 and 0.165 μm3, respectively.
First, room-temperature photoluminescence (PL) studies were carried out using an excitation source with a 193 nm wavelength. The resulting PL spectrum had an emission peak at 246 nm with a 20 nm bandwidth, indicating a high 70% Al composition and good Al uniformity. A second peak at about 210 nm indicated that AlN shells formed on the AlGaN nanowire sidewalls, which help suppress nonradiative surface recombination.
Next, electrically injected laser diodes were fabricated via photolithography and metallization. Room-temperature electroluminescent spectra both below and above the laser threshold were measured. Below-threshold operation resulted in a broad emission spectrum. As the current reached the 0.35 mA threshold, the 239 nm laser line began to emerge. At threshold, the linewidth was about 0.9 nm, but gradually rose to about 1.4 nm as the current was raised to about 1.4 mA.
The researchers previously had demonstrated electrically injected AlGaN nanowire deep-UV lasers emitting at 262 and 289 nm. These lasers had much-higher compositional modulation, resulting in quantum-dot-like structures that produced a very low threshold current of only tens of microamperes.2,3 But because the high compositional modulation prevents laser emission at shorter wavelengths, the group had to boost the compositional uniformity to create laser emission at 239 nm. The higher uniformity caused the quantum-dot-like nature to be lost and raised the threshold current to 0.35 mA. However, this threshold current at 239 nm is still quite low, and could help make laser-based, battery-powered deep-UV instrumentation viable.
REFERENCES
1. S. Zhao et al., Appl. Phys. Lett., 109, 191106 (2016).
2. S. Zhao et al., Nano Lett., 15, 7801 (2015).
3. S. Zhao et al., Appl. Phys. Lett., 107, 043101 (2015).
