New laser diode: can emit deep ultraviolet

- 2021-12-21-

Background

Laser diodes that emit short-wavelength ultraviolet rays, also known as "UV-C (wavelength range between 200 nanometers and 280 nanometers)," can be used for disinfection in the medical and healthcare field, treatment of skin diseases such as psoriasis, and analysis of gas and DNA.

Innovation

According to a paper published in the "Applied Physics Express" magazine, scientists from Nagoya University in Japan and Asahi Kasei Co., Ltd. successfully designed a laser diode that emits deep ultraviolet light.

The far-field pattern of the UV-C laser projected on the phosphor screen (Image source: Nagoya University and Asahi Kasei Co., Ltd.)

Professor Chiaki Sasaoka of the Future Electronics Integration Research Center of Nagoya University said: “Our laser diode emits the world’s shortest laser wavelength (271.8 nanometers) through pulsed current injection at room temperature.” Sasaoka explained that the previous report on ultraviolet laser diodes The research only achieved laser emission as low as 336 nanometers.

Technology

The deep ultraviolet laser diode of Nagoya University overcomes several problems encountered by scientists in their efforts to develop these semiconductor devices.

The team used a high-quality aluminum nitride (AIN) substrate as the basis for their construction of the laser diode layer. They said this is necessary because the lower quality AIN contains a large number of defects, which ultimately affects the efficiency of the laser diode's active layer to convert electrical energy into light energy.

In laser diodes, the "p-type" and "n-type" layers are separated by a "quantum well". When current flows through the laser diode, the positively charged holes in the p-type layer and the negatively charged electrons in the n-type layer combine toward the middle, releasing energy in the form of photons.

Researchers designed this kind of quantum well that emits deep ultraviolet light. This p-type layer and n-type layer are made of aluminum gallium nitride (AlGaN). The cladding layer is also made of AlGaN and is mounted on the p-type layer or the n-type layer. The cladding layer under the N-type layer contains silicon impurities, and this process is also called "doping". Doping is used as a technique to modify the properties of materials. The cladding layer on the p-type layer has undergone distributed polarization doping, and it will not add impurities when doping this layer. The aluminum content in the p-side cladding layer is designed, the bottom is the highest, and it decreases toward the top. Researchers believe that this gradient in aluminum content improves the flow of positively charged holes. Finally, a top contact layer made of magnesium-doped p-type AlGaN was added.

Cross-sectional structure of UV-C semiconductor laser diode (photo source: Nagoya University and Asahi Kasei Co., Ltd.)
The researchers found that the polarization doping of the p-side cladding layer means that the "shortest wavelength so far" laser emission requires a pulse current of "very low operating voltage (13.8 volts)."

Emission characteristics under pulse operation (Image source: Nagoya University and Asahi Kasei Co., Ltd.)




The team is currently working with Asahi Kasei Co., Ltd. to achieve continuous deep ultraviolet laser emission at room temperature to develop UV-C semiconductor laser products.