New etched-mirror fabrication process for aluminum gallium nitride laser diodes

Ultraviolet laser diodes (LDs) at the 100 to 280 nanometer range – the wavelength range considered “deep ultraviolet,” or “UV-C” – have attracted attention as light sources that could replace less efficient solid-state lasers for sterilization equipment and chemical sensors.

Developing LDs requires forming two parallel vertical mirror facets. Mirror formation via dry etching on whole wafers is a way to reduce production costs, but drawbacks to dry etching include poor etching control and low reflection quality.

Sakai et al. addressed this issue by integrating dry- and wet-etching fabrication processes to produce UV-C LDs made from aluminum gallium nitride (AlGaN) that operate under current injection at room temperature. The result was smooth, vertical AlGaN facets on whole wafers.

The researchers conducted reactive-ion etching (i.e. dry etching) on a two-inch aluminum nitride (AlN) wafer to expose the cladding layer and etch a five-micrometer-wide stripe. The wafer was immersed in an alkaline-based selective wet-etching solution to expose smooth planes of the AlGaN heterostructure. The wet etching was reasonably uniform in the lateral direction, despite the different compositions of the various materials in the LD structure.

Using atomic layer deposition, the researchers then uniformly coated the etched facets with a distributed Bragg reflector consisting of hafnium oxide and aluminum oxide films.

“We saw a significant reduction of the lasing threshold current density, owing to the high reflectivity of the etched and coated mirror facets,” said author Maki Kushimoto. “This shows the combined fabrication process as a viable AlGaN-based UV-C LD production method that could also be compatible with highly integrated optoelectronics based on AlN substrates.”

Source: “On-wafer fabrication of etched-mirror UV-C laser diodes with the ALD-deposited DBR,” by Tadayoshi Sakai, Maki Kushimoto, Ziyi Zhang, Naoharu Sugiyama, Leo J. Schowalter, Yoshio Honda, Chiaki Sasaoka, and Hiroshi Amano, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/1.5145017 .