Ultraviolet-C Vertical-Cavity Surface-Emitting Lasers with Precise Cavity Length Control

In vertical-cavity surface-emitting lasers (VCSELs), the cavity length defines the resonance wavelength, which is directly related to the laser detuning, that is, the difference between resonance wavelength and gain peak. A low detuning maximizes the modal gain leading to a reduction of the threshold. Therefore, controlling the cavity length of VCSELs is of great importance. Here optically pumped ultraviolet-C (wavelength $\le$ 280 nm) VCSELs with precise cavity length control are demonstrated. The VCSEL structure is formed by an AlN cavity with 5 $\times$ Al_0.40Ga_0.60/Al_0.70Ga_0.30N quantum wells and a top HfO₂ spacer layer with dielectric SiO₂/HfO₂ distributed Bragg reflectors on both sides of the cavity. To access the N-face side of the cavity, a new methodology referred to as photo-assisted electrochemical etching is employed for substrate removal. Across a 0.9 mm $\times$ 1.2 mm area, the lasing wavelength varies a maximum of 1.17 nm between different UVC VCSELs, exhibiting threshold pump power densities from 0.7 MW/cm² to 3.7 MW/cm² and detuning values between 0 to 2 nm. The results show that VCSELs with a cavity length variation lower than 1$\%$ can be obtained with this technology.