Air-Cladding Blue Laser Diodes

Low refractive index contrast in long-wavelength nitride laser diodes (LDs) limits optical confinement, motivating new architectural approaches. Here, we report the first electrically driven edge-emitting LDs featuring top and bottom air-claddings. To enable the top air cladding, the architecture employs a tunnel junction, which converts the current flow from holes to electrons. This allows for low series resistance lateral current flow and placement of the metal contact on the side of the LD mesa. The bottom air-cladding is realized postepitaxy through lateral electrochemical etching (ECE) of a highly doped InGaN:Ge sacrificial layer. Depending on the geometry of the openings for electrolyte access, wing-like and membrane LD devices are obtained. Very high backside smoothness of the membrane has been achieved thanks to an abrupt doping profile and excellent selectivity in material removal by ECE. Synchrotron-based scanning X-ray diffraction microscopy shows that laser membranes exhibit slight elastic relaxation, which results in bending of the LDs by a few nanometers over a distance of a dozen microns. LDs with dual air-claddings operated in pulse mode at a wavelength of λ = 456 nm with a slope efficiency of 0.4 W/A, similar to their reference counterparts without under-etching. This architecture is expected to provide greater benefits of refractive index engineering for longer wavelength LDs, where high refractive index contrast is more challenging. Moreover, the work highlights ECE as an extremely effective method for device liftoff, enabling GaN substrate reuse and facilitating transfer and integration of LDs into advanced photonic platforms, including therapeutic applications.