AlGaN-Based Ultraviolet Laser Diodes

The current status of UV lasers and laser diodes (LDs) based on group III-nitrides is reviewed. The focus is on the design, fabrication, and performance of AlGa(In)N laser heterostructures grown by metal–organic vapor phase epitaxy (MOVPE) on high-quality bulk AlN substrates. The review begins with the fundamentals of laser diode operation and identifies the challenges to realize short-wavelength devices with wide band gap materials. In particular, simultaneously achieving high material quality and good p-type conductivity becomes increasingly challenging with higher aluminum concentrations in the epitaxial films. Using low defect density bulk AlN substrates is a good strategy to realize high internal quantum efficiencies and, ultimately, high gain within the active zone. Polarization-assisted hole generation with a short-period superlattice for the cladding layer is a viable approach to overcome the limitations of thermally activated p-type doping. Topics include LD processing considerations that are relevant for the high band gap materials, issues related to efficient carrier injection at the high current densities required for LD operation, and specific approaches to improve the functionality of the electron blocking layer. Next, results are presented for optically pumped UV lasers, with wavelengths down to λ = 237 nm and low lasing thresholds, and design options are described to manipulate the polarization of the emitted laser light. The review concludes with a discussion of alternative laser designs to realize deep-UV laser emission with nitride semiconductors.