Shortest wavelength AlGaN based semiconductor lasers
Laser diodes (LDs) with emission wavelengths near 270 nm are attracting attention for healthcare applications such as virus inactivation and sterilization because DNA absorbs this wavelength range. The challenges to realize UVC LDs, the epitaxial growth of high-quality AlGaN active layers and the lack of carrier injection capability due to poor activation of Mg-doped AlGaN, were solved by pseudomorphic growth of epitaxial layers on freestanding AlN single-crystal substrates and distributed polarization doping technique (DPD) without impurity doping of the p-side cladding layer. These technologies have achieved breakthroughs in realizing AlGaN-based UVC LDs during pulsed-current operation. However, the threshold current density was high at the beginning of development, and various efforts have been carried out to reduce the threshold current density. As a result the suppression of inhomogeneous emission, improvement of optical confinement, and reduction of voltage, continuous-wave lasing has been demonstrated.