2020 Volume 69 Issue 10 Pages 721-726
III-nitride semiconductors with wide-bandgap energy are promising materials for optoelectronic devices. Particularly, blue and green light-emitting diodes (LEDs) based on InGaN/GaN multi quantum wells are commercially available at present. To realize a highly-efficient red LED, we have focused on Eu-doped GaN (GaN:Eu) and demonstrated GaN:Eu-based LED (maximum external quantum efficiency of 9.2%) with a high output power of 1.25 mW at 20 mA. As a next challenge of GaN:Eu-based optical devices, we have paid attention to fabricate laser diodes (LDs). In this paper, we focus on two-dimensional photonic crystal (2D-PhC) nanocavities with high-Q-factors and extremely small modal volumes as a prominent candidate towards laser oscillation. Based on finite-difference time-domain (FDTD) simulations, photonic band structures of 2D-PhC with hexagonal air-holes, which are suitable for wurtzite crystals, and possibility of laser oscillation of GaN:Eu are investigated. We find that appropriate lattice constants and radii of the air-holes form sufficiently-wide photonic bandgap like the case of circular air-holes conventionally used for cubic crystals. Then, we choose the line-defect cavity (LN cavity), widely used in 2D-PhC LDs, as nanocavity structures, and the Q-factors and material gain thresholds (gth), which are at least required material gain for GaN:Eu lasing, are calculated. We clarify that long cavities show high Q-factors and low gth. For a L6 cavity, the gth is calculated to be 12 cm-1, which is lower than experimentally estimated optical gain of GaN:Eu at room temperature (19 cm-1). This result indicates that appropriate 2D-PhC cavity-design allows laser oscillation of GaN:Eu.
