No abstract

III-nitride-based two-dimensional photonic crystal (2D-PhC) cavities with high-quality factors (Q-factors) have a large potential application, however realized Q-factors in the visible wavelength regime have been relatively moderate. In this study, we demonstrate the design and fabrication of 2D-PhC cavities to achieve high Q-factors, especially in the visible range. From the comparison of numerical calculations and the experimental results, we discuss the dominant optical losses that limit the Q-factor of H3-type cavities formed in an Eu,O-codoped GaN film. Based on these results we designed 2D-PhC cavities which can effectively suppress these dominant losses. We fabricated 2D-heterostructures and show a high Q-factor of 10500 at a resonant wavelength of ∼660 nm, which is considerably larger than any existing GaN-based nano/micro-resonators in the visible region. This study provides design guidelines for the realization of high Q-factors in photonic crystal nanocavities based on III-nitride semiconductors.

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 timedomain (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.

We report an analysis of one-dimensional rod-based photonic crystal nanocavities. These cavities offer opportunities for dielectric materials which lack a matching low-refractive index substrate or are limited in under-etching possibilities to create slab-based PhC cavities. They offer high theoretical Q -values exceeding 10 6 for transverse magnetic polarized modes with modal volumes below 2.5 ( λ / n ) 3 . For practical implementations, we propose embedding these structures in a low-refractive index polymer. An analysis of intentionally introduced variations in a rod diameter reveals which design directions should be followed in order to create cavities that are most robust for fabrication-induced variations.

Luminescence efficiency of Eu-related emission from Eu, O-codoped GaN (GaN:Eu, O) strongly depends on the local structure of Eu ions. Growth at relatively low temperature (∼960 °C) not only enables high Eu doping concentration but also elevates Eu-clustering due to its low diffusion coefficient, which results in formation of a large number of inefficient luminescent sites. We have studied the impact of post-growth thermal annealing at high temperatures on elimination of Eu clusters by photoluminescence measurements. These clarify that thermal annealing at high temperatures induces changes in the structural conformation and converts inefficient luminescent sites to efficient ones. As a result, the sample annealed at 1100 °C shows increased luminescence efficiency with a maximum of 5.1 times that of the as-grown sample. Post-growth thermal annealing offers a way to improve the efficiency of GaN:Eu, O further for practical application in III-nitride-based monolithic three-primary colors' light-emitting diodes.