Yiyu Ou

Surface plasmon coupling dynamics in InGaN/GaN quantum-well structures and radiative efficiency improvement

Surface plasmonics from metal nanoparticles have been demonstrated as an effective way of improving the performance of low-efficiency light emitters. However, reducing the inherent losses of the metal nanoparticles remains a challenge. Here we study the enhancement properties by Ag nanoparticles for InGaN/GaN quantum-well structures. By using a thin SiN dielectric layer between Ag and GaN we manage to modify and improve surface plasmon coupling effects, and we attribute this to the improved scattering of the nanoparticles at the quantum-well emission wavelength. The results are interpreted using numerical simulations, where absorption and scattering cross-sections are studied for different sized particles on GaN and GaN/SiN substrates.

Antireflective sub-wavelength structures for improvement of the extraction efficiency and color rendering index of monolithic white light-emitting diode.

We have theoretically investigated the influence of antireflective sub-wavelength structures on a monolithic white light-emitting diode (LED). The simulation is based on the rigorous coupled wave analysis (RCWA) algorithm, and both cylinder and moth-eye structures have been studied in the work. Our simulation results show that a moth-eye structure enhances the light extraction efficiency over the entire visible light range with an extraction efficiency enhancement of up to 26 %. Also for the first time to our best knowledge, the influence of sub-wavelength structures on both the color rendering index (CRI) and the correlated color temperature (CCT) of the monolithic white LED have been demonstrated. The CRI of the monolithic white LED could be improved from 92.68 to around 94 by applying a cylinder structure, and the CCT could be modified in a very large range with appropriate design of the cylinder structure.

Donor-acceptor-pair emission characterization in N-B doped fluorescent SiC

In the present work, we investigated donor-acceptor-pair emission in N-B doped fluorescent 6H-SiC, by means of photoluminescence, Raman spectroscopy, and angle-resolved photoluminescence. The photo ...

Fluorescent SiC as a new material for white LEDs

Current III–V-based white light-emitting diodes (LEDs) are available. However, their yellow phosphor converter is not efficient at high currents and includes rare-earth metals, which are becoming scarce. In this paper, we present the growth of a fluorescent silicon carbide material that is obtained by nitrogen and boron doping and that acts as a converter using a semiconductor. The luminescence is obtained at room temperature, and shows a broad luminescence band characteristic of donor-to-acceptor pair recombination. Photoluminescence intensities and carrier lifetimes reflect a sensitivity to nitrogen and boron concentrations. For an LED device, the growth needs to apply low-off-axis substrates. We show by ultra-high-resolution analytical transmission electron microscopy using aberration-corrected electrons that the growth mechanism can be stable and that there is a perfect epitaxial relation from the low-off-axis substrate and the doped layer even when there is step-bunching.

Broadband and omnidirectional light harvesting enhancement of fluorescent SiC

In the present work, antireflective sub-wavelength structures have been fabricated on fluorescent 6H-SiC to enhance the white light extraction efficiency by using the reactive-ion etching method. Broadband and omnidirectional antireflection characteristics show that 6H-SiC with antireflective sub-wavelength structures suppress the average surface reflection significantly from 20.5 % to 1.01 % over a wide spectral range of 390-784 nm. The luminescence intensity of the fluorescent 6H-SiC could be enhanced in the whole emission angle range. It maintains an enhancement larger than 91 % up to the incident angle of 70 degrees, while the largest enhancement of 115.4 % could be obtained at 16 degrees. The antireflective sub-wavelength structures on fluorescent 6H-SiC could also preserve the luminescence spectral profile at a large emission angle by eliminating the Fabry-Pérot microcavity interference effect.

Broadband Antireflection and Light Extraction Enhancement in Fluorescent SiC with Nanodome Structures

We demonstrate a time-efficient and low-cost approach to fabricate Si3N4 coated nanodome structures in fluorescent SiC. Nanosphere lithography is used as the nanopatterning method and SiC nanodome structures with Si3N4 coating are formed via dry etching and thin film deposition process. By using this method, a significant broadband surface antireflection and a considerable omnidirectional luminescence enhancement are obtained. The experimental observations are then supported by numerical simulations. It is believed that our fabrication method will be well suitable for large-scale production in the future.

Broadband light-extraction enhanced by arrays of whispering gallery resonators

We demonstrate a light-extraction approach using a whispering gallery resonators array. The wavelength-scale resonant dielectric nanospheres support whispering gallery modes, which can be coupled with the confined waveguide modes inside the bulk material, thus dramatically improving light extraction. Broadband light-extraction enhancement across the entire visible spectral range is achieved by exciting three low-order and low-quality-factor resonances. As an example, the broadband extraction enhancement of about 50% is obtained for the emission of fluorescent SiC at all the tested angles. The experimental results are supported by numerical simulations. Our light-extraction strategy could enable the manufacturing of high-throughput, nondestructive, and affordable optical coating in a variety of optical devices.

Internal quantum efficiency enhancement of GaInN/GaN quantum-well structures using Ag nanoparticles

We report internal quantum efficiency enhancement of thin p-GaN green quantum-well structure using self-assembled Ag nanoparticles. Temperature dependent photoluminescence measurements are conducted to determine the internal quantum efficiency. The impact of excitation power density on the enhancement factor is investigated. We obtain an internal quantum efficiency enhancement by a factor of 2.3 at 756 W/cm2, and a factor of 8.1 at 1 W/cm2. A Purcell enhancement up to a factor of 26 is estimated by fitting the experimental results to a theoretical model for the efficiency enhancement factor.

Omnidirectional luminescence enhancement of fluorescent SiC via pseudoperiodic antireflective subwavelength structures

In the present work, an approach of fabricating pseudoperiodic antireflective subwavelength structures (ARS) on fluorescent SiC by using self-assembled etch mask is demonstrated. By applying the pseudoperiodic (ARS), the average surface reflectance at 6° incidence over the spectral range of 390-785 nm is dramatically suppressed from 20.5% to 1.62%, and the hydrophobic surface with a large contact angle of 98° is also achieved. The angle-resolved photoluminescence study presents a considerable omnidirectional luminescence enhancement with an integral intensity enhancement of 66.3% and a fairly preserved spatial emission pattern.

Formation and characterization of varied size germanium nanocrystals by electron microscopy, Raman spectroscopy, and photoluminescence

Germanium nanocrystals are being extensively examined. Their unique optical properties (brought about by the quantum confinement effect) could potentially be applied in wide areas of nonlinear optics, light emission and solid state memory etc. In this paper, Ge nanocrystals embedded in a SiO2 matrix were formed by complementary metal-oxide-semiconductor compatible technology, e.g. plasma enhanced chemical vapour deposition and annealing. Different sizes of the Ge nanocrystals were prepared and analyzed by transmission electron microscopy with respect to their size, distribution and crystallization. The samples of different size Ge nanocrystals embedded in the SiO2 matrix were characterized by Raman spectroscopy and photoluminescence. Interplayed size and strain effect of Ge nanocystals was demonstrated by Raman spectroscopy after excluding the thermal effect with proper excitation laser power. It was clarified that two strong emission peaks at 3.19 eV and 4.40 eV are from the interface between Ge nanocrystals and SiO2 matrix.