Yulong Feng

Fabrication and Effects of Ag Nanoparticles Hexagonal Arrays in Green LEDs by Nanoimprint

In this letter, the Ag nanoparticles (NPs), which are located inside the hexagonal photonic crystals (PhCs) array holes, are successfully fabricated in green light-emitting diode (LED) by nanoimprint and lift-off techniques. The photoluminescence intensity of the green LED is increased by 4.5 folds compared with that of the bare LED due to the PhCs effect and the localized surface plasmon (LSP) multiple quantum wells coupling effect, which is further confirmed by the enhanced decay rate of LSP-functioned LED. In the simulation of 3-D finite difference time domain, it reveals that the morphology of Ag NP will affect the LSP resonant strength and the light scattering efficiency besides the periodic structure.

Study on Light Extraction from GaN-based Green Light-Emitting Diodes Using Anodic Aluminum Oxide Pattern and Nanoimprint Lithography.

An anodic aluminum oxide (AAO) patterned sapphire substrate, with the lattice constant of 520 ± 40 nm, pore dimension of 375 ± 50 nm, and height of 450 ± 25 nm was firstly used as a nanoimprint lithography (NIL) stamp and imprinted onto the surface of the green light-emitting diode (LED). A significant light extraction efficiency (LEE) was improved by 116% in comparison to that of the planar LED. A uniform broad protrusion in the central area and some sharp lobes were also obtained in the angular resolution photoluminescence (ARPL) for the AAO patterned LED. The mechanism of the enhancement was correlated to the fluctuations of the lattice constant and domain orientation of the AAO-pattern, which enabled the extraction of more guided modes from the LED device.

Silane controlled three dimensional GaN growth and recovery stages on a cone-shape nanoscale patterned sapphire substrate by MOCVD

Three dimensional (3D) growth induced by silane was performed on cone-shape nano-scale patterned sapphire substrates (NPSS) by metal organic chemical vapor deposition (MOCVD). The growth evolution for the silane controlled 3D growth process and the recovery stage were investigated by a series of growth interruptions. The GaN epilayers grown on the templates with different 3D growth conditions were characterized by X-ray diffraction (XRD), Raman scattering, and atomic force microscopy (AFM) measurements. The full width at half maximums (FWHMs) of the (002) and (102) reflections in the XRD rocking curves were 267 and 324 arcsec, respectively, for the sample on NPSS with 600 s of 3D growth. An extremely smooth surface was achieved with an average roughness of 0.10 nm over 3 × 3 μm2. All the above data were superior to those for the planar sample or the NPSS ones without the optimized 3D growth time. The silane addition caused effective 3D growth. The size, homogeneity, and faceted sidewalls of the islands by the 3D growth led to a high crystalline quality, much strain relaxation and a specular surface for the GaN epilayers.

The Coupling Behavior of Multiple Dipoles and Localized Surface Plasmons in Ag Nanoparticles Array

In this work, the coupling behavior of multiple dipoles and localized surface plasmons (LSPs) in Ag nanoparticle arrays is explored based on experimental results and 3D finite difference time domain (FDTD) simulations. The Ag nanoparticles (NPs) located inside the hexagonal photonic crystal (PhC) array holes are embedded in a green light-emitting diode (LED), which enhances emission efficiency significantly. In the simulation of the 3D FDTD, five spaced x-polarized dipoles are approximated as five quantum wells. The internal quantum efficiency (IQE) and light extraction efficiency (LEE) of the LSP-coupled LED are deduced respectively from the original IQE of the bare LED and the FDTD simulation results. Besides, the dynamic LSP-dipole coupling behavior is also explored considering the interaction of the five dipoles and their feedback effect to LSP, which lead to the magnification of the LSP-dipole coupling enhancement effect and the reduction of energy dissipation in Ag NPs.

Capability of GaN based micro-light emitting diodes operated at an injection level of kA/cm2

Different size InGaN/GaN based micro-LEDs (μLEDs) are fabricated. An extremely high injection level above 16 kA/cm2 is achieved for 10 μm-diameter LED. The lateral current density and carrier distributions of the μLEDs are simulated by APSYS software. Streak camera time resolved photoluminescence (TRPL) results show clear evidence that the band-gap renormalization (BGR) effect is weakened by strain relaxation in smaller size μLEDs. BGR affects the relaxation of free carriers on the conduction band bottom in multiple quantum wells (MQWs), and then indirectly affects the recombination rate of carriers. An energy band model based on BGR effect is made to explain the high-injection-level phenomenon for μLEDs.

Study on the Coupling Mechanism of the Orthogonal Dipoles with Surface Plasmon in Green LED by Cathodoluminescence

We analyzed the coupling behavior between the localized surface plasmon (LSP) and quantum wells (QWs) using cathodoluminescence (CL) in a green light-emitting diodes (LED) with Ag nanoparticles (NPs) filled in photonic crystal (PhC) holes. Photoluminescence (PL) suppression and CL enhancement were obtained for the same green LED sample with the Ag NP array. Time-resolved PL (TRPL) results indicate strong coupling between the LSP and the QWs. Three-dimensional (3D) finite difference time domain (FDTD) simulation was performed using a three-body model consisting of two orthogonal dipoles and a single Ag NP. The LSP–QWs coupling effect was separated from the electron-beam (e-beam)–LSP–QW system by linear approximation. The energy dissipation was significantly reduced by the z-dipole introduction under the e-beam excitation. In this paper, the coupling mechanism is discussed and a novel emission structure is proposed.

Effect of dipole polarization orientation on surface plasmon coupling with green emitting quantum wells by cathodoluminescence

Ag nanoparticles (NPs) are fabricated on the cross-section of green emitting quantum wells (QWs). The effect of the dipole polarization orientation on the localized surface plasmon (LSP)-QW coupling can be studied by setting the incident direction of the electron beam parallel to the plane of the QWs. Cathodoluminescence (CL) measurements on the QWs show that the intensity with the Ag NPs is enhanced 6.1 times compared with that without the Ag NPs. Total energy loss profiles for an electron beam in the GaN and Ag NP are accurately simulated using a Monte Carlo program (CASINO). The orientations of the in-plane dipoles in the QWs can vary from 0° to 360°. Through a two-step simulation process using the three-dimensional (3D) finite difference time domain (FDTD) method, the weighted average of CL intensities are simulated for QWs with the Ag NPs. The simulation results agree well with the experimental results. Lastly, the dipole orientation dependent LSP-QW coupling process is discussed.