Pei-Ying Shih

Efficiency improvement of a vertical light-emitting diode through surface plasmon coupling and grating scattering

The enhancement of output intensity, the generation of polarized output, and the reduction of the efficiency droop effect in a surface plasmon (SP) coupled vertical light-emitting diode (LED) with an Ag nano-grating structure located between the p-GaN layer and the wafer bonding metal for inducing SP coupling with the InGaN/GaN quantum wells (QWs) are demonstrated. In fabricating the vertical LED, the patterned sapphire substrate is removed with a photoelectrochemical liftoff technique. Based on the reflection measurement from the metal grating structure and the numerical simulation result, it is found that the localized surface plasmon (LSP) resonance induced around the metal grating crest plays the major role in the SP-QW coupling process although a hybrid mode of LSP and surface plasmon polariton can be generated in the coupling process. By adding a surface grating structure to the SP-coupled vertical LED on the n-GaN side, the output intensity is further enhanced, the output polarization ratio is further increased, and the efficiency droop effect is further suppressed.

Further reduction of efficiency droop effect by adding a lower-index dielectric interlayer in a surface plasmon coupled blue light-emitting diode with surface metal nanoparticles

Further reduction of the efficiency droop effect and further enhancements of internal quantum efficiency (IQE) and output intensity of a surface plasmon coupled, blue-emitting light-emitting diode (LED) by inserting a dielectric interlayer (DI) of a lower refractive index between p-GaN and surface Ag nanoparticles are demonstrated. The insertion of a DI leads to a blue shift of the localized surface plasmon (LSP) resonance spectrum and increases the LSP coupling strength at the quantum well emitting wavelength in the blue range. With SiO2 as the DI, a thinner DI leads to a stronger LSP coupling effect, when compared with the case of a thicker DI. By using GaZnO, which is a dielectric in the optical range and a good conductor under direct-current operation, as the DI, the LSP coupling results in the highest IQE, highest LED output intensity, and weakest droop effect.

Regularly patterned non-polar InGaN/GaN quantum-well nanorod light-emitting diode array

The growth and process of a regularly patterned nanorod (NR)- light-emitting diode (LED) array with its emission from sidewall non-polar quantum wells (QWs) are demonstrated. A pyramidal un-doped GaN structure is intentionally formed at the NR top for minimizing the current flow through this portion of the NR such that the injection current can be effectively guided to the sidewall m-plane InGaN/GaN QWs for emission excitation by a conformal transparent conductor (GaZnO). The injected current density at a given applied voltage of the NR LED device is similar to that of a planar c-plane or m-plane LED. The blue-shift trend of NR LED output spectrum with increasing injection current is caused by the non-uniform distributions of QW width and indium content along the height on a sidewall. The photoluminescence spectral shift under reversed bias confirms that the emission of the fabricated NR LED comes from non-polar QWs.

Dependencies of the emission behavior and quantum well structure of a regularly-patterned, InGaN/GaN quantum-well nanorod array on growth condition

To achieve green emission from the sidewall non-polar quantum wells (QWs) of a GaN nanorod (NR) light-emitting diode, regularly patterned InGaN/GaN QW NR arrays are grown under various growth conditions of indium supply rate, QW growth temperature, and QW growth time for comparing their emission wavelength variations of the top-face c-plane and sidewall m-plane QWs based on photoluminescence and cathodoluminescence (CL) measurements. Although the variation trends of QW emission wavelength by changing those growth conditions in the NR structure are similar to those in the planar structure, the emission wavelength range of the QWs on an NR is significantly shorter than that in a planar structure under the same growth conditions. Under the growth conditions for a longer NR QW emission wavelength, the difference of emission wavelength between the top-face and sidewall QWs is smaller. Also, the variation range of the emission wavelength from the sidewall QWs over different heights on the sidewall becomes larger. On the other hand, strain state analysis based on transmission electron microscopy is undertaken to calibrate the average QW widths and average indium contents in the two groups of QW of an NR. The variation trends of the calibrated QW widths and indium contents are consistent with those of the CL emission wavelengths from various portions of NR QWs.

Behaviors of Surface Plasmon Coupled Light-Emitting Diodes Induced by Surface Ag Nanoparticles on Dielectric Interlayers

The enhanced surface plasmon (SP) coupling effects in a blue light-emitting diode (LED) with regularly patterned (REG) surface Ag nanoparticles (NPs) on a dielectric interlayer (DI) of a lower refractive index overgrown on p-GaN are demonstrated. Without a DI, the surface Ag NPs-induced SP coupling with the quantum wells (QWs) in the LED can lead to the increases of internal quantum efficiency and LED output intensity, the reduction of the external quantum efficiency droop effect, and the enhancement of modulation response. By adding a DI, the SP coupling effect is enhanced, resulting in the further improvements of all the aforementioned factors. We compare the SP coupling effects in the LEDs with REG Ag NPs on DIs to those of randomly distributed (RAN) Ag NPs previously reported. Although the variation trends of the localized surface plasmon (LSP) resonance peaks and hence the SP coupling behaviors of REG and RAN Ag NPs are similar, their LSP resonance strengths at the QW emission wavelength are different due to their different spectral patterns of LSP resonance. In other words, although the REG Ag NPs can produce stronger collective LSP resonance with a narrower spectral width, the SP coupling effect depends mainly on the LSP resonance strength at the QW emission wavelength.

Surface plasmon coupled light-emitting diode: Experimental and numerical studies

First, the experimental implementations and theoretical/numerical investigations of surface plasmon (SP) coupled InGaN/GaN quantum-well light-emitting diodes (LEDs) are reviewed. If the p-GaN layer in an LED can be thin, surface metal nanoparticle (NP) is an inexpensive structure for inducing effective SP coupling. When the p-GaN layer is thick, a few metal structures, including metal protrusion, buried metal NP, and embedded metal NP, can be used for effective SP coupling. In the numerical study, an algorithm, including the feedback effect of the induced SP resonance on the radiating behavior of the source dipole, has been proposed for studying the SP coupling effects with an embedded metal NP, a surface metal NP, and a metal protrusion. Then, the theoretical formulations and numerical algorithms for evaluating the radiated power enhancement in the coupling process between two radiating dipoles and the localized surface plasmon (LSP) induced on a nearby Ag NP are built. Three mechanisms are considered in the coupling process for radiated power enhancement, including the interference of the two phase-retarded radiation contributions from the two dipoles, the interaction between the two dipoles, and the LSP resonant coupling.

Independent variations of applied voltage and injection current for controlling the quantum-confined Stark effect in an InGaN/GaN quantum-well light-emitting diode.

A reverse-biased voltage is applied to either device in the vertical configuration of two light-emitting diodes (LEDs) grown on patterned and flat Si (110) substrates with weak and strong quantum-confined Stark effects (QCSEs), respectively, in the InGaN/GaN quantum wells for independently controlling the applied voltage across and the injection current into the p-i-n junction in the lateral configuration of LED operation. The results show that more carrier supply is needed in the LED of weaker QCSE to produce a carrier screening effect for balancing the potential tilt in increasing the forward-biased voltage, when compared with the LED of stronger QCSE. The small spectral shift range in increasing injection current in the LED of weaker QCSE is attributed not only to the weaker QCSE, but also to its smaller device resistance such that a given increment of applied voltage leads to a larger increment of injection current. From a viewpoint of practical application in LED operation, by applying a reverse-biased voltage in the vertical configuration, the applied voltage and injection current in the lateral configuration can be independently controlled by adjusting the vertical voltage for keeping the emission spectral peak fixed.

Strain reduction and crystal improvement of an InGaN/GaN quantum-well light-emitting diode on patterned Si (110) substrate

The comparisons of the morphology, material property, and optical characteristics of a crack-free InGaN/GaN quantum-well light-emitting diode (LED) structure of completely coalesced overgrowth on an a-axis-oriented one-dimensional trench-patterned Si (110) substrate with the other two samples grown on flat Si (110) and Si (111) substrates are demonstrated. This sample on patterned Si (110) substrate shows the highest crystal quality, weakest tensile strain, largest internal quantum efficiency, strongest LED output intensity, lowest device resistance, and smallest spectral shift range in increasing injection current. The small spectral shift range indicates the weak quantum-confined Stark effect. The advantages of this sample are attributed to the small lattice mismatch between Si and GaN along the m-axis, the reduced thermal stress along the m-axis, and the minimized upward-propagating dislocation density.

Localized Surface Plasmon Coupled Light-Emitting Diodes With Buried and Surface Ag Nanoparticles

Two sets of light-emitting diodes (LEDs) based on two epitaxial structures of different p-GaN layer thicknesses for demonstrating the effects of localized surface plasmon (LSP) coupling with the quantum wells (QWs) in the LEDs are compared. In the first set based on the epitaxial structure of thick p-GaN, to reduce the distance between the Ag nanoparticles (NPs) and QWs for increasing the LSP coupling strength, Ag NPs are filled into a hole array fabricated on the p-GaN layer. In the second set based on the epitaxial structure of thin p-GaN, Ag NPs are fabricated on the top surface of the p-GaN layer. The LSP-coupled LEDs show the significant enhancements of internal quantum efficiency and LED output intensity even though the coverage of the transparent conductor, GaZnO, red-shifts the LSP resonance peak such that the LSP resonance at the QW emission wavelength becomes weaker.

Void Structures in Regularly Patterned ZnO Nanorods Grown with the Hydrothermal Method

The void structures and related optical properties after thermal annealing with ambient oxygen in regularly patterned ZnO nanrorod (NR) arrays grown with the hydrothermal method are studied. In increasing the thermal annealing temperature, void distribution starts from the bottom and extends to the top of an NR in the vertical (c-axis) growth region. When the annealing temperature is higher than 400°C, void distribution spreads into the lateral (m-axis) growth region. Photoluminescence measurement shows that the ZnO band-edge emission, in contrast to defect emission in the yellow-red range, is the strongest under the n-ZnO NR process conditions of 0.003 M in Ga-doping concentration and 300°C in thermal annealing temperature with ambient oxygen. Energy dispersive X-ray spectroscopy data indicate that the concentration of hydroxyl groups in the vertical growth region is significantly higher than that in the lateral growth region. During thermal annealing, hydroxyl groups are desorbed from the NR leaving anion vacancies for reacting with cation vacancies to form voids.