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Enhanced light emission from AlGaN/GaN multiple

quantum wells using the localized surface

plasmon effect by aluminum nanoring patterns

KYUNG ROCK SON,

BYEONG RYONG LEE,

MIN HO JANG,

HYUN CHUL PARK,

YONG HOON CHO,

AND

TAE GEUN KIM

School of Electrical Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul 02841, South Korea

Department of Physics and KI for the NanoCentury, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea

*Corresponding author: tgkim1@korea.ac.kr

Received 12 September 2017; revised 11 November 2017; accepted 11 November 2017; posted 15 November 2017 (Doc. ID 305263);

published 14 December 2017

We investigate the localized surface plasmon (LSP) effect by Al nanorings on the AlGaN/GaN multiple quantum

well (MQW) structure emitting at 365 nm. For this experimen t, first, the size of Al nanorings is optimized to

maximize the energy transfer (or coupling) between the LSP and MQW using the silica nanospheres. Then, the Al

nanorings with an outer diameter of 385 nm, which exhibit a strong absorption peak in the near-ultraviolet

region, are applied to the top surface of the AlGaN/GaN MQW. The photoluminescence (PL) intensity of

the MQW structure with Al nanorings increased by 227% at 365 nm compared to that without Al nanorings.

This improvement is mainly attributed to an enhanced radiative recombination rate in the MQWs through the

energy-matched LSPs by the temperature-dependent PL and time-resolved PL analyses. The radiative lifetime was

about two times shorter than that of the structure without Al nanorings at room temperature. In addition, the

measured PL efficiency at room temperature of the structure with Al nanorings was 33%, while that of the struc-

ture wit hout Al nanorings was 19%, implying that LSP-QW coupling together with the nanoring array pattern

itself played important roles in the enhancement.

OCIS codes: (240.6680) Surface plasmons; (230.3670) Light-emitting diodes; (250.5230) Photoluminescence.

https://doi.org/10.1364/PRJ.6.000030

1. INTRODUCTION

Ultraviolet (UV) light-emitting diodes (LEDs) have been used

in several application s such as chemical sensors for the detection

of biohazardous and heavy metals, optical communication, and

water purification systems [1–5]. However, the external quan-

tum efficiency (EQE) of UV LEDs is still much lower than that

of blue and green LEDs. This is because UV LEDs have

AlGaN layers in their active layers, which are strongly influ-

enced by the nonradiative recombination process via the high

density of threading dislocations and polarization fields related

to the quantum-confined stark effect (QCSE). These effects

lower the internal quantum efficiency (IQE) of UV LEDs

and eventually result in low EQEs [6–8]. Thus, to fundamen-

tally improve the EQE of UV LEDs, it is important to enhance

the IQE in addition to the light extraction efficiency of

UV LEDs.

The study of improving the IQE of UV LEDs is closely

related to the improvement of crystal quality by reducing

the polarization charges existing in the epitaxial layer structure

[i.e., via control of the QCSE in quantum wells (QWs) or

reduction of lattice mismatch using GaN bulk substrates]

[9–11]. Through these efforts, the IQE of deep-UV LEDs

has improved up to 80% or more at room temperature [9].

However, the maximum EQE of UV LEDs is still low—for

example, approximate ly 30% [4] for 365 nm near-UV LEDs

and 20% [12] for 275 nm deep-UV LEDs.

Many research groups have investigated various methods

that can enhance both the IQE and EQE simultaneously using

localized surface plasmons (LSPs) [13–18] and/or embedded

photonic crystals [19–21] using various shapes of metal nano-

particles or nanostructures. Hong et al. reported the enhance-

ment of light emission and IQE of near-UV LEDs by using

colloidal silver nanoparticles on a p-GaN spacer layer to in-

duce the LSPs [15]. Huang et al. showed a significant increase

in the photoluminescence (PL) of AlGaN-based multiple QW

(MQW) LEDs using cube-shaped Al nanoparticles when

the light waves are coupled to the LSPs [16]. Yin et al. also

reported the enhancement of deep-UV emission by introduc-

ing the energy-matched coupling of LSPs from Al-based tri-

angular prism nanostructures with hot excitons in QWs [17].

Vol. 6, No. 1 / January 2018 / Photonics Research

Research Article

On the other hand, among the studies on nanostructure pat-

terns for LSPs, nanoring patterns have been reported to gen-

erate high local electric fields from LSP modes intensively

coupled between the inner and outer surface, due to resonant

excitation of its dominant dipole-active plasmon resonance and

uniformly distributed field in the cavity inside the ring. Also,

these structures create two plasmonic modes (a low-energy

“bonding” mode and high-energy “anti-bonding” mode), re-

sulting in a high degree of tunability [22,23].

In this study, we investigate the effect of an Al nanoring

structure on the light extraction efficiency from AlGaN/

GaN MQWs of 365 nm LEDs through the LSP coupling ef-

fect. Note that the SP energy of Al ranges from 3.4 to 4 eV

(310–365 nm) [24]. First, we investigate the LSP resonance

absorption spectra for different size of Al nanorings. Then,

we chose the appropriate Al nanoring size that could interact

with the light emitted from the 365 nm LEDs and confirmed

that the PL intensity was increased. In addition, temperature-

dependent PL (TDPL) and time-resolv ed PL measurements

were conducted to investigate the carrier dynamics of the

recombination mechanism. These systematic analyses, includ-

ing the PL efficiency and carrier lifetime, indicate that the

improvement in IQE can be attributed to the LSP-QW cou-

pling and the nanoring pattern form ed at the sample surface.

2. EXPERIMENTAL DETAILS

A. Methods of Characterization

The morphology of the Al nanoring array was characterized

using scanning electron microscopy (SEM, Hitachi, s-4300).

The morphology and height distribution of the Al nanoring

array were measured by atomic force microscopy (AFM,

Park XE 15). Cross-sectional images of the AlGaN/GaN

MQW structure and Al nanorings were obtained using a

high-angle annular dark-field (HAADF) scanning transmission

electron microscope (STEM) with energy dispersive X-ray spec-

troscopy (EDS) analysis at an accelerating voltage of 200 kV

(FEI, Talos F200X). The absorbance spectra of the Al nanoring

array were measured using a UV-visible spectrophotometer

(Perkin Elmer, Lambda 35). PL measurements were performed

using a pulse laser operating at a wavelength of 266 nm with an

average output power of 10 mW. Time-resolved PL measure-

ments were performed using a mode-locked femtosecond

pulsed Ti:sapphire laser (Coherent, Cham eleon Ultra II) and

a streak camera detector (Hamamatsu, C7700-01). The exci-

tation source at 266 nm was prepared by the third-harmonic-

generation method, and the width and repetition of the laser

were 150 fs and 4 MHz, respectively. We used a cryostat to

control the temperature between 12 and 300 K.

B. Fabrication of 365 nm AlGaN/GaN LEDs with Al

Nanorings

The light-emitting structure was grown via metal organic

chemical vapor deposition on c-plane sapphire substrates. After

the growth of a 50 nm thick AlN buffer layer and 1000 nm

thick undoped Al

0.1

0.9

N layer on the sapphire substrates,

a 2000 nm thick n-type Al

0.1

0.9

N layer was grown. An

AlGaN MQW array, which consisted of four periods of

2.5 nm thick, undoped GaN wells and 10 nm thick n-type

0.1

0.9

N barriers, was subsequently grown. Finally, a

10 nm thick undoped-Al

0.1

0.9

N EBL was grown and

covered with a 20 nm thick p-type Al

0.2

0.8

N layer and

150 nm thick p-type Al

0.1

0.9

N capping layer. The fringing

electric field of LSPs has a limiting distance, which is Z

(penetration depth) and can be given by

Z  λ∕2πε

− ε

∕ε



1∕2

; (1)

where ε

and ε

represent the real part of the dielectric constant

of the semiconductor and metal, respectively [13]. In our case,

Z  16 nm was calculated at a wavelength of 365 nm for Al

(ε

 −19) on the GaN layer (ε

 8.9). Thus, we removed all

p-type layers on the EBL layer using the inductively coupled

plasma (ICP) reactive-ion etching (RIE) process. In order to

fabricate the Al nanorings, the nanosphere lithography

(NSL) method was used. The NSL method has the advantages

of being inexpensive and a simple technique for producing size-

tunable metal nanoparticles with controllable plasmon resonan-

ces in the deep-UV to infrared region [25,26]. Figure 1(a)

shows a flow chart illustrating the Al nanoring array fabrication

process on the near-UV LED AlGaN/GaN MQW structure

using the NSL method.

Initially, we synthesized SiO

nanospheres with diameters of

300 nm using tetraethyl orthosilicate, ethanol (C

OH),

ammonia (NH

), and deionized water by the solgel method

at room temperature. Then, we arranged the SiO

nanosphere

monolayer on top of the MQWs by using the spin coating

method. After spin coating, a 25 nm thick Al film was depos-

ited on the prepared wafer with the SiO

nanospheres using the

radio-frequency sputtering technique. Next, the wafer was

exposed to Cl

and Ar etching gas by using the ICP-RIE proc-

ess. Finally, the exposed SiO

nanospheres were etched under

and SF

etching gas by using the RIE process, resulting in

the Al nanoring patterns.

C. Finite-Difference Time-Domain Simulation

Three-dimensional finite-difference time-domain (3D FDTD)

simulations using the FullWAVE software package (Synopsys

and Rsoft Group, Inc.) were performed to obtain the absorb-

ance spectrum and near-electromagnetic field distribution of

the Al nanoring array to analyze the plasmonic modes by

LSP resonance. The structure for the simulation consisted of

an Al nanoring with a width of 65 nm, height of 60 nm,

and outer diameter of 385 nm with a hexagonal close-packed

(HCP) unit cell. A pulse-mode source with TE and TM waves

was used to obtain the wavelength-resolved response for the Al

nanoring structure.

3. RESULTS AND DISCUSSION

Figures 1(b)–1(d) show SEM top-view images of the Al nanor-

ing single layers with the HCP structure, fabricated by the NSL

and subsequent etching process using SiO

nanospheres. In this

process, some of the top layers were etched unintentionally

with SiO

nanospheres. This might be able to influence the

light extraction efficiency from MQWs. The Al nanorings were

prepared uniformly and closely with average diameters of 165,

385, and 520 nm. Figures 1(e)– 1(g) present the absorbance

spectra of the experimental results. A blueshift of the corre-

sponding peak is observed as the size of the Al nanorings

Research Article

Vol. 6, No. 1 / January 2018 / Photonics Research 31

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