Download Localized surface plasmon-enhanced near

Localized surface plasmon-enhanced nearultraviolet emission from InGaN/GaN lightemitting diodes using silver and platinum
nanoparticles
Sang-Hyun Hong,1 Chu-Young Cho,2 Sang-Jun Lee,2 Sang-Youp Yim,3 Wantae Lim,4
Sung-Tae Kim,4 and Seong-Ju Park1,2,*
1
Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju 500712, South Korea
School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712,
South Korea
3
Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, South
Korea
4
Samsung Electronics Co. LTD., Suwon 443-742, South Korea
*
[email protected]
2
Abstract: We demonstrate localized surface plasmon (LSP)-enhanced nearultraviolet light-emitting diodes (NUV-LEDs) using silver (Ag) and
platinum (Pt) nanoparticles (NPs). The optical output power of NUV-LEDs
with metal NPs is higher by 20.1% for NUV-LEDs with Ag NPs and 57.9%
for NUV-LEDs with Pt NPs at 20 mA than that of NUV-LEDs without
metal NPs. The time-resolved photoluminescence (TR-PL) spectra shows
that the decay times of NUV-LEDs with Ag and Pt NPs are faster than that
of NUV-LEDs without metal NPs. The TR-PL and absorbance spectra of
metal NPs indicate that the spontaneous emission rate is increased by
resonance coupling between excitons in the multiple quantum wells and
LSPs in the metal NPs.
©2013 Optical Society of America
OCIS codes: (230.0230) Optical devices; (230.3670) Light-emitting diodes; (240.6680) Surface
plasmons.
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Received 2 Aug 2012; revised 28 Dec 2012; accepted 25 Jan 2013; published 1 Feb 2013
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1. Introduction
Recently, near-ultraviolet (NUV) light-emitting diodes (LEDs) have been used in many
applications such as light sources for photometric detecting systems, biochemical sensors,
medical equipment, counterfeit bill detection, and pumping sources of white LEDs [1–5].
However, the internal quantum efficiency (IQE) of NUV-LEDs is much lower than those of
blue and green LEDs [1, 6], because the NUV-LEDs have a reduced potential confinement of
carriers due to the low indium (In) composition in InGaN quantum wells (QWs) compared
with blue and green LEDs. Much research has been performed to improve the IQE of NUVLEDs by improving crystal quality [7–10]. However, it is still necessary to investigate means
to enhance the IQE for high efficiency NUV-LEDs.
It was reported that the efficiency of GaN-based blue and green LEDs can be enhanced by
using localized surface plasmon (LSP) resonance in the visible region [11, 12]. Surface
plasmons (SPs) are collective oscillations of free electrons in metals at the interfaces between
metals and dielectric materials [13, 14]. In particular, the collective oscillations of electrons in
metal nanoparticles (NPs) embedded in a dielectric material are LSPs. In LEDs with NPs,
excitons in the QW can be affected by a resonant coupling to SP modes within the penetration
depth of the SP fringing field. The spontaneous emission rate in QW can be enhanced by the
influence of the local electric field from the SP mode [15]. Silver (Ag) and platinum (Pt) NPs
have been reported to have SP characteristic wavelengths for blue and ultraviolet emission by
photoluminescence (PL) and absorption measurements [11, 16]. However, investigations on
the coupling between NUV emission of NUV-LEDs and LSPs in metal NPs have not been
reported even though these are expected to improve the IQE of NUV-LEDs. In this study, we
successfully demonstrated the LSP-enhanced NUV-LEDs using Ag and Pt NPs embedded in
p-GaN layer. The NUV-LEDs with p-GaN containing either Ag or Pt showed good electrical
properties and the increased optical output power. These results were attributed to an increase
in IQE by LSPs resonance coupling between excitons in the multiple quantum wells (MQWs)
and metal NPs in the p-GaN layer. Pt NPs showed more advantages over Ag NPs with respect
to the particle density and the SP characteristic peak. The high density of Pt NPs, which can
increase the penetration depth, could be easily achieved and a closer match between the SP
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characteristic wavelength of Pt NPs and an emission wavelength of NUV-LEDs was highly
desirable for high efficiency NUV-LEDs.
2. Experiments
The NUV-LEDs were grown on a c-plane (0001) sapphire by metal-organic chemical vapor
deposition (MOCVD). Figure 1(a) shows a schematic diagram of LSPs-enhanced NUV-LEDs
with metal NPs. After the growth of a 25 nm-thick GaN nucleation layer at 550 °C, a 2 μmthick undoped GaN layer and a 2 μm-thick Si-doped n-GaN layer were grown at 1020 °C.
Then, five periods of InGaN/GaN NUV-MQWs consisting of 3 nm-undoped InGaN wells and
12 nm-Si doped GaN barriers were grown at 770 °C, followed by a 20 nm-thick p-GaN layer
grown at 950 °C as a spacer layer between MQWs and metal NPs. To estimate the effective
thickness of p-GaN spacer layer between MQWs and metal NPs layer, the penetration depth
of the SP fringing field was first calculated from Z = (λ/2π)[(ε´GaN - ε´metal)/ε´metal2]1/2, where
ε´GaN and ε´metal are the real part of the dielectric constants for GaN and metal, and λ is the
wavelength of the SP resonance [13]. The penetration depth of SP fringing field for Ag and Pt
was calculated as 41.0 and 38.9 nm for SP-QW coupling, respectively, at an NUV emission
wavelength of 406 nm when the real parts of the dielectric constants for GaN and metal were
used [17, 18]. However, Jang et al. showed that the penetration depth of Ag NPs surrounded
by dielectric materials is estimated to be in the range of 8-18 nm with increasing the number
of Ag NPs from single NP to 7 NPs [19]. In this study, the NUV-LEDs with metal NPs and a
p-GaN spacer layer with a thickness of less than 10 nm showed a poor electrical performance
presumably due to the rapid thermal annealing (RTA) process for metal NP formation and the
subsequent overgrowth of 200 nm-thick p-GaN on metal NPs. To study the resonance
coupling effect of LSPs and MQWs of NUV-LEDs with stable operation, a 20 nm-thick pGaN was used as a spacer layer in this study. The Ag or Pt layer was deposited on the p-GaN
spacer layer by e-beam evaporation and metal NPs were formed by RTA process. A p-GaN
layer was overgrown on the metal NPs and metal NPs were embedded in the 200 nm-thick pGaN layer. To fabricate NUV-LEDs, a dry etching process was performed until the n-GaN
layer was exposed for n-type ohmic contact by an inductively-coupled plasma etching process
using Cl2/CH4/H2/Ar source gases. Then, the NUV-LEDs, each of size 300 × 300 µm2, were
fabricated by using 150 nm-thick indium tin oxide as a transparent current spreading layer
and Cr (50 nm)/Au (150 nm) layers were deposited as n- and p-contact by e-beam
evaporation. Figure 1(b) and 1(c) show scanning electron microscopy (SEM) images of metal
NPs on the p-GaN spacer layer after the RTA process, performed at 600 °C for 5 min under
an N2 ambient gas prior to the over-growth of the p-GaN layer on the metal NPs. The thin
metal layer was transformed into NPs via the Ostwald ripening process. The diameter and
density of Ag NPs are in the range of 35-75 nm and 2 × 109 cm−2, respectively, whereas those
of Pt NPs are in the range of 10-40 nm and 4 × 1010 cm−2, respectively. The density of Pt NPs
was always higher by ~20 times than Ag NPs on p-GaN layer under the same deposition and
annealing conditions.
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Fig. 1. (a) Schematic diagram of the LSP-enhanced NUV-LEDs with metal NPs in p-GaN
layer. SEM images of (b) Ag NPs and (c) Pt NPs on the 20 nm-thick p-GaN spacer layer after
a rapid thermal annealing process.
3. Results and discussion
Figure 2(a), 2(b), and 2(c) show the PL spectra at 10 K and 300 K of NUV-LEDs with and
without metal NPs, respectively. The IQE of NUV-LEDs was estimated by comparing the
integrated PL intensities, assuming that the IQE is 100% at 10 K [13]. The IQEs of NUVLEDs with Ag, Pt, and without metal NPs are calculated to be 52.2%, 60.5% and 41.8%,
respectively, and the IQEs of NUV-LEDs with Ag and Pt NPs are increased by 24.9% and
44.7%, respectively, compared with that of NUV-LEDs without metal NPs. It has been
reported that the probability of coupling between the excitons in MQWs and LSPs increases
with increasing density of metal NPs in the p-GaN layer [20]. The fact that the enhancement
in IQE of NUV-LEDs with Pt NPs is higher than that of NUV-LEDs with Ag NPs is
attributed partly to the particle density of Pt which is higher than that of Ag NPs. Figure 2(d)
shows the PL enhancement ratio of the PL intensity of NUV-LEDs with metal NPs to that of
NUV-LEDs without metal NPs. The inset of Fig. 2(d) shows the absorbance of metal NPs as
a function of wavelength. As shown in the inset of Fig. 2(d), the peak position of absorbance
of Pt NPs is closer to a 406 nm emission wavelength of NUV-LEDs compared with that of
Ag NPs. The absorbance spectra of Pt NPs is much broader than that of Ag NPs due to the
different nature of SPs in Pt and Ag nanostructures [21] and high density of Pt NPs. The high
density of Pt NPs can also cause a large penetration depth in MQWs, resulting in a strong SP
resonance with excitons in MQWs. For this reason, the PL enhancement ratio of NUV-LEDs
with Pt NPs is higher than that of NUV-LEDs with Ag NPs as shown in Fig. 2(d) because the
peak position of absorbance of Pt NPs, which represents the SP characteristic wavelength of
Pt NPs is closer to the 406 nm emission wavelength of NUV-LEDs and the density of Pt NPs
is higher than that of Ag NPs.
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Fig. 2. PL spectra at 10 K and 300 K of the NUV-LEDs; (a) without metal NPs, (b) with Ag
NPs, and (c) with Pt NPs. (d) PL enhancement ratio of PL intensity of the NUV-LEDs with Ag
and Pt NPs to that of the NUV-LED without metal NPs. The inset shows the absorbance of Ag
and Pt NPs as a function of wavelength.
Figure 3 shows time-resolved PL (TR-PL) spectra of NUV-LEDs with and without metal
NPs. The light source was a Ti:sapphire laser (MaiTai, Spectra Physics) with 100 fs pulse
width, 710 nm wavelength, and repetition rate of 80 MHz, which was further reduced to 4.44
MHz using a pulse picker to obtain a temporal window as long as 200 ns. The beam was
frequency doubled to 355 nm by using a β-BaB2O4 (BBO) crystal. The collected PL was
dispersed by a 30-cm monochromator and detected by a streak scope (C10627, Hamamatsu
K.K.) to obtain PL decay curves. The TR-PL measurement was performed at 10 K to remove
non-radiative recombination process that competes with the LSPs coupling. The decay
lifetime τ is defined such that the ideal radiative decay curve can be shown as I(t) = Ae-t/τ,
where A is the normalization constant. The fast part of each decay curve is fitted with a
monoexponential decay model to investigate the resonance coupling between excitons in the
MQWs and LSPs of the metal NPs. The decay time of NUV-LEDs with Ag and Pt NPs is
3.77 ns and 3.68 ns, respectively, whereas that of NUV-LEDs without metal NPs is 4.26 ns.
The faster decay time of NUV-LEDs with metal NPs can be attributed to resonance coupling
between MQWs and LSPs. When the energy of the excitons in QW is close to the electron
vibrational energy of the LSP in the metal NPs, the recombination rate of excitons can be
increased via fast recombination path due to the energy transfer from excitons to LSPs [13,
14].
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Fig. 3. Time resolved-PL spectra of the NUV-LEDs with and without metal NPs at 10 K. (a)
without metal NPs, (b) with Ag NPs, and (c) with Pt NPs.Sample figure.
Figure 4(a) shows the current-voltage (I-V) curves of NUV-LEDs with and without metal
NPs. The forward voltages of NUV-LEDs with and without metal NPs are nearly the same as
3.9 V at 20 mA. Additionally, the series resistance of NUV-LEDs with Ag, Pt NPs, and
without metal NPs is 16.1 Ω, 16.0 Ω, and 16.1 Ω, respectively. These results indicate that the
electrical properties of NUV-LEDs with metal NPs are not degraded by the metal NPs
embedded in p-GaN.
Figure 4(b) shows the optical output power of the NUV-LEDs with and without metal
NPs as a function of injection current. The optical output power of the NUV-LEDs was
measured at a distance of 5 cm from the top of the NUV-LEDs by Si photodiode. As shown
in Fig. 4(b), the optical output power of the NUV-LEDs with metal NPs is higher than that of
the NUV-LEDs without metal NPs. Compared with NUV-LEDs without metal NPs, the
optical output power of NUV-LEDs with Ag and Pt NPs is increased by 20.1% and 57.9% at
an injection current of 20 mA, respectively. The measurements of IQE and TR-PL of the
NUV-LEDs show that the enhancement of optical output power is attributed to the increase in
the spontaneous emission rate by resonance coupling between excitons in the MQWs and
LSPs in the metal NPs.
Fig. 4. (a) I-V curves of the NUV-LEDs with and without metal NPs. (b) Optical output power
of the NUV-LEDs with and without metal NPs as a function of injection current.
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4. Summary
In summary, we investigated the LSP-enhanced NUV-LEDs by using Ag or Pt NPs
embedded in p-GaN layer. The optical output power of NUV-LEDs with metal NPs is
increased by 20.1% for NUV-LEDs with Ag NPs and 57.9% for NUV-LEDs with Pt NPs at
20 mA compared with that of NUV-LEDs without metal NPs. The IQE of NUV-LEDs with
metal NPs is increased by 24.9% for NUV-LEDs with Ag NPs and 44.7% for NUV-LEDs
with Pt NPs compared with that of NUV-LEDs without metal NPs. The TR-PL measurement
shows the PL decay time is decreased in NUV-LEDs with metal NPs embedded in p-GaN
layer. These results indicate that the increase in optical output power of the NUV-LEDs with
metal NPs is attributed to an increase in IQE by LSPs resonance coupling between excitons in
the MQWs and metal NPs in the p-GaN layer. In addition, Pt NPs are found to be more
suitable for LSP-enhanced NUV-LEDs due to the easy formation of high density NPs and a
closer match between the SP characteristic wavelength of Pt NPs and an emission wavelength
(406 nm) of NUV-LEDs, compared to Ag NPs.
Acknowledgments
This work was supported by Korea Science and Engineering Foundation (KOSEF) NCRC
grant funded by the Korea government (MEST) (Project No. R15-2008-006-02001-0), the
Samsung LED Research Center at GIST, and partially supported by APRI-Research Program
through a grant provided by GIST.
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Received 2 Aug 2012; revised 28 Dec 2012; accepted 25 Jan 2013; published 1 Feb 2013
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