December 1, 2013 / Vol. 38, No.

23 / OPTICS LETTERS 5055

ITO/Ag/ITO multilayer-based transparent conductive

electrodes for ultraviolet light-emitting diodes
Jae Hoon Lee, Kie Young Woo, Kyeong Heon Kim, Hee-Dong Kim, and Tae Geun Kim*
School of Electrical Engineering, Korea University, Seoul 136-701, South Korea
*Corresponding author: tgkim1@korea.ac.kr
Received September 3, 2013; revised October 17, 2013; accepted October 24, 2013;
posted October 25, 2013 (Doc. ID 196923); published November 22, 2013
ITO/Ag/ITO (IAI) multilayer-based transparent conductive electrodes for ultraviolet light-emitting diodes are
fabricated by reactive sputtering, optimized by annealing, and characterized with respect to electrical and optical
properties. Increasing the annealing temperature from 300°C to 500°C decreased the sheet resistance and increased
the transmittance. This may result from an observed improvement in the crystallinity of the IAI multilayer and a
reduction in the near-UV absorption coefficient of Ag. We observed the lowest sheet resistance (9.21 Ω∕sq) and the
highest optical transmittance (88%) at 380 nm for the IAI multilayer samples annealed in N2 gas at 500°C. © 2013
Optical Society of America
OCIS codes: (230.3670) Light-emitting diodes; (310.4165) Multilayer design; (310.6860) Thin films, optical properties;
(310.7005) Transparent conductive coatings.
http://dx.doi.org/10.1364/OL.38.005055

GaN-based near-ultraviolet (UV) light-emitting diodes properties of IAI-based TCEs can be improved by varying
(LEDs) in the wavelength range 380–400 nm have appli- the thickness of the metal layer between the oxide layers.
cations in various fields such as analytical instrumenta- In a well-designed IAI multilayer, the large difference in
tion, water purification, medical treatments, biological the refractive index between ITO and Ag suppresses the
agent identification, and white LED displays for general reflection from the metal layer, which can then become
lighting [1–3]. However, unlike blue LEDs, there are few selectively transparent in the visible and UV regions.
transparent conductive electrodes (TCEs) suitable for Nevertheless, IAI-based TCEs in UV LEDs have not yet
near-UV LEDs. Using incompatible TCE materials de- been fully realized, because the metal tends to agglomer-
creases the external quantum efficiency (EQE) of UV ate during the annealing processes that are inevitable in
LEDs [4]. In particular, because of the high sheet resis- LED fabrication [15]. Therefore, to use IAI-based TCEs in
tance and low carrier mobility of the p-type thin GaN UV LEDs, a method must be found to allow IAI to sustain
layer over the whole range of emitted wavelengths, LEDs its properties after annealing.
made from this material exhibit current crowding and We propose TCEs based on IAI multilayers. Herein, we
low current spreading. Therefore, the p-type electrode demonstrate their high conductivity and high transmit-
must have low contact resistance on a p-GaN layer and tance, acceptable for UV LED applications. In addition,
high transmittance in the near-UV region [5]. we discuss the possible conduction mechanisms in the
Indium-tin oxide (ITO) is widely used as a TCE IAI-based TCEs as a function of the annealing tempera-
material in optoelectronic devices, including flat dis- ture, and the role of Ag in determining the transmission
plays, thin film transistors, and solar cells, because it properties of the films.
provides high transparency in the visible region and good The p-GaN layer of the 380 nm UV LEDs used in this
electrical conductivity [6–8]. However, ITO suffers from study was grown on c-face sapphire substrates by metal
several problems because of absorption in UV region. organic chemical vapor deposition (MOCVD). The UV LED
Several methods have been reported to reduce the light structure consisted of a 20-nm-thick GaN low-temperature
absorption by TCEs in UV LEDs. One method is to use a buffer layer, a 2.6-μm-thick undoped GaN layer, a 4.6-μm-
thin ITO layer, with only a few tens of nanometers [9]. thick n-type GaN layer, five AlGaInN/InGaN multiple quan-
Although this method improves the transmittance of the tum well (MQW) layers, a 30-nm-thick p-type Al0.2 Ga0.8 N
ITO, it abruptly increases the sheet resistance by reduc- layer, and a 95-nm-thick p-type GaN capping layer. Before
ing the thickness of the thin ITO films. Another approach the fabrication of the IAI multilayer, the surfaces of p-GaN
is to use carbon-based TCEs such as graphene and single- were ultrasonically degreased with acetone, methanol,
walled carbon nanotubes (SWNTs) [5,10]. The feasibility deionized (DI) water, and a mixture of sulfuric acid
of carbon-based TCEs with low sheet resistance and high (H2 SO4 ) and hydrogen peroxide (H2 O2 ) for 5 min in each
UV transmittance has been demonstrated [11]. However, step to remove the surface oxides formed on GaN. After
before carbon-based TCEs can be widely incorporated this treatment, a 15-nm-thick bottom ITO film was depos-
into UV LEDs, crucial manufacturing challenges such ited by RF magnetron sputtering onto the p-GaN surface at
as ensuring that the carbon nanomaterial film has high 5 mTorr pressure and 100 W. After deposition of the
quality and is uniform and processed in a manner com- bottom ITO film, the Ag interlayer with a thickness of
patible with the UV LED fabrication must be overcome. 7 nm was deposited by RF magnetron sputtering onto the
Recently, a continuous metal film has also exhibited ITO film at 2 mTorr pressure and 100 W. The top ITO film
high performance for the broadband light transparency was then deposited using the same conditions as for the
as TCEs [12,13]. On the other hand, Girtan reported an bottom ITO film. The resulting IAI multilayer was annealed
ITO/Ag/ITO (IAI) multilayer as an alternative TCE [14]. in a rapid thermal annealing (RTA) system in N2 ambient
According to this report, the electrical and optical gas. The sheet resistance of the IAI multilayer was
0146-9592/13/235055-04$15.00/0 © 2013 Optical Society of America
5056 OPTICS LETTERS / Vol. 38, No. 23 / December 1, 2013

measured at room temperature using a four-point probe

(Advanced Instrument Technology, CMT-SR1000N). Cur-
rent–voltage (I–V ) characteristics were measured at the
interface between IAI-TCEs and p-GaN using a Keithley
4200 measurement system. The surface morphology of
the IMI multilayer, as a function of annealing temperature,
was analyzed via transmission electron microscopy
(TEM). The transmittance of the IAI multilayer was also
measured over the wavelength range 280–800 nm using
a UV/visible spectrometer (PerkinElmer, Lambda 35).
Fig. 2. I–V characteristics of the IAI multilayer as a function of
We first investigated the sheet resistance and resistiv- annealing temperature.
ity of the IAI multilayer as a function of the annealing
temperature. Figure 1 shows the effect of postannealing
on the sheet resistance and resistivity of the IAI multi- calculated a figure of merit, ϕTC , from the 380 nm trans-
layer. It is well established that the resistivity of thin ITO mittance and the sheet resistance. The ϕTC values are
films may vary greatly [16,17]. The thin ITO films in this defined by Haacke as follows [21]:
study exhibited sheet resistance of 130 Ω∕sq and resistiv-
ity of 1.08 × 10−2 Ω · cm. For the IAI multilayer, the resis-
T 10
tivity decreases from 9.67 × 10−4 to 7.66 × 10−4 Ω · cm φTC
; (1)
after annealing at temperatures of up to 500°C; however, Rsh
at temperatures over 600°C, the resistivity increases
drastically to 2.73 × 10−3 Ω · cm. On the other hand, the where T is the transmittance and Rsh is the sheet resis-
sheet resistance decreases from 11.62 to 9.21 Ω∕sq at an tance for the films. The figure of merit measured for the
annealing temperature of 500°C, which is attributed to as-deposited sample and those annealed at 300°C, 400°C,
the improved crystal quality of ITO films and efficient 500°C, 600°C, and 700°C are 8.4 × 10−3 , 10.0 × 10−3 ,
protection from Ag-oxide formation [18,19]. However, 20.0 × 10−3 , 31.7 × 10−3 , 10.0 × 10−3 , and 0.2 × 10−3 Ω−1 , re-
metal agglomeration causes a sudden increase in sheet spectively. The samples annealed at 500°C exhibited the
resistance to 32.77 Ω∕sq at annealing temperatures highest figure of merit (31.7 × 10−3 Ω−1 ), much higher
over 600°C. than that of a typical thin ITO film (2.8 × 10−3 Ω−1 ) [22].
Figure 2 shows the typical I–V characteristics of Figure 4 shows the optical transmittance spectra mea-
IAI-TCE on p-GaN layers as a function of the annealing sured for the IAI multilayer on quartz substrates as a
temperature. The slope of the I–V curve increases with function of annealing temperature. The optical transmit-
annealing temperature until 500°C, after which it decreases. tance of the as-deposited IAI multilayer is approximately
The as-deposited sample and the samples annealed at 79% at 380 nm. At 500°C, its transmittance increases from
300°C, 400°C, 500°C, 600°C, and 700°C exhibit current 79% to 88%, an increase of 9% compared to a typical thin
values of 3.7 × 10−2 , 4.1 × 10−2 , 4.3 × 10−2 , 4.9 × 10−2 , ITO film. However, the transmittance may differ depend-
3.2 × 10−2 , and 1.6 × 10−2 A at 1 V, respectively. The ing on the wavelength; this phenomenon may be related
improvement in conduction observed for the samples
annealed at or below 500°C is consistent with the interfacial
morphology of the IAI multilayer, as shown by the TEM
cross-sectional images (Fig. 3). While the as-deposited
sample shows an amorphous phase [Fig. 3(a)], the samples
annealed at 500°C show increased crystallinity and colum-
nar grains in both ITO and Ag layers [Fig. 3(b)]. This helps
explain the improvement in the electrical properties [20].
However, Fig. 3(c) shows that the Ag films agglomerate
when annealed at 700°C. The agglomeration increases the Fig. 3. Cross-sectional TEM image in the interface region of
sheet resistance of the IAI multilayer films. IAI multilayer (a) before and after postannealing at (b) 500°C
and (c) 700°C.
To determine the best conditions for fabricating the IAI
multilayer as a transparent p-electrode in UV-LEDs, we

Fig. 1. Sheet resistance and resistivity of IAI multilayer as a Fig. 4. Optical transmittance spectra for IAI multilayer as a
function of annealing temperature. function of annealing temperature.
 December 1, 2013 / Vol. 38, No. 23 / OPTICS LETTERS 5057

to the optical properties of the Ag film. The absorption energy gap E g can be described with the following
coefficient of Ag is determined by interband electronic formula [25]:
transitions, which are the excitation of electrons from
the d-band to the Fermi surface [23]. The absorption co- αhv ∝ hv − Eg1∕2 ; (2)
efficient of Ag is high in the longer wavelength near the
red part of the visible spectrum; therefore, the transmit- where hν is the photon energy. The optical band gap of
tance of the IAI multilayer decreases after annealing. On ITO is ∼3.78–3.8 eV [26,27]. In Fig. 5, the calculated
the other hand, in the near-UV region, the transmittance optical band gap energy of ITO is ∼3.8 eV; however, the
of IAI multilayers increases after annealing, probably band gap energy of the IAI multilayer blue shifts with an
owing to the smaller absorption coefficient of Ag in the increase in the postannealing temperature up to 500°C.
near-UV region. However, at temperatures over 600°C, This phenomenon can be explained by the Burstein–
the transmittance decreases again because the Ag films Moss effect [28]. In other words, the carrier concentra-
are agglomerated to increase the scattering loss. In addi- tion of the multilayers increases with annealing temper-
tion, an increase in transmission is observed with ature, filling the energy levels in the bottom conduction
decreasing wavelength in the longer-wavelength regions. band [25], thereby shifting the position of the absorption
It is believed that the transport of free carriers may be edge to higher energies.
weakened at UV wavelengths, reducing the absorption In summary, IAI multilayer-based TCEs with high
coefficient of Ag [23]. Not shown here, we investigated conductivity and good transmittance for UV LED appli-
the thickness dependence of Ag on the electrical and cations were fabricated by magnetron sputtering, and
optical properties of the IAI structure to determine the optimized via annealing. The optimal annealing temper-
optimal Ag thickness. Before annealing, the transmit- ature was determined to be 500°C, where the sample
tance of the ITO 15 nm∕Ag 7 nm∕ITO 15 nm was 79% exhibited a sheet resistance of 9.21 Ω∕sq and a resistivity
at 380 nm, whereas those of the ITO 15 nm∕Ag 5 nm∕ of 7.66 × 10−4 Ω⋅cm, while the transmittance was as high
ITO 15 nm and ITO 15 nm∕Ag 14 nm∕ITO 15 nm as 88% at 380 nm. The multilayer also showed a
films were ∼67% at 380 nm. This is because each material figure of merit of 31.7 × 10−3 Ω−1 . These low-resistivity
composing the IAI multilayer has a different refractive and high-transmittance properties were thought to result
index; accordingly, high transmittance is observed at a from the reduction in grain boundary scattering, optical
specific film thickness [24]. On the other hand, after reflection, losses in the ITO films, and the quality of the
annealing at 600°C, a highest transmittance of 82% continuous Ag films during optimization of the annealing
was also observed for the ITO 15 nm∕Ag 7 nm∕ process. These IAI-multilayer-based transparent electro-
ITO 15 nm film at the same wavelength, whereas the des are expected to increase the EQE of GaN-based
ITO 15 nm∕Ag 5 nm∕ITO 15 nm film showed a UV-A LEDs.
strong absorption peak at 400 nm due to metal agglom- This work was supported by a National Research
eration. The ITO 15 nm∕Ag 14 nm∕ITO 15 nm film Foundation of Korea (NRF) grant funded by the Korean
showed lower transmittance than the ITO 15 nm∕ Government (MEST) (No. 2011-0028769). This work was
Ag 7 nm∕ITO 15 nm film due to the increased light also partially supported by LG Innotek Co., Korea.
absorption in Ag layers. On the other hand, the sheet re-
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