Lester Eastman Conference on High Performance Devices (LEC) 2012, 1-4, 7-9 Aug. 2012, dx.doi.org/10.1109/lec.2012.6410988
GaN-based Light Emitting Diode with Embedded
SiO2 Pattern for Enhanced Light Extraction
Wenting Hou, Liang Zhao, Xiaoli Wang, Shi You, Theeradetch Detchprohm, and Christian Wetzel
Future Chips Constellation, Department of Physics, Applied Physics, and Astronomy
Rensselaer Polytechnic Institute
Troy, NY 12180 USA
houw2@rpi.edu
Abstract—The n-GaN layer of c-plane GaInN/GaN light
emitting diodes (LEDs) on sapphire was modified to contain a
pattern of SiO2 nanorods. This embedded pattern of 300 nm long
rods and diameter of 200 - 400 nm was created by thermal
agglomeration of a Ni mask layer and subsequent dry-etching.
The light output power (LOP) and external quantum efficiency
(EQE) of the resulting LEDs increased both by some 25% to 40%
depending on current density over the unpatterned structure.
The increase is thought to be primarily the result of enhanced
light extraction efficiency induced by enhanced scattering at the
substrate-side interface.
Index Terms—green light emitting diode, light extraction
efficiency, nano-pattern n-GaN substrate, Ni self assembly.
I. INTRODUCTION
Group-III nitride compound semiconductors are the leading
material system for consumer and utility scale energy savings
in lighting by help of light-emitting diodes (LEDs) [1,2], laser
diodes [3,4], and rapidly expand into utility scale power
generation by help of solar cells [5]. While GaInN/GaN
materials have been extensively used, the output power in
particular of longer wavelength LEDs needs to be improved.
The external quantum efficiency (EQE) is the product of
internal quantum efficiency (IQE) and light extraction
efficiency (LEE). Within state-of-the-art LEDs, IQE is limited
by high threading dislocation (TD) densities in GaN junction
layers, generally induced by the lattice mismatch with sapphire
substrate. Light extraction, in turn is governed by the optical
refractive index of GaN (nGaN ≅2.5) which induces total internal
reflection (TIR) beyond an extraction cone of θc=23° opening
angle. This results in a first path LEE of only 4%. Unless light
scatters internally to fall into an extraction cone, the remainder
must be lost in absorption.
It has been reported that patterned sapphire substrate (PSS)
on a length scale of a few micrometers can both, reduce the TD
density [6] and improve the LEE [7]. It was found that TDs
initiated at the bottom of an etched substrate can mostly be
stopped by open voids while TDs originating from inclined
facets of the pattern can change direction to propagate only
within the growth plane and annihilate. Primarily TDs
originating in the un-etched portions of the substrate seem to
propagate up with growth towards the ensuing active region [8].
LEE improvement can be attributed to light scattering at the
patterned GaN/sapphire interface. Consequently, the use of
micrometer-sized PSS has widely been adopted in the LED
industry. Furthermore, with patterning on the nanometer length
scale, we recently demonstrated 3.4 times enhanced efficiency
of green LEDs, which by detailed quantitative analysis could
be attributed to about equal enhancement of IQE (2.24-fold)
and LEE (1.58-fold) [8].
Patterning of a template for homoepitaxial overgrowth by
SiO2 patterns was applied by T. Zheleva et al to GaN and
showed substantial reduction in dislocation densities on SiC [9].
In turn, patterning of the substrate itself, e.g. by a pattern
aiming at a photonic crystal by J. Park et al showed an
enhanced LOP of some 20% [10]. K. Kwon et al showed blue
LEDs with a patterned n-GaN substrate with SiO2 nano sized
patterns, and found 33% improvement in LOP [11].
In this paper, we present our work in GaN-based LEDs with
embedded SiO2 pattern in n-GaN.
II. EXPERIMENTS
Epitaxial layers of GaN were grown on c-plane sapphire
substrate by metal organic vapor phase epitaxy (MOVPE).
After growth of a low temperature GaN buffer layer, a 5 µm
thick u-GaN/n-GaN layer was grown successively by MOVPE.
The n-GaN sample was then taken to undergo the nanorod
fabrication process. A layer of 300 nm SiO2 was deposited on
the top surface of n-GaN by plasma enhanced chemical vapor
deposition (PECVD), followed by a 20 nm Ni layer by electron
beam evaporation (Fig. 1(a)). The Ni-coated sample was
subsequently subjected to rapid temperature annealing (RTA)
in nitrogen ambient at 850 °C for 1 min to form self-assembled
Ni metal clusters [12] (Fig. 1(b)). The nano-pattern generated
by the Ni mask is then transferred into the SiO2 layer by
etching. The SiO2 layer was etched down to the n-type GaN
layer by inductively coupled plasma-reactive ion etching (ICPRIE) to form nanorods. Then the sample was dipped into a
heated nitric acid solution (HNO3) for 10 min to remove the Ni
nanomask (Fig. 1(c)).
Figure 2(a) shows a scanning electron microscopy (SEM)
image of the SiO2 patterns on the n-type GaN layer. The
diameters of the SiO2 pattern columns range from 200 nm to
400 nm, and the height is 300 nm. After the SiO2 nano sized
columns were formed on the surface of the n-type GaN layer,
the full LED structure was regrown (Fig. 1(d)) including nGaN, MQW, and p-GaN. The cross-section SEM image of the
LED structure is shown in Fig. 2(b). The regrown n-GaN layer
was 3.5 µm thick, which covers the SiO2 patterns to become a
smooth layer (Fig. 2(c)). The active layers were grown on the
Lester Eastman Conference on High Performance Devices (LEC) 2012, 1-4, 7-9 Aug. 2012, dx.doi.org/10.1109/lec.2012.6410988
smooth n-GaN, which consisted of ten periods of GaInN
quantum well and GaN barrier with a period of about 17.5 nm
(Fig. 2(d)). A reference LED with the same structure without
embedded SiO2 pattern was prepared for comparison.
Fig. 1.
Schematic illustration of nano-patterned LED process
flowchart. (a) A SiO2 and Ni layer deposited on the n-GaN surface. (b) Ni
clusters formed by RTA. (c) ICP-RIE process to fabricate SiO2 nanorods
followed by removal of the Ni mask in heated HNO3. (d) Regrown nGaN and LED structure.
Fig. 2. SEM image of the LED with embedded SiO2. (a) SiO2 pattern
on n-GaN surface before LED regrown. (b) Cross-section image of LED.
(c) Cross-section image of SiO2 pattern in LED. (d) Cross-section image
of MQW structure in LED.
III. RESULTS AND DISCUSSION
In an x-ray diffraction analysis of the two samples, no
significant difference was found between the samples. The line
width of the rocking curve along 002 direction showed 323 and
351 arcsec (full width at half maximum, FWHM) for the
reference LED and LED with embedded SiO2 pattern
(patterned), respectively. The linewidth of the rocking curve
along 102 direction showed 319 and 415 arcsec for the
reference LED and patterned LED, respectively. These
indicated that the embedded SiO2 layer does not change the
crystal quality of the GaN layer.
The samples were inspected in the microscope under UV
illumination (Fig. 3). The reference LED showed uniform light
across the sample with some dark spots attributed to dislocation
bunches (Fig. 3(a)). The patterned LED showed bright spots
under UV illumination (Fig. 3(b)). Analysis of these bright
spots showed that they had similar size to the SiO2 nanopatterns (Fig. 3(b) insert). We can deduce from the comparison
that the bright spots came from the scattering from the SiO2
pattern. This indicated an improvement in light extraction due
to the increase in scattering.
Fig. 3. Microscope picture under UV illumination: (a) reference LED
on unpatterned n-GaN; (b) patterned LED with embedded SiO2 pattern
(insert: SEM image of SiO2 pattern on the same scale).
Electroluminescence measurement was performed at room
temperature (RT) using Indium contacts with about 1 mm2 in
size from the substrate side. Both LEDs show a single green
emission line. The LOP, EQE, voltage, and wavelength as a
function of current are shown in Figure4. The LOP clearly
increased in the patterned LED (Fig.4 (a)). The overall L–I
curve for the patterned LED was a smooth curve with no abrupt
slope change, indicating the absence of anomalies such as
instability. An improvement of 38% was obtained from the
patterned LED under a current of 20 mA (2.5 A/cm2), and 25%
at 100 mA (12.7 A/cm2). EQE maximum was achieved at 5 mA
(0.6 A/cm2) for both LEDs, which was 8.8% for the patterned
LED, and 5.8% for the reference LED. A significant EQE
enhancement of 52% was achieved at 5 mA, 39% at 20 mA,
and 26% at 100 mA. The wavelength for the LED with
embedded SiO2 is 517 nm at 20 mA, and 511 nm at 100 mA,
slightly higher than the reference LED. The voltage for the
patterned LED is higher, 11 V at 100 mA compared to 7 V for
the reference LEDs.
It is likely that the embedded SiO2 layer in the n-GaN
substrate affects both, the wavelength and the I-V
characteristics through a variation of thermal properties during
both, growth and operation of the LED. The layer is likely to
cause an increase of series resistance within the n-GaN layer
and lead to a higher voltage drop during operation, which in
turn could lead to a larger junction temperature increase with
current in comparison to the unpatterned structure. This
Lester Eastman Conference on High Performance Devices (LEC) 2012, 1-4, 7-9 Aug. 2012, dx.doi.org/10.1109/lec.2012.6410988
resulted in a red shift of wavelength and rapid decrease in EQE
under high current injection for the patterned LED.
35
Reference
30
IQE (%)
25
Patterned
20
15
10
5
0
1
10
100
1000
10000
2
Laser Intensity (W/cm )
Fig. 5.
IQE as a function of laser intensity for the patterned LED (red)
and refrence LED (black).
IV. CONCLUSION
In conclusion, we demonstrate green GaInN LEDs using a
patterned n-GaN substrate with nano sized SiO2 columns
fabricated by Ni self assembly process. The LEDs with
embedded SiO2 pattern were compared to reference LEDs on
planar n-GaN. Both, the LOP and EQE of the LEDs with
embedded SiO2 pattern increase about 40% at 2.5 A/cm2, and
about 25% at 13 A/cm2 over the unpatterned structure.. The
increase is primarily attributed to enhanced light extraction, by
increased light scattering at the nano sized SiO2 pattern
integrated inside n-GaN in the LED structcture.
Fig. 4. Light output power, EQE, forward voltage, and peak
wavelength as a function of current for the patterned LED (red) and
reference LED (with 1 mm2 Indium dot contacts).
For an upper limit estimate of IQE, the photoluminescence
(PL) efficiency as a function of excitation power density was
measured at RT and at 4.2 K [13]. IQE at RT was determined
by scaling to the point of highest PL efficiency at low
temperature (Fig. 5). By choosing an excitation wavelength of
408 nm, within the bandgap of GaN, photo carriers are
generated directly within the QWs.
IQE varies strongly with excitation power density. The
patterned LED showed a maximum of 30% at 2.3 kW/cm2,
while the reference LED reached 35% at 0.9 kW/cm2. Under
low and moderate laser excitation power below 10kW/cm2,
IQE of the patterned LED is lower than that of the LED on
regular n-GaN. IQE of the patterned LED surpass that of the
reference LED for high laser excitation power over 10kW/cm2.
While EQE for the patterned LED is higher, IQE is lower
than the reference LED for low and moderate laser intensity.
This indicates the improvement in EQE is mostly due to the
improvement of LEE. The improvement of LEE can be
contributed to the improvement in light-escape due to the
scattering of light caused by the SiO2 nano sized columns
inside the n-type GaN. Further optimization of the growth
condition can be optimized and has the potential to decrease the
dislocation density in the LED with embedded SiO2, and
improve IQE as well.
ACKNOWLEDGMENT
This work was supported by a DOE/NETL Solid-State
Lighting Contract of Directed Research under DE-EE0000627.
This work was also supported by the National Science
Foundation (NSF) Smart Lighting Engineering Research
Center (# EEC-0812056).
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