Effect of packing density and packing geometry on
light extraction of III-nitride light-emitting
diodes with microsphere arrays
Peifen Zhu
1,2,3
and Nelson Tansu
1,4
1
Center for Photonics and Nanoelectronics, Department of Electrical and Computer Engineering, Lehigh University,
Bethlehem, Pennsylvania 18015, USA
2
Department of Physics and Engineering Physics, The University of Tulsa, 800 South Tucker Drive, Tulsa,
Oklahoma 74104, USA
3
e-mail: pez311@lehigh.edu
4
e-mail: tansu@lehigh.edu
Received February 27, 2015; revised May 23, 2015; accepted June 17, 2015;
posted June 19, 2015 (Doc. ID 235348); published July 22, 2015
The finite-difference time-domain method was employed to calculate light extraction efficiency of thin-film
flip-chip InGaN/GaN quantum well light-emitting diodes (LEDs) with TiO
2
microsphere arrays. The extraction
efficiency for LEDs with microsphere arrays was inv estigated by focusing on the effect of the packing density,
packing configuration, and diameter-to-period ratio. The comparison studies revealed the importance of having a
hexagonal and close-packed monolayer microsphere array configuration for achievin g optimum extraction effi-
ciency, which translated into a 3.6-fold enhancement in light extraction compar ed to that for a planar LED. This
improvement is attributed to the reduced Fresnel reflection and enlarged light escape cone. The engineering of
the far-field radiation patterns was also demonstrated by tuning the packing density and packing configur ation of
the microsphere arrays. © 2015 Chinese Laser Press
OCIS codes: (230.0230) Optical devices; (250.0250) Optoelectronics.
http://dx.doi.org/10.1364/PRJ.3.000184
1. INTRODUCTION
Significant advances have been achieved in the field of
III-nitride semiconductor materials and devices. The applica-
tions of these III-nitride technologies have impacted solid-
state lighting [
1–6], thermoelectricity [7,8], lasers [9,10], and
solar energy conversion [
11]. The application of III-nitride-
based light-emitting diodes (LEDs) has been implemented for
solid-state lighting, specifically as blue emitters in white
LED configurations. The key advances in III-nitride LEDs have
also been recognized for the 2014 Nobel Prize in physics
[
12]. The advances in the fields of nitride-based LEDs were
driven strongly by innovations in material epitaxy [
13] and
nanostructure active region engineering [
14–17] for improved
internal quantum efficiency, solutions for addressing the
efficiency-droop issue [
18–21], and approaches to achieving
improved extraction [
22–25] in LEDs. The optimizations of
both internal and extraction efficiencies in LEDs are instru-
mental in achieving optimized external quantum efficiency
for LED emitters.
Approaches that address the light extraction limitation in
a cost-effective and scalable manner are instrumental for
addressing the extraction limitation in LEDs for wide imple-
mentation. The flip-chip technology was developed to
avoid the absorption of semitransparent metal of conventional
top emitting devices, resulting in a 1.6-fold improvement
over that of the conventional top-emitting LED [
22]. Other
methods such as surface roughening [
26], embedded photonic
crystals [
27–29], self-assembled lithography p-GaN patterning
[
30], GaN microdomes [31,32], TiO
2
micropillars [33],
nanopyramids [
34], and shape design [35] were employed
to improve light extraction efficiency as well. Sapphire micro-
lenses [
36,37], oblique mesa sidewalls [38], nanowires [39],
and the graded refractive index [
40] were also employed to
enhance the extraction efficiency. To further enhance the
light extraction efficiency, a thin-film flip-chip (TFFC) LED
structure can provide surface brightness and flux output ad-
vantages over conventional flip-chip LEDs [
23,24], and this
technology is widely used in industry today. Approaches to
improve the extraction efficiency in TFFC LEDs have been
pursued by using surface roughening and photonic crystal.
Though the surface roughening approach results in a 2- to
3-fold enhancement in light extraction efficiency, the photo-
chemical etching process led to a nonuniform surface and pro-
vided no control over the far-field radiation pattern of the
extracted light. The electrical properties of LEDs could also
be degraded via the use of photochemical etching [
41–43].
The photonic crystal method results in state-of-the-art results
for TFFC LEDs, with a 2.3-fold enhancement in light extrac-
tion efficiency [
44]; however, this approach requires a rela-
tively expensive e-beam lithography fabrication step.
Recently, we have demonstrated the use of colloidal-based
microsphere and microlens arrays deposited via a scalable
rapid convective deposition (RCD) process, resulting in
improved light extraction efficiency for GaN-based LEDs
[
45–52] with a cost-effective approach. By using a binary dep-
osition and heat treatment process, SiO
2
∕polystyrene micro-
lens arrays can be formed [
48]. The use of these colloidal
microsphere/microlens arrays has also been implemented in
184 Photon. Res. / Vol. 3, No. 4 / August 2015 P. Zhu and N. Tansu
2327-9125/15/040184-08 © 2015 Chinese Laser Press