OPTIC 2013
Simulation of Light-Emitting
Diodes and Solar Cells
Yen-Kuang Kuo, Jih-Yuan Chang, Miao-Chan Tsai,
Tsun-Hsin Wang, Yi-An Chang, Fang-Ming Chen,
and Shan-Rong Li
Department of Physics, College of Science
National Changhua University of Education
1
Outline
 Introduction
 III-nitride Light-emitting diodes (LEDs)
 III-nitride solar cells
 Recent research results
1.
2.
3.
4.
5.
6.
Blue InGaN based LEDs
Green InGaN based LEDs
Ultraviolet AlInGaN based LEDs
Dual-wavelength emission LEDs
Superlattice solar cells
GaN-based Tunnel Junctions (TJ)
 Conclusion
2
III-Nitride Materials
Fig. 1. Energy bandgaps of III-nitrides as a function of in-plane lattice constant.
3
III-Nitride LEDs
p-contact
p-GaN
p-AlGaN (EBL)
InGaN/GaN
(MQW)
n-GaN
Fig. 3. Schematic plot of the energy band diagrams.
n-contact
u-GaN
Sapphire (100 μm)
Fig. 2. Schematic plot of the GaN-based
LED structure.
 GaN-based LEDs have extensive
applications in full-color displays,
liquid-crystal display backlighting,
and solid-state illumination.
 The LED market expands quickly
due to many advantages, such as
low-power consumption, long
lifetime, and small size.
4
III-Nitride Solar Cells
Built-in field
–
Solar Irradiation
p
hν
p-contact
Eg
+
p-GaN
Fig. 5. Schematic plot of the energy band diagrams.
InGaN
 The energy bandgap of InGaN
alloys can cover most of the solar
spectrum.
(absorption layer)
n-GaN
u-GaN
n
n-contact
Sapphire (100 μm)
Fig. 4. Schematic plot of the GaN-based
solar cell structure.
 Many superior photovoltaic (PV)
characteristics: high absorption
coefficient, high carrier mobility,
high saturation velocity, and high
radiation resistance.
5
Recent research result No. 1
Numerical Study of the Suppressed
Efficiency Droop in Blue InGaN LEDs
with Polarization-Matched
Configuration
(Opt. Lett., vol. 38, p. 3158, 2013)
6
Fig. 2. IQEs of the original LED structure with six
QWs (QW width is 2.5 nm) and of the 5.0 nm QW
LED with two and six QWs.
Fig. 1. Band profiles and subband wave functions of
the InGaN QWs with (a) 2.5 nm and (b) 5.0 nm
widths, (c), (d) carrier concentrations and (e), (f)
recombination rates of the 5.0 nm QW LED with two
and six QWs at 100 A∕cm2.
In blue InGaN LEDs, the Auger
recombination can be reduced by reducing
the carrier density.
 Increasing the number of quantum wells
(QWs) and thickening the width of wells,
suffer from nonuniform carrier distribution
and more severe spatial separation of
electron and hole wave functions.
 The severe quantum-confined Stark effect
(QCSE) will reduce the radiative efficiency
and hence cause an overall reduction in IQE. 7
Fig. 4. IQE characteristics of the original six-QW LED (QW
width is 2.5 nm) and the LEDs with two 5.0 nm thick QWs
and a single 10.0 nm thick polarization-matched well.
 The
Fig. 3. (a) Band profile of the LED with two 5.0 nm
thick polarization-matched QWs at 100 A∕cm2. (b)
Enlarged drawing of the first QW in (a), in which the
subband wave functions (C1 and HH1) are included.
Band profile and subband wave functions (c) C1 and
HH1 and (d) C2 and HH2 of the LED with a single 10.0
nm thick polarization-matched well at 100 A∕cm2.
purpose of reducing the number of
quantum wells is to mitigate the additional
compressive strain in polarization matched
AlGaInN barriers. With the proposed LED
structure, the QCSE can be markedly
eliminated.
 Moreover, the Auger recombination can
also be largely suppressed as a result of the
uniformly distributed and dispersed
carriers. The IQE and the efficiency droop
8
can thus be largely improved.
Recent research result No. 2
Investigation of Green InGaN LightEmitting Diodes with Asymmetric AlGaN
Composition-Graded Barriers and
without an Electron Blocking Layer
(Appl. Phys. Lett., vol. 100, p. 251102, 2013 )
9
= 508 nm
6 QWs
Fig. 2. Simulated L-I-V characteristics and IQEs as a
function of injection current for the three LEDs.
 The
Fig. 1. Schematic plot of the green LEDs under study.
Structure A is a conventional green LED. Structure B has
identical layer structure with structure A except that the GaN
barriers are replaced by asymmetric AlGaN CGBs. Structure
C has identical layer structure with structure B except that the
AlGaN EBL is removed.
Al compositions in AlGaN barriers:
13-11%, 11-9%, 9-7%, 7-5%, 5-3%,
3-1%, 1-0%
characteristics of the green InGaN
LEDs with asymmetric AlGaN CGBs
and without the commonly used AlGaN
EBL are systematically investigated.
 Green InGaN LEDs with asymmetric
AlGaN composition-graded barriers
and without the use of an AlGaN
electron blocking layer is presented to
possess markedly enhanced optical and
electrical performance.
10
 Specifically,
Fig. 3. Energy band diagrams near the active region of the
three LEDs at 100 mA.
the energy band
diagrams, carrier transportation,
and distribution in the active
region are studied.
 It is observed in Figs. 3 (b) and 3
(c) that the energy bands become
flatter, electrons can be confined
in the active region more
effectively, and holes can be
injected into the active region
more easily, especially when the
LED is with the asymmetric
AlGaN CGBs and without an
AlGaN EBL.
 The
Fig. 4. Carrier concentrations near the active regions of the
three LEDs at 100 mA.
simulation results suggest
that
the
improved
device
performance is due mainly to the
markedly enhanced injection of
holes into the active region.
11
Recent research result No. 3
Design and Characterization of
Polarization-Reversed AlInGaN Based
Ultraviolet Light-Emitting Diode
(IEEE J. Quantum Electron., vol. 49, p. 553, 2013 )
12
 The
effect of using polarizationreversed AlInGaN based quantum
well active region in ultraviolet
light emitting diode is numerically
investigated.
 By
Fig. 1. Schematic diagram of the three UV LEDs under study.
employing Al0.54In0.26Ga0.20N
and Al0.83In0.17N as barrier and
electron blocking layers, which
ably reverse the direction of the
polarization in quantum wells and
provide
sufficient
potential
barrier height to confine electrons
in the conduction band.
Fig. 2. (a) L-I-V curve and (b) IQE as a
function of injection current density for
the three UV LEDs under study.
13
 For
structure B, the integration of
the wave function overlap (Γe−h) is
only 23.31%. In structure C, which
indicates that the polarization of
Al0.005In0.02Ga0.975N QWs is reversed
by using the Al0.54In0.26Ga0.20N as
barrier layer. Moreover, the value of
Γe−h is increased to 47.65%.
 The
design of polarization reversed
QW active region is significant
because more electrons can be
effectively confined in the QWs due
to the tilting-up band profile and
the markedly enhanced overlap
between Φe and Φh.
Fig. 3. Energy band diagrams near the active
regions of (a) structure B and (b) structure C at an
injection current density of 277.8 A/cm2. The band
profiles of the second QWs of structures B and C
are expanded in (c) and (d), respectively.
14
 It
is observed that more carriers are
confined in the QWs, which contributes
to the radiative recombination more
efficiently since the electron leakage
current is suppressed markedly when the
original Al0.19Ga0.81N EBL is replaced by
a high-bandgap Al0.83In0.17N layer in
structures B and C. Fewer carriers are
confined in the QWs of structure A,
which in turn results in the lowest
radiative and Auger recombinations.
Fig. 4. (a) Percentage of electron leakage current. (b)
Integrated Auger recombination rate distributed in the
QWs as a function of injection current density for the
three UV LEDs under study. The insets of (a) and (b)
show the normalized current density flowing along the
growth direction near the active region and the Auger
recombination rate distributed in the QWs,
respectively, when the injection current density is
277.8 A/cm2.
15
Recent research result No. 4
Spectral Competition of Chirped DualWavelength Emission in Monolithic InGaN
Multiple-Quantum Well Light-Emitting
Diodes
(Appl. Phys. Lett., vol. 102, p. 171112, 2013 )
16
 In
this work, spectral competition of
chirped dual-wavelength emission in
monolithic InGaN MQW LEDs is
studied numerically.
 In
Fig. 1. Schematic diagrams of green-violet LED, violet-green
LED, strain reduced LED, and broad-band LED.
addition to the crystalline quality
that is generally desired for good
LED performance, the simulation
results
show
that
effective
suppression
of
piezoelectric
polarization is another key issue
toward the realization of efficient
monolithic
dual-wavelength
emission in InGaN MQW LEDs.
17
 The
green-violet LED suffers from large polarization
field induced by the large polarization mismatch
between the green QWs and GaN barriers and thus
the insufficient injection efficiency of holes in green
QWs near n-side is detrimental for spectral balance
and the realization of dual-wavelength emission.
 For
the violet-green LED, the injection efficiency of
holes in the green QWs is improved with the price of
reduced injection efficiency of holes in the violet
QWs.
 The
strain-reduced LED relaxes the strain between
the QWs and barriers effectively, which results in a
flatter band diagram, the phenomenon of insufficient
injection of holes still exists.
 For
broad-band LED, the first QW near n-side
cannot contribute to spontaneous emission
effectively; however, the reduced band-bending
phenomenon is beneficial for the injection of holes in
both green QWs and violet QWs.
Fig. 2. Energy band diagrams near the active region of (a) green-violet LED, (b) violet-green LED, (c) strainreduced LED, and (d) broad-band LED at 100 mA.
18
 For
green-violet LED, the spontaneous emission rates
of green QWs and violet QWs increase
simultaneously with the increase of injection current.
 For
the violet-green LED with reversed sequence of
QWs, although the spontaneous emission of green
QWs is enhanced by moving the green QWs toward
the p-side, the light output from the violet QWs
deteriorates seriously and hence only green emission.
 The
total light output of the strain-reduced LED is
improved compared to that of the green-violet LED;
however, the spontaneous emission of green QWs is
always higher than that of violet ones, which indicates
that the carriers distributed in the dual-wavelength
QWs are not balanced.
 For
the broad-band LED, the green QWs and violet
QWs have similar emission performance and
approximately equal intensity of the dual-wavelength
emission is achieved at 100 mA.
Fig. 3. Energy band diagrams near the active region of (a) green-violet LED, (b) violet-green LED, (c) strainreduced LED, and (d) broad-band LED at 100 mA.
19
Recent research result No. 5
Advantages of InGaN Solar Cells with
P-doped and High-Al-Content
Superlattice AlGaN Barriers
(IEEE Photonics Technol. Lett., vol. 25, p. 85, 2013)
20
14-pair SL
 When
the In content in InGaN QWs
is increased from 21% to 40%, the
conversion efficiency can be increased
from 1.13% to 1.38%.
 The
improvements
in
the
In0.4Ga0.6N/Al0.14Ga0.86N SL solar cell
can be attributed to the markedly
increased Jsc by increasing In content
in InGaN QWs for long wavelength
absorption, although the Voc is
reduced accordingly.
 Meanwhile, since the depth of the
In0.4Ga0.6N QWs becomes deeper, the
carriers generated by the photon
absorption in In0.4Ga0.6N QWs are
relatively difficult to escape.
 By
Fig. 1. Experimental and simulated photovoltaic J-V
characteristics of the In0.21Ga0.79N and In0.4Ga0.6N SL solar
cells with undoped and p-type doped Al0.14Ga0.86N barriers
under AM 1.5 G illumination.
introducing p-type doping in
Al0.14Ga0.86N barriers, it is noted
worthily that the Jsc can be further
enhanced to 1.82 mA/cm2, the FF can
also be slightly improved, and
accordingly the conversion efficiency
is increased to 1.65%.
21
Fig. 3. Electron and hole current densities distributed in the
In0.4Ga0.6N SL solar cells with undoped Al0.14Ga0.86N,
undoped and p-type doped Al0.2Ga0.8N barriers.
 The
Fig. 2. Photovoltaic J-V characteristics of the In0.4Ga0.6N SL
solar cells with undoped Al0.14Ga0.86N, undoped and p-type
doped Al0.2Ga0.8N barriers under AM 1.5 G illumination.
simulation results suggest that
the conversion efficiency can be
markedly enhanced by introducing ptype doping and more Al content in
AlGaN barriers, which is mainly
attributed to the improved capability
of carrier transport, hence increasing
the carrier collection efficiency.
22
Recent research result No. 6
Low Resistivity GaN-Based
Polarization-Induced Tunnel Junctions
(IEEE J. Lightwave Technol., vol. 31, p. 3575, 2013)
23
Performance of TJ Structure as a Function of InGaN
Composition and Thickness of the I-layer (T1)
(a)
 The TJ resistivity can be reduced from
the original value of 69.6 Ω·cm2 down to
7.8 × 10−3 Ω·cm2 (which is needed for a
reasonable voltage drop of <0.3 V) by the
introduction of several functional layers.
Fig. 1. (a) Schematic diagram of the n-i-p TJ for simulation (refer to
T1). J–V curves in log scale of different TJ structures showing the
dependence on (b) InGaN composition and (c) thickness of the i-layer.
2,3 nm
High In
Defects
Optical
absorption
In = 0.2
24
Dependence on [Mg] and [Si] Doping Densities (T2)
(a)
 Firstly, inserting two thin (5 nm) highly-doped
p- and n-GaN layers below and above the
InGaN layer to support more space charges.
 With 5 nm layers of p-GaN and n-GaN with
[ND ] = 3 × 1019 cm−3 and [NA] = 2 × 1019 cm−3
(denoted as T2), the resistivity of the TJ is
reduced to 5 × 10−2 Ω·cm2.
 Further increasing the Mg doping level from 2
to 3 × 1019 cm−3 does not increase the tunneling
current appreciably, while increasing the Si
doping changes the current density by almost
an order of magnitude. Because of the large
conduction band offset of the nitride
materials, increasing the Si doping density is
more effective than increasing the Mg doping
density for improving the tunneling.
Fig. 2. (a) Schematic diagram of the n-i-p TJ with the
insertion of thin layers of different doping levels (refer to T2).
(b) J–V curves in log scale.
25
Hybrid Use of both AlGaN and InGaN Layers
(T3, T4, T5)
T3
T4
T5
 Three new layer strategies in the
n-GaN/InGaN/p-GaN TJ structure:
(1)An intermediate AlGaN layer within
the i-InGaN layer to produce increased
polarization charges for high electric
fields.
(2)Replacing the highly-doped p-GaN
layer with a p-AlGaN layer, which
increases the polarization charges at
the AlGaN/InGaN interface compared
to the GaN/InGaN interface while the
increase in p-type valence band offset
(Ev) is relatively small.
(3)Utilization of InGaN as the n-type
layer, in which the decrease in the ntype conduction band energy is
Fig. 3. (a), (c), and (e) Tunneling current density at a reverse bias effective for shallow well trapping of
of 1 V of the different TJ structures (b), (d), and (f), respectively charge carriers for tunneling.
(refer to T3, T4, T5).
26
Hybrid Use of both AlGaN and InGaN Layers
(T3, T4, T5)
 As shown in Fig. 4, a kink
occurs at −0.30 V for T3 and
T4, and at −0.23 V for T5 in the
J-V curves, indicating the onset
of a reduction in tunneling
current due to suppressed
carrier collection.
Fig. 4. Tunneling current curves as a function of
reverse bias for the different TJ structures. The inset
shows the schematic diagram of the proposed
optimized TJ structure (T5).
 With practical In composition
and doping levels, the proposed
optimized TJ structure (T5) has
a tunneling current density of
more than 80 A/cm2 at –1 V and
a resistivity of as low as 7.8 ×
10–3 Ω·cm2.
27
Conclusion
1. Some specific designs on band structure near the active
region in the blue, green, UV, and dual-wavelength LEDs
are investigated with the APSYS simulation program.
2. Simulation results show that, with appropriate designs,
the optical performance may be effectively improved due
to the increase of hole injection efficiency, enhancement
of electron confinement, or uniform distribution of
carriers in the active region.
3. Some specific methods may also be employed in IIInitride solar cells to enhance the device performance.
4. The tunnel junction design principles introduced here
may enable a new range of high efficiency GaN-based
devices, such as the enhanced multijunction solar cells,
optoelectronic and electronic devices.
28
More papers in 2013




Jih-Yuan Chang, Shih-Hsun Yen, Yi-An Chang, and Yen-Kuang Kuo*,
“Simulation of high-efficiency GaN/InGaN p-i-n solar cell with
suppressed polarization and barrier effects,” IEEE Journal of
Quantum Electronics, Vol. 49, No. 1, 17–23, January 2013.
Jih-Yuan Chang, Shih-Hsun Yen, Yi-An Chang, Bo-Ting Liou, and
Yen-Kuang Kuo*, “Numerical investigation of high efficiency InGaNbased multi-junction solar cell,” IEEE Transactions on Electron
Devices, Vol. 60, No. 12, pp. 4140–4145, December 2013.
Jih-Yuan Chang, Yi-An Chang, Fang-Ming Chen, Yih-Ting Kuo, and
Yen-Kuang Kuo*, “Improved quantum efficiency in green InGaN
light-emitting diodes with InGaN barriers,” IEEE Photonics
Technology Letters, Vol. 25, No. 1, 55–58, January 2013.
Jih-Yuan Chang and Yen-Kuang Kuo*, “Advantages of blue InGaN
light-emitting diodes with composition-graded barriers and electronblocking layer,” Physica Status Solidi (a), Vol. 210, No. 6, 1103–1106,
published online 12 February 2013.
29
Some of Our Previous Papers

Yen-Kuang Kuo*, Tsun-Hsin Wang, and Jih-Yuan Chang,
“Advantages of InGaN light-emitting diodes with InGaNAlGaN-InGaN barriers,” Applied Physics Letters, Vol. 100,
No. 3, 031112, 2012.
(Editor's choice: one of the Best Papers of 2012)

Yen-Kuang Kuo*, Jih-Yuan Chang, Miao-Chan Tsai, and
Sheng-Horng Yen, “Advantages of blue InGaN multiplequantum well light-emitting diodes with InGaN barriers,”
Applied Physics Letters, Vol. 95, No. 1, 011116, 2009.
(High-download and highly cited paper)
30
Blue Laser Lab, NCUE, Taiwan
Thank you for your attention!
31