rXXXX American Chemical Society
dx.doi.org/10.1021/nl2011164|Nano Lett. XXXX, XXX, 000–000
Room Temperature Current Injection Polariton Light Emitting Diode
with a Hybrid Microcavity
Tien-Chang Lu,*,†Jun-Rong Chen,†Shiang-Chi Lin,†Si-Wei Huang,†Shing-Chung Wang,*,†and
†Department of Photonics, National Chiao Tung University, Hsinchu 300, Taiwan
‡E. L. Ginzton Laboratory, Stanford University, Stanford California 94305, United States
§National Institute of Informatics, Hitotsubashi, Chiyoda-ku, Tokyo 101-8430, Japan
twenty years due to their potential to probe fundamental physics
and create practical devices. Exciton-polaritons are bosonic
particles with very small effective mass (typically 10?4times
the bare exciton mass) and controllable energy-momentum
dispersion curves by appropriate detuning.1These unique prop-
erties have led to demonstration of a wealth of experimental
results, including cavity quantum electrodynamics,2dynamical
Bose?Einstein condensates (BEC) or polariton lasers,3?9and
polariton parametric amplifiers.10Thus far the polariton BEC or
lasing has been demonstrated in GaAs,5,6CdTe,3organic
materials,7and GaN.8,9Nevertheless, these results are mostly
based on optical pumping and low-temperature experiments.
Toward a polariton optoelectronic device for practical applica-
room-temperature would be an important step toward that goal.
An electrically pumped microcavity light-emitting diode
(LED) has been demonstrated for organic semiconductors.11A
mid-infrared polariton LED based on a GaAs/AlGaAs quantum
cascadestructure has beenreported.12More recently,electrically
strated at temperature from 10 to 315 K.13?16However, no
current injection wide bandgap semiconductor polariton LED
possesses several unique advantages over other material systems.
First, GaN has a small Bohr radius and large exciton binding
energy (40 meV for quantum well), so that exciton-polaritons
can exist at high temperatures.17,18Second, a GaN exciton has a
fast phonon-assisted relaxation rate, which can effectively sup-
press the relaxation bottleneck and achieve the efficient
thermalization.19Third, a GaN exciton has a large oscillator
trong light-matter interactions in semiconductor high-Q
microcavities have attracted much attention over the past
Rabi oscillation frequency.20Besides, in the exciton-polariton
and the nonradiative decay can be suppressed by the strong
coupling between cavity photons and localized excitons. This is
an important advantage for a highly inhomogeneous material
system such as GaN. Nevertheless, for realization of an electri-
cally pumped GaN-based polariton LED, there are several
technical difficulties including the growth of high-reflectivity
nitride-based distributed Bragg reflectors (DBRs), high-conduc-
In this paper, we report the first realization of an electrically
carefully designed nanostructures within the microcavity to
achieve high optical quality and high cavity reflectivity. The
GaN-based microcavity, shown schematically in Figure 1a, con-
sists of a 29-pair AlN/GaN bottom DBR and a 5λ-thick optical
cavity layer composed of an n-type GaN, 10 In0.15Ga0.85N/GaN
multiple quantum wells (MQWs) at an antinode position, and a
p-type GaN layer. The thicknesses of quantum well and barrier
are 2 and 8 nm, respectively. The grown 29-pair AlN/GaN
bottom DBR showed a reflectivity of R = 99.4% with a spectral
bandwidth of ∼25 nm. In order to reduce the tensile strain
between the AlN and GaN to achieve high optical quality and
five DBR periods at first twenty pairs of bottom DBR. Then the
superlattice was inserted into each three DBR periods for the
April 4, 2011
June 9, 2011
to produce half-matter/half-light quasiparticles, exciton-polaritons. The exciton-polaritons have very small
effective mass and controllable energy-momentum dispersion relation. These unique properties of polaritons
provide the possibility to investigate the fundamental physics including solid-state cavity quantum electro-
dynamics, and dynamical Bose?Einstein condensates (BECs). Thus far the polariton BEC has been
demonstrated using optical excitation. However, from a practical viewpoint, the current injection polariton
devices operating at room temperature would be most desirable. Here we report the first realization of a
current injection microcavity GaN exciton-polariton light emitting diode (LED) operating under room
temperature. The exciton-polariton emission from the LED at photon energy 3.02 eV under strong coupling
condition is confirmed through temperature-dependent and angle-resolved electroluminescence spectra.
KEYWORDS: GaN, exciton-polariton, light emitting diode (LED), Rabi splitting
dx.doi.org/10.1021/nl2011164 |Nano Lett. XXXX, XXX, 000–000
remaining nine pairs of bottom DBR.22The thicknesses of AlN
and GaN layers are about 5 and 2.8 nm, respectively, in the SL
structure andthe totalthickness ofSL have tobegrown carefully
to match the phase of DBR design. The epitaxially grown
structure was then processed to form the intracavity coplanar
p- andn-contacts for current injection.A 0.2μm thick SiNxlayer
was used as the mask to form a current injection and light
emitting aperture of 30 μm in diameter. A 30 nm thick
indium?tin-oxide (ITO) layer was then deposited on the
current aperture to serve as the transparent contact layer
ambient to reduce the contact resistance as well as to increase
metal contact layer was deposited by the electron beam evapora-
1000 nm) as the n-type electrode and p-type electrode to form
coplanar intracavity contacts, respectively. Finally an 8-pair
Ta2O5/SiO2dielectric top DBR with a reflectivity of R = 99%
was deposited to achieve a reasonable quality factor (Q-factor =
400). The cross-sectional transmission electronic microscopy
lighter layers represent AlN layers while the darker layers
represent GaN layers. The interfaces between AlN and GaN
pairs AlN/GaN SL insertion layers under high magnification.
V-shaped surfaces observed on top of AlN layers were formed in
order to partially release the crystal stress because the thickness
of AlN layer in DBRs has been larger than the critical thickness.
However, the residual tensile stress in DBRs accumulates alone
play an important role for the reduction of remaining in-plane
improvement in the reflectivity of the DBRs.
Two commonly used experimental techniques are employed to
confirm the strong coupling and the anticrossing behavior of the
QW excitons and cavity photons in the fabricated InGaN/GaN
microcavity device. One is the temperature-dependent electrolumi-
nescence and the other is angle-resolved electroluminescence.23,24
The former technique mainly relies on the temperature-dependent
the cavity photon energy. The latter technique uses the parabolic
dispersion with increasing emission angle to adjust the detuning
parameter between the cavity photon and the QW exciton. The
angle-resolved electroluminescence measurement can prevent col-
couple to one cavity mode. The temperature-dependent measure-
ments are performed in a temperature-controlled, closed-cycle,
carried out by using a 600 μm core UV optical fiber mounted on a
attached to a 320 mm single monochromator with a spectral
resolution of about 0.2 nm.
Figure 2a shows experimental electroluminescence spectra
from the GaN-based microcavity for different temperatures
Figure 1. Schematic sketch of the electrically pumped InGaN-based
polariton LED. (a) The GaN-based hybrid microcavity consists of a 29-
pair AlN/GaN bottom DBR and a 5λ optical thickness microcavity
composed of a n-type GaN, 10 pairs In0.15Ga0.85N/GaN MQWs, a
An enlarged active region including an ITO layer, MQWs, and p- and
n-type GaN layers. (c) Cross-sectional TEM image of the superlattice
DBR structure. (d) Cross-sectional TEM image of one set of 5.5-pairs
AlN/GaN SL insertion layers under high magnification.
Figure 2. Polariton electroluminescence emission as a function of
temperatures. (a) Experimentally measured temperature-dependent
electroluminescence spectra from 180 to 300 K when the input current
is 2 mA. (b) Theoretically calculated temperature-dependent transmis-
sion spectra from 180 to 300 K. The detuning between exciton mode
(X) and cavity photon mode (C) changes with increasing temperature.
The spectra shown in (a,b) are normalized for easily identifying the
emission peaks. Both results show the characteristic anticrossing beha-
vior. Rabi splitting of 7.4 meV is observed at 280 K. (c) Extracted peaks
energy curves of uncoupled exciton and photon modes and lower and
upper polariton modes.
dx.doi.org/10.1021/nl2011164 |Nano Lett. XXXX, XXX, 000–000
the input current of 2 mA. The emission spectrum at 180 K
exhibits two peaks of exciton-like upper polariton branch (UPB)
and photon-likelower polaritonbranch (LPB).Thiscondition is
commonly termed as negative detuning. With increasing the
temperature, the decrease in the QW exciton energy resulted
from a reduction of the bandgap energy, overwhelms the
decrease in the cavity photon energy due to the temperature
is much more significantly affected by the temperature change
negative to positive detuning with increasing temperature from
180 to 300 K. An anticrossing dispersion is clearly exhibited,
which confirms that the cavity system is in a strong coupling
regime even at 300 K. To understand the temperature-depen-
dent electroluminescence spectra obtained from the experiment,
we employed the transfer matrix method coupled with a Lorentz
oscillator model to calculate the transmission spectra of the
microcavity structure. The shift of the cavity photon energy with
increasing temperature is estimated to be ∼0.054 meV/K25and
the temperature-dependent QW excitons energy follows the
modified Varshni formula including the localization effect
E(T)=Em(0) ? [(RT2)/(T + β)] ? [(σ2)/(kBT)], where E(T)
β are Varshni’s fitting parameters, kBis the Boltzmann constant,
and σ is related with localization effect.26In this numerical
simulation, we use R = 0.435 meV/K, β = 900 K, and σ =
17.5 meV. These values are estimated from the independent
measurements of our bare InGaN/GaN MQWs and are close to
the values reported in recent literatures.26,27The QW exciton
was modeled by a coupled harmonic oscillator dispersive di-
electric function, taking into accounts the homogeneous and
inhomogeneous broadening line width.28,29The simulation
results of temperature-dependent spectra from 180 to 300 K
are shown in Figure 2b.It is noteworthy that the almost identical
evolution of the polariton emission spectra as the experimental
spectra is obtained from the numerical simulation by using the
exciton damping rate of 7 meV and the inhomogeneous broad-
ening line width of 8 meV for the InGaN QW excitons.29,30The
emission intensity at 180 K is dominated by photon-like LPB.
With increasing temperature, cavity mode couples with exciton
mode and shares identical polariton emission at resonance. The
estimated oscillator strength of the InGaN/GaN QW exciton is
as that of bulk GaN (0.03?0.04 eV2) and GaN/AlGaN QW
value extracted in our data compared that of the bulk GaN could
mainly be due to the well-known strong built-in piezoelectric
quantum confinement of the InGaN QW. In addition, the actual
device structure and crystal quality of QWs could be the
contributing factors in our relatively small value of the oscillator
strength. The simulation results reveal that the condition of zero
exciton-photon detuning at zero angle is reached at a tempera-
ture of 280 K and the normal mode splitting at zero detuning is
about 7.4 meV as shown in Figure 2c.
Angle-resolved electroluminescence measurements were per-
the negative detuning at normal incidence angle for probing the
anticrossing behavior as a function of angle. Figure 3a shows the
measured angle-resolved electroluminescence spectra, which
reveals the well-resolved upper and lower polariton modes and
exhibits the anticrossing behavior of the strong coupling regime.
A zero detuning is realized at 7.4? and the corresponding normal
mode splitting is about 8.3 meV, which is very close to that
obtained from the temperature-dependent experiment at 280 K.
The calculated angle-resolved spectra (Figure 3b) are in good
agreement with the measured results (Figure 3a). To get better
understanding of the dispersive features of the two polariton
branches, the color maps of the angular dispersion of measured
spectra from 0 to 13? are shown in Figure 3c. At small angle
emission intensity since the cavity mode always dominates the
emission in a microcavity. As the cavity mode crossing the
exciton mode, the high energy line has transformed into
photon-like UPB and the intensity emitted from the exciton-like
LPB vanishes gradually with increasing angle. This evolution of
the characteristics of strong coupling. The data from the two
measurement techniques agree well each other indicating the
strong coupling survive in our device at room temperatures. The
relatively small normal mode splitting compared with the pre-
viously reported values18by optical pumping experiment could
originate from the longer cavity length, smaller number of
Figure 3. Angle-resolved polariton electroluminescence spectra. (a)
Experimentally measured angle-resolved electroluminescence spectra
calculated angle-resolved transmission spectra from 0 to 13?. The
detuning between exciton mode (X) and cavity photon mode (C)
changes with increasing angle both results show the characteristic
anticrossing behavior. The spectra shown in (a,b) are normalized for
easily identifying the emission peaks. (c) Color map of the measured
polariton angular dispersion. The horizontal dotted lines are the bare
exciton mode and the curve dotted lines are the cavity mode. Rabi
splitting of 8.3 meV is observed at 7.4?.
dx.doi.org/10.1021/nl2011164 |Nano Lett. XXXX, XXX, 000–000
MQWs or weak optical field overlap with MQWs, and smaller
oscillator strength due to the built-in piezoelectric field in
We further measured the current-dependent electrolumines-
cence spectra at zero degree of the angle and under the
temperature of 240 K when the detuning was closed to the zero.
Figure 4a shows the electroluminescence spectra as a function of
clearly resolved polariton peaks separated by 9.1 meV are
observed. With increasing injection current, the two polariton
peaks are progressively close to each other, leading to the
decrease in normal mode splitting from 9.1 to 6.8 meV. To
estimate the carrier density, we assume that all injected carriers
are trapped into the quantum wells, and thus n = jτ/e where e is
the electron charge, j is the current density, and τ is the average
carrier lifetime. For 4 mA injection current, the estimated
polariton density is n ∼ 1.87 ? 1012cm?2if we assume τ =
0.53 ns.33This value is much lower than the Mott density in
InGaN/GaN QWs (∼1 ? 1013cm?2).33Figure 4b presents the
could originate from the increase of homogeneous broadening
due to the enhanced exciton?exciton scattering. The apparent
red shift trend should be noted as the current stems from the
device heating under CW current injection. Additional measure-
effects. Nevertheless, the decrease in Rabi splitting can also be
of exciton?exciton scattering should dominate the bleaching
mechanism of strong coupling. The further increase in injection
current was not conducted due to the device heat dissipation
current injection range, we observed that the integrated electro-
luminescence intensities for both UPB and LPB as functions of
to the strong coupling between cavity photons and excitons in a
fast Rabi oscillation that suppresses the nonradiative decay.15,16
In conclusion, we have demonstrated an electrically pumped
InGaN-based exciton-polariton LED operating at room tem-
perature. Both temperature-dependent electroluminescence
spectra and angle-resolved electroluminescence spectra show
the existence of anticrossing in the strong coupling regime.
Further optimization of device design, including the decrease
in ITO thickness, the optimization of ITO annealing condition,
the increase in QW number, and the decrease in cavity length,
would enhance the device performance. In addition, the im-
provement of heat dissipation should allow higher injection
current operation and possible achievement of a room-tempera-
electrically pumped GaN-based polariton LED at room tem-
suppress the nonradiative decay. In addition, the significantly
lower density of states of polaritons makes extra low-threshold
polariton lasers possible as compared to conventional semicon-
ductor lasers based on GaN materials.20,34
*E-mail: (T.-C.L.) email@example.com; (S.-C.W.) scwang@
This work has been supported in part by the MOE ATU
program and in part by the National Science Council of Taiwan
under Contracts NSC99-2221-E-009-035-MY3, NSC99-2120-
M-009-007, and NSC98-2923-E-009-001-MY3. J.R.C., S.C.L.
and S.W.H. acknowledge C. K. Chen, S. W. Chen, Z. Y. Li, and
Professor H. C. Kuo of National Chiao Tung University for
sample preparation at an early stage. Y.Y. acknowledges the
support from the FIQST-Quantum Information Processing
(1) Weisbuch, C.; Nishioka, M.; Ishikawa, A.; Arakawa, Y. Phys. Rev.
Lett. 1992, 69, 3314–3317.
(2) Vahala, K. J. Nature 2003, 424, 839–846.
(3) Kasprzak, J.; Richard, M.; Kundermann, S.; Baas, A.; Jeambrun,
P.; Keeling, J. M. J.; Marchetti, F. M.; Szyma? nska, M. H.; Andr? e, R.;
Staehli, J. L.; Savona, V.; Littlewood, P. B.; Deveaud, B.; Dang, L. S.
Nature 2006, 443, 409–414.
(4) Deng,H.;Weihs,G.;Santori,C.;Bloch, J.;Yamamoto,Y.Science
2002, 298, 199–202.
(5) Deng, H.; Weihs, G.; Snoke, D.; Bloch, J.; Yamamoto, Y. Proc.
Nat. Acad. Sci. U.S.A. 2003, 100, 15318–15323.
(6) Balili, R.; Hartwell, V.; Snoke, D.; Pfeiffer, L.; West, K. Science
2007, 316, 1007–1010.
(7) K? ena-Cohen, S.; Forrest, S. R. Nat Photonics 2010, 4, 371–375.
(8) Christopoulos, S.; Von H€ ogersthal, G. B. H.; Grundy, A. J. D.;
Lagoudakis, P. G.; Kavokin, A. V.; Baumberg, J. J.; Christmann, G.;
Butt? e, R.; Feltin, E.; Carlin, J.-F.; Grandjean, N. Phys. Rev. Lett. 2007,
(9) Christmann, G.; Butt? e, R.; Feltin, E.; Carlin, J.-F.; Grandjean, N.
App. Phys. Lett. 2008, 93, 051102.
Figure 4. Current-dependent polariton electroluminescence spectra.
(a) The normalized polariton electroluminescence spectra as a function
two clearly resolved polariton peaks are evident. With increasing
injection current, the two polariton peaks are progressively close to
each other,leading tothe decrease inRabisplittingfrom 9.1to 6.8 meV.
(b) Extracted lower and upper polariton peak energies by Gaussian
E Download full-text
dx.doi.org/10.1021/nl2011164 |Nano Lett. XXXX, XXX, 000–000
(10) Saba,M.;Ciuti,C.;Bloch,J.;Thierry-Mieg,V.;Andr? e,R.;Dang,
B. Nature 2001, 414, 731–735.
(11) Tischler, J. R.; Bradley, M. S.; Bulovi? c, V.; Song, J. H.; Nurmikko,
A. Phys. Rev. Lett. 2005, 95, 036401.
(13) Tsintzos, S. I.; Pelekanos, N. T.; Konstantinidis, G.; Hatzopoulos,
Z.; Savvidis, P. G. Nature 2008, 453, 372–375.
(14) Tsintzos,S.I.;Savvidis, P.G.;Deligeorgis, G.;Hatzopoulos, Z.;
Pelekanos, N. T. App. Phys. Lett. 2009, 94, 071109.
(15) Khalifa, A. A.; Love, A. P. D.; Krizhanovskii, D. N.; Skolnick,
M. S.; Roberts, J. S. App. Phys. Lett. 2008, 92, 061107.
(16) Bajoni, D.; Semenova, E.; Lema^itre, A.; Bouchoule, S.; Wertz,
E.; Senellart, P.; Bloch, J. Phys. Rev. B 2008, 77, 113303.
(17) Kornitzer, K.; Ebner, T.; Thonke, K.; Sauer, R.; Kirchner, C.;
Schwegler, V.; Kamp, M.; Leszczynski, M.; Grzegory, I.; Porowski, S.
Phys. Rev. B 1999, 60, 1471–1473.
(18) Christmann, G.; Butt? e, R.; Feltin, E.; Mouti, A.; Stadelmann,
P. A.; Castiglia, A.; Carlin, J.-F.; Grandjean, N. Phys. Rev. B 2008,
(19) € Ozg€ ur,€ U.;Bergmann,M.J.;Casey,H.C.;Everitt,H.O.;Abare,
A. C.; Keller, S.; DenBaars, S. P. App. Phys. Lett. 2000, 77, 109–111.
(20) Malpuech, G.; Carlo, A. D.; Kavokin, A.; Baumberg, J. J.;
Zamfirescu, M.; Lugli, P. App. Phys. Lett. 2002, 81, 412–414.
(21) Lu, T.-C.; Chen, J.-R.; Chen, S.-W.; Kuo, H.-C.; Kuo, C.-C.;
Lee, C.-C.; Wang, S.-C. IEEE J. Sel. Top. Quantum Electron. 2009,
(22) Huang, G.-S.; Lu, T.-C.; Yao, H.-H.; Kuo, H.-C.; Wang, S.-C.;
Lin, C.-W.; Chang, L. Appl. Phys. Lett. 2006, 88, 061904.
(23) Houdr? e, R.; Weisbuch, C.; Stanley, R. P.; Oesterle, U.; Pellandini,
P.; Ilegems, M. Phys. Rev. Lett. 1994, 73, 2043–2046.
(24) Sellers, I. R.; Semond, F.; Leroux, M.; Massies, J.; Zamfirescu,
M.; Stokker-Cheregi, F.; Gurioli, M.; Vinattieri, A.; Colocci, M.;
Tahraoui, A.; Khalifa, A. A. Phys. Rev. B 2006, 74, 193308.
(25) Wang, S.-C.; Lu, T.-C.; Kao, C.-C.; Chu, J.-T.; Huang, G.-S.;
Kuo, H.-C.; Chen, S.-W.; Kao, T.-T.; Chen, J.-R.; Lin, L.-F. Jpn. J. Appl.
Phys. 2007, 46, 5397–5407.
(26) Eliseev, P. G.; Perlin, P.; Lee, J.; Osi? nski, M. App. Phys. Lett.
1997, 71, 569–571.
(27) Lee, J.-C.; Wu, Y.-F.; Wang, Y.-P.; Nee, T.-E. J. Cryst. Growth
2008, 310, 5143–5146.
(28) Kavokin, A. V.; Baumberg, J. J.; Malpuech, G.; Laussy, F. P.
Microcavities; Oxford University Press, Inc.: New York, 2007.
(29) Houdr? e, R.; Stanley, R. P.; Ilegems, M. Phys. Rev. A 1996,
(30) Tawara, T.; Gotoh, H.; Akasaka, T.; Kobayashi, N.; Saitoh, T.
Phys. Rev. Lett. 2004, 92, 256402.
(31) Antoine-Vincent, N.; Natali, F.; Byrne, D.; Vasson, A.; Disseix,
P.; Leymarie, J.; Leroux, M.; Semond, F.; Massies, J. Phys. Rev. B 2003,
(32) Ollier, N.; Natali, F.; Byrne, D.; Disseix, P.; Mihailovic, M.;
Vasson, A.; Leymarie, J.; Semond, F.; Massies, J. Jpn. J. Appl. Phys. 2005,
(34) Solnyshkov, D.; Petrolati, E.; Carlo, A. D.; Malpuech, G. App.
Phys. Lett. 2009, 94, 011110.