Effects of reduced exciton diffusion in
InGaN/GaN multiple quantum well nanorods
Bin Jiang, 1 Chunfeng Zhang, 1* Xiaoyong Wang, 1 Fei Xue, 1 Min Joo Park,2
Joon Seop Kwak,2,4 and Min Xiao1, 3,5
1
2
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing
210093, China
Department of Printed Electronic Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, South
Korea
3
Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA
4
jskwak@sunchon.ac.kr
5
mxiao@uark.edu
*cfzhang@nju.edu.cn
Abstract:We investigate the effects of reduced exciton diffusion on the
emission properties in InGaN/GaN multiple-quantum-well nanorods. Timeresolved photoluminescence spectra are recorded and compared in dryetched InGaN/GaN nanorods and parent multiple quantum wells at various
temperatures with carrier density in different regimes. Faster carrier
recombination and absence of delayed rise in the emission dynamics are
found in nanorods. Many effects, including surface damages and partial
relaxation of the strain, may cause the faster recombination in nanorods.
Together with these enhanced carrier recombination processes, the reduced
exciton diffusion may induce the different temperature-dependent emission
dynamics characterized by the delayed rise in time-resolved
photoluminescence spectra.
©2012 Optical Society of America
OCIS codes: (300.6500) Spectroscopy, time-resolved; (230.3670) Light-emitting diodes.
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1. Introduction
Recently, nano-texturing has been widely performed on InGaN/GaN multiple-quantum-wells
(MQWs) to optimize the light-emitting-diode (LED) devices [1–7]. Compared with the planar
MQWs, InGaN/GaN MQW nano-LEDs show better performance with improved brightness,
luminescent efficiency, and color tunability over the whole visible spectral range [3–7]. The
impacts of nanofabrication, including the light extraction enhancement, the strain relaxation,
and the ease of sample growth with high-indium concentration, have been regarded as major
factors for these improvements [1–10].
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The recombination of localized states is generally accepted as an origin of the high
luminescent efficiency [11–16] in InGaN/GaN heterostructures. It is efficient to compete with
nonradiative recombination channels induced by the threading dislocations, strain induced
quantum confined Stark effect (QCSE), and other effects [17–19]. Of the same importance of
localization, the carrier transport process is also crucial for the emission properties [20–23].
Thermally-activated carrier transport processes drive the carriers to recombine through the
strongly-localized centers in planner MQWs [20–23]. In nano-textured samples, the aspect of
carrier transport should be different due to the size effect. For example, InGaN nanorod LEDs
reported so far have diameters mostly in the scale of 102 nanometers [4–8]. The dimensions
are close or even smaller than the carrier diffusion length [20–23]. The sized confinement will
lead a reduction of the exciton diffusion reduction, which would have impacts on the
emission properties in InGaN/GaN nanostructures; however, these impacts remain
unexplored.
In this work, we investigated the nanofabrication impacts on the emission dynamics in
InGaN/GaN MQW nanorods with emphasis on the effects of reduced exciton diffusion. We
carry out time-integrated (TI) and time-resolved (TR) photoluminescence (PL) experiments
on dry-etched MQW nanorod LED structures and the parent MQWs at various temperatures
with a broad range of carrier density. We observe different temperature-dependent behaviors
of steady and transient emission in nanorods and MQWs. While the emission intensity of
TIPL in nanorods decreases monotonically with increasing temperature, the emission
intensity in MQWs remains nearly unchanged with temperature up to 120 K. The thermallyactivated carrier transport exhibits delayed-rise in TRPL traces recorded from MQWs when
temperature increases [24], but the delayed-rise characteristic is not present in the TRPL
traces recorded from nanorods. This difference of temporal evaluation is a result of the
reduced exaction diffusion in nanorods.
2. Experimental procedures
The LED samples used in this study are grown on sapphire substrates by metal-organic
chemical vapor deposition. The InGaN/GaN MQW LED structures consist of a GaN buffer,
an n-type GaN layer, five pairs of 2.5 nm-thick In0.1Ga0.9N quantum wells sandwiched
between 13 nm GaN barriers, a p-type of GaN layer, as well as a p-type capping layer. The
InGaN/GaN NRs with average diameters of 200 nm are fabricated by inductively-coupledplasma etching using a novel mask of self-assemble ITO-based nanodots [25]. Scanning
electron image of the nanorods and transmission electron image of a single nanorod are
shown in Fig. 1. Frequency doubled optical pulses generated from a Ti: Sapphire regenerative
amplifier (Libra, Coherent, 1 kHz, 90 fs) are used for pumping. The pumping wavelength at
400 nm only excites the InGaN well layers from backside with excitation flux in the range of
7-117 µJ/cm2 (corresponding to carrier density of 1016-1018 cm−3). Due to different scattering
or reflection in the MQWs and NRs, the equal excitation density does not mean necessarily
the same carrier density in the two samples. Time-integrated emission spectra are measured
with a fiber spectrometer (USB 2000 + , Ocean-Optics). The technique of optical Kerr gating
is utilized to do TRPL studies with Kerr medium of 5 mm thick carbon disulfide cell. The
temporal resolution is about 2 ps, which is sufficient to study the dynamic feature of reduced
exciton diffusion in the first 200 ps temporal window. A PMT (5784-20, Hamamatsu) was
used to record the TRPL traces with band pass optical filters at 450 nm with 40 nm bandwidth
or with wavelength selection by a monochrometer. The slow component of emission decay is
checked with a fast oscilloscope with a fast PMT with nanosecond resolution. The emission
spectra at different decay time are recorded with a 0.5 m spectrograph (Sp 2500i, Princeton
Instruments) equipped a liquid nitrogen cooled charge coupled devices. A liquid helium
cryostat (MicroCryostatHe, Oxford) with a transparent sample holder is used to do
temperature-dependent measurements from 5 K to 300 K.
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Fig. 1. Scanning electron microscopy image of the morphologies of the InGaN/GaN nanorods.
Inset shows the transmission electron microscopy image of the cross section of a single
nanorod.
3. Results and discussion
TIPL spectra in nanorods show some common emission features of InGaN/GaN
heterostructures (Fig. 2). With increasing excitation density, the emission shifts to shorter
wavelength band in the spectra recorded at room temperature [Fig. 2(a)]. This blue-shift has
been explained by the combination of band-tail filling effect and reversal QCSE with high
density of photo-excited carriers [18,26–29]. With decreasing temperature, PL intensity
increases dramatically with slight blue-shift of the emission band [Fig. 2(b)]. Compared with
the parent MQWs, emission wavelength is shorter [Fig. 2(c)] and emission intensity is much
higher in NRs [Fig. 2(f)] under same experimental configurations (i.e., the same excitation
power and temperature). Temperature-dependence of emission properties show diverges in
two samples as compared in Figs. 2(c) and 2(d). With increasing temperature, the emission
peak shifts to the red side monotonically in NRs [Fig. 2(c)]. In MQWs, the emission peak
shifts to the blue side with temperature above 150 K, but further increasing temperature leads
to a red-shift of emission peak [Fig. 2(c)]. While the emission intensity in nanorods decreases
monotonically with increasing temperature, the emission intensity in MQWs remains nearly
unchanged with temperature up to 120 K [Fig. 2(d)]. To understand the detailed mechanisms,
we probe the carrier dynamic behaviors with TRPL spectra.
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Intensity (a.u.)
117 µJ/cm
2
90 µJ/cm
2
55 µJ/cm
2
35 µJ/cm
2
25 µJ/cm
2
12 µJ/cm
420
440
460
480
420
440
Wavelength (nm)
Normalized Intensity (a.u.)
Photon Energy (eV)
2.78
2.76
2.74
2
25 µJ/cm , NRs
2
25 µJ/cm , MQWs
2
117 µJ/cm , NRs
2
117 µJ/cm , MQWs
2.72
2.70
0
50
100
150
200
250
(d)
0.8
2
0.6
25 µJ/cm , NRs
2
25 µJ/cm , MQWs
2
117 µJ/cm , NRs
2
117 µJ/cm , MQWs
0.4
300
0
50
150
200
250
300
(f)
(e)
300 K, NRs
5 K, NRs
300 K, MQWs
5 K, MQWs
10
2.76
2.74
300 K, NRs
5 K, NRs
300 K, MQWs
5 K, MQWs
2.72
2.70
20
40
60
80
100 120
2
Excitation density (µJ/cm )
Intensity (a.u.)
Photon Energy (eV)
100
Temperature (K)
2.78
0
480
1.0
Temperature (K)
2.80
460
Wavelength (nm)
(c)
2.80
5K
25 K
45 K
85 K
130 K
200 K
250 K
300 K
(b)
2
25 µJ/cm
NRs
2
Intensity (a.u.)
(a)
300 K
NRs
5
0
20
40
60
80
100
120
2
Excitation Density (µJ/cm )
Fig. 2. Emission properties of InGaN/GaN nanorods. (a) Room-temperature emission spectra
of InGaN/GaN nanorods recorded with different excitation densities; (b) Emission spectra of
InGaN/GaN nanorods recorded at different temperature with excitation density of 25 µJ/cm2;
(c) Photon energy of Emission peaks from nanorods and MQWs is plotted as functions of
temperature with excitation densities of 25 µJ/cm2 and 117 µJ/cm2, respectively; (d) Intensities
of emission from nanorods and MQWs are plotted as functions of temperature with excitation
densities of 25 µJ/cm2 and 117 µJ/cm2, respectively. (e) and (f) plot the photon energy of
emission peak and the emission intensity recorded in the samples of nanorods and MQWs as
function of excitation density at 5 K and 300 K, respectively.
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10
0
0
117 µJ/cm
2
55 µJ/cm
2
35 µJ/cm
2
25 µJ/cm
2
12 µJ/cm
-1
Experimental
SE-fitting
Exp-fitting
2
-1
10
-2
10
0.5
0.0
0
1
2
Delay Time (ns)
3
1
2
3
0
(c)
0
-1
10
-2
Experimental
SE-fitting
Exp-fitting
-2
0
10
0
Delay Time (ns)
1
2
Delay Time (ns)
3
(d)
8
(e)
1.0
NRs
MQWs
6
4
β
τ0 (ns)
10
20
30
40
Delay Time (ns)
10
Band pass
Wavelength selected
Intensity (a.u.)
Intensity (a.u.)
1.0
10
(b)
10
(a)
Intensity (a.u.)
Intensity (a.u.)
10
0.5
2
NRs
MQWs
0
0
20
40
60
80
100
120
0
2
Delay Time (ns)
Delay time (ns)
0
1
2
3 (f)
430
20
40
60
80
100
2
120
Excitation Density (µJ/cm )
Excitaiton Density (µJ/cm )
0
0.10
1
0.32
1.00
2
3 (g)
440
450
Wavelength (nm)
460
430
440
450
460
Wavelength (nm)
Fig. 3. (a) Logarithmic plot of TRPL traces recorded from InGaN/GaN nanorods at room
temperature with different excitation densities. Inset shows a comparison of the TRPL traces
recorded with a band pass filter and wavelength selected at the TIPL peak by a
monochrometer. (b) and (c) show a TRPL trace from nanorods recorded with the methods of
fast electrical recording (ns-resolution) and Kerr gate technique (ps-resolution), respectively.
The fitting curves to the mono-exponential (Exp) and stretched exponentiall (SE) function of
the decay component are also present. The SE fitting parameters (τ0 and β) of the TRPL traces
recorded from nanorods and MQWs are plotted as functions of excitation densities in (d) and
(e). Logarithmic plot of emission intensity as a function of emission wavelength and decay
time in parent MQWs (f) and NRs (g) with excitation density of 117 µJ/cm2 at room
temperature.
Figure 3 displays the TRPL results recorded from the InGaN nanorods at room
temperature. We first check the difference of TRPL traces recorded with the band pass optical
filter and wavelength selected at the emission peak with a monochrometer [inset, Fig. 3(a)].
The difference between the two curves is small, so that the curve recorded with the band pass
filter is a good average of the dynamics at the emission peak in the sample. Compared to the
wavelength selection measurement, the signal to noise ratio of the curves recorded with the
#164337 - $15.00 USD Received 7 Mar 2012; revised 21 Apr 2012; accepted 23 May 2012; published 31 May 2012
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4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 13483
optical filter is much better. In the following sections, we'll present these results as a good
approximation to illustrate the carrier dynamics.
The excitation-power dependent TRPL traces are present in Fig. 3(a). Due to the limited
length of the optical delay line, the maximum temporal window of TRPL traces recorded by
the method of optical Kerr gating is about 3.5 ns in our study. To check the emission
evolution in a longer dynamic range, a TRPL trace recorded with a fast PMT is shown in Fig.
3(b). The TRPL traces recorded from the InGaN nanorods show highly non-exponential
decay, especially, in the early stage [Fig. 3(c)].The recombination rate is strongly dependent
on the excitation densities. To interpret the nonexponential decay dynamics, several models
have been proposed in the last decade including a bi-dimensional donor-acceptor like
recombination model [30], a charge-separated dark state model [16], and a frequently-used
phenomenological stretched exponential (SE) function [31]. For simplicity, we will use the
SE function to describe the carrier dynamics in this work. The SE function in the form of
β
I (t ) = I 0e − (t /τ 0 ) is a good mathematic description of the TRPL traces. In the function, τ0 is
the SE decay lifetime, and β expresses the distribution of rates with values between 0 (broad
distribution) and 1 (narrow distribution).
Figures 3(d) and 3(e) compare the best fitted parameters of the TRPL traces in the NRs
and MQWs at room temperature with different excitation densities. The carrier recombination
rate is generally faster in nanorods than in MQWs [Figs. 3(d), 3(f) and 3(g)]. As discussed in
previous reports [2, 32], the surface trapping effect may be a reason for the faster
recombination since the etching process can cause surface damages in nanorods. If we
checked the internal efficiency as the ratio of emission intensity at room temperature to that at
5 K approximately, the internal efficiency in the nanorods is lower than that in the parent
QWs, which might be another signature of increased surface recombination. Also, other
effects induced during nanofabrication can change the dynamical behaviors of the PL
emission in nanorods. The QCSE associated to the strain relaxation after nano-texturing may
also lead to the faster recombination in nanorods. The QCSE, induced by the strain-induced
internal electrical field, causes the spatial separation of confined electrons and holes in
InGaN/GaN heterostructures and slows down the carrier recombination [18,29,33]. In the
nanorods, the strain may be partially relaxed [1,2,8,9] and less internal electrical field will be
present, leading to the faster relaxation in nanorods [2], which is consistent with the powerdependent TIPL measurement as shown in Fig. 2(e). With increasing excitation density, blueshift of the emission peak, which is frequently considered as a signature of reversal QCSE
(free carrier screening), is observed in both MQWs and nanorods. However, smaller blueshifts were observed in the nanorod samples. These results indicate that the strain-induced
QCSE still exists in the nanorods, in agreement with a recent study [34], but the strain may be
weaker in comparison to the parent MQWs.
As shown in the Figs. 3(d) and 3(e), the carrier recombination dynamics shows strongly
dependence on the excitation density. The recombination lifetime decreases rapidly with
carrier density below a critical value but the change becomes smaller and converges to a
value at high carrier density in both samples. The reversal QCSE and the band-tail filling
effect are frequently assigned to the power-dependent variations of recombination rate. With
increasing excitation power, the internal electrical field may be screened and compensated by
photon-excited carriers, leading to the reversal QCSE with the observed decrease of carrier
recombination lifetime [18,29]. This effect also exhibits the time-dependent descreening
phenomena with red-shift of PL spectra over the decay time [33]. Initially, the high density
carriers induced by the intense laser pulses compensate the internal field efficiently. The
carrier density decreases over the delay time so that the QCSE becomes more important and
the recombination slows down [18,29,33]. In Figs. 3(f) and 3(g), we compare the counterplotted image of the intensity as function of delay time and emission wavelength in the
nanorods and MQWs, respectively, with excitation density of 117
. Less red-shift of
the emission over time can be observed in nanorods, indicating less strain in nanofabricated
samples.
#164337 - $15.00 USD Received 7 Mar 2012; revised 21 Apr 2012; accepted 23 May 2012; published 31 May 2012
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Band tail filling effect may also cause the temperature-dependent variation of
recombination rate [11,24,26]. In general, this effect should not cause the discrimination of
carrier dynamics in nanorods and the parent MQWs, since the band structure change is
neglectable in the nanorods with diameters much larger than the effective Bohr radius (~3
nm) [27]. Nevertheless, considering the confinement of carrier transport, the filling of lower
energy levels can lead to the different recombination rates between the nanorods and MQWs.
In InGaN MQWs, it has been observed that excitation transfers from the weakly-localized
states to the strongly-localized state with increasing temperature [11,24]. This thermallyactivated carrier transport drives the excitons filling the low energy levels prior to the
interband transition. The diameters of nanorods are close or smaller than the carrier diffusion
length [21–23], so that the carrier transport will be partially eliminated in nanorods.
4
(a)
4
MQWs
NRs
3
Intensity (a.u.)
Intensity (a.u.)
3
(b)
2
1
2
1
5K
65 K
100 K
160 K
200 K
300 K
0
0
5K
65 K
100 K
160 K
200 K
300 K
0
1
2
Delay Time (ns)
3
0
1
2
Delay Time (ns)
3
Fig. 4. Temperature-dependent TRPL traces recorded from MQWs (a) and nanorods (b) with
excitation density of 25 µJ/cm2. The curves are vertically shifted for clarity. The dashed lines
are horizontal references to clarify the delayed rise of TRPL traces recorded from the MQWs.
The influence of carrier transport confinement on the emission properties in nanorods is
evidenced by the transient spectral results. With relatively low carrier density, the
temperature-dependent TRPL spectra recorded from the MQWs and nanorods are compared
in Figs. 4(a) and 4(b). Instead of immediate decay post-excitation, slight rise of the emission
over time can be distinct in the TRPL curves recorded from MQW samples. This delayed-rise
becomes more explicit with increasing the temperature up to 160 K. It is observable till the
temperature goes up to 250 K, above which the non-radiative decay process dominates. The
delayed rise originates from intra-well carrier transport from weakly-localized states (higher
potential minima) to strongly-localized states (lower potential minima) [11,24]. This
temperature dependence indicates the carrier transport is activated thermally. The carrier
transport process, in a time scale of sub-nanosecond [21–23], enables that photon-excited
electrons and holes to recombine from the strongly-localized states in MQWs. In nanorods,
the size effect has profound impacts on carrier transport process and the temporal evolution
behavior of the emission in nanorods diverges from that in MWQs. As shown in Fig. 4(b),
with increase of the temperature, the delayed-rise is not observable in the TRPL traces in
#164337 - $15.00 USD Received 7 Mar 2012; revised 21 Apr 2012; accepted 23 May 2012; published 31 May 2012
(C) 2012 OSA
4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 13485
nanorods. This difference is a clear signature that the thermally-activated carrier transport
process [11,24] has been strongly modified and partially eliminated by the nanofabrication
processes.
In principle, the delayed rise appears when the population accumulation at the emitting
levels caused by the carrier transport can overcome the removal process including radiative
and non-radiative recombination processes [11,24,32]. At the low temperature (5 K), thermal
energy is insufficient for carriers to conquer the weak potential barriers. When the
temperature increases, thermal energy will assist the carriers to “escape” from the weaklylocalized states and transport to the strongly-localized states, which causes the delayed-rise in
the TRPL curves in the MQWs. To understand the absence of the delayed rise in nanorods,
we will analyze the mechanisms on both the carrier accumulation and removal processes.
For carrier transport in InGaN samples, the exciton diffusion length is a critical scale. The
diffusion lengths of 60 nm [22], 100 nm [20], and 200-500 nm [21] have been evaluated in
the previous studies with techniques of polarized photoluminescence and spatially-resolved
catholuminescence. These dimensions are in a same scale of the average diameter of the
nanorods. Assuming a diffusion length of ~100 nm and a uniform distribution of the carrier
density, the distance from localization to the nearest boundary will be shorter than the
diffusion length. In consequence, the carrier diffusion may be significantly reduced in the
nanorods. The carriers generated in the nanorods cannot diffuse to the strongly localized
states if they exist originally outside the boundary. The population accumulation at the
emitting levels in the parent MQWs originally contributed by the states outside the boundary
is eliminated in the nanorods. Thus, the residual carrier transport in the nanorods may be not
sufficient to support the carrier accumulation, which could be one reason of the absence of
delayed rise in the TRPL traces.
Nevertheless, if the reduced exciton diffusion is the only origin, the distribution of
emission centers in the MQWs should be narrower than that in the nanorods. However, the
distribution factor [Fig. 2(e)] doesn't show significant difference between the two samples,
indicating that other factors may be also involved in the absence of delayed rise in the
nanorods. The enhanced removal processes either radiatively or non-radiatively may be the
reason. As we discussed above, the surface state recombination and the reduced QCSE will
lead to faster recombination in the nanorods. These processes may also cause the absence of
delayed rise in the nanorods.
Hence, multiple mechanisms may be involved in the different emission dynamics
observed in nanorods and MQWs with faster PL recombination and different temperaturedependent temporal evolutions in nanorods. Many effects, including the surface damages, the
partial relaxation of strain and the band filling effect, could contribute to the faster
recombination in nanorods. Besides these enhanced carrier removal processes, the
discrimination of temperature dependence, characterized by the delayed-rise in the TRPL
traces, should be also attributed to the reduced exciton diffusion. With higher power
excitation, free carriers dominate in both MQWs and nanorods, so that the delayed rise
process is absent in TRPL traces recorded from both samples.
With the above discussions on carrier dynamics, we can get a better understanding of the
mechanisms on TI emission properties in nanorods. The nanofabrication influences the carrier
recombination processes and causes the faster carrier recombination with a shorter emission
wavelength in the nanorods [2,11,24,26], resulting in different peak shifts of PL emission in
the nanorods and MQWs with increasing excitation density or decreasing temperature. The
blue shift of the emission peak with excitation power [Fig. 2(e)] has been regarded as a result
of many effects including the QCSE and band filling effect. In general, these effects co-exist
in InGaN samples. The slope of excitation-density-dependence may indicate different
mechanisms. In nanorods, the rapid shift at the relatively low power may be attributed to cocontribution of the QCSE and band filling effect while the slow shift at the high power may
be a signature of the full compensation of QCSE. Similar results have been observed in the
previous literatures [18]. With the measured data, one can estimate the internal electrical field
by solving the Poisson equation and the Schrödinger equation ([18] and ref. in) or more
#164337 - $15.00 USD Received 7 Mar 2012; revised 21 Apr 2012; accepted 23 May 2012; published 31 May 2012
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accurately with numerical simulation [2]. These detailed numerical tasks are beyond the
scope of this study. Roughly, we evaluate the internal electrical field as the saturated
compensation field induced by photon-induced carriers. The magnitude of electrical field with
accuracy in order is estimated to be ~200 kV/cm in the nanorods and more than ~400 kV/cm
in the parent samples, which is lower than the samples with thick wells. With an increasing
temperature, the thermally-activated carrier transport enables most of carriers to recombine
from the strongly-localized states in MQWs. This carrier transport may compensate the
increasing non-radiative recombination with temperature up to 120 K [Fig. 2(d)]. In nanorods,
the carrier transport process is partially eliminated and the emission intensity drops
monotonically with increasing temperature. This causes a stronger temperature dependence of
PL intensity in nanorods. The modification of the carrier transport in nanorods could cause
relatively low quantum efficiency in nanorods at room temperature [Fig. 2(d)]. Nevertheless,
the light extraction efficiency is significantly improved [1,2,10]. Higher power of PL
emission is observed in nanorods [Fig. 2(f)], even though the number of photo-excited
carriers is much less since the filling factor is about 50% in nanorods, which suggests the
nanorod structure can be an alternative approach for roughening the LED devices. Since the
total output power is the ultimate goal of LED design, optimizing NR diameters is of promise
to improve the device performance.
4. Conclusions
In conclusion, we have performed a systematic study on emission properties of InGaN/GaN
nanorod LED structures by TRPL and TIPL experiments at different power densities and
temperatures. We observe different temperature behaviors of steady and transient emission
properties in nanorods and MQWs. Besides the contributions of surface states and strain
relaxation effect, the reduced exciton diffusion may also play an important role to these
differences. The impacts of nanofabrication on carrier dynamics revealed in this study are key
factors for the nano-LED design. Device performance can be improved with a tradeoff
between internal quantum efficiency and light extraction efficiency in nanorods.
Acknowledgments
This work is supported by the Program of International S&T Cooperation (2011DFA01400,
MOST), the National Basic Research Program of China (2012CB921801, and
2011CBA00205), National Science Foundation of China(61108001, and 11021403), NRF of
Korea grant funded by the Korea government (MEST) (K2011-0017325). The author C.Z.
acknowledges financial support from New Century Excellent Talents program (NCET-090467), PAPD and Fundamental Research Funds for the Central Universities (1107020420,
1118020406, 1104020403, and 1115020404). The author J.S.K acknowledges financial
support from WCU program in SCNU.
#164337 - $15.00 USD Received 7 Mar 2012; revised 21 Apr 2012; accepted 23 May 2012; published 31 May 2012
(C) 2012 OSA
4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 13487