Download Near-red emission from site-controlled pyramidal InGaN quantum dots

APPLIED PHYSICS LETTERS 87, 163121 共2005兲
Near-red emission from site-controlled pyramidal InGaN quantum dots
V. Pérez-Solórzano,a兲 A. Gröning, and M. Jetter
4. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany
T. Riemann and J. Christen
Institut für Experimentelle Physik, Otto-von-Guericke Universität, Universitätsplatz 2,
39160 Magdeburg, Germany
共Received 20 April 2005; accepted 24 August 2005; published online 14 October 2005兲
We have fabricated InGaN nanostructures on top of GaN hexagonal pyramids by selective
metalorganic vapor-phase epitaxy. With this approach, we are able to exactly control the position of
the emitting quantum dot, which is an essential requirement for functionalized single-photon
emitters. The emission properties as well as the relaxation and recombination mechanisms were
investigated using spectroscopic methods. Regions of different confinement were identified, with the
photoluminescence emission from the InGaN quantum dots around 2.03 eV and a decay time of
1.4 ns. The constant temperature behavior of the radiative decay time confirms its zero-dimensional
character. Spatially resolved cathodoluminescence measurements attribute this emission to the apex
of the pyramid. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2108126兴
Local composition fluctuations acting as quantum dots
共QDs兲 in ternary InGaN alloys have been proposed to be
responsible for the high efficiency of blue-green lightemitting devices.1 The growth of such QDs has been demonstrated via the Stranski-Krastanov growth mode2,3 and other
routes.4 However, these methods do not allow the control of
the position and even the size of the nanostructures, which is
an essential requirement for applications such as singlephoton emitters. Photon antibunching observed from sitecontrolled InGaAs/ AlGaAs QDs grown by selective epitaxy
on patterned substrates5 confirms this method as an alternative to self-assembly. For GaN hexagonal pyramids, optically
pumped laser action6 and more than three different groups of
optical resonance modes have been observed,7 making them
suitable as high-finesse cavities via total internal reflection
inside the pyramids. Recent studies investigating the optical
properties of selectively grown 5 ⫻ InGaN / GaN QD layers8
showed narrow emission peaks 共⬃650 ␮eV兲 originating in
the pyramid apex with a decay lifetime of about 0.5 ns.
In this Letter we report on the growth and characterization of site-controlled InGaN QDs located on GaN micropyramids. A SiO2 mask deposited on 1 ␮m high-quality GaN
grown by metalorganic vapor-phase epitaxy was patterned
using electron-beam lithography. The subsequent overgrowth
with GaN results in hexagonal pyramidal structures with facets formed by 兵101គ 1其 planes, arising from the wurzite structure. InGaN with a nominal thickness of 3 nm was deposited
on the micropyramids, and afterwards a GaN barrier was
grown under the same conditions. The growth method has
been described in detail elsewhere.9 An InGaN quantum well
grown under the same conditions on planar GaN has an In
content of about 18%.
Time-integrated and time-resolved single-photon counting photoluminescence 共PL兲 measurements were performed
using a frequency-doubled Ti:sapphire laser at 325 nm
共200 fs pulse width兲 which was focused to a spot-size of
approximately 20 ␮m in a temperature-controlled cryostat.
The PL spectra were collected using a 0.32 m Jobin-Yvon
monochromator with a 1800 grooves/ mm grating and a fast
multichannel plate photomultiplier tube.
Cathodoluminescence 共CL兲 imaging was carried out in a
scanning electron microscope 共SEM兲, operated at 5 keV and
an electron-beam current of 400 pA, with a spatial resolution
of 40 nm. CL spectra shown here have been taken under
continuous-wave 共cw兲 excitation.
Figure 1 shows the side-view SEM image of the completely overgrown sample under investigation. The pyramids
are connected to the substrate only by the material deposited
in the predefined windows. When they are filled, the lateral
growth over the SiO2 mask begins with a much lower growth
rate compared to the vertical direction, which allows the formation of structures of more than 2 ␮m height. In the
present case, holes with 1 ␮m diameter and a separation between centers of 5 ␮m were patterned. The selectivity of the
SiO2 is also excellent, as we could not observe any material
deposition apart from the openings.
a兲
FIG. 1. SEM side view of the sample under investigation. Material deposition was observed in the openings of the SiO2 mask only.
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0003-6951/2005/87共16兲/163121/3/$22.50
87, 163121-1
© 2005 American Institute of Physics
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163121-2
Perez-Solorzano et al.
Appl. Phys. Lett. 87, 163121 共2005兲
FIG. 2. PL spectra over the emission energy for different optical excitation
powers. With increasing excitation power 共I0 = 0.05 mW兲 the low-energy
part of the spectra saturates and peaks at the high-energy side occur. The
lines are guides to the eyes.
In Fig. 2 the power-dependent PL of this sample is
shown. As the excitation density is increased, the emission
peak always broadened, indicating a possible state filling
effect.10 This signifies that if the ground state is completely
filled, the injected carriers can reach states with higher energy, where a radiative recombination is also possible. If the
excitation density is high enough, even states of the expected
quantum wire or quantum well on the pyramid could be
reached. Such an effect is clear evidence of the presence of
zero-dimensional structures within the pyramid.
Figure 3共a兲 shows the low-temperature time-integrated
PL measured by an excitation power of 15 mW, displaying a
wide emission peak at 2.17 eV. In principle, one would expect the formation of a quantum well on the side-walls of the
pyramid, a quantum wire at its edges, and a quantum dot on
the top as it has been observed for other material systems.11
Further optimization of the growth could make just one of
these structures to be dominant in the emission. Considering
this we identified three different well-defined peaks within
the broad spectrum, with energies of 2.03, 2.20, and 2.35 eV.
Time-resolved PL measurements at different temperatures were performed to obtain further information about the
recombination behavior of our structures. The PL emission
demonstrated a clear nonexponential decay. We fitted our
data with a stretched exponential model for heavily disordered materials,12
I共t兲 = I0 exp关− 共t/␶*兲␤兴,
where ␶* is the time constant and ␤ is the stretching parameter, giving a measure of the degree of disorder. This model
fits in the case of the nitride material system because of the
presence of piezoelectric fields, which have a large influence
on the overlap of the electron-hole wave function through the
quantum-confined Stark effect. For our investigated structures the value of ␤ ranges between 0.7 and 0.9. This value is
FIG. 3. 共a兲 Low-temperature time-integrated PL spectra consisting of three
different emission peaks. 共b兲 Radiative decay time vs temperature of the
low- and high-energy emissions from Fig. 3共a兲. The dashed-lines are fits,
taking zero-dimensional excitonic behavior or two-dimensional excitonic
behavior with transfer from the barriers into account.
higher compared to the other published data for InGaN lowdimensional structures,13,14 and is a result of reduced piezoelectric fields in the pyramid structure.15
Figure 3共b兲 shows the variation of the radiative decay
time with the temperature for the peaks at 2.03 eV 关zerodimensional 共0D兲兴 and 2.35 eV 关two-dimensional 共2D兲兴. Assuming that at 5 K only radiative emission is observed,16 the
radiative decay times can be calculated from the experimental decay times, taking the intensities of the emission peaks
into account. The decay time at T = 5 K for the peak at
2.03 eV is 1.5 ns, and for the peak at 2.35 eV we observed
0.9 ns. For the emission at 2.03 eV, the radiative decay time
is independent of the temperature whereas the 2.35 eV luminescence peak varies by a factor of 10. It is well known that
the radiative lifetime of a zero-dimensional exciton is nearly
constant over temperature, while the two-dimensional excitonic lifetime varies linearly with the temperature.17 This ex-
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163121-3
Appl. Phys. Lett. 87, 163121 共2005兲
Perez-Solorzano et al.
plains the temperature dependence of the emission peak at
2.03 eV 共0D兲, but in order to clarify the behavior of the
2.35 eV luminescence peak, we have to take the transport of
the carriers into the quantum well on the sidewalls of the
pyramid into account.
Assuming that the carriers in the quantum well and in
the barriers are in thermal equilibrium during the recombination process and the number of emitted electrons and holes in
the barriers are equal, the rate equations for this system describing the carrier dynamics in the low-injection regime can
be solved, resulting in an effective lifetime,18
␶eff =
␶ex
1 + C冑T exp关− 共EA/2kT兲兴
.
␶ex is the temperature-dependent excitonic lifetime mentioned above. The constant C is containing the ratio ␶ex / ␶bar
of the excitonic lifetime and the lifetime in the barrier, as
well as the effective masses of the electrons and holes in the
quantum well and barriers, respectively. EA represents the
activation energy of the excitons out of the quantum well.
With this model, we have achieved good agreement with our
experimental data 关dashed lines in Fig. 3共b兲兴. In this lowinjection regime we can derive from the obtained value of
126 meV for EA a total confinement energy ⌬E of 252 meV,
which is in the range of the band offset of a comparable
InGaN / GaN quantum well.19 From C we have extracted the
lifetime ratios ␶ex / ␶bar between 100 and 900. Presumably
these large values are due to the presence of nonradiative
decays channels that shorten the lifetime in the barriers. This
was confirmed by examinations of the temperature behavior
of the radiative and nonradiative decays 共not shown here兲.
We found that the radiative decay dominates for both emissions at 2.03 and 2.35 eV for low temperatures up to 80 K.
The constant temperature behavior of the radiative decay
with long experimental decay times confirms the zerodimensional character of the low-energy emission peak. A
tentative assignment attributes the peak at 2.2 eV to the onedimensional 共1D兲 structure 共quantum wire兲 as expected from
the sample geometry. However, the temperature dependence
of the radiative decay time does not shows unambiguously
1D character, probably due to the carrier transport processes
between the structures of different dimensionality. A more
detailed study of the pyramid edges is required in the future.
In order to clarify the origin of these emissions within
the pyramidal structure, spatially resolved CL measurements
were performed. In Fig. 4 the CL intensity collected for two
different wavelength ranges is displayed. One can clearly see
that the emission between 490 and 510 nm is mainly coming
from the sidewalls of the pyramid, whereas between 580 and
640 nm only emission from the apex of the pyramid is observable. This is in excellent agreement with our PL results
where we could determine the zero-dimensional behavior
around 610 nm.
In summary, we have performed time-integrated, timeresolved, temperature-dependent photoluminescence, and
FIG. 4. 共Color online兲 CL mapping of a single InGaN pyramid for two
different wavelength ranges. Only the emission between 580 and 640 nm
originates in the apex of the pyramid.
spatially resolved cathodoluminescence measurements on
site-controlled pyramidal InGaN quantum dots. We found a
strong green emission 共around 2.35 eV兲 originating from the
sidewalls of the pyramids, and also emission at 2.03 eV corresponding to a QD. The combination of PL and CL measurements supports the fact that an InGaN quantum dot is
formed at the apex of the pyramid, having an emission wavelength in the orange-red spectral range.
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