Monolithic integration of InGaN segments emitting in the blue, green, and
red spectral range in single ordered nanocolumns
S. Albert, A. Bengoechea-Encabo, X. Kong, M. A. Sanchez-Garcia, E. Calleja et al.
Citation: Appl. Phys. Lett. 102, 181103 (2013); doi: 10.1063/1.4804293
View online: http://dx.doi.org/10.1063/1.4804293
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APPLIED PHYSICS LETTERS 102, 181103 (2013)
Monolithic integration of InGaN segments emitting in the blue, green,
and red spectral range in single ordered nanocolumns
S. Albert,1 A. Bengoechea-Encabo,1 X. Kong,2 M. A. Sanchez-Garcia,1 E. Calleja,1
and A. Trampert2
1
ISOM and Dept. Ingenierıa Electr
onica, ETSI Telecomunicaci
on, Universidad Polit
ecnica de Madrid,
Ciudad Universitaria s/n, 28040 Madrid, Spain
2
Paul-Drude-Institut f€
ur Festk€
operelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
(Received 21 January 2013; accepted 24 April 2013; published online 6 May 2013)
This work reports on the selective area growth by plasma-assisted molecular beam epitaxy and
characterization of InGaN/GaN nanocolumnar heterostructures. The optimization of the In/Ga and total
III/V ratios, as well as the growth temperature, provides control on the emission wavelength, either in
the blue, green, or red spectral range. An adequate structure tailoring and monolithic integration in a
single nanocolumnar heterostructure of three InGaN portions emitting in the red-green-blue colors lead
C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4804293]
to white light emission. V
The monolithic integration of high efficient, tunable,
and temperature stable red, green, and blue (RGB) emitters
for white light generation is a challenge in solid-state lighting. Group III-nitride semiconductors are most convenient
for this application, in particular, InGaN alloys whose
bandgap covers the whole visible range.1–3 So far, most
research efforts focused on c-plane (polar) InGaN/GaN
quantum wells (QWs) structures as the active region of light
emitting diodes (LEDs). However, the reduced radiative
recombination rate due to spontaneous and piezoelectric
polarizations (Quantum Confined Stark effect) and the
increasing density of non-radiative defects with In content
pose severe limitations for InGaN layers application for red
and green emissions. Though some work has been done on
semi-polar and non-polar structures aiming to avoid/reduce
the internal electric field, the high defect density is still a
major issue.
In a different approach, it has been shown that dislocation- and strain-free group-III nitrides can be grown on a variety of substrates4–8 as self-assembled nanocolumns (NCs).
Aside from their very high crystal quality, NC-based LEDs
would benefit from better light extraction efficiency as compared to standard LEDs. White-emitting LEDs based on selfassembled InGaN/GaN nanocolumnar structures have been
demonstrated.9,10 However, self-assembled InGaN/GaN
NC-based LEDs usually show an inhomogeneous distribution of In% caused by geometry dispersion and strain distribution (radial and axial) within the active region.9,11 In
addition to planarization difficulties, dark spots may generate
from defective NCs (crystal defects upon NCs merging) and
an inhomogeneous current injection leads to a poor electroluminescence (EL) yield.12
The recently developed selective area growth (SAG) of
GaN NCs13–16 produces geometrically ordered arrays with
very little morphology dispersion among NCs. In the case of
SAG of c-plane GaN NCs, the growth front (topside) is generally formed by semi-polar planes (r-planes) and a topmost
c-plane, leading to a “pencil-like” profile, though this morphology depends strongly on the III/V ratio used.17 Aiming
for white light emission, the planar monolithic integration of
0003-6951/2013/102(18)/181103/4/$30.00
four distinct SAG InGaN/GaN LEDs emitting in the green
and orange spectral range has been demonstrated recently.18
This work reports on the SAG of InGaN/GaN NC structures on GaN/sapphire templates by plasma assisted molecular beam epitaxy (PAMBE) using a Ti-mask. With an
appropriate selection of growth conditions, InGaN NCs emitting in the RGB spectral range have been fabricated. Further,
white light emission at room temperature (RT) is achieved
by monolithic integration of RGB emitting InGaN sections
within each NC.
The PAMBE system was equipped with a rf-plasma
source providing active nitrogen and standard Knudsen cells
for Ga and In. All samples were grown on nanohole titanium
masks fabricated by colloidal lithography on (0001) GaN/
sapphire templates. Details on the mask and substrate preparation can be found elsewhere.19 Nanoholes were arranged in
a compact hexagonal lattice, with an average pitch (distance
between center points of two adjacent holes) of around
270 nm and a diameter of 190 nm. Impinging molecular
fluxes were calibrated in (0001) GaN (for Ga and N) and
(0001) InN (for In) growth rate units (nm/min).20 In wurtzite
GaN and InN, the areal densities referring to 1 monolayer
are 1.14 1015 GaN/cm2 and 9.17 1014 InN/cm2, respectively.21,22 The samples were characterized by scanning
electron microscopy (SEM), high resolution transmission
electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), low-loss electron energy-loss
spectroscopy (low-loss EELS), and photoluminescence (PL)
excited by a He-Cd laser with a power density of 1 W/cm2.
A problem, which needs to be addressed in order to grow
InGaN/GaN nanocolumnar heterostructures, is the loss of selectivity caused by the lower temperatures used during the
growth of InGaN as compared to the growth of GaN. A solution for this problem is the use of shadowing effects, i.e., the
reduction of the amount of Ga, In, and active nitrogen arriving
on the mask due to a high NC length to pitch ratio, in order to
prevent nucleation on the mask. For this purpose, 600 nm long
GaN NCs, sufficiently long for the given mask geometry, were
grown in a two step way: (i) 1 h growth of GaN NCs at 880 C
to start selective growth16 (Ga-flux, UGa, of 16 nm/min and a
102, 181103-1
C 2013 AIP Publishing LLC
V
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181103-2
Albert et al.
Appl. Phys. Lett. 102, 181103 (2013)
N-flux, UN, of 5 nm/min) with a growth rate of 3 nm/min, and
(ii) 1 h of GaN NCs growth at 840 C (UGa ¼ 8 nm/min and
UN ¼ 14 nm/min) with a growth rate of 7 nm/min. The second
growth step was used for practical reasons, such as the reduction of Ga consumption as well as an increased growth rate. In
order to achieve emission in the blue, green, and red spectral
range, a first series A (T1, T2, and T3) of InGaN NCs was
grown on top of the GaN NCs under the growth conditions
described in Table I. A second series B of two samples (T4
and T5) was grown under the same conditions for the GaN
part as in series A, but using a RGB structure formed by stacking the three InGaN sections (T1, T2, and T3) in the InGaN
part in order to achieve white light emission.
In both samples of series B, the blue emitting part T1 was
grown first, followed by the green emitting part T2, and the red
emitting part T3, to minimize the decomposition of the high
In% InGaN section. The overall growth time of the InGaN
RGB region was 1 h, but while in sample T4 each InGaN section was grown for 20 min (20 min/20 min/20 min); in sample
T5, the growth times were modified to 41 min/11 min/8 min to
tune the emission towards real white emission. The growth was
stopped for 5 min at every interface to adapt the growth conditions of the respective sections T1 to T3 (Table I).
Figure 1 shows cross-section and top view SEM pictures
of series A samples (T1 to T3) as well as photographs of the
emission color. Taking into account the height of the GaN
part grown first, i.e., 600 nm, the height of the InGaN regions
can be estimated from the cross-sectional SEM pictures in
Figure 1, being 280 nm, 390 nm, and 230 nm for samples T1,
T2, and T3, respectively. Photographs, taken while exciting
PL at T ¼ 12 K, show red-orange, green, and blue emissions.
Notice that both high In/Ga ratios and low growth temperatures (sample T3) promote the incorporation of In.
According to the varying growth conditions for each sample,
different top morphologies can be observed, i.e., pyramidal
top for T1 and T2 and flat top for T3, where the InGaN section resembles a truncated pyramid with a hexagonal c-plane
top facet (better seen in top view of sample T3 in Figure 1).
This might relate to a transition from nitrogen rich (T1 and
T2) to metal rich (T3) growth, as reported before for the case
of SAG of GaN NCs.17 The overall nanocolumnar shape also
changes from sample to sample due to their specific growth
conditions, even showing some merging (sample T2).
Low temperature (LT) and RT PL spectra from samples
T1 to T3 are shown in Figure 2(a). The In content estimated
from the PL peak position at LT23 is 17% for T1 (the peak
at 2.77 eV has been considered for the calculation), 27%
for T2, and 38% for T3. The FWHM values of the main
FIG. 1. Top view SEM, cross-sectional SEM, and PL emission pictures
from InGaN/GaN NCs.
InGaN peaks at T ¼ 12 K show no clear trend with In content
(141 meV for T1; 147 meV for T2; and 99 meV for T3) being
all of them significantly lower than those corresponding to
self-assembled InGaN/GaN NCs reported before,24 indicating a higher homogeneity in In distribution among the NCs.
Therefore, the observed broadening of the InGaN peaks
most likely originates from In distribution inside NCs,
either caused by “lattice pulling”25,26 and/or thermal
decomposition.24
Figure 2(b) shows LT and RT PL spectra of sample T4
in which each segment of the RGB structure was grown for
20 min. A blue-white emission is observed both at LT and
RT, though a significant intensity quenching occurs at RT,
particularly strong for the green-blue range. In addition, the
spectrum does not show a significant contribution in the red
region.
In order to tune the RGB structure towards real white
light emission, sample T5 was grown using different growth
times for each color section, namely, 8 min for the blue,
11 min for the green, and 41 min for the red, aiming to
enhance the low-energy (red) emission. The growth times of
the respective InGaN regions were calculated based on the
integrated room temperatures PL intensities of the single
color emissions of samples T1-T3 (Figure 2(a)) with the goal
to achieve a more equal spectral contribution of the three
regions. Figure 2(c) shows LT and RT-PL spectra of sample
T5 covering the energy range from 1.8 to 2.8 eV (almost the
TABLE I. Growth conditions for the InGaN NCs grown on top of GaN NCs.
Impinging fluxes in nm/min ( 1014 atoms/(s cm2))
Sample number
T1
T2
T3
Ga
In
N
III/V ratio
In/Ga ratio
Tsample in C
4.3
(3.17)
4.3
(3.17)
2.3
(1.7)
4.3
(2.27)
6.3
(3.33)
6.3
(3.33)
14
(10.3)
17
(12.5)
14
(10.3)
0.5
0.7
700
0.5
1
700
0.5
2
650
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181103-3
Albert et al.
FIG. 2. Room- and low temperature (12 K) PL spectra of (a) samples T1,
T2, T3, (b) T4, and (c) T5 (insets show photographs of the emission color
taken during the PL experiments; numbers in brackets correspond to the
magnification factor of the respective spectrum).
whole visible spectrum) yielding white color. Still a significant intensity quenching at RT is observed at the green-blue
region of the spectrum. This behavior, reported before for
self assembled InGaN/GaN NCs,24,27 is speculated to be
Appl. Phys. Lett. 102, 181103 (2013)
related to carrier diffusion to regions of higher In% (higher
localization) upon thermal excitation.
Figure 3 shows the results of the structural investigations
performed by TEM on sample T5. The bright-field scanning
(S)TEM image in Figure 3(a) reveals the morphology of the
NCs in cross-section demonstrating their high uniformity in
the pencil-like shape and the absence of threading dislocations
or other extended defects in most observed NCs. The diameter
is slightly increased (as indicated by the black arrows) at the
transition region from GaN to the InGaN layer, which is identified by the dark strain contrast. This is a clear indication for
elastic surface relaxation of the coherent InGaN. The highangle annular dark-field (HAADF) STEM imaging is applied
in Figure 3(b) in order to resolve the different InGaN regions
by observing an increase in bright contrast, which is directly
correlated to the In content. In fact, three regions with distinct
bright contrast can be identified and marked as (I), (II), and
(III) in Figure 3(c). However, well-defined boundaries
between the regions cannot be determined but for an r-plane
at the InGaN/GaN interface. The reasons for these blurred
transitions can be both a pulling effect that widens the interface region and the pencil-like morphology that results in an
umbrella-shaped heterostructure, which leads to an overlapping from regions with different In content.
In order to get more information about the local In concentration and distribution inside the NCs of sample T5 spatially resolved low-loss EELS28 was carried out along the
axial and radial directions using a beam diameter of about
10 nm. Figure 4(a) displays the bright-field TEM image of a
selected NC including the positions of the EELS measurements marked by small dots. Along the axial direction (see
Fig. 4(b)), an increase of the In concentration is observed
reaching up to 38% to 45% at the NC tip, which corresponds
to region III. The initial low In% of about 5% (points 1 to 3)
further increasing most likely represents region I, where the
In% inhomogeneous distribution is speculated to arise from
lattice pulling effects.28 The In% change between points 4
and 5 can be attributed to region II.
The radial scan shown in Figure 4(c) reveals an In concentration variation along the NC radius being maximum at
the edges and minimum in the center. This result most likely
reflects the scan line going through areas of different In%
inside region I. The inhomogeneities are assumed to be
caused by lattice pulling effects inside the pencil like structure. The specific pencil-like morphology of the heterostructures as well as the ambiguities arising from the projection
requirements of TEM in combination with the thin foil preparation make a more detailed EELS analysis difficult.
Nevertheless, the results demonstrate the presence of regions
FIG. 3. (a) Bright-field STEM image of the
NCs array of sample T5 in cross-section, (b)
corresponding HAADF Z-contrast image
identifying the InGaN regions on the upper
part of the columns. (c) Magnified part of
the top area with the different InGaN regions
marked by I, II, and III (inset: appropriate
bright-field STEM of the NC; dashed line
corresponds to GaN/InGaN interface).
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181103-4
Albert et al.
Appl. Phys. Lett. 102, 181103 (2013)
FIG. 4. Bright-field TEM image along
[1100] direction of a single InGaN/GaN
nanorod (a), and axial (b) and radial (c) lowloss EELS scans as indicated by dots in the
bright-field image.
with different In concentration, which is in agreement with
PL measurements.
In summary, ordered InGaN/GaN NCs were successfully grown by PAMBE on GaN/sapphire templates. The
control of In incorporation by means of growth temperature,
In/Ga ratio, and III/V ratio allowed the fabrication of blue,
green, and red light emitters and provided control of the
emission wavelength. The monolithic integration of three
InGaN portions emitting in the red-green-blue colors in a
single nanocolumnar heterostructure leads to white light
emission. It is shown that the emission color can be tuned by
playing with the relative “weight” of the colors by means of
a thickness variation of the respective InGaN regions.
We acknowledge partial financial support by the EU
FP7 Contracts SMASH 228999-2, GECCO 280694-2, the
Initial Training Network RAINBOW (PITN-GA-2008213238), and by the Spanish projects CAM/P2009/ESP-1503
and MICINN MAT2011-26703.
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