Journal of the Korean Physical Society, Vol. 45, December 2004, pp. S840∼S843
Self-Assembled GaN Quantum Dots in GaN/Alx Ga1−x N Structures
Grown by PAMBE
G. N. Panin,∗ Y. S. Park and T. W. Kang
Quantum-functional Semiconductor Research Center and Department of Physics, Dongguk University, Seoul 100-715
T. W. Kim
Advanced Semiconductor Research Center, Division of Electrical and Computer Engineering, Hanyang University, Seoul 133-791
K. L. Wang and M. Bao
Device Research Laboratory and Department of Electrical Engineering,
University of California, Los Angeles, CA 90095-1594
High-resolution scanning electron microscopy (HRSEM), cathodoluminescence (CL) and electron
beam induced current (EBIC) techniques were used to investigate the structural, optical and electrical properties of GaN/Alx Ga1−x N structures with self-assembled quantum dots (QDs) grown by
molecular beam epitaxy (MBE). The results of the spatially resolved investigations show that GaN
QD assemblies divided laterally by a 2D GaN layer can be formed on Al0.4 Ga0.6 N surface. The size
and the density of QD assemblies are significantly affected by the surface polarity and the thickness
of the GaN epilayer. The CL spectra of the QDs show blue-shifted GaN near-band-gap emission,
and the shift is stronger for the Ga-face GaN epilayers. The EBIC measurements indicate reduced
non-radiative charge-carrier recombination in the QD regions.
PACS numbers: 81.07.Ta; 68.37.Hk; 78.67.Hc
Keywords: Quantum Dots, GaN/AlGaN structures, CL, EBIC
bility of adjusting the size and density of the assemblies
by using different polarities of the GaN surface and the
thickness of GaN epilayer was investigated.
I. INTRODUCTION
Semiconductor QDs have attracted considerable attention due to their promising applications in optoelectronic
and electronic devices, such as lasers, photodetectors [1–
9] and single-electron transistors [10] operating at low
current and high temperature. Until now, QD structures have been mainly fabricated from materials grown
under compressive stress in the Stranski-Krastanov (SK)
growth mode, due to the strain-induced self-organizing
mechanism. The SK growth mode requires a large lattice mismatch between the wetting and the barrier materials to achieve elastic relaxation through the formation of QDs [11]. The formation of QDs from slightly
stressed semiconductors by using a surfactant [12], an
ion-sputtering technique [13], or hydrogenation treatment [14] has also been previously demonstrated.
In this paper, we report on the self-formation of GaN
nanometer-sized dots in local regions of GaN/AlGaN
structures by plasma-assisted molecular beam epitaxy
(PAMBE). HRSEM, CL and EBIC measurements were
carried out to investigate the optical and electrical properties of the structures with QD assemblies. The possi∗ E-mail:
II. EXPERIMENTS
The GaN/AlGaN structures were grown on sapphire
substrates by using a PAMBE system. An inductively
coupled radio-frequency (RF) plasma source provided reactive nitrogen gas with a purity of 99.9999 % while Ga
and Al with purities of 99.9999 % were evaporated by
using a conventional effusion cell. Prior to GaN film
growth, the surfaces of chemically cleaned substrates
were exposed to an activated nitrogen beam for 10 min,
so that they were completely covered with nitridated layers. The deposition of the GaN epilayers on Al0.4 Ga0.6 N
layers grown at 550 ◦ C was carried out at a substrate
temperature of 750 ◦ C. The thicknesses of GaN epilayers
grown on an Al0.4 Ga0.6 N buffer layer of 100-nm thickness
were 10 and 20 nm. Ga-face and N-face GaN epilayers
were grown by a technique described previously [15].
III. RESULTS AND DISCUSSION
g panin@dongguk.edu
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Self-Assembled GaN Quantum Dots in GaN/Alx Ga1−x N Structures· · · – G. N. Panin et al.
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Fig. 1. SEM images of GaN QD assemblies in GaN layer
grown on Al0.4 Ga0.6 N surface in (a) CL, and (b) SE modes.
The inset shows a HRSEM image of QDs within the assembly.
The QD width is 30 nm.
Fig. 3. (a) EBIC image of the GaN/AlGaN structure with
a QD assembly, and (b) a line EBIC profile acquired through
the assembly.
Fig. 2. Cathodoluminescence images of GaN nanodot assemblies grown on Al0.4 Ga0.6 N layers with different surface
polarities and thicknesses of the GaN epilayers: Ga-face GaN
epilayers with thickness of (a) 10 nm (the dot assembly density is 8 × 106 cm−2 ), and (b) 20 nm (the dot assembly density is 2 × 106 cm−2 ) and N-face GaN epilayers with thickness
of (c) 10 nm (the dot assembly density is 4 × 107 cm−2 ), and
(d) 20 nm (the dot assembly density is 6 × 106 cm−2 ).
HRSEM-CL examination of the GaN/AlGaN structures revealed assemblies of GaN nanodots, which are
formed on the Al0.4 Ga0.6 N surface. The size and the
density of the GaN QD assemblies divided by a 2D GaN
layer are significantly affected by the surface polarity and
the thickness of the GaN epilayer. Figure 1 shows SEM
images of GaN QD assemblies in GaN/AlGaN structure
in CL (Figure 1 (a)) and SE (Figure1 (b)) modes. The
inset shows a HRSEM image of GaN QDs with width
less than 30 nm. CL images of GaN nanodot assemblies for different surface polarities and thicknesses of
the GaN layers are shown in Figure 2. The assemblies
with diameters of approximately 2.5- 4 µm were formed
in the Ga-face GaN epilayer, while the 1-2 µm assemblies
of QDs were formed in the N-face GaN. The thin GaN
layer shows small size and high density of nanodot assem-
blies and smaller width of dots. The assemblies with size
of approximately 100 nm consisting of GaN QDs with
width less than 10 nm are observed for the 10-nm GaN
layer. The highest density of assemblies (up to 4 × 107
cm−2 ) is observed for the 10-nm N-face GaN epilayers. It
should be noted that the GaN nanodots show enhanced
luminescence, whereas small Ga droplets, which could be
immerged during the initial stage of growth, show strong
non-radiative charge carrier recombination (Figure 2 (a):
the small dark dots).
Figure 3 shows an EBIC image of the GaN/
Al0.4 Ga0.6 N structure with a QD assembly as a circle
of 3-µm size with bright contrast. The enhanced EBIC
signal of QDs indicates low non-radiative recombination
of charge carriers inside the QDs. This is in agreement
with the enhanced luminescence of the QD assemblies
revealed by CL measurements.
Figure 4 shows ultra violet CL spectra of GaN QD assemblies for Ga-face and N-face of GaN epilayers. Three
peaks at 3.5, 4.12 and 3.9 eV are observed in the spectra
for N-face samples. The peak at 3.5 eV is attributed to
neutral donor-bound exciton (Do , X) emissions in GaN
[16]. The position of the peak does not depend on the
GaN-layer polarity. The peak at 4.12 (4.22) eV is attributed to the Al0.4 Ga0.6 N near-band-gap exciton emission. Some difference in energy position of the peak for
the Ga-face sample is attributed to different strains in
the Ga-face and N-face GaN structures, which are revealed by XRD measurements. The peak centered at
3.9 eV is related with GaN QD emissions. The peak
position depends on the thickness and polarity of the
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Fig. 4. 80-K ultra violet CL spectra obtained inside single
assemblies in GaN epilayers with different polarities. Circles
and rectangles represent the spectra for the Ga-face GaN and
the N-face GaN epilayers, respectively.
GaN epilayer. The QD luminescence is blue-shifted for
the Ga-face GaN epilayer. A peak at 4.07 eV is detected for the Ga-face GaN epilayer of 20-nm thickness.
The blue-shift increases as the GaN epilayer thickness
decreases. This indicates the formation of smaller QDs
with stronger quantum confinement of carriers. It should
be noted that the CL spectra of the QD assemblies show
broad bands as a result of the QD size dispersion.
In order to investigate the origin of QD assembly formation, deep-level CL measurements of the structures
were performed. Figure 5 shows deep-level CL spectra
obtained at 80 K for (a) the N-face GaN, and (b) the Gaface GaN epilayers with different thickness. Three peaks
centered at 2.43, 2.07 and 1.9 eV were observed for the
Ga-face GaN epilayers. Their peak intensities depend
strongly on the surface polarity and the thickness of the
GaN epilayer. The relative intensity of the 2.07-eV luminescence emission increases with decreasing thickness
of the GaN epilayer, for both the Ga-face and the N-face
GaN layer. For N-polarity of the GaN epilayer, red-shift
of the CL peak (2.02 eV) was observed. XRD measurements on the samples indicate a larger tensile strain in
the N-face GaN/AlGaN structure. In addition, the CL
peak at 1.9 eV shows a relatively enhanced intensity for
the N-face GaN epilayers. The peaks at 2.43, 2.07 and
1.9 eV are attributed to defect-related deep-level luminescence [17–19]. Although the origin of this luminescence is not completely understood, different proposed
models [20–22] suggest that the emission is due to native
point defects and impurities. Our results indicate that
impurity-assisted assembly formation takes place. The
formation of the assemblies divided by a 2D GaN layer
Journal of the Korean Physical Society, Vol. 45, December 2004
Fig. 5. 80-K deep-level CL spectra for (a) the N-face GaN,
and (b) the Ga-face GaN epilayers with different thickness.
Filled and open symbols represent the spectra for GaN epilayers with thickness of 10 nm and 20 nm, respectively.
grown on Alx Ga1−x N surface could be explained by the
inhomogeneous stress induced by impurities. The localimpurity induced strain leads to nanodot formation. The
lattice mismatch between Al0.4 Ga0.6 N and GaN layers
is not enough for effective elastic relaxation through the
formation of dots by SK mode. However, the effect of impurities acting as surfactant [12] and strain source [19,23]
could result in nanodot formation inside the GaN layer
regions with an enhanced impurity concentration. The
impurities distributed inhomogeneously in slightly mismatched AlGaN/GaN structures may provide the additional strain leading to the formation of the GaN nanodot
assemblies. The density and size of the GaN nanodot assemblies depend on the thicknesses of the GaN epilayers.
This correlates with the thickness dependences of the impurity and strain distributions. Gettering of impurities
by structural defects might lead to an inhomogeneous impurity distribution and an increase of local strains, with
subsequent relaxation through the formation of the QD
assemblies.
The GaN QD assemblies in the GaN/AlGaN structures show blue-shift of near-GaN-band-gap luminescence, which indicates a small size of dots. These assemblies strongly affect the luminescence and transport
properties of the GaN/AlGaN structures. The controllable direct QD formation in local regions can be used
for device fabrication.
IV. CONCLUSIONS
In summary, the QD assembly formation in
GaN/AlGaN structures with different polarity of the
Self-Assembled GaN Quantum Dots in GaN/Alx Ga1−x N Structures· · · – G. N. Panin et al.
GaN layer was investigated through spatially-resolved
HRSEM, CL and EBIC measurements. The results show
that GaN QD assemblies with enhanced CL and EBIC
are formed in local regions of the GaN layer grown on
the Al0.4 Ga0.6 N surface. The size and the density of
the assemblies and nanodots are affected strongly by the
polarity and thickness of the GaN epilayer. Blue-shift
of GaN near-band-gap luminescence in small GaN QDs
with reduced non-radiative charge-carrier recombination
is observed.
ACKNOWLEDGMENTS
This work was supported by the Korean Science
and Engineering Foundation through the Quantumfunctional Semiconductor Research Center at Dongguk
University.
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