Transferrable GaN Layers Grown on ZnO-Coated Graphene layers for Optoelectronic Devices · 2010....
Transcript of Transferrable GaN Layers Grown on ZnO-Coated Graphene layers for Optoelectronic Devices · 2010....
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Supporting Online Material for
Transferrable GaN Layers Grown on ZnO-Coated Graphene layers for Optoelectronic Devices
Kunook Chung,1 Chul-Ho Lee,1,2 Gyu-Chul Yi1*
*To whom correspondence should be addressed. E-mail: [email protected]
Published 29 October 2010, Science 330, 655 (2010)
DOI: 10.1126/science.1195403
This PDF file includes:
Materials and Methods
Figs. S1 to S6
References
SUPPLEMENTARY ONLINE INFORMATION
Transferrable GaN Layers Grown on ZnO-Coated Graphene layers for Optoelectronic Devices Kunook Chung1, Chul-Ho Lee1,2, and Gyu-Chul Yi1*
1National Creative Research Initiative Center for Semiconductor Nanorods and Department of Physics
and Astronomy, Seoul National University, Seoul 151-747, Korea
2Department of Materials Science and Engineering, POSTECH, Pohang, Gyeongbuk 790-784, Korea
*To whom correspondence should be addressed. E-mail: [email protected]
SUPPORTING FIGURES AND DESCRIPTION
Figure S1 shows scanning electron microscopy (SEM) images of GaN grown on
bare graphene layers. On using low-temperature GaN buffer layer, GaN islands can be grown
readily along the naturally formed step-edges (Fig. S1(A)). To increase GaN nucleation sites,
many step-edges were artificially created by oxygen-plasma etching. Although the oxygen-
plasma treatment increased the GaN island density, GaN films were poly-crystalline with
random c-axis orientations, and their surfaces were rough and irregular as shown in Fig.
S1(B). The results indicate that even the typical use of a low-temperature GaN buffer layer
did not improve the film morphology or crystallinity.
A B
Fig. S1. SEM images of GaN directly grown on bare graphene layers (A) without any
treatment and (B) after oxygen-plasma treatment.
For the growth of high-quality GaN films on graphene layers, ZnO nanowalls were
employed as an intermediate layer. The details of ZnO nanowalls growth are previously
reported (S1). We further investigated the cross-sectional TEM image and selective area
electron diffraction (SAED) patterns of ZnO nanostructures grown on graphite substrates. As
shown in Figs. S2(B) and S2(C), SAED patterns clearly show that the (0002) and (112―
0)
planes of ZnO are parallel to those of the graphite [i.e., ZnO(0002)║C(0002) and ZnO(112―
0)║C(112―
0)], indicating that ZnO nanostructures were heteroepitaxially grown on graphite
with an in-plane alignment as well as c-axis orientation.
Fig. S2. (A) Cross-sectional TEM image of ZnO nanostructures grown on graphite and
corresponding SAED patterns for (B) ZnO and (C) graphite. The SAED patterns were
obtained from two areas: one from ZnO just above the interface (red circle), and the other
from graphite (blue circle).
We investigated the surface morphologies of GaN layers grown on ZnO nanowalls
with different ZnO nanowalls densities to examine lateral growth. As shown in the SEM
images of Fig. S3(A), the GaN film exhibited a flat surface due to lateral overgrowth of GaN
on high-density ZnO nanowalls. However, when the density of the ZnO nanowalls (top of Fig.
S3(B)) was too low for complete coalescence of GaN micropyramids, a rough surface
morphology with hexagonal pyramidal facets was observed (bottom of Fig. S3(B)). GaN
layers are heteroepitaxially grown on ZnO nanowalls because both GaN and ZnO have a
wurtzite crystal structure with small lattice misfits that are within 2%. The clean interface
between GaN and ZnO layers were confirmed using high-resolution TEM (S2, S3). This
result clearly indicates that lateral overgrowth of GaN on ZnO nanowalls plays a critical role
in forming high-quality GaN films with very smooth surface morphology.
1 μm
B
2 μm
1 μm
2 μm
A Before GaN growth Before GaN growth
After GaN growth After GaN growth
Fig. S3. SEM images of high-density (A) and low-density (B) ZnO nanowalls (top) and
corresponding GaN films (bottom) grown on the nanowalls.
The crystal structure and growth orientation of GaN thin films on ZnO-coated
graphene layers was investigated by x-ray diffraction (XRD) and transmission electron
microscopy (TEM). Figure S4 shows a typical θ–2θ scan result of GaN thin films grown on
ZnO-coated graphite substrates. The 2θ peaks of the thin films were observed at 34.58° and
72.90°, which correspond to the (0002) and (0004) diffraction peaks of wurtzite GaN,
respectively. Besides the c-plane XRD peaks of GaN and graphite, no other peaks were
observed in the measured range of 20–80°, indicating that the films were grown with a
preferred c-axis normal to the graphite substrates. In addition, Phi scans of GaN layers
exhibited repeated XRD peaks with 60 degree shifts, indicating six-fold symmetry of GaN
layers. These XRD scan results strongly suggest that GaN films are heteroepitaxially grown
on ZnO-coated graphite substrates.
Fig. S4. XRD θ−2θ scan of a GaN thin film grown on a ZnO-coated graphite substrate.
20 30 40 50 60 70 80
102
103
104
GaN(0004)
Graphite(004)GaN(0002)
Graphite(002)
Inte
nsity
(arb
.uni
ts)
2θ (deg.)
Further structural analysis was performed using TEM. Figure S5(A) shows cross-
sectional TEM images of GaN thin films on ZnO-coated graphene layers. Although low-
magnification TEM image shows that flat GaN films were grown on graphene layers without
significant microstructural defects such as voids or cracks, threading dislocations are clearly
shown, similar to those of the GaN layers grown on single crystal sapphire substrates.
Additionally, the high-resolution TEM image of GaN films in Fig. S5(B) reveals a well-
ordered crystal lattice array, and the lattice spacing between the adjacent planes was
measured to be 0.52 nm, corresponding to the d-spacing of GaN(0001) planes. The electron
diffraction pattern exhibits a regular spot array as shown in the inset of Fig. S5(B). These
TEM results suggest that high-quality GaN films were grown with c-axis orientation on
graphene layers.
500 nm
A (0002)
(1010)�
0.52 nm
2 nm
B
Fig. S5. (A) Low-magnification and (B) high-resolution TEM images and diffraction
patterns of a GaN thin film grown on ZnO-coated graphene layers.
Figure S6 shows temperature-dependent PL spectra of GaN thin films on graphene
layers in the range of 20–210 K. At the low temperature of 20 K, the dominant emission peak
was observed at 3.467 eV, which is attributed to excitons. Additionally, donor-acceptor pair
(DAP) recombination and its longitudinal optical-phonon replica emission peaks were
observed at 3.269 and 3.190 eV, respectively. With increasing temperature, the DAP emission
exhibited at blue shift, which originated from the emission related to free electron-bound
acceptor transitions, and finally quenched and vanished at the high temperature of 210 K (S4).
Furthermore, there was no emission peak associated with carbon impurities around 2.8 eV in
the low-temperature PL spectra (S5). This strongly suggests that carbon atoms in graphene
layers were not incorporated into GaN films during the growth.
3.0 3.1 3.2 3.3 3.4 3.5102
103
104
105
PL
inte
nsity
(arb
.uni
ts)
Photon energy (eV)
3.467
3.2693.190
20 K30 K40 K50 K60 K80 K100 K120 K140 K160 K180 K210 K
Fig. S6. Temperature-dependent PL spectra of GaN thin films grown on graphene layers
in the range of 20–210 K.
METHODS
Preparation of ZnO nanowalls on plasma-treated graphene layers
ZnO nanowalls were grown on graphene layers using catalyst-free metal–organic
chemical vapor deposition (MOCVD). Graphene layers were mechanically exfoliated from a
graphite powder using a simple Scotch tape method and transferred onto Al2O3(0001)
substrates. The typical size of graphene layers was in the range of 1 µm to 2 mm. Before
growth of the ZnO nanowalls, oxygen-plasma treatment was performed at an oxygen partial
pressure of 100 mTorr and an applied current of 50 mA. For ZnO growth, high-purity
diethylzinc (DEZn) and oxygen (>99.9999%) were used as the reactants for Zn and O, and
high-purity argon (>99.9999%) as the carrier gas. The flow rates of DEZn and oxygen were
20 and 40 standard cubic centimeters per minute (sccm), respectively. The reactor pressure
and temperature during the growth were kept at 6 Torr and 700°C, respectively.
Growth of GaN thin films and multiple InxGa1-xN/GaN quantum structures on ZnO-
coated graphene layers
GaN thin films were grown on ZnO-coated graphene layers using MOCVD. Prior to
GaN film growth, a thin GaN intermediate layer was grown at 600°C to prevent degradation
of ZnO nanowalls and prohibit reactions between ZnO and GaN layers at a higher
temperature (S2, S3). Trimethylgallium and high-purity ammonia (>99.999%) were
employed as the reactants. Nitrogen was used as an ambient gas and the growth pressure was
kept at 200 Torr for the low-temperature GaN intermediate layer. After growing the
intermediate layer, the growth temperature was raised to 1080–1100°C for the growth of
epitaxial GaN layers. At this stage, hydrogen was used as an ambient gas and carrier gas, and
the reactor pressure was kept at 100 Torr. Typical GaN thin film thicknesses were in the
range of 2 to 5 μm.
GaN-based p–n homojunction LED structures with InxGa1-xN/GaN MQWs were
grown sequentially after the preparation of GaN thin films using conventional GaN MOCVD.
Following the Si-doped n-GaN layer deposition, three-period InxGa1-xN/GaN MQWs with a
2-nm-thick well and 15-nm-thick barrier layers were grown at 760 and 850°C, respectively,
from which we expected to observe visible-light emissions from thin-film LED devices.
Subsequently, an Mg-doped p-GaN layer with a thickness of 300 nm was deposited on the
top of the GaN quantum barrier layer at 1000°C.
Surface morphology and crystal structure characterizations
Surface morphological analysis was performed using a SEM (JEOL 6510) and
crystal structures of GaN layers were investigated using TEM (TECNAI F20) and high-
resolution XRD (Bruker D8 Discover).
LED fabrication
To fabricate LED devices, semi-transparent Ni (10 nm)/Au (10 nm) bi-layers were
deposited onto the top surface of p-GaN using thermal evaporation. Then, to obtain the ohmic
contact to p-GaN, a rapid thermal annealing process was performed in ambient air at 500°C
for 3 min. Additionally, the graphene layer underneath the n-GaN was used for the bottom
electrode in the vertical geometry of the devices.
Electrical and optical characterization
The EL and I–V characteristic of the devices were measured by applying the DC
voltage to the device using a source meter (Keithley 2400). The EL and PL spectra were
measured using a detection system equipped with a monochromator and a charge-coupled
device.. A He-Cd laser (325 nm) and a pulsed Nd:YAG laser (355 nm) were employed as
optical excitation sources for the PL spectroscopy. The temperature-dependent PL
measurements were performed in the range of 20–300 K using a He Displex refrigerating
system. Details of the PL measurement have been reported elsewhere (S6).
REFERENCES
S1. Y.-J. Kim, J. H. Lee, G.-C. Yi. Appl. Phys. Lett. 95, 213101 (2009).
S2. S. J. An et al., Appl. Phys. Lett. 84, 3612–3614 (2004).
S3. Y. J. Hong et al., New J. Phys. 11, 125021 (2009).
S4. M. A. Reshchikov, H. Morkoç, J. Appl. Phys. 97, 061301 (2005).
S5. R. Armitage et al., Appl. Phys. Lett. 82, 3457–3459 (2003).
S6. W. I. Park, G.-C. Yi, and H. M. Jang, Appl. Phys. Lett. 79, 2022– 2024 (2001).