www.sciencemag.org/cgi/content/full/330/6004/655/DC1

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: gcyi@snu.ac.kr

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: gcyi@snu.ac.kr

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).