Conformal Al2O3 dielectric layer deposited by atomic layer deposition for graphene-based nanoelectronics

Article abstract of DOI:10.1063/1.2928228

We present a facile route which combines the functionalization of a highly oriented pyrolytic graphite surface with an atomic layer deposition (ALD) process to allow for conformal Al₂O₃ layers. While the trimethylaluminum (TMA) H2 O process caused selective deposition only along step edges, the TMA O3 process began to provide nucleation sites on the basal planes of the surface. O3 pretreatment, immediately followed by the ALD process with TMA O3 chemistry, formed Al₂O₃ layers without any preferential deposition at the step edges. This is attributed to functionalization of graphene by ozone treatment, imparting a hydrophilic character which is desirable for ALD deposition.

Full text of DOI:10.1063/1.2928228

Conformal Al 2 O 3 dielectric layer deposited by atomic layer deposition for graphene-
based nanoelectronics
Bongki Lee, Seong-Yong Park, Hyun-Chul Kim, KyeongJae Cho, Eric M. Vogel, Moon J. Kim, Robert M. Wallace
, and Jiyoung Kim
Citation: Applied Physics Letters 92, 203102 (2008); doi: 10.1063/1.2928228
View online: http://dx.doi.org/10.1063/1.2928228
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APPLIED PHYSICS LETTERS 92, 203102 ͑2008͒
Conformal Al2O3 dielectric layer deposited by atomic layer deposition
for graphene-based nanoelectronics
Bongki Lee, Seong-Yong Park, Hyun-Chul Kim, KyeongJae Cho, Eric M. Vogel,
Moon J. Kim, Robert M. Wallace, and Jiyoung Kima͒
Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson,
Texas 75080, USA
͑Received 15 February 2008; accepted 24 April 2008; published online 20 May 2008͒
We present a facile route which combines the functionalization of a highly oriented pyrolytic
graphite surface with an atomic layer deposition ͑ALD͒ process to allow for conformal Al2O3
layers. While the trimethylaluminum ͑TMA͒/H2O process caused selective deposition only along
step edges, the TMA/O3 process began to provide nucleation sites on the basal planes of the surface.
O3 pretreatment, immediately followed by the ALD process with TMA/O3 chemistry, formed Al2O3
layers without any preferential deposition at the step edges. This is attributed to functionalization of
graphene by ozone treatment, imparting a hydrophilic character which is desirable for ALD
deposition. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2928228͔
Carbon atoms in nanocrystalline structures of a honey-
comb network with sp2 bonding, such as carbon nanotubes
͑CNTs͒ and graphene, have attracted much attention due to
their superior electrical transport characteristics. Extraordi-
narily high carrier mobility on the order of 10 000 cm2/V s
can be achieved even at room temperature.1–3 Up until now,
carbon nanotube ͑CNT͒-based devices have been extensively
investigated for next generation nanoelectronics because of
potential scaling benefits originating from its one dimen-
sional geometric structure. Despite the above assertion,
nanotube technology has confronted significant issues in
terms of integration into high density devices due to its ran-
dom nature, causing difficulties in purification, controllabil-
ity, and alignment. Two dimensional graphene is expected to
be compatible with conventional device processing based on
flat substrates. However, in order to realize graphene-based
devices, various challenging issues, such as dielectrics and
contact for graphene devices, need to be overcome. Since the
carrier concentration and polarity in the graphene layer is
modulated by an electric field,4,5 it is vital to develop an
atomically uniform gate dielectric deposition technique so as
to provide uniform electric field over an active graphene
area, while avoiding damage to the graphene surface and
potentially high interface defect density ͑Dit͒ between the
graphene and the dielectric.
To date, most graphene research work has been per-
formed by using a back gate structure typically with a blan-
ket SiO2 layer on top of a degenerated Si wafer.1,2,4,5 This
method cannot provide a localized field essential for surface
channel field effect transistor ͑FETs͒ applications. To make
use of top-gated devices, a high quality dielectric on top of
graphene is required. Although a few papers report gate di-
electrics by physical vapor deposition, such as sputtering or
evaporation, such methods likely result in damage to a
graphene sheet only a few monolayers thick. Lemme et al.6
reported a significant reduction in both electron and hole
mobilities when using a 20 nm top-gated dielectric deposited
by evaporation. On the other hand, chemical vapor deposi-
tion, particularly atomic layer deposition ͑ALD͒, allows
atomically precise control over thickness and uniformity
while averting physical damage to the interface by energized
particles. Unfortunately, the hydrophobic characteristics of
graphene basal planes do not provide appropriate nucleation
sites for the ALD precursor, whereas step edges of graphene
are selectively decorated by ALD.7 In the case of CNT FETs,
the gate dielectric layer was nucleated and grown starting
from the substrate rather than a CNT. In this method, the
dielectric layer with nonuniform thickness covers the tube.8
In order to improve the coverage of the ALD film on CNT
devices, two approaches have been extensively attempted to
date. The first approach involves a non-covalent bonding
method, such as DNA and NO2 functionalization. These
were applied for single wall CNT applications.9,10 Williams
et al.11 implemented the NO2 pretreatment technique into
deposition of the Al2O3 layer on top of graphene for top-
gated structures. The second involved covalent bonding
driven by chemical processes: fluorination, ozonolysis, and
osmylation and organic functionalization on the CNT-based
structure.12 However, only a few studies have reported func-
tionalization of the two dimensional ͑2D͒ graphene layer for
gate dielectric deposition. In this letter, we present a facile
route which combines the functionalization of highly ori-
ented pyrolytic graphite ͑HOPG͒ surface with an ALD pro-
cess to allow for an extremely smooth and uniform Al2O3
layer on the HOPG surface. We investigate the evolution of
morphology and chemical binding states with an Al2O3 thin
film on top of the HOPG surface using atomic force micros-
copy ͑AFM͒, transmission electron microscopy ͑TEM͒, and
monochromatic x-ray photoelectron spectroscopy ͑XPS͒.
The sample was 1-mm-thick of HOPG of grades SPI-1
or 2. An atomically smooth surface with sharp step edges
was prepared by peeling off several top layers of graphite by
using scotch tape. Immediately after the mechanical exfolia-
tion, the HOPG samples were transferred into a commercial
ALD reactor ͑Cambridge Nanotech Inc., Savannah 100͒. The
chamber is pumped down to a pressure of 0.3 torr with N2
purging of 20 sccm. Next, Al2O3 layers in 50–200 cycles
were deposited on top of the HOPG at a temperature of
200 °C. For the Al2O3 deposition, trimethylaluminum
a͒
Author to whom correspondence should be addressed. Electronic mail:
jiyoung.kim@utdallas.edu.
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203102-2
Lee et al.
Appl. Phys. Lett. 92, 203102 ͑2008͒
FIG. 1. ͑Color online͒ ͑a͒ AFM image of fresh HOPG surface ͑b͒ the sample
with Al2O3 layer deposited from TMA/H2O ͑200 cycles͒ process ͑c͒ with
Al2O3 layer from TMA/O3 process ͑50 cycles͒.
FIG. 2. ͑Color online͒ AFM images of ͑a͒ the HOPG surface treated by
ozone pretreatment, ͑b͒ the ALD-Al2O3 dielectric ͑50 cycles͒ surface on the
ozone-treated HOPG, and ͑c͒ the cross-sectional HR-TEM image of the Al
metal/Al2O3 ͑100 cycles͒/the ozone-treated HOPG.
͑TMA͒ and water ͑H2O͒ or ozone ͑O3͒ were used as reac-
tants. Each pulse of TMA and H2O or O3 was separated with
N2 purging time of 7 s. The deposited Al2O3 layers and fresh
HOPG were characterized by AFM, TEM, and ex situ XPS.13
For XPS analysis, the peak energies were calibrated by as-
signing the major C 1s peak at 284.6 eV.14
on the basal plane while a detectable amount of chemisorbed
oxygen atoms ͑by XPS, not shown here͒ may slightly in-
crease in rms. The progress of the uniform and conformal
Al2O3 deposited on the HOPG surface can be seen in Fig.
2͑b͒, which exhibits the AFM image of the HOPG surface
treated by the ozone prior to 50 cycles of TMA/O3 ALD
process. Clearly, the Al2O3 layer deposition is consistent
with an ideal ALD mechanism with an almost 2D growth
mode showing an atomically smooth and uniform surface
with a small rms roughness of ϳ0.17 nm on the basal planes
between step edges, which is similar to that observed on a
HF-last Si substrate.15 This is also observed at the obvious
step edges on the HOPG surface after 50 cycles of the Al2O3
layer from the line profile in Fig. 2͑b͒. In contrast to Figs.
1͑b͒ and 1͑c͒, no preferential Al2O3 deposition on the step
edges was observed, as shown in Fig. 2͑b͒. It is also ob-
served that no significant damage occurs on the top graphene
layer during Al2O3 dielectric deposition with ozone pretreat-
ment by a high resolution TEM ͑HR-TEM͒ image, as shown
in Fig. 2͑c͒. In addition, the growth rate of the Al2O3 layer
on the HOPG treated by ozone was confirmed to be
ϳ0.095 nm/cycle from the TEM image, which is frequently
reported as a growth rate for the TMA/ozone ALD process.16
We performed ex situ XPS analysis of the Al2O3 layer on
ozone-treated HOPG to explore the binding energies of Al
2p, O 1s, and C 1s. Figure 3͑a͒ shows the XPS spectra of the
Figure 1͑a͒ shows the topography image of fresh HOPG
surface obtained by mechanical exfoliation. It consists of
atomically smooth basal planes with a rms roughness of
0.04 nm and sharp step edges with different heights ranging
from 0.3 to 0.8 nm, indicating steps with a monolayer or a
few monolayers of graphene. Figure 1͑b͒ illustrates the
AFM topography image of a HOPG surface on which the
Al2O3 layers were deposited by ALD with 200 cycles of
TMA/H2O process at a substrate temperature of 200 °C. In
agreement with previous reports by Xuan et al.,7 it was ob-
served that the Al2O3 layer was deposited only along with
the sharp step edges and not on the basal planes having a
chemically inert surface. From the line profile, it was con-
firmed that the Al2O3 nanolines at the step edges are
ϳ100 nm in width and ϳ20 nm in height. In addition, the
height of the Al2O3 nanolines indicates a vertical growth rate
of roughly 0.1 nm/cycle. From Fig. 1͑b͒, it can be concluded
that the Al2O3 layer deposition preferentially occurs on top
of the ALD-Al2O3 already present on the step edges. This
phenomenon has been explained by the fact that an atomi-
cally sharp step edge with a high chemical reactivity acts as
a one dimensional nucleation site for initial ALD growth due
to dangling bonds of broken ␴-␴ bonds.7 Physisorption of
the water layer which is present on the basal planes is easily
removed by a purge step with N2 gas in the ALD process.
However, the Al2O3 layer ͑50 cycles͒ from TMA/O3 process
shows a different growth behavior on the HOPG surface
compared to that from TMA/H2O, as shown in Fig. 1͑c͒.
From the line profile indicated in Fig. 1͑c͒, the thickness of
the Al2O3 layer after a 50 cycle deposition was measured to
be 4–5 nm. It should be noted that the Al2O3 layer deposi-
tion occurred not only at the step edges but also on the basal
planes, making it possible to enhance the Al2O3 coverage on
the HOPG surface. This result suggests that conformal and
uniform thin films can be achieved by an increase in nucle-
ation sites on the basal planes. Nevertheless, Fig. 1͑c͒ shows
some regions on the basal planes where no Al2O3 deposition
was observed.
In order for the HOPG surface to be rendered reactive
with ALD precursor molecules, ozone treatment was carried
out into ALD reactor prior to Al2O3 deposition. In Fig. 2͑a͒,
AFM analysis of the samples treated by ozone indicates the
very smooth surface of the basal planes was maintained
͑rms: ϳ0.1 nm͒ as well as the atomic step edges without any
significant pits on the basal plane. This indicates that the
FIG. 3. ͑Color online͒ XPS spectra of the Al2O3 layer deposited in
50 cycles on top of HOPG treated by ozone ͑a͒ O 1s/Al 2p ͑c͒ C 1s. ͑b͒
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ozone pre-treatment does not cause any detectable roughness
XPS spectra of C 1s on the fresh HOPG.
On: Tue, 29 Apr 2014 13:24:23

203102-3
Lee et al.
Appl. Phys. Lett. 92, 203102 ͑2008͒
O 1s and Al 2p lines of the film with ozone pretreatment
followed by the Al2O3 layer in 50 cycles. The XPS spectra
clearly show that the Al 2p feature consists of a single peak
centered at about 74.8 eV and the binding energy difference
͑O 1sBE–Al 2pBE͒ is measured to be 456.6 eV, which is
consistent with fully oxidized O–Al–O bonding.17 In addi-
tion, the O 1s line can be deconvoluted into three distinct
components: a main peak ͑531.4 eV͒, at low binding energy
corresponding to Al–O bonds ͑as indicated earlier͒, and two
peaks at higher binding energy relative to the main peak of O
1s, which are assigned to correspond to oxygen bonding in-
volved in epoxide ͑or ethers͒ and carbonyl group, located at
ϳ533.4 and 532.4 eV, respectively.18,19
atomic step edges as well as on the basal planes of the HOPG
surface. This facile route could be directly applied to fabri-
cation of top-gated transistor for graphene-based nanoelec-
tronics.
The authors thank Dr. Y. Chabal ͑UT-Dallas͒ and Dr. L.
Colombo ͑Texas Instruments͒ for valuable discussions and
Mr. G. Mordi for his technical support. We acknowledge the
financial support by KETI through the international collabo-
ration program of COSAR ͑funded by MKE in Korea͒ and
the SWAN program funded by the GRC-NRI. H.-C. Kim
thanks the financial support by the Korea Research Founda-
tion Grant ͑KRF-2007-357-D00155͒.
Next, Figs. 3͑b͒ and 3͑c͒ present the XPS spectra of C 1s
in the fresh HOPG surface and the spectra from an Al2O3
layer of 50 cycles after an ozone pretreatment, respectively.
The C 1s core level of the fresh HOPG is dominated by a
single peak ͑284.6 eV͒ and a peak shifted higher relative to
the main C 1s peak at ϳ285 eV was also observed. The
origin of this feature is not yet clear and we speculate that it
may be due to a surface core-level shift20 or imperfect crys-
talline structure.21 Note that the C 1s feature of the sample
prepared by 50 cycle of Al2O3 deposition with an ozone
treatment ͓Fig. 3͑c͔͒ shows the presence of oxidized carbon
species, such as epoxide ͑ϳ286 eV͒, carbonyl ͑ϳ287 eV͒,
and carboxylic ͑ϳ289 eV͒ groups.19,22–24 This observation is
also supported by the two peaks in the O 1s line at higher
binding energies, as indicated above. Considering the rela-
tive concentration of these carbon species calculated from
XPS measurements, we observe small increases ͑ϳ3%͒ in
the concentrations of the carbonyl and carboxylic groups in
the sample with the Al2O3 layer compared to the fresh
HOPG sample, while an increase in the concentration of the
epoxide functional group is about 10% after the ALD Al2O3
deposition. This result suggests TMA/O3 ALD deposition
immediately following ozone treatment would render the
chemically inert HOPG surface to be hydrophilic mainly
through epoxide functionalization, which is required to form
an extremely smooth and uniform Al2O3 layer. In addition,
we were not able to observe any noticeable difference in C
1s XPS spectra between a fresh HOPG surface and that im-
mediately after ozone treatment ͑not shown here͒, implying
that ozone pretreatment alone does not appear to result in a
detectable perturbation of sp2 bonding and/or etching
effects.
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