Atomic-layer-deposited nanostructures for graphene-based nanoelectronics

Article abstract of DOI:10.1063/1.2828338

Graphene is a hexagonally bonded sheet of carbon atoms that exhibits superior transport properties with a velocity of 108 cms and a room-temperature mobility of >15 000 cm2 V s. How to grow gate dielectrics on graphene with low defect states is a challenge for graphene-based nanoelectronics. Here, we present the growth behavior of Al2 O3 and Hf O2 films on highly ordered pyrolytic graphite (HOPG) by atomic layer deposition (ALD). To our surprise, large numbers of Al2 O3 and Hf O2 nanoribbons, with dimensions of 5-200 nm in width and >50 μm in length, are observed on HOPG surfaces at growth temperature between 200 and 250 °C. This is due to the large numbers of step edges of graphene on HOPG surfaces, which serve as nucleation sites for the ALD process. These Al2 O3 and Hf O2 nanoribbons can be used as hard masks to generate graphene nanoribbons or as top-gate dielectrics for graphene devices. This methodology could be extended to synthesize insulating, semiconducting, and metallic nanostructures and their combinations.

Full text of DOI:10.1063/1.2828338

Atomic-layer-deposited nanostructures for graphene-based nanoelectronics
Y. Xuan, Y. Q. Wu, T. Shen, M. Qi, M. A. Capano, J. A. Cooper, and P. D. Ye
Citation: Applied Physics Letters 92, 013101 (2008); doi: 10.1063/1.2828338
View online: http://dx.doi.org/10.1063/1.2828338
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APPLIED PHYSICS LETTERS 92, 013101 ͑2008͒
Atomic-layer-deposited nanostructures for graphene-based nanoelectronics
Y. Xuan, Y. Q. Wu, T. Shen, M. Qi, M. A. Capano, J. A. Cooper, and P. D. Yea͒
School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University,
West Lafayette, Indiana 47907, USA
͑Received 1 November 2007; accepted 4 December 2007; published online 2 January 2008͒
Graphene is a hexagonally bonded sheet of carbon atoms that exhibits superior transport properties
with a velocity of 108 cm/s and a room-temperature mobility of Ͼ15 000 cm2/V s. How to grow
gate dielectrics on graphene with low defect states is a challenge for graphene-based
nanoelectronics. Here, we present the growth behavior of Al2O3 and HfO2 films on highly ordered
pyrolytic graphite ͑HOPG͒ by atomic layer deposition ͑ALD͒. To our surprise, large numbers of
Al2O3 and HfO2 nanoribbons, with dimensions of 5–200 nm in width and Ͼ50 ␮m in length, are
observed on HOPG surfaces at growth temperature between 200 and 250 °C. This is due to the large
numbers of step edges of graphene on HOPG surfaces, which serve as nucleation sites for the ALD
process. These Al2O3 and HfO2 nanoribbons can be used as hard masks to generate graphene
nanoribbons or as top-gate dielectrics for graphene devices. This methodology could be extended to
synthesize insulating, semiconducting, and metallic nanostructures and their combinations. © 2008
American Institute of Physics. ͓DOI: 10.1063/1.2828338͔
Recently, the discovery of superior transport properties
in mechanically exfoliated graphene sheets or epitaxial
graphene grown on SiC substrate has generated enormous
interests in academy and industry.1–4 Graphene is a hexago-
nal network of carbon atoms connected by strong covalent
␴-␴ bonds that exhibits nondispersive transport characteris-
tics. The electron velocity in graphene is ϳ108 cm/s and the
mobility is Ͼ15 000 cm2/V s at room temperature, which is
much higher than those from most of semiconductor materi-
als. These properties make a possible new class of carbon-
based transistors and integrated circuits with performance su-
perior to silicon, germanium, and III-V compound semicon-
ductors. While the prospect of controlling the band gap in
graphene by forming nanoribbons using nanolithography is
an important step toward graphene devices,5,6 one remaining
challenge for graphene-based transistors is the formation of
high-quality, ultrathin dielectrics with low interface trap den-
sity.
Atomic layer deposition ͑ALD͒ is a thin-film growth
technique for depositing uniform and conformal films with
atomic precision. ALD is a special modification of the
chemical vapor deposition ͑CVD͒ method with the distinc-
tive feature of alternate and cyclic supply of each gaseous
precursors to the reactor chamber at relatively low tempera-
ture ͑Ͻ350 °C͒. The alternate pulses of the precursors are
chemisorbed on the exposed substrate surface and the ex-
cesses are removed by a purge step. The key feature of the
ALD methods is the saturation of all the reaction and purging
steps, which makes film growth self-limiting on the surface.
The precision achieved with ALD allows processing of na-
nometer scale films in a controlled manner. ALD methods
and applications have developed rapidly over the last few
years, in particular, for high-k gate oxides.7 ALD high-k Hf-
based oxide has become a manufacturing process to replace
SiO2 in 45 nm node Si complementary metal-oxide-
semiconductor ͑CMOS͒ digital integrated circuits starting in
the second half of 2007.8 It is natural to investigate oxide
growth on graphene using ALD, the state-of-the-art CMOS
technology. But ALD process on graphene, similar to sus-
pended single-walled carbon nanotubes ͑SWCNTs͒, is ex-
pected to be very different from the deposition on conven-
tional semiconductors or oxides, because the perfect graphite
basal surface is chemically inert.9,10 However, it is of great
interest to study the growth mechanism of ALD high-k di-
electrics on this novel electronic channel material. In this
letter, we study the growth behavior of ALD Al2O3 and HfO2
films on highly ordered pyrolytic graphite ͑HOPG͒ substrate
at different growth temperatures. Field emission scanning
electron microscope ͑FESEM͒ and atomic force microscope
͑AFM͒ are used for characterization.
HOPG is a periodic stack of two-dimensional graphene
layers along the c axis. Each graphene layer is weakly ␲-␲
bonded to its neighbors leading to graphene layers that can
easily be cleaved or peeled off, providing an atomically
smooth fresh surface with step edges. Fresh HOPG surfaces
could be used to reasonably mimic ideal graphene surfaces.
After freshly cleaved, HOPG ͑grade ZYH͒ surfaces were
prepared by peeling off several top layers of graphite with
scotch tape, the samples were immediately transferred into
an ASM F-120 ALD reactor. It took about 1 h to ramp up to
the growth temperature from room temperature at the pres-
sure of 2–6 Torr of N2 purging. 1–35 nm ALD Al2O3 films
were deposited at growth temperatures of 200–300 °C by
alternating pulses of trimethyl aluminum ͑TMA͒ and water
͑H2O͒ as precursors. The pulse length and N2 purge time are
0.8 s and 2.0 s for TMA, and 1.0 s and 2.0 s for H2O, re-
spectively. For HfO2 ALD growth, hafnium chloride ͑HfCl4͒
and H2O were used as Hf source and oxidant, respectively.
The pulse length and N2 purge time are 1.4 s and 2.0 s for
HfCl4, and 1.0 s and 2.0 s for H2O, respectively. The films
were characterized by Hitachi S-4800 FESEM and Veeco
Dimension 3100 AFM.
Figure 1͑a͒ shows the AFM topography image and line
profile of fresh HOPG surface with an atomically smooth
͑rms=0.04 nm͒ carbon surface and atomically sharp step
a͒
Author to whom correspondence should be addressed. Electronic mail:
yep@purdue.edu.
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0003-6951/2008/92͑1͒/013101/3/$23.00 92, 013101-1 © 2008 American Institute of Physics
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Xuan et al.
Appl. Phys. Lett. 92, 013101 ͑2008͒
FIG. 1. ͑Color online͒ ͑a͒ AFM images of HOPG surface with graphene
terraces and ͑b͒ similar surface after 1 nm thick ALD Al2O3 growth.
FIG. 2. ͑Color online͒ High-quality nanoribbons are formed in the condi-
tions of ͑a͒, ͑b͒, and ͑d͒ on HOPG surface, while very rough and discon-
tinuous films are formed out of ALD process window, as shown in ͑c͒, ͑e͒,
and ͑f͒.
edges created by broken graphene layers. From the line pro-
file, the step height is measured to be 0.3–0.4 nm, confirm-
ing that it is generated by a single graphene layer. Figure
1͑b͒ shows the AFM image and line profile of a similar area
after 10 cycles, i.e., 1 nm of ALD Al2O3 process at 250 °C
growth temperature. From the line profile, the graphene sur-
face remains bare and atomically smooth with a rms rough-
ness of 0.04 nm, indicating that no Al2O3 films are grown on
graphene surface. This demonstrates that conventional
H2O-based ALD process cannot grow two-dimensional, con-
tinuous, and isotropic oxide films directly on graphene sur-
faces. On the other hand, Al2O3 nanoribbons with ϳ1.5 nm
height and several nanometers wide were clearly observed at
step edges, indicating that ALD Al2O3 films only grow along
the step edges. These results are also supported by the fact
that H2O-based ALD films cannot grow on SWCNTs, oppose
to NO2 and TMA based ALD technique, where strong ab-
sorption of NO2 and TMA on chemically inert SWCNTs sur-
face results in the nucleation of ALD precursors.11 The re-
sults indicate that ALD could also be used as a decoration
method for nanoscale features, in particular, when a low-
temperature process is required.
1D dangling bonds is of great importance to passivate or
functionalize these edges. Due to the possible different bond-
ing energy of C-dangling bonds from armchair or zigzag
edges, the chemical reaction of ALD could be selective with
these two types of edges. This selectivity could be within a
narrow process window. Nevertheless, some indications of
this selectivity are observed, i.e., as highlighted by arrows in
Fig. 1͑b͒. The step edges in certain directions do not serve as
nucleation centers for ALD process in certain process condi-
tions. More studies on the selectivity based on edge orienta-
tions are ongoing.
Figure 2 shows the SEM images of ALD Al2O3 and
HfO2 nanoribbons grown on HOPG at different growth tem-
peratures. Al2O3 and HfO2 nanoribbons with dimensions of
5–200 nm in width and Ͼ50 ␮m in length are obtained in a
controlled fashion at growth temperatures of 200–250 °C, as
illustrated in Figs. 2͑a͒, 2͑b͒, and 2͑d͒. For growth tempera-
tures above 250 °C, rough and discontinuous films are
formed, as shown in Figs. 2͑c͒, 2͑e͒, and 2͑f͒. This experi-
ment demonstrates an approach to generate relatively regular
nanoribbons on HOPG using well-controlled gas-phase ALD
if the growth temperature is within the right ALD process
window. This approach is not limited to oxide or insulator
nanoribbons. The work could be expanded easily to create
semiconductor nanoribbons or metallic nanoribbons or their
combinations, since ALD processes of semiconductors ͑i.e.,
ZnS and GaP͒ and metals ͑i.e., Ru, Pd, Ir, and Pt͒ are
well-established.23 These semiconductor or metallic nanorib-
bons can be transferred to other substrates for further device
fabrication. Meanwhile, the insulating nanoribbons could be
used as a hard mask for dry etching to create graphene nan-
oribbons underneath. For example, high mobility epitaxial
graphene can be formed on SiC surface with regular, dense,
and atomic sharp step edges.4,24 The combination of insulat-
ing nanoribbons and graphene nanoribbons is the ideal build-
ing block for top-gate graphene transistors.
Interestingly enough, atomically sharp step edges on
graphene provides one-dimensional ͑1D͒ broken carbon
bonds along the terraces, which serves as 1D nucleation cen-
ter for the initial ALD process. It provides a perfect material
system with 1D dangling bonds by nature. Graphene sheets
are comprised of a hexagonal lattice of carbon bonded by a
strong ␴ bonding in the a-b plane. When cutting through an
infinite graphene sheet, two semi-infinite graphene sheets
with 1D edge are obtained. Therefore, two types of edges
͑armchair or zigzag͒ exist depending on the cutting direction.
Because of the dangling bonds, these edges have much
higher chemical or catalytic reactivity than inert basal
graphene planes.9 Theoretical calculation or experimental re-
sults show that small molecules, such as X2 ͑X=F, Cl, and
Br͒,10 H2,12 O2,13 SO2,14 and NO ͑Ref. 15͒ gases, are chemi-
cally adsorbed on the armchair and zigzag edges at room
temperature, but only a weak physical adsorption takes place
on the basal planes. H2O is also preferentially adsorbed at
these edges resulting in nucleated ALD films at edges. Simi-
lar results are widely reported in physical vapor deposited
metals,16,17 CVD SiO2 ͑Ref. 18͒ and Pt,19 and electrochemi-
cal deposited Mo ͑Ref. 20͒ and Bi2Te3 ͑Ref. 21͒ systems,
sometimes called as “step edge decoration.” Since the build-
ing blocks for future graphene-based transistors are a few
nanometers wide graphene ribbons, the electronic transport
properties are mainly determined by the edge states of these
The lateral size of ribbons has an almost linear depen-
dence on ALD cycles with a constant lateral growth rate, as
shown in Fig. 3͑a͒. The lateral growth rate is temperature
dependent. For example, the lateral growth rate at 200 °C for
ALD Al2O3 is about 50% larger than that at 250 °C. Within
the ALD process window, the higher the growth temperature
is, the more dynamic the TMA molecules are with less
chance to be susceptible on clean graphene surface, and the
lower the lateral growth rate is. The vertical growth rate is a
perfect ALD growth of ϳ0.1 nm/cycle independent on
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graphene nanoribbons.22 Surface chemistry study on these
growth temperature, as shown in Fig. 3͑b͒. The observed
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Xuan et al.
Appl. Phys. Lett. 92, 013101 ͑2008͒
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formed on graphene by the implementation of NO or NO2
and TMA as noncovalent functionalization layers on
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common in metal-organic CVD growth of other materials. It
should also depend on chemical properties of precursors and
detailed ALD process, such as HfCl4 versus tetakis͑ethylm-
ethylamido͒hafnium. Figures 3͑c͒ and 3͑d͒ illustrate the pos-
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nanostructures are formed at the graphene step edges. Wedge
shape structures are expected for a typical ALD process, as
shown in Fig. 3͑c͒. But the SEM and AFM studies confirm
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not very clear at this moment how the 1D-type ALD growth
mode should be.
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synthesizing oxide nanoribbons on HOPG surface with the
control of nanoribbon dimensions. This methodology could
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and metallic nanostructures or combinations thereof since a
wide variety of materials can be grown by ALD. Two-
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