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Direct Large-Area Growth of Graphene on Silicon for Potential Ultra-
Low-Friction Applications and Silicon-Based Technologies
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2020
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Direct Large
-
Area Growth of Graphene on Silicon
for
Potential
Ultra
-
Low
-
Friction
Applications
and
Silicon
-
Based
Technolog
ies
Wei
-
Shiuan Tseng
1,2
, Yen
-
Chun Chen
3
, Chen
-
Chih Hsu
1
, Chen
-
Hsuan Lu
4
, Chih
-
I Wu
5
,
and Nai
-
Chang Yeh
1,
*
1
Department of Physics, California Institute of Technology, Pasadena, CA, 91125, USA
2
College of Photonics, National Chiao
-
Tung University, Hsin
-
Chu 30013, Taiwan
3
Department of Physics, National Tsing
-
Hua University, Hsin
-
Chu 30013, Taiwan
4
Department of Applied Physics and Materials Science, California Institute o
f Technology, Pasadena,
CA 91125, USA
5
Graduate Institute of Photonics and Optoelectronics and Depart
ment of Electrical Engineering,
National Taiwan University, Taipei 106, Taiwan
E
-
mail:
ncyeh@caltech.edu
Received xxxxxx
Accepted for publication xxxxxx
Published xxxxxx
Abstract
Deposition of layers of graphene on silicon has the potential for a
wide range of optoelectronic and mechanical applications.
However, direct growth of graphene on silicon has been difficult due to the inert, oxidized silicon surfaces. Transferring
graphene from metallic growth substrates to silicon is not a good solution
either, because most transfer methods involve multiple
steps that often lead to polymer residues or degradation of sample quality. Here we report a single
-
step method for large
-
area
direct growth of continuous horizontal graphene sheets and vertical graphe
ne nano
-
walls on silicon substrates by plasma
-
enhanced chemical vapor deposition (PECVD) without active heating. Comprehensive studies utilizing Raman spectroscopy,
X
-
ray/ultraviolet photoelectron spectroscopy
(XPS/UPS)
, atomic force microscopy
(AFM)
, scan
ning electron microscopy
(SEM)
and
optical transmission are carried out to characterize the quality and properties of these samples. Data gathered by the
residual gas analyzer
(RGA)
during the growth process further
provide information about
the synthesis
mechanism.
Additionally, ultra
-
low friction (with a frictional coefficient ~ 0.015) on multilayer graphene
-
covered silicon surface is
achieved, which is approaching the superlubricity limit (for frictional coefficients < 0.01). Our growth method therefore
opens
up
a new pathway
towards
scalable
and direct
integration of graphene into silicon technology
for
potential applications
ranging
from structural superlubricity
to nano
electronics, optoelectronics, and even the next
-
generation lithium
-
ion batteries.
K
eywords:
graphene
-
on
-
silicon, PECVD, AFM, superlubricity, XPS/UPS, RGA
1.
Introduction
Graphene, a monolayer of carbon atoms forming a two
-
dimensional honeycomb structure, is known for its
extraordinary electronic, optical, thermal, magnetic and
mechanical properties [1
−
5
]. Recently, several research
groups have demonstrated that mesoscale graphite, which
consists of many layers of graphene, offers superior properties
as a solid lubricant that is promising for significantly reducing
the wear and energy cons
umption in mecha
nical systems [6
,
7
]. The structural superlubricity of graphite may be attributed
to the weak van der Waals (vdW) interaction of graphene
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layers [
8
] with most materials and the lateral mec
hanical
stiffness of graphite [9
] when it forms incommensurable rigi
d
crystalline contacts with most solid surfaces. Nevertheless, the
aforementioned beneficial properties of graphite may not be
realizable in the case of monolayer graphene because the
morphology and characteristics of the substrate that supports
the monola
yer graphene will likely play a significant role in
determining the friction between the monolayer graphene and
other material surfaces [
10,11
]. Another major challenge
associated with the realization of superlubricity is its
scalability; superlubricity wi
ll cease to exist when the contact
area scales up to the extent where disorder and imperfections
become unavoidable. Thus, most superlubricity reported to
date have only been achieved for submicron
-
scale contact
areas [1
2
,1
3
].
A feasible approach to ta
ke advantage of the weak vdW
interactions of graphene layers to achieve ultra
-
low friction for
sliding on a variety of substrates is to deposit multilayer large
-
area graphene sheets on the substrates. Although the contact
between the sliding object and mul
tilayer graphene may not
be as perfect as that on atomically flat monolayer graphene,
any deformation from the surface corrugation of the
underlying substrate can be considerably damped by the
graphene layers if the thickness of multilayer graphene
exceeds
a characteristic penetration depth. This approach can
therefore achieve an almost identical sliding condition on the
very top layer of the graphene sheets independent of the
substrate effects, which may serve as a promising solution for
real
-
world applica
tions.
However, a major challenge for using multilayer graphene
as solid lubricant on substrates involves complicated and time
-
consuming transfer processes. The most common technique
for transferring graphene from its growth substrate to a target
substrate
involves a polymer
-
supported method, which always
leads to polymer residues on the transferred graphene surfaces
and therefore degraded performance of the graphene
-
incorporated devices [1
4
−
1
7
]. Other transfer methods without
using polymers also involve mu
ltiple procedures such as
copper etching, solution transfer, residue removal, and
annealing [1
8
−
22
], which generally lead to compromised
graphene quality, including contaminations, damages,
formation of wrinkles and bubbles,
etc.
Therefore, it is highly
de
sirable to explore direct growth of graphene on common
substrates, such as silicon, for better integration of graphene
into existing industrial technology.
In particular
, direct growth
of graphene on SiO
2
/Si substrates
can enable
fully
CMOS
-
compatible
optoelectronic devices
by
exploit
ting
various
unique properties of graphene
,
including
the
high electron
mobility, high modulation depth, large Kerr coefficient (~ 10
10
--
10
13
m
2
W
-
1
), low heat dissipation at GHz operation speeds,
ultrafast pho
todetection, electrostatic gate
-
tunable broadband
absorption and novel broadband photoluminescence
[2
3
-
2
9
]
,
which are
promising for
the development of
high performance
photodetectors, optical modulators and
hybrid optical
interconnects
.
To date, there
have been few techniques
developed for
direct growth
of
graphene on silicon
. Among the existing
approaches, all of them require
high
-
temperature (from 900
C to 1550
C)
processing [
30
], and
the resulting products are
small graph
ene islands/flakes
typically
of sub
-
micrometer
lateral scales [
31
−
33
]. In this work, we report the development
of a new scalable method by means of plasma enhanced
chemical vapor deposition (PECVD) to directly grow
gr
aphene with full coverage on large
-
area (~ 1 cm
2
) substrates
of silicon (Si), silicon dioxide (SiO
2
), and diamond
-
like
carbon (DLC) without the need of active heating. Systematic
friction studies of graphene multilayers on Si substrates
further indicate th
at ultra
-
low friction approaching the
superlubricity limit has been achieved at micrometer scales.
Given the importance of Si in modern technologies, our
method of direct and scalable graphene growth on Si provides
a pathway towards integrating graphene in
to Si
-
based
technologies for applications ranging from
structural
superlubricity in
nano
/
micro
-
electromechanical devices
and
hard drives
[
34
]
,
optoelectronic
devices
[
2
3
−
2
8
,
3
5
−
42
],
to
next
-
gener
ation lithium
-
ion batteries [4
3
−
4
6
].
2.
Experimental
2
.1
Graphene Synthesis
Figure S
1
shows a schematic illustration of our PECVD
system, which consists of a plasma source, a plasma cavity, a
quartz growth tube, gases cylinders (containing CH
4
, Ar and
H
2
), valves for mass flow control (MFC), vacuum gauges, and
vacuum pumps. Immediately before placing silicon substrates
into the processing tube, we used hydrofluoric acid (HF) to
etch away the native oxides on the substrate surface (for
around 20 minutes) t
o ensure that most of the oxides were
removed. This step helped temporarily passivate the reactive
silicon surface by forming silicon
-
hydrogen bonds on the
substrate surface to minimize surface oxidation. Before
turning on the plasma, we introduced all nec
essary gases into
the tube at the same time and reached the desired partial
pressures for all gases by using the MFCs to control the gas
flow rates. When all gas flows reached a steady state, the
plasma source was turned on to ionize gas molecules into
ene
rgetic ions and radicals to induce reactions with the
substrate.
For this single
-
step PECVD growth process, the type of
final graphene products is determined by three critical
parameters: the ratio of methane
-
to
-
hydrogen flow rates, the
plasma power,
and the growth time. In this study, the plasma
power was fixed at ~ 70 Watts over ~ 1 cm
3
volume and the
growth time required was 10 minutes. By controlling the
methane and hydrogen rates, we could fabricate either
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horizontal graphene sheets or vertical gr
aphene nano
-
walls.
Specifically, when the ratio of CH
4
-
to
-
H
2
flow rates was less
than 5.5, we usually obtained horizontal graphene sheets on
the silicon surface. On the other hand, vertical graphene nano
-
walls could be synthesized by controlling the ratio
of CH
4
to
H
2
gas flows to a value greater than 5.5.
2
.2
Characterization
Raman spectra were obtained via a Renishaw M1000 micro
-
Raman spectrometer system using a 514.3 nm laser (2.41 eV)
as the excitation laser source. SEM images were taken using
FEI Nova
600 NanoLab.
Studies of the x
-
ray photo
electron
spectroscopy
(
XPS
)
and
ultraviolet
photoe
lectron
spectroscopy (
UPS
)
were
carried out
via the Kratos
-
Ultra
-
XPS model
,
which employed a magnetic immersion lens with
a spherical mirror and concentric hemispherical analyzers
with a delay
-
line detector for both imaging and spectroscopy.
The UPS were measured via He I (21.2 eV) as the excitation
source
under
a base pressure o
f
~
10
-
1
0
Torr.
P
hotoelectrons
emitted from the sample were recorded by a hemispherical
analyzer with an overall resolution of 0.05 eV, as determined
from the width of the Fermi step measured on a gold substrate
cleaned by Ar ion sputter. The vacuum levels
of the sam
ples
were derived from the secondary
-
electron cutoff
of the UPS
spectra at the high binding energy sides.
The energy difference
between the secondary
-
electron cutoff and the Fermi level on
the spectra was obtained as the
value of the
work function.
Al Kα (1.486 keV) monochromatic X
-
rays and He I (21.2
eV) were used as the excitation sources for XPS
and UPS
measurements, respectively, in an ultrahigh vacuum chamber
with a base pressure lower than 2 × 10
−
10
Torr.
Optical
t
ransmission spectra were collected using a Cary 5000
absorption spectrometer with an integrating sphere
, and
q
uartz
substrates were
used
for the samples
.
The procedure for
preparing graphene
samples
for
the
optical measurement
involved the following steps
:
First, t
he graphene
-
covered Si
substrate was
placed
into a Teflon beaker filled with buffered
oxide etch (BOE). The Si substrate
r
emained afloat
on the
liquid due to
surface tension. After at least 24 hours, the
surface oxide between graphene and Si
was
fully
etched away
so that the
graphene
sample
was fre
ed from the substrate and
suspended
on the liquid surface.
Here we note
that the
typical
substrate
s
used
for these experiments
were
~ (
0.5 cm
0.5 cm
)
in dimension, which was sufficiently small
to
ensure fast
exfoliation.
Next, t
he
liquid
-
exfoliated
graphene
samples were
scooped
off
from the liquid surface and
transferred to another
beaker filled with DI water to rinse
off
the residual BOE. This
process was repeated
several
times to
ensure
that
the graphene
was
completely
free of che
mical residues
. A
quartz
substrate
was then
inserted to
the bottom of the beaker
,
and DI water
was slowly
drained
until graphene
landed
on
top of
the
substrate
.
Finally, a
15
-
minute
mild annealing
of the graphene
sample on quartz substrate
was
carried out to
remove residual
DI water.
AFM images and friction measurements were performed
on a Bruker Dimension Icon AFM. Two modes of AFM were
used: the tapping mode and the contact mode. The tapping
mode was used t
o acquire the surface morphology and the
contact mode was used to analyze the frictional force. For the
friction measurements, a triangular cantilever made of SiN
and coated with reflective gold (Bruker DNP
-
10) was used for
the lateral force microscopy (LF
M) tests. The thickness,
length, width, and spring constant of the cantilever were 0.6
μm, 205 μm, 25 μm, and 0.06 N/m, respectively. The height
and radius of the tip on the cantilever were 5.5 μm and 20 nm,
respectively. To extract the applied normal forc
e from the
voltage signal of the AFM system, the following formula was
used: Normal force (N) = [voltage (V)]
[deflection
sensitivity (nm/V)]
[spring constant (N/m)]. The deflection
sensitivity was calibrated every time by doing a force
-
curve
scan befo
re the friction measurements. To convert the
measured voltage signal to the friction force, the following
formula was applied [
47
]:
풇
=
ퟎ
.
ퟒ풉
푲
푳
푽
푳
푺
푫풊풇
Here
h
is the height of the tip,
L
is the length of the cantilever,
V
is the measured voltage signal,
S
Dif
is the deflection
sensitivity, and the
K
L
is given by the following expression:
푲
푳
=
푮푾
풕
ퟑ
ퟑ푳
(
ퟏ
풉
+
풕
/
ퟐ
)
ퟐ
where
G
is the shear modulus (= 1.69 x 10
11
Pa),
W
is the
width of th
e cantilever, and
t
is the thickness of the
cantilever.
3
.
Results and discussion
3
.1
Characterization of PECVD
-
grown Graphene Sheets
on Silicon
In
f
ig
ure
1(a) we show a representative scanning electron
microscopy (SEM) image of as
-
grown graphene sheets fully
covering the underlying Si substrate after only 10 minutes of
PECVD growth. Details of the synthesis conditions are
described in the Experimental sect
ion, and experimental setup
of the PECVD system has been reported previousl
y [
4
8
,
4
9
]
.
The SEM image indicates that the as
-
grown graphene layers
generally exhibit multi
-
domain distributions with varying
thicknesses. The lateral dimension of each graphene l
ayer is
typically much larger than 1 μm although it is difficult to
identify the borders associated with individual sheets
because
they mostly overlap
each other.
To characterize the quality of the graphene multilayers on
Si, we performed Raman spectroscop
ic studies on various
random areas of each sample and found consistent spectra
throughout, implying uniform quality of the as
-
grown
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graphene multilayers on Si. As shown in
f
ig
ure
1(b), the
representative Raman spectrum taken on a sample of
multilayer graph
ene
-
on
-
Si reveals typical graphene Raman
modes of the 2D, G
-
and D
-
bands with narrow FWHM
[
50
,
51
].
The intensity of the 2D
-
band at (2696 ± 2) cm
−
1
in the
Raman spectrum was larger than that of the G
-
band at (1583
± 2) cm
−
1
, and an intense D
-
band was also
observed due to the
presence of many boundaries around the edges of graphene
sheets, which was further accompanied by the D
-
band at
(1615± 2) cm
−
1
as the result of an i
ntense D
-
band. The optical
micrographs shown in the inset of
f
ig
ure
1
(
b
)
were taken on a
(1 cm
1 cm)
area of a
Si substrate before and after the
PECVD graphene growth. Evidently the Si substrate after
graphene growth
was still
somewhat
reflective
,
which
indicated
that the graphene layers on Si were relatively thin
and semi
-
t
ransparent.
Additional characterizations of the physical and chemical
properties of the graphene
-
covered Si samples were
investigated by X
-
ray and ultraviolet photoemission
spectroscopy (XPS
and UPS). The XPS spectrum in f
ig
ure
1(c)
revealed a clean sampl
e surface with only C
-
1s and O
-
1s
signals. The dominant C
-
1s peak (> 95%) was from the as
-
grown graphene multilayers, whereas the presence of a small
O
-
1s peak (< 5%) may be attributed to broken Si
-
O bonds
from the substrate surface, which will be further
elaborated in
the context of the growth mechanism later.
In particular, we
note that no Si peaks could be found in the XPS spectrum,
which implied
a
full coverage of graphene on the Si substrate
Figure 1. (a) SEM image taken on an as
-
grown sample of multilayer graphene fully covering a Si substrate.
The
sample were
synthesized with the following growth parameters:
applied
p
lasma power
,
70 Watts
; plasma volume, ~ 1
cm
3
;
growth time
,
10
minutes
;
and ratio of
CH
4
-
to
-
H
2
flow rates
,
< 5.5.
(b) Main panel: Raman spectrum of the sample shown in (a), revealing typical
Raman modes of graphene with the 2D
-
, G
-
and D
-
bands. The inset are optical micrographs of the substrate before and after
graphene growth. (c) XPS analysis of the as
-
grown graphe
ne
-
on
-
Si sample, showing a dominant C
-
1s (> 95%) peak and a
secondary O
-
1s (< 5%) peak. The complete absence of any Si peaks implied full graphene coverage on the Si substrate. (d)
UPS spectrum of the as
-
grown graphene
-
on
-
Si sample, showing a work function
(4.45 ± 0.05) eV consistent with that of pristine
graphene without doping. (e)
-
(f) Raman spectra of as
-
grown graphene sheets on
(e)
SiO
2
and (f) diamond
-
like carbon (DLC)
substrates, respectively. Both spectra reveal distinct D, G and 2D bands of graphene
Raman modes with slightly different D
-
band intensities and 2D/G ratios
.
and also corroborated the finding from the SEM image in
f
ig
ure
1(a).
The work function of the graphene layers was also
examined through U
PS and the
result was shown in f
ig
ure
1(d). The value deduced from the secondary electron cutoff of
the UPS spectrum was (4.45 ± 0.05) eV, which was in good
agreement with the typical value of pristine graphene without
doping. Direct PECVD growth of graphene on SiO
2
and DLC
substrates was also achieved with slightly different growth
parameters, as shown in
f
ig
ure
1(e) and 1(f). These results
suggest that our PECVD growth method is a universal and
scalable technique for depositing graphene on a wide variety
of substrat
es ranging from metallic to insulating materials. In
this work we mainly focus on the studies of graphene grown
on Si because of the broad range of Si
-
based applications.
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3
.
2
Morphology and Thickness Studies
To investigate the surface morphology and thickness of the as
-
grown graphene layers on Si, tapping mode atomic force
microscopy (AFM) was conducted on a Si substrate both
before and after graphene growth. Figure 2(a) shows a high
-
sensor AFM image of the gr
aphene
-
covered Si surface, which
revealed multiple domains with varying heights throughout
the micrometer
-
scale area. The layered structures on the
surface were the as
-
grown graphene sheets with an averaged
lateral dimension larger than 1 μm. The thickness
of the sheets
was examined by studying the cross
-
sectional profiles. As the
cross
-
sectional profiles r
evealed in the lower panels of f
ig
ure
2(a), we observed different steps along lines a, b, and c.
The
cross
-
section along line a showed a
1.4 nm step that
corresponded to 4 layers of graphene
, whereas that along line
b revealed
a 0.7 nm step that corresponded to two layers of
graphene thickness
.
These results suggested that the surface
consisted of integer numbers of graphene layers within the
resolution of
the AFM
[
5
2
,
5
3
]
, which were also in agreement
with the SEM image shown in
f
ig
ure
1. More importantly, we
note that along line c in
f
ig
ure
2(a), the absence of any
Figure 2. (a) AFM image and cross
-
sectional profiles of
a fully graphene
-
covered Si surface.
Here t
he
growth parameters for
the sample were:
applied
plasma power
,
70 Watts
; plasma volume, ~ 1
cm
3
;
growth time
,
10 minutes
;
ratio of
CH
4
-
to
-
H
2
flow
rates
,
< 5.5.
(b) AFM image and cross
-
sectional profiles of a bare Si
(100)
surface.
discernible height variations within instrumentation resolution
suggested that the profile was taken on an atomically flat
surface. We note that such ato
mically flat regions were found
to have a lateral dimension larger than a few to tens of
micrometers, suggesting that the individual graphene layers
grown on Si by the PECVD method were several orders of
magnitude larger than the sub
-
micrometer scale flake
s
obtained by others in the past [
31
−
3
3
].
In addition to understanding the surface topography of
graphene
-
covered Si substrate, we performed the same studies
on bare Si surface as a reference for comparison
, and the
results are shown in f
ig
ure
2(b). From
the cross
-
sectional
profiles, we found that the roughness of the Si surface was
mostly within 1 nm. However, a few pits along lines a and b
with depths larger than 1 nm were found, a
s shown in the
lower panels of f
ig
ure
2(b). Additionally, sharp particles
with
heights taller than 1 nm were observed along lines b and c in
the cross
-
sectional profiles. Combining
the findings from
f
ig
ure
2(a) and 2(b), we suggest that the as
-
grown graphene
layers can effectively smooth out small pinholes and deposits
on the Si surface, resulting in reduced roughness as
exemplified by the relatively flat morphology along line c on
the graphene
-
covered surf
ace. The reduction of surface
roughness can contribute to lower sliding friction, which has
been independently verified by studies of the friction and will
be discussed in the last part of this paper.
3
.3
Synthesis and Characterization of Graphene Nano
-
Wa
lls
In general, the growth conditions inside the PECVD chamber
can be modified significantly through varying the plasma
power, growth time, and methane (CH
4
) to hydrogen (H
2
)
ratio. This flexibility led to the successful synthesis of vertical
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