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Communication
Atomic-scale Structural and Chemical Characterization
of Hexagonal Boron Nitride Layers Synthesized at
the Wafer-Scale with Monolayer Thickness Control
Wei-Hsiang Lin, Victor W. Brar, Deep Jariwala, Michelle C. Sherrott,
Wei-Shiuan Tseng, Chih-I Wu, Nai-Chang Yeh, and Harry A Atwater
Chem. Mater.
,
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• Publication Date (Web): 22 May 2017
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Atomic-scale Structural and Chemical Characterizati
on of Hexagonal Boron Nitride Layers
Synthesized at the Wafer-Scale with Monolayer Thick
ness Control
WeiHsiang Lin
1
, Victor W. Brar
1,2,3
, Deep Jariwala
1,4
, Michelle C. Sherrott
1,4
, WeiShiuan Tseng
5
,
ChihI Wu
6
, NaiChang Yeh
2, 5
, and Harry A. Atwater
1, 2,4*
* haa@caltech.edu
1. Thomas J. Watson Laboratory of Applied Physics, C
alifornia Institute of Technology, Pasadena, CA 9112
5,
United States
2. Kavli Nanoscience Institute, California Institut
e of Technology, Pasadena, CA 91125, United States
3. Department of Physics, University of WisconsinMa
dison, Madison, WI 53711, United States
4. Resnick Sustainability Institute, California Inst
itute of Technology, Pasadena, CA 91125, USA.
5. Department of Physics, California Institute of Te
chnology, Pasadena, CA 91125, United States
6. Graduate Institute of Photonics and Optoelectroni
cs and Department of Electrical Engineering, Nation
al
Taiwan University, Taipei, Taiwan, Republic of Chin
a
Abstract
Hexagonal boron nitride (hBN) is a promising twod
imensional insulator with a large band
gap and low density of charged impurities that is i
sostructural and isoelectronic with graphene. Here
we report the chemical and atomicscale structure o
f CVDgrown waferscale (~25 cm
2
) hBN
sheets ranging in thickness from 120 monolayers. A
tomicscale images of hBN on Au and
graphene/Au substrates obtained by scanning tunneli
ng microscopy (STM) reveal high hBN
crystalline quality in monolayer samples. Further c
haracterization of 120 monolayer samples
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indicates uniform thickness for waferscale areas;
this thickness control is a result of precise contr
ol
of the precursor flow rate, deposition temperature
and pressure. Raman and infrared spectroscopy
indicate the presence of BN bonds and reveal a lin
ear dependence of thickness with growth time.
Xray photoelectron spectroscopy (XPS) shows the fi
lm stoichiometry, and the B/N atom ratio in our
films is 1 ± 0.6% across the range of thicknesses.
Electrical current transport in metal/insulator/met
al
(Au/hBN/Au) heterostructures indicates that our CV
Dgrown hBN films can act as excellent tunnel
barriers with a high hardbreakdown field strength.
Our results suggest that largearea hBN films
are structurally, chemically and electronically uni
form over the wafer scale, opening the door to
pervasive application as a dielectric in layered na
noelectronic and nanophotonic heterostructures.
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Introduction
Since the isolation of graphene on an insulating su
bstrate
17
, a variety of other layered
materials have been isolated and characterized, and
have opened up an exciting field of research
811
.
However the electronic quality of twodimensional (
2D) active layers in devices is highly sensitive
to their immediate environment owing to the large s
urface to volume ratio. Therefore a crystalline
2D material that can serve the role of an insulatin
g substrate, encapsulating layer or gate dielectric
is
highly desirable in device applications. Hexagonal
boron nitride (hBN) has emerged as a promising
material for these applications. It has been observ
ed experimentally that encapsulating other 2D
materials in hBN not only enhances the performance
of devices but also extends their durability to
long timescales
1213
. However most previous studies have used mechanica
lly exfoliated hBN
derived from bulk crystals which does not allow for
control over layer thickness and sample lateral
size. While numerous reports about chemical vapor d
eposition (CVD) synthesis of hBN already
exist
1432
ranging from low pressure CVD (LPCVD) for monolaye
r growth to ambient pressure CVD
(APCVD) for multilayer growth, most of these approa
ches do not yield precise thickness control
from the monolayer scale to thick multilayers over
large areas. Many previous experiments have
reported on use of solid ammoniaborane as the boro
n and nitrogen source. The sublimation of a
solid source gives poor control of precursor flow r
ate and partial pressure in the growth chamber
over large length scales. Further, monolayer to sub
2nm growth of hBN is a process catalyzed by
the Cu surface and requires low growth pressures. H
owever the catalytic activity of Cu surface is
diminished after growth of 12 nm hBN on its surfa
ce and thus growth of thicker hBN films occurs
solely via van der Waals epitaxy, requiring higher
growth pressures. No system has yet been
developed that can achieve both of these growth con
dition regimes; therefore most prior work on
CVD grown hBN has been able to control both BN thi
ckness and lateral thickness uniformity over
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technologically relevant areas. Our CVD methods fea
tures precursor flow and pressure control
systems which combines the merits of both LPCVD and
APCVD by allowing precise control over
the precursor flow rate and partial pressure of the
precursor in the growth zone over a wide range of
growth chamber pressure and temperatures. This perm
its growth of high quality mono to sub 5layer
hBN films on a Cu foil which require low growth pr
essures as well as thicker hBN films on Cu
foils, by switching to APCVD mode.
We image the atomic structure of monolayer hBN/Au
sheets and monolayer
hBN/graphene/Au heterostructures using scanning tu
nneling microscopy (STM). Atomic STM
images of CVDgrown monolayer hBN/Au film monolaye
rs and hBN/graphene heterostructures
indicate high crystalline quality. The crystal stru
cture and chemical composition of the resulting
hBN film are also characterized by atomic force mi
croscopy (AFM), Raman spectroscopy,
infrared
transmission measurements
and xray photoelectron spectroscopy (XPS). The el
ectrical properties
of the thin and thick hBN films were systematicall
y measured on metalinsulatormetal (MIM)
tunneling devices to understand the dielectric prop
erties of these films. Our results demonstrate a
new standard and stateoftheart for large area sy
nthesis of hBN potentially enabling applications
in nanoelectronic and nanophotonic devices.
Results and Discussion
To grow ultrathin (< 1.5 nm) hBN films, we use a g
rowth pressure of ~ 2 Torr. Figure 1b
shows an optical photograph and atomic force micros
copy (AFM) analysis of a waferscale hBN
monolayer which has transferred onto a 285 nm SiO
2
/Si substrate. Raman spectra acquired over six
different spots on the sample (denoted by x), as se
en in the adjacent plot suggests that the sample
thickness and quality is uniform over the entire cm
2
scale. Likewise, the growth mode can be
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switched from a slow growth rate of ~0.3 nm/min in
catalytically controlled CVD at low pressures to
a high growth rate of 1 nm/min at near atmospheric
pressures (~500 Torr). Figure 1c illustrates the
large area uniformity of ~ 15 monolayer hBN as inf
erred from the optical micrograph and
corresponding Raman spectra. To validate the precis
e control of layer thickness and growth
uniformity, a series of growth experiments were per
formed for varying times at both low and
ambient pressures to produce hBN films of varying
layer number/thicknesses. These hBN films
were then characterized with several spectroscopic
techniques to characterize their structure and
chemical composition. Atomic scale structure and ar
eal homogeneity of hBN films were revealed
by scanning tunneling microscopy (STM) for CVDgrow
n monolayer hBN. Despite numerous
reports on CVD synthesis of hBN, little is known a
bout atomic scale electronic structure for layers
grown on polycrystalline Cu foils, due to the rough
ness of the Cu foil substrate which renders STM
difficult. Also, the film thickness inhomogeneity s
een in most prior reports
3337
would prevent image
formation by direct electron tunneling through hBN
. Notably, hBN is difficult to characterize using
STM due to its insulating character. To enable adeq
uate sample conductance, many research groups
have used a graphene/ hBN heterostructure which ex
ploits the conductance of graphene to visualize
the atomic structure of the hBN layer underneath t
he graphene. In contrast, we are able to use STM
to directly image our CVDgrown monolayer hBN film
s on Au (111)/mica substrates transferred
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using polymer free transfer method
38
. Figure 2a is a schematic of the STM measurement
configuration for monolayer hBN sheets on Au (111)
substrates. Figure 2b shows a representative
STM image of the monolayer hBN sheet on Au (111) w
ithout any post transfer annealing. The
atomicallyresolved hBN honeycomb structure is cle
arly visible, superimposed on the Au (111)
herringbone reconstruction pattern. The appearance
of the distorted hexagonal lattice in the STM
image indicates strong surface tension originating
from the interaction between boron nitride atoms
and the Au (111) substrate herringbone reconstructi
on. Figure 2c shows schematic of STM
measurements of the monolayer hBN sheet on monolay
er graphene/Au (111) substrate and Figure
2d, e show topographic STM images acquired from two
different areas of the singlelayer hBN on
graphene. Both the atomic lattices of hBN and long
er range moiré patterns can be clearly revealed.
The moiré pattern is formed by interference between
the hBN layer and underlying graphene/Au
(111) substrate, and can be attributed to their lat
tice mismatch (a
= 0.252 nm, b= 0.246 nm) and
rotational misalignment. The hBN
and graphene sheets interact through van der Waals
forces, and
display the same topographic conformal mapping to t
he underlying Au (111). However, the relative
rotation angle between the graphene and hBN sheets
can be modified by tiny wrinkles and bubbles
are inevitably introduced during the transfer proce
ss of hBN onto graphene as seen in Fig. 2d (left)
and original grain boundary of hBN and graphene.
Changing the rotation angles between the
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hBN and graphene lattices, leads to moiré patterns
with different periodicities and orientations as
observed in Figure 2d and e (center). The periodici
ty of moiré pattern presented in Figure 2d is 3.4
nm and 2.0 nm in Figure 2e. The twist angle between
the hBN and graphene lattices can be
ascertained from the structure of the Moire pattern
and is given by
=
[1 −
−
( − )
2
]
where a is the hBN lattice constant, b is the grap
hene lattice constant, and
is the periodicity of
the moiré pattern. Thus, the twist angle
of Figure 2d and 2e is found to be (4
±
0.1)
°
and (7
±
0.1)
°
, respectly. Alternatively, the twist angle
can also be extracted by performing a fast Fourier
transform (FFT) analysis of the STM images, as show
n in the inset of Figure 2d and 2e. The outside
set of spots corresponds to the reciprocal lattice
of hBN, while the inner set of spots are assigned
to
the moiré pattern stemming from the rotation betwee
n the monolayer hBN and graphene substrate.
The atomic resolution STM images presented here can
be achieved in image locations over a wide
area of the waferscale sample, indicating excellen
t hBN sheet surface quality for layers transferred
with the polymerfree
38
transfer method and supporting the existence of hi
gh crystalline quality in
CVDgrown hBN down to the monolayer level. The STM
results provide information about the
atomic scale structure and crystallinity of monolay
er hBN. However they do not provide
information about macroscopicscale hBN film thick
ness homogeneity, composition or structure.
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Raman spectra of an hBN layer typically have two
active E
2g
modes, one at 1366 cm
l
which is
strong and corresponds to vibrations of B and N mov
ing against each other in the plane and another
at 51.8 cm
l
, which is attributed to sliding between whole plan
es. However the lower frequency
mode is more difficult to observe because of proxim
ity to the Rayleigh diffusion, as well as
the
presence of a fluorescence background. The width, i
ntensity and position of these Raman features
are sensitive to hBN thickness, and these dependen
cies were determined by combining Raman
measurement with STM results and AFM results. We us
e monolayer hBN as a thickness calibration
for every run and verify the integrated intensity o
f this calibration point with the value marked with
an arrow in Figure 3b (Top). The integrated intensi
ty of the hBN layers determined using this
protocol gives a reasonable estimate of the layer t
hickness and matches very well with the estimates
given by the AFM line profiles (see supporting info
rmation S1). Figure 3a shows Raman spectra of
monolayer hBN to 15 layers hBN films, showing the
Raman intensity increases
with the number of
layers. The integrated intensity is plotted as a fu
nction of the number of layers in Figure 3b (Top).
Figure 3b (Bottom) suggests that the peak position
have a redshift as layers decrease and the FWHM
become shaper as the layer number decreases. While
Raman spectroscopy suggests the presence of
BN bonds and a linear dependence of thickness with
growth time, they do not provide any
information on the stoichiometry of the grown films
. To probe the chemical composition, we used
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Xray photoelectron spectroscopy (XPS) to determine
the B/N ratio. Figure 3c shows the XPS
spectra of asgrown hBN films on a Cu foils with a
film thickness varying from monolayer to 30
layers. It has been previously reported
39
that bulk boron nitride with hexagonal phase exhib
its a B 1s
core level at 190.1 eV. Figure 3c (left) shows XPS
B1s core level spectra with a peak center at 190.2
eV, which is very close to the hBN bulk phase valu
e. Figure 3c (right) shows that the N 1s peak is
located at 397.7 eV, similar to the reported positi
on of the N1s spectrum (398.1 eV) for hBN. Both
the B 1s and N 1s spectra indicate that the configu
ration for B and N atoms is the BN bond,
implying that the hexagonal phase is the phase of o
ur BN films. Further, it can also be seen that the
intensities of the B1s and N1s increases with incre
asing layer thickness of our films. In addition, we
also observe that the intensity of the Cu2p peak we
akens with the increasing thickness of the hBN
films (see supporting information S3) further corro
borating our precise thickness controlled growth.
Finally, quantitative analysis of the B1s and N1s s
pectra indicates that the B/N atom ratio in our
films was 1 ± 0.6%across the range of thicknesses.
These results evidently confirm growth of high
quality hBN layer and continuous film on Cu foil u
sing ammonia borane by our optimized CVD
process. The large area uniformity and high crystal
line quality of our CVD grown hBN films makes
them ideal for applications as ultrathin dielectric
s in optoelectronic devices
38, 4041
. Therefore, to
evaluate the dielectric strength and leakage throug
h our hBN films, we investigate the electronic
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properties of tunnel junctions in which hBN acts a
s a barrier layer between two gold electrodes. The
dielectric properties of the CVD hBN films with di
fferent thicknesses were measured by fabricating
metal/hBN/metal (MIM) capacitors and measuring cur
rent–voltage (IV) characteristics. Figure 4a
shows a schematic diagram of an Au/hBN/Au capacito
r. We use template stripped gold films as our
bottom electrode with root mean square (RMS) roughn
ess less than 0.5nm. The hBN films of
various thicknesses were transferred onto the templ
ate stripped gold. Then, we used standard
electron beam deposition techniques to deposit 100n
m gold through a shadow mask to define the top
electrodes. The contact area was 10μm×10μm. Figure
4b shows IV measurements of hBN layers
with various thicknesses from 1 to 15 layers. Mono
, bi, and 4layer samples show measurable
lowbias conductance, which we ascribe to direct tu
nneling. Thicker samples are insulating at low
bias and show sharp increases at a breakdown voltag
e that increases with thickness. The inset in the
Figure 4b shows the conductance as a function of sa
mple thickness, which decays exponentially, as
expected for direct tunneling. The current densitie
s at the two metal electrodes and through the hBN
layers of different thicknesses were investigated a
s a function of voltage, as plotted in Figure 4c. A
s
shown in Figure 4c, the measured currents of the th
in hBN films agreed well with the PooleFrenkel
(PF) emission model, indicating that a trapassiste
d PF emission mechanism dominated the transport
mechanism for the leakage current in our hBN films
. The Figure 4c shows the PF plot using the
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following equation:
(
)
=
!"
#
$
$
%
−(Φ
'
−
(
)*
+
*
,
)
-.
/
0
0
1
where A, q, N
c
, μ, Φ
T
, V, d and h are the effective area, electron charg
e, density of state in the
conduction band, electronic mobility in the oxide,
trap energy level in the hBN, voltage, hBN
thickness and Planck’s constant respectively. Final
ly, for thicker (> 1 nm) hBN films we performed
irreversible dielectric breakdown measurements to d
etermine the hardbreakdown voltage (see
supporting information S 4) and corresponding field
strength of the ultrathin hBN. Figure 4 (d)
plots the breakdown field strength as a function of
the hBN thickness. In hBN films with a
thicknesses less than 5 nm, the breakdown voltage i
ncreased linearly with hBN thickness, indicating
very high quality films at the fewlayer limit. Bre
akdown field strength approaching ~ 4.3 MV/cm
were observed for 4.5 nm thick BN films.
Conclusions
We have imaged the atomicscale structure of monol
ayer hBN sheets on Au, and moiré patterns on
monolayer hBN/graphene heterostructures using scan
ning tunneling microscopy (STM). Atomic
STM images of monolayer hBN film and moiré pattern
s on monolayer hBN/graphene
heterostructures show the high crystalline quality
of the CVD grown hBN up to the atomic level.
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We also introduced a hybrid LP and APCVD system tha
t uses controlled precursor to grow uniform,
layer by layer thickness controlled wafer scale hB
N films with thicknesses ranging from monolayer
to 10 nm. Spectroscopic characterization suggests t
hat the films are stoichiometric and highly
uniform over waferscale areas. Electrical measurem
ents for metalinsulatormetal (Au/hBN/Au)
structures indicate that our CVDgrown hBN films c
an act as an excellent tunnel barrier with a high
hardbreakdown field strength. Successful large are
a CVD growth of hBN films defines a new
stateoftheart for application of this material i
n future largearea, electronic and photonic device
s.
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Methods
Pre-treatment of copper foil.
Copper foil (25 μm, 99.999% pure, Alfa Aesar, item
no. 10950) was
soaked and sonicated in acetone and isopropyl alcoh
ol (IPA) for 30 min consecutively to remove
organic impurities. Then, it was washed with deioni
zed water and dried with nitrogen gas.
Synthesis of mono- and multilayer h-BN.
The hBN films were grown using a homebuilt hybrid
CVD setup. Figure 1a shows the schematic of the gro
wth setup. The setup comprises of a 52 mm
inner diameter (I.D.) horizontal split tube furnace
(MTI Corporation). The solid precursor
ammoniaborane (NH
3
BH
3
) powder, (97% purity, SigmaAldrich) is contained
in a homemade
quartz container, attached to the main growth chamb
er (22 mm I.D. quartz tube) via a leak valve and
heated separately from the quartz tube via use of a
resistive heating belt. Cu foils (25 μm, 99.999%
pure, Alfa Aesar) are used as the catalytic growth
substrates. The pressure in the growth chamber can
be independently controlled via an angle valve at t
he vacuum pump while the pressure in the
precursor bubbler can be controlled via the leak va
lve and carrier gas flow rates. The monolayer and
multilayer hBN was synthesized using a pressure co
ntrollable CVD system. The copper foil was
inserted into the center of a 22 mm I.D. quartz tub
e, heated by a horizontal splittube furnace. The
Ammonia borane (NH
3
BH
3
) (97% purity, from SigmaAldrich), stable in an at
mospheric
environment, was used as the precursor. It was load
ed in a homemade quartz container which is
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isolated from the main CVD system with a leak valve
to control the flow rate. The quartz tube inlet
and outlet were blocked by the filters to prevent t
he BN nanoparticles from diffusing into the gas
line. First, the quartz tube was pump down to 5×10
3
torr, and then ultrahigh purity grade hydrogen
gas was introduced during the temperature rampup o
f the furnace (pressure ~100mtorr, flow rate
~50 sccm). The copper foil was annealed at 950°C in
hydrogen for 60 min to obtain a smooth
surface. After annealing, the ultrahigh purity arg
on gas (300 sccm) was introduced into the system
and waited for 30min to stable the tube environment
. The precursor was heated to 130°C and
decomposed to hydrogen gas, monomeric aminoborane,
and borazine gas. After the precursor
temperature reach 130°C, the manual valve between q
uartz tube outlet and the pump was slowly
closed and stopped until the pressure reach 20 torr
. When the desired pressure was achieved, the leak
valve to the precursor was open. The typical growth
time is 3 min for monolayer hBN layer and 20
min for the 20 nm thickness hBN layer. To atomical
ly control the thickness of hBN layer, it is very
important to have a leak valve to control the flow
rate of the precursor. Also, we can change the
growth rate of the hBN by changing the pressure of
the growth environment. After growth, the
tube furnace was cooled down with the cooling rate
~16°C/min.
Transfer of mono- and multilayer h-BN.
To transfer hBN onto a target substrate, the conv
entional
poly(methyl methacrylate) (PMMA) transfer method an
d polymerfree transfer method
41
were
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applied based on different purposes.
Characterization.
Atomic Force Microscopy (AFM) (Bruker Dimension Ic
on), were done using
tapping mode to characterize the surface morphology
of the hBN film transferred on the SiO
2
/Si
substrate. The quality of the hBN film was charact
erized using the Raman spectroscopy, Xray
photoemission spectroscopy (XPS) and Scanning tunne
ling microscopy. The Raman spectra were
taken with a Renishaw M1000 microRaman spectromete
r system using a 514.3 nm laser (2.41 eV)
as the excitation source. A 50X objective lens with
a numerical aperture of 0.75 and a 2400
lines/mm grating were chosen during the measurement
to achieve better signaltonoise ratio.
XPS was performed under 10
9
torr with a Surface Science Instruments MProbe in
strument utilizing
Al K
α
Xrays and a hemispherical energy analyzer. STM wa
s carried out with an Omicron system at
room temperature.
Acknowledgements
The authors gratefully acknowledge support from the
Department of Energy, Office of Science
under Grant DEFG0207ER46405 (W.H.L. and H.A.A.) a
nd for use of facilities of the DOE
“
LightMaterial Interactions in Energy Conversion
”
Energy Frontier Research Center
(DESC0001293). D.J. and M.C.S. acknowledge additi
onal support from Resnick Sustainability
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Institute Graduate and Postdoctoral Fellowships whe
reas V.W.B. acknowledge additional support
from the Kavli Nanoscience Postdoctoral Fellowship.
The authors thank Prof. George Rossman
for access to the Raman and FTIR tools. The authors
thank ITe Lu for the discussion about the
moiré pattern calculation. The authors acknowledge
support from the Beckman Institute of the
California Institute of Technology to the Molecular
Materials Research Center.
Competing financial interests
The authors declare no competing financial interest
s.
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Figure 1.
Figure 1. (a) Schematic diagram of hybrid atmospher
ic pressure and low pressure CVD system used
for hBN grow. (b) Photograph of a large and unifor
m monolayer hBN film on a 285 nm thick
SiO
2
/Si substrate, and corresponding AFM image and Rama
n spectra. (c) Photograph of a large and
uniform multilayer hBN film on a 285 nm thick SiO
2
/Si substrate, and corresponding AFM image
and Raman spectra.
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