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Atomic-scale Structural and Ch
emical Characterization of
Hexagonal Boron Nitride Layers
Synthesized at the Wafer-Scale with Monolayer Thickness Control
Wei-Hsiang Lin
1
, Victor W. Brar
1,2,3
, Deep Jariwala
1,4
, Michelle C. Sherrott
1,4
, Wei-Shiuan Tseng
5
,
Chih-I Wu
6
, Nai-Chang Yeh
2, 5
, and Harry A. Atwater
1, 2,4*
* haa@caltech.edu
1. Thomas J. Watson Laboratory of Applied Physics,
California Institute of Technology, Pasadena, CA 91125,
United States
2. Kavli Nanoscience Institute, California Institute
of Technology, Pasadena, CA 91125, United States
3. Department of Physics, University of Wisc
onsin-Madison, Madison, WI 53711, United States
4. Resnick Sustainability Institute, California Ins
titute of Technology, Pasadena, CA 91125, USA.
5. Department of Physics, California Institute
of Technology, Pasadena, CA 91125, United States
6. Graduate Institute of Photonics and Optoelectronics
and Department of Electrical Engineering, National
Taiwan University, Taipei, Taiwan, Republic of China
Abstract
Hexagonal boron nitride (h-BN) is a promising
two-dimensional insulator with a large band
gap and low density of charged impurities that is
isostructural and isoelectr
onic with graphene. Here
we report the chemical and atomic-scale st
ructure of CVD-grown wafer-scale (~25 cm
2
) h-BN sheets
ranging in thickness from 1-20
monolayers. Atomic-scale images
of h-BN on Au and graphene/Au
substrates obtained by scanning tunneling microscopy
(STM) reveal high h-BN crystalline quality in
monolayer samples. Further characterization of 1-
20 monolayer samples indicates uniform thickness
for wafer-scale areas; this thickness control is a result
of precise control of
the precursor flow rate,
deposition temperature and pressure. Raman and infr
ared spectroscopy indicate the presence of B-N
bonds and reveal a linear dependen
ce of thickness with
growth time. X-ray phot
oelectron spectroscopy
(XPS) shows the film stoichiometry, and the B/N at
om ratio in our films is 1 ± 0.6% across the range
of thicknesses. Electrical current transport in me
tal/insulator/metal (Au/h-BN/Au) heterostructures
indicates that our CVD-grown h-
BN films can act as excellent
tunnel barriers with a high hard-
breakdown field strength. Our result
s suggest that large-area h-BN films are structurally, chemically
and electronically uniform over the wafer scale,
opening the door to perv
asive application as a
dielectric in layered nanoelectr
onic and nanophotonic heterostructures.
Introduction
Since the isolation of graphe
ne on an insulating substrate
1-7
, a variety of othe
r layered materials
have been isolated and characterized, and
have opened up an exciting field of research
8-11
. However
the electronic quality of two-dimensional (2D) activ
e layers in devices is highly sensitive to their
immediate environment owing to the large surface to
volume ratio. Therefore a crystalline 2D material
that can serve the role of an insulating substrate, encapsulating layer or gate dielectric is highly
desirable in device applications. Hexagonal boron nitr
ide (h-BN) has emerged as a promising material
for these applications. It has been observed experi
mentally that encapsulating other 2D materials in h-
BN not only enhances the performance of devices
but also extends their
durability to long time-
scales
12-13
. However most previous studies
have used mechanically exfo
liated h-BN derived from bulk
crystals which does not allow for
control over layer thickness and sa
mple lateral size. While numerous
reports about chemical vapor deposition
(CVD) synthesis of h-BN already exist
14-32
ranging from low
pressure CVD (LPCVD) for monolayer growth to
ambient pressure CVD (APCVD) for multilayer
growth, most of these approaches
do not yield precise thickness c
ontrol from the monolayer scale to
thick multilayers over large areas. Many previous e
xperiments have reported on use of solid ammonia-
borane as the boron and nitrogen source. The sublim
ation of a solid source gives poor control of
precursor flow rate and partial pressure in the
growth chamber over large length scales. Further,
monolayer to sub-2nm growth of h-BN is a pr
ocess catalyzed by the Cu surface and requires low
growth pressures. However the catalytic activity of
Cu surface is diminished
after growth of 1-2 nm
h-BN on its surface and thus growth of thicker h-BN
films occurs solely vi
a van der Waals epitaxy,
requiring higher growth pressures. No system has ye
t been developed that can achieve both of these
growth condition regimes; therefore most prior wo
rk on CVD grown h-BN has been able to control
both BN thickness and lateral thickness uniformity
over technologically relevant areas. Our CVD
methods features precursor flow
and pressure control systems whic
h combines the merits of both
LPCVD and APCVD by allowing precise control over th
e precursor flow rate and partial pressure of
the precursor in the growth zone over a wide range
of growth chamber pressure and temperatures. This
permits growth of high quality mono to sub 5-laye
r h-BN films on a Cu foil
which require low growth
pressures as well as thicker h-BN films
on Cu foils, by switching to APCVD mode.
We image the atomic structure of monol
ayer h-BN/Au sheets and monolayer h-
BN/graphene/Au heterostructures using scanning
tunneling microscopy (STM). Atomic STM images
of CVD-grown monolayer h-BN/Au
film monolayers and h-BN/graphene
heterostructures indicate
high crystalline quality. Th
e crystal structure and chemical compos
ition of the resulting h-BN film are
also characterized by atomic force microscopy (AFM), Raman spectroscopy,
infrared transmission
measurements
and x-ray photoelectron spectroscopy (XPS). Th
e electrical properties of the thin and
thick h-BN films were systemati
cally measured on metal-insulator-
metal (MIM) tunneling devices to
understand the dielectric properties
of these films. Our results dem
onstrate a new standard and state-
of-the-art for large area synthesi
s of h-BN potentially enabling appl
ications in nanoelectronic and
nanophotonic devices.
Results and Discussion
To grow ultrathin (< 1.5 nm) h-BN films, we
use a growth pressure of ~ 2 Torr. Figure 1b
shows an optical photograph and atomic force micr
oscopy (AFM) analysis of a wafer-scale h-BN
monolayer which has transf
erred onto a 285 nm SiO
2
/Si substrate. Raman spectra acquired over six
different spots on the sample (denoted by x), as s
een in the adjacent plot suggests that the sample
thickness and quality is uni
form over the entire cm
2
scale. Likewise, the growth mode can be
switched from a slow growth rate
of ~0.3 nm/min in catalytically c
ontrolled CVD at low pressures to
a high growth rate of 1 nm/min at near atmosphe
ric pressures (~500 Torr). Figure 1c illustrates the
large area uniformity of ~ 15
monolayer h-BN as inferred fr
om the optical micrograph and
corresponding Raman spectra. To validate the pr
ecise control of layer thickness and growth
uniformity, a series of growth experiments were
performed for varying times at both low and
ambient pressures to produce h-BN films of vary
ing layer number/thicknesses. These h-BN films
were then characterized with several spectroscopi
c techniques to characterize their structure and
chemical composition. Atomic scale
structure and areal homogeneity
of h-BN films were revealed
by scanning tunneling microscopy (STM) for CVD-
grown monolayer h-BN. Despite numerous
reports on CVD synthesis of h-BN, little is known about
atomic scale electronic structure for layers
grown on polycrystalline Cu foils, due to the roug
hness of the Cu foil substrate which renders STM
difficult. Also, the film thickness inho
mogeneity seen in most prior reports
33-37
would prevent image
formation by direct electron tunneling through h-BN.
Notably, h-BN is difficult to characterize using
STM due to its insulating
character. To enable adequate samp
le conductance, many research groups
have used a graphene/ h-BN hetero
structure which exploits the conduc
tance of graphene
to visualize
the atomic structure of the h-BN
layer underneath the graphene. In contrast, we are able to use STM
to directly image our CVD-grown monolayer h-BN
films on Au (111)/mica substrates transferred
using polymer free transfer method
38
. Figure 2a is a schematic of the STM measurement
configuration for monolayer h-BN
sheets on Au (111) substrates. Fi
gure 2b shows a representative
STM image of the monolayer h-BN sheet on Au
(111) without any post tr
ansfer annealing. The
atomically-resolved h-BN honeycomb structure is
clearly 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 fr
om the interaction between boron nitride atoms
and the Au (111) substrate herringbone recons
truction. Figure 2c shows schematic of STM
measurements of the monolayer h-BN sheet on m
onolayer graphene/Au (111) substrate and Figure
2d, e show topographic STM images acquired from tw
o different areas of th
e single-layer h-BN on
graphene. Both the atomic lattices of h-BN and l
onger range moiré patterns
can be clearly revealed.
The moiré pattern is formed by interference betw
een the h-BN layer and
underlying graphene/Au
(111) substrate, and can be attrib
uted to their lattice mismatch (a
= 0.252 nm, b= 0.246 nm) and
rotational misalignment. The h-BN
and graphene sheets interact th
rough van der Waals forces, and
display the same topographic conformal mapping to
the underlying Au (111). However, the relative
rotation angle between the graphe
ne and h-BN sheets can be modi
fied by tiny wrinkles and bubbles
are inevitably introduced during the transfer process
of h-BN onto graphene as
seen in Fig. 2d (left)
and original grain boundary of h-BN
and graphene. Changing the rotation angles between the h-
BN and graphene lattices, leads to
moiré patterns with different pe
riodicities and orientations as
observed in Figure 2d and e (center)
. The periodicity of mo
iré pattern presented in Figure 2d is 3.4
nm and 2.0 nm in Figure 2e. The twist angle be
tween the h-BN and graphene lattices can be
ascertained from the structure of
the Moire pattern and is given by
ݏ݋ܿൌ ߠ
ିଵ
ሾ1െ
ܽ
ܾ
ߣെ
ሻܽെܾሺ
ߣܾܽ2
where a is the h-BN lattice constant, b
is the graphene lattice constant, and
ߣ
is the periodicity of the
moiré pattern. Thus, the twist angle
ߠ
of Figure 2d and 2e is found to be (4
0.1)
°
and (7
0.1)
°
, respectly. Alternatively, the twist angle
ߠ
can also be extracted by performing a fast Fourier
transform (FFT) analysis of the STM images, as s
hown in the inset of Figur
e 2d and 2e. The outside
set of spots corresponds to
the reciprocal lattice of
h-BN, while the inner set
of spots are assigned to
the moiré pattern stemming from th
e rotation between the monolayer
h-BN and graphene substrate.
The atomic resolution STM images presented here
can be achieved in image locations over a wide
area of the wafer-scale sample, indi
cating excellent h-BN sheet surface
quality for layers transferred
with the polymer-free
38
transfer method and supporting the ex
istence of high crystalline quality in
CVD-grown h-BN down to the monol
ayer level. The STM results provide information about the
atomic scale structure and crystallinity of monolay
er h-BN. However they do not provide information
about macroscopic-scale h-BN film thickne
ss homogeneity, composition or structure.
Raman spectra of an h-BN layer typically have two active E
2g
modes, one at 1366 cm
-l
which is
strong and corresponds to vibrations of B and N m
oving against each other in the plane and another
at 51.8 cm
-l
, which is attributed to slid
ing between whole planes. Howe
ver the lower frequency mode
is more difficult to observe because of pr
oximity to the Rayleigh diffusion, as well as
the presence of
a fluorescence background. The width, intensity and pos
ition of these Raman features are sensitive to
h-BN thickness, and these dependencies were de
termined by combining Raman measurement with
STM results and AFM results. We use monolayer h-
BN as a thickness calibration for every run and
verify the integrated intensity of this calibration po
int with the value marked
with an arrow in Figure
3b (Top). The integrated intensity of the h-BN layers
determined using this protocol gives a reasonable
estimate of the layer thickness and matches very well with the estimates given by the AFM line profiles
(see supporting information S1). Figure 3a shows Ra
man spectra of monolaye
r h-BN to 15 layers h-
BN films, showing the Raman intensity increases
with the number of layers. The integrated intensity
is plotted as a func
tion of the number of layers in Figure 3b
(Top). Figure 3b (Bot
tom) 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 B-N 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 X-ray photoelectron spectroscopy (XPS) to
determine the B/N ratio. Figure 3c
shows the XPS spectra of as-grown
h-BN 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 exhibits a B 1s core
level at 190.1 eV. Figure 3c
(left) shows XPS B1s
core level spectra with a peak cen
ter at 190.2 eV, which is very clos
e to the h-BN bulk phase value.
Figure 3c (right) shows that the N 1s peak is lo
cated at 397.7 eV, similar to the reported position of
the N1s spectrum (398.1 eV) for h-BN. Both the B 1s
and N 1s spectra indicate
that the configuration
for B and N atoms is the B-N bond, implying that the
hexagonal phase is the phase of our BN films.
Further, it can also be seen that the intensitie
s of the B1s and N1s increases with increasing layer
thickness of our films. In addition,
we also observe that the intensity of the Cu2p peak weakens with
the increasing thickness of
the h-BN films (see supporting inform
ation S3) further corroborating our
precise thickness controlled growt
h. Finally, quantitative analysis of
the B1s and N1s spectra indicates
that the B/N atom ratio in our films was 1 ±
0.6%across the range of th
icknesses. These results
evidently confirm growth of high quality h-BN
layer and continuous film
on Cu foil using ammonia
borane by our optimized CVD process. The large area
uniformity and high crys
talline quality of our
CVD grown h-BN films makes them ideal for applica
tions as ultrathin dielect
rics in optoelectronic
devices
38, 40-41
. Therefore, to evaluate the dielectric stre
ngth and leakage through
our h-BN films, we
investigate the electronic properties
of tunnel junctions in which h-BN
acts as a barrier layer between
two gold electrodes. The dielectric properties of th
e CVD h-BN films with different thicknesses were
measured by fabricating metal/h-BN/metal (MIM)
capacitors and measuring current–voltage (I-V)
characteristics. Figure 4a shows a schematic diagra
m of an Au/h-BN/Au capacitor. We use template
stripped gold films as our bottom
electrode with root mean squa
re (RMS) roughness less than 0.5nm.
The h-BN films of various thicknesses were transfe
rred onto the template stri
pped gold. Then, we used
standard electron beam deposition techniques to
deposit 100nm gold through a shadow mask to define
the top electrodes. The contact
area was 10μm×10μm. Figure 4b show
s I-V measurements of h-BN
layers with various thicknesses from 1 to 15 layers
. Mono-, bi-, and 4-layer samples show measurable
low-bias conductance, which we ascribe to direct
tunneling. Thicker samples are insulating at low bias
and show sharp increases at a breakdown voltage that
increases with thickness. The inset in the Figure
4b shows the conductance as a function of sample
thickness, which decays exponentially, as expected
for direct tunneling. The current dens
ities at the two metal electrodes
and through the h-BN layers of
different thicknesses were investigated as a function
of voltage, as plotted in
Figure 4c. As shown in
Figure 4c, the measured currents of the thin h-BN
films agreed well with the Poole-Frenkel (PF)
emission model, indicating that
a trap-assisted PF emission mechanism dominated the transport
mechanism for the leakage current in our h-BN fi
lms. The Figure 4c shows the PF plot using the
following equation:
ܫ
ܸ
௉ி
ܰݍܣൌ
݌ݔܸ݁݀ߤ
ۏ
ێ
ێ
ۍ
െݍሺΦ
ܸ݀ݍ
ߝߨ
ߝ
ܶ݇
ے
ۑ
ۑ
ې
where A, q, N
c
, μ,
Φ
T
, V, d and h are the effective area, el
ectron charge, density
of state in the
conduction band, electronic mobility in the oxide,
trap energy level in the h-BN, voltage, h-BN
thickness and Planck’s constant respectively. Finally
, for thicker (> 1 nm) h-
BN films we performed
irreversible dielectric breakdow
n measurements to determine th
e hard-breakdown voltage (see
supporting information S 4) and corresponding field
strength of the ultrathin h-BN. Figure 4 (d)
plots the breakdown field strength as a function of
the h-BN thickness. In h-BN films with a
thicknesses less than 5 nm, the breakdown voltage incr
eased linearly with h-BN
thickness, indicating
very high quality films at the few-layer limit.
Breakdown field strength
approaching ~ 4.3 MV/cm
were observed for 4.5 nm thick BN films.
Conclusions
We have imaged the atomic-scale structure of
monolayer h-BN sheets on Au, and moiré patterns on
monolayer h-BN/graphene heterost
ructures using scanning tunne
ling microscopy (STM). Atomic
STM images of monolayer h-BN film and
moiré patterns on monol
ayer h-BN/graphene
heterostructures show the high crystalline quality of
the CVD grown h-BN up to the atomic level. We
also introduced a hybrid LP and APCVD system that
uses controlled precursor
to grow uniform, layer
by layer thickness controlled
wafer scale h-BN films with thickne
sses ranging from monolayer to 10
nm. Spectroscopic characterization suggests that the
films are stoichiometric and highly uniform over
wafer-scale areas. Electrical measurements for
metal-insulator-metal (Au/h-BN/Au) structures
indicate that our CVD-grown h-BN
films can act as an excellent
tunnel barrier wi
th a high hard-
breakdown field strength. Successful
large area CVD growth of h-BN
films defines a new state-of-
the-art for application of this material in fu
ture large-area, electr
onic and photonic devices.
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 al
cohol (IPA) for 30 min consecutively to remove
organic impurities. Then, it was washed with de
ionized water and dried with nitrogen gas.
Synthesis of mono- and multilayer h-BN.
The h-BN films were grown using a home-built hybrid
CVD setup. Figure 1a shows the schematic of the gr
owth setup. The setup comprises of a 52 mm inner
diameter (I.D.) horizontal split tube furnace (MTI
Corporation). The solid precursor ammonia-borane
(NH
3
-BH
3
) powder, (97% purity, Sigma-Aldrich) is c
ontained in a home-made quartz container,
attached to the main growth chamber (22 mm I.D. qua
rtz tube) via a leak valve and heated separately
from the quartz tube via use of a resistive heat
ing belt. Cu foils (25 μm, 99.999% pure, Alfa Aesar)
are used as the catalytic growth substrates. The pr
essure in the growth cham
ber can be independently
controlled via an angle valve at th
e vacuum pump while the pressure
in the precursor bubbler can be
controlled via the leak valve and
carrier gas flow rates. The monolayer and multilayer h-BN was
synthesized using a pressure cont
rollable CVD system. The copper foil
was inserted into
the center of
a 22 mm I.D. quartz tube, heated by a horizont
al split-tube furnace. The Ammonia borane (NH
3
-BH
3
)
(97% purity, from Sigma-Aldrich), stable in an a
tmospheric environment, was used as the precursor.
It was loaded in a homemade quart
z container which is isolated from the main CVD system with a
leak valve to control the flow ra
te. The quartz tube inlet and outlet
were blocked by the filters to
prevent the BN nanoparticles from diffusing into th
e gas line. First, the qua
rtz tube was pump down
to 5×10
-3
torr, and then ultrahigh pu
rity grade hydrogen gas was introduced during the temperature
ramp-up of the furnace (pressure ~1
00mtorr, flow rate ~50 sccm).
The copper foil was annealed at
950°C in hydrogen for 60 min to obtain a smooth surf
ace. After annealing, the ultrahigh purity argon
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 r
each 130°C, the manual valve between quartz tube
outlet and the pump was slowly closed and stopped un
til the pressure reach 20 torr. When the desired
pressure was achieved, the leak valve to the precur
sor was open. The typical growth time is 3 min for
monolayer h-BN layer and 20 min for the 20 nm th
ickness h-BN layer. To
atomically control the
thickness of h-BN layer, it is very important to ha
ve a leak valve to control the flow rate of the
precursor. Also, we can change the growth rate of
the h-BN 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 h-BN onto a targ
et substrate, the conventional
poly(methyl methacrylate) (PMMA) transfer
method and polymer-free transfer method
41
were applied
based on different purposes.
Characterization.
Atomic Force Microscopy (AFM) (Br
uker Dimension Icon), were done using
tapping mode to characterize
the surface morphology of the h-BN
film transferred on the SiO
2
/Si
substrate. The quality of the h-BN film was characterized using the Raman spectroscopy, X-ray
photoemission spectroscopy (XPS) and Scanning t
unneling microscopy. The Raman spectra were
taken with a Renishaw M1000 micr
o-Raman spectrometer 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 signa
l-to-noise ratio.
XPS was performed under 10
-9
torr with a Surface Science Instru
ments M-Probe instrument utilizing
Al K
α
X-rays and a hemispherical energy analyzer. ST
M was 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 DE-FG02-07ER46405 (W.H.L. and H.A.A.)
and for use of facilities of the DOE
Light-
Material Interactions in Energy Conversion
Energy Frontier Research
Center (DE-SC0001293).
D.J. and M.C.S. acknowledge additional support from
Resnick Sustainability Institute Graduate and
Postdoctoral Fellowships whereas V.W.B.
acknowledge additional support from the Kavli
Nanoscience Postdoctoral Fellowsh
ip. The authors thank Prof. Ge
orge Rossman for access to the
Raman and FTIR tools. The authors thank I-Te Lu
for the discussion a
bout the moiré pattern
calculation. The authors acknowledge support from the
Beckman Institute of th
e California Institute
of Technology to the Molecular Materials Research Center.
Competing financial interests
The authors declare no compe
ting financial interests.
Figure 1.
Figure 1. (a) Schematic diagram of hybrid atmosphe
ric pressure and low pressure CVD system used
for h-BN grow. (b) Photograph of a large and
uniform monolayer h-BN film on a 285 nm thick
SiO
2
/Si substrate, and corresponding AFM image and
Raman spectra. (c) Phot
ograph of a large and
uniform multilayer h-BN film on a 285 nm thick SiO
2
/Si substrate, and
corresponding AFM image
and Raman spectra.
Figure 2.
Figure 2. (a) STM measurement schematics on monol
ayer h-BN film. (b) The representative STM
image of the h-BN after transfer to
the Au (1 1 1) substrate, with V
sample
=0.5 V and I
tunnel
= 0.5 nA.
The image size is 25 nm×25 nm (Left) and 5 nm
× 5 nm (Right). (c) STM measurement schematics
on monolayer h-BN/graphene heterostructure.
(d) Topography of a moir
é superlattice with
periodicity of 3.7nm and a 4
°
twist between h-BN and graphene. The image was acquired under
V
sample
=0.5 V and I
tunnel
= 0.5 nA and image size is 15 nm ×15 nm (Left) and 5 nm × 5 nm (Right). (e)
Topography of a moiré superlattice with
periodicity of 2.0 nm and a 7
°
twist between h-BN and
graphene. The image was acquired under V
sample
= 0.5 V and I
tunnel
= 0.5 nA and image size is 15 nm
×15 nm (Left) and 5 nm × 5 nm (Right). The insets
of (d) and (e) are Four
ier transform patterns of
corresponding STM images.
Figure 3.
Figure 3. (a) Raman spectra for h-BN layers with
1-15 atomic layers. (b) (T
op) Integrated intensity
shows a steady increase with increase in the laye
r number of h-BN. (b) (Bottom left) The position of
E
2g
peak vs. number of monolayers
of h-BN, showing the blue shift
with increased number of h-BN
layers. (Bottom right) The Full widt
h at half maximum vs. number of
h-BN layers, showing a steady
increase of the FWHM with h-BN thickness. (c)
X-ray photoelectron spectra (XPS) from different
thickness of h-BN layers on Cu foil. (d) High re
solution B1s and N1s peak
s corresponding to the
thickness from 1 ~ 30 layers h-BN films on Cu foil. (Right) Quantitative analysis of the B1s and N1s
spectra indicates that the B/N atom ratio in our
films was 1 ± 0.6% across
the range of thicknesses.
Figure 4.
Figure 4. (a) A schematic diagram of the Au /
h-BN / Au (MIM) capacitors fabricated on a Si
substrate. (b) Characteristic I
V curves for Au / h-BN / Au devices
with different thicknesses of BN
insulating layer: red curve, monolay
er of BN; orange, bilayer; green,
four layer; navy, 10 layers; and
purple, 15 layers. The inset of (a)
is typical J-V characteristics of
a MIM capacitor, described by the
field-assisted tunneling model. Th
e h-BN thickness range was less th
an 5 nm. The inset shows a PF
emission plot (J/V versus 1/V
1/2
). (d) The breakdown characteristics
as a function of
the h-BN film
thickness.
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Supporting information
AFM images and photographs of bi-l
ayer h-BN to 30-layer h-BN on SiO
2
/Si substrate a
nd the height
profile, infrared phonon of h-BN w
ith the film thickness ranging from
30 layers to 1 layer, X-ray
photoelectron spectra - Cu2p
spectrum of 1 ~ 30 layers
h-BN films, irreversible
dielectric breakdown
measurements to determine the hard-breakdown volta
ge, film thickness with re
spect to the Borazine
partial pressure, growth mechanism of the
h-BN synthesis by APCVD and LPCVD, SEM images
showing triangular ad-layer with different doma
in size, cross – section TEM image of 5-layer
h-BN, and AFM image of 5-
layer h-BN with 0.69 nm
root mean square roughness.