Supporting Information
Space-charge limited transport in large-area monolayer
hexagonal boron nitride
Farzaneh Mahvash1,2, Etienne Paradis3,4, Dominique Drouin3,4, Thomas Szkopek1, Mohamed Siaj2*
1
Department of Electrical and Computer Engineering, McGill University, Montréal, Quebec, H3A
2A7, Canada
2
3
Department of Chemistry, Université du Québec à Montréal, Montréal, Quebec, H3C 3P8, Canada
Institut Interdisciplinaire d'Innovation Technologique (3IT), Universite de Sherbrooke, Sherbrooke,
Quebec, J1K 0A5, Canada
4
Laboratoire Nanotechnologies Nanosystemes (LN2), Universite de Sherbrooke, Sherbrooke, Quebec,
J1K 0A5, Canada
1
hBN growth and transfer
After washing the Cu foils (Alfa Aesar) with warm acetic acid (60 ºC) for 15 minutes followed by
rinsing with deionized water and isopropanol alcohol (IPA), they were placed in a one end closed
quartz tube as shown in Fig. S1. This small quartz tube was placed in the center of another long tube
with its dead-end facing the flow of precursor and carrier gases. In this way, high quality hBN layer
can be obtained. The temperature of furnace was set to 1000 ºC while the whole system was under
vacuum (10-4 Torr). To avoid the oxidation of copper foils, carrier gases of H2 (5 Sccm) and Ar (5
Sccm) were introduced into the growth chamber through the course of temperature raising. This
introduction would raise the pressure of system to 1 Torr. After annealing the copper foils at 1000 ºC
in Ar/H2 flowing (1 Torr) for 15 minutes, the amount of carrier gases was reduced to obtain a pressure
of 600 mTorr. In the meanwhile, the ammonia borane (Sigma Aldrich) was heated using heating belt
and the temperature of heating belt was measured using thermocouple thermometer. When the
temperature reached 60 ºC, the precursor was introduced into the growth chamber. It should be noted
that the heating of the precursor continued till 100 ºC. After 30 minutes, heating and introduction of
the precursor were stopped. The furnace was cooled down rapidly to room temperature under Ar/H2
flowing.
2
Supporting Figure S1: Schematic of the CVD system for h-BN growth. Setup for growing hBN films is composed of a quartz tube sitting in a split-tube furnace connecting from one side to
the precursor and the other side to diffusion and mechanical pumps. The pressure of system was
monitored using a multi gauge controller using convectron and ionization gauges. The flow of
carrier gases (H2 and Ar) was controlled by mass flow controller.
The transfer of CVD grown hBN on Cu foils to desired substrates is needed for further
characterisations as well as device fabrication. This transfer starts with spin coating poly (methyl
methacrylate) (PMMA) onto the surface of h-BN-coated Cu. Then the sample was baked at 100
ºC for 1 minute to stiffen the PMMA handle. In order to etch away the Cu foils, the sample was
immersed in Cu etchant (ammonium persulfate (0.1 M)). After completely etched away Cu, the
hBN layer covered by PMMA was gently transferred to a deionized water bath to wash away the
etchant residues. Then, the sample floating on deionized water must be placed on the target
substrate. The selected target substrates are quarts and heavily doped Si with 300 nm of thermal
3
oxide. The samples were then kept in desiccator to dry for 48 hours. The final step is to remove
the PMMA on top of the h-BN film. This step was done by dissolving the sample in warm
acetone for 48 hours followed by rinsing in deionized water and IPA.
4
hBN characterization
a
O 1s
Intensity (a. u.)
C 1s
N 1s
Cu 3p
Cu 3s
Si 2s B 1s
Si 2p
0
100
200
300
400
500
Binding energy (eV)
c
b
B 1S
186
Intensity (a. u.)
Intensity (a. u.)
N 1S
188
190
192
194
394
Binding energy (eV)
396
398
400
Binding energy (eV)
Supporting Figure S2: X-ray photoelectron spectroscopy (XPS) analysis of hBN on Cu. a)
Survey spectra. The B/N ratio of 1.1 was derived. The existence of additional carbon and oxygen
atoms stems from the exposure of hBN film to air prior to XPS measurement. b) B1S spectra, the
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experimental data points are fitted by the Gaussian function with the peak position of 190.1 eV.
c) N1S spectra, the experimental data points are fitted by the Lorentzian function with the peak
position of 397.6 eV.
As illustrated in Fig. S3, the CL spectrum of suspended hBN at 5 K shows significant emission
at energies 2.6 eV and 2.8 eV below bandgap. Monocrystal hBN CL emission is primarily
concentrated at the 5.8 eV band edge with a shoulder extending down to 3.8 eV1, while
luminescence peaks far below the absorption edge have been routinely observed in hBN
quantum dots2, polycrystalline hBN3, and hBN powders4 and attributed to transitions involving
impurity atoms or structural defects. We conclude that the monolayer hBN used in this work is
polycrystalline, and may incorporate impurity atoms.
Wavelength (nm)
620
496
413
354
3.0
3.5
CL intensity (a. u.)
Suspended hBN
2.0
2.5
Energy (eV)
Supporting Figure S3: Cathodoluminescence spectrum of suspended hBN at 5 K.
6
0.6
1.0
0.4
0.8
0.2
0.6
0.0
0.4
-0.4
0.0
Ig(pA)
Is(pA)
-0.2
0.2
-0.6
-0.2
-0.8
-0.4
-1.0
-0.6
-60
-40
-20
0
20
40
60
-1.2
Vs(V)
Supporting Figure S4: I-V curve of a device without hBN. Is-Vs (black curve- left axis) and IgVs (Blue Curve- right axis) of the device without hBN (Only SiO2) with L = 8 μm and W = 100
μm.
7
3.0
Device labels
F4- 10 um-3,4
F4- 10 um-4,5
F4- 8 um-1,2
F4- 8 um-8,9
F4- 8 um-9,10
G4- 4 um-7,8
F4- 4 um-2,3
F4- 8 um-2,3
F4- 8 um-7.8
α
2.5
2.0
1.5
0.002
0.004
0.006
0.008
0.010
1/T (1/K)
Supporting Figure S5: α versus 1/T for various devices. The voltage dependent of spacecharge limited current is given by Is ∝ Vsα. The value of α is extracted at different temperatures
from the IS – VS curves of Fig. 3c. As shown here, α increases as device temperature T decreases.
This behaviour is in accord with space charge conduction in solids, with an observed trend of
!
𝛼 = ( !! ) + 1 where TC is a characteristic temperature relating to charge trap distribution as first
described by A. Rose (1954)5.
8
Supporting Figure S6: Back gate dependence. The channel length for this device is 8 µm.
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(5)
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