Transport properties of ultrathin black phosphorus on hexagonal boron nitride
Rostislav A. Doganov, Steven P. Koenig, Yuting Yeo, Kenji Watanabe, Takashi Taniguchi, and Barbaros
Özyilmaz
Citation: Applied Physics Letters 106, 083505 (2015); doi: 10.1063/1.4913419
View online: http://dx.doi.org/10.1063/1.4913419
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APPLIED PHYSICS LETTERS 106, 083505 (2015)
Transport properties of ultrathin black phosphorus on hexagonal boron
nitride
Rostislav A. Doganov,1,2,3 Steven P. Koenig,1,2 Yuting Yeo,1,2 Kenji Watanabe,4
1,2,3
€
Takashi Taniguchi,4 and Barbaros Ozyilmaz
1
Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore,
6 Science Drive 2, 117546 Singapore
2
Department of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore
3
Graduate School for Integrative Sciences and Engineering (NGS), National University of Singapore,
28 Medical Drive, 117456 Singapore
4
National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
(Received 25 November 2014; accepted 5 February 2015; published online 24 February 2015)
Ultrathin black phosphorus, or phosphorene, is a two-dimensional material that allows both high
carrier mobility and large on/off ratios. Similar to other atomic crystals, like graphene or layered
transition metal dichalcogenides, the transport behavior of few-layer black phosphorus is expected
to be affected by the underlying substrate. The properties of black phosphorus have so far been
studied on the widely utilized SiO2 substrate. Here, we characterize few-layer black phosphorus
field effect transistors on hexagonal boron nitride—an atomically smooth and charge trap-free
substrate. We measure the temperature dependence of the field effect mobility for both holes and
electrons and explain the observed behavior in terms of charged impurity limited transport. We
find that in-situ vacuum annealing at 400 K removes the p-doping of few-layer black phosphorus
C 2015
on both boron nitride and SiO2 substrates and reduces the hysteresis at room temperature. V
AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4913419]
The transport properties of few-layer semiconducting
2D materials are being extensively investigated for applications in electronic,1 optoelectronic,2 spintronic,3 and thermoelectric devices.4 The most widely studied 2D
semiconducting materials so far have been the layered transition metal dichalcogenides (TMDCs), and in particular,
MoS2.5 Recently, ultrathin black phosphorus (bP), or phosphorene, has emerged as an exciting addition to the family of
2D semiconductor materials.6–10 Phosphorene is the second
known monotypic van der Waals 2D material, and its band
gap allows for both larger on/off ratios than in graphene transistors and higher field-effect mobility than in TMDCs and
organic semiconductors.
In 2D material based field effect transistors (FETs), the
electronic transport is confined within a few atomic layers
that are supported by a macroscopic substrate. The properties
of the substrate often drastically alter the transport behavior
of the 2D crystal and the overall characteristics of the device.
For example, in the case of graphene, the disorder on standard SiO2 substrates results in the formation of electron-hole
puddles, which lead to inferior mobility compared to the
intrinsic one.11,12 MoS2 devices fabricated on polymethyl
methacrylate (PMMA) substrates have been shown to exhibit
ambipolar behavior different from what is normally observed
on SiO2.13 Understanding how the substrate and the associated transfer and fabrication steps, such as annealing, affect
the transport properties is crucial before potential applications of 2D materials.
In this letter, we study the transport properties of fewlayer bP supported on hexagonal boron nitride (hBN). hBN
is a 2D dielectric that forms an atomically smooth surface
with lower roughness than SiO2 and is free of dangling bonds
and surface charge traps.11 It has already been utilized to
improve the transport properties of graphene and MoS2
0003-6951/2015/106(8)/083505/5/$30.00
devices.11,14 Being a 2D crystal itself, hBN is arguably the
most important substrate and gate dielectric for potential
few-layer transparent flexible FETs. Here, we measure the
charge transport properties of annealed ultrathin bP/hBN
FETs and study the temperature dependence of the fieldeffect mobility. We explain the observed behavior in terms
of charged impurity (CI) limited transport. We compare the
subthreshold behavior of bP FETs on hBN and SiO2 and
demonstrate that in situ vacuum annealing also removes the
p-doping and reduces the room temperature (RT) hysteresis
in bP samples on SiO2. This strongly suggests that vacuum
annealing at 400 K removes oxidation and fabrication residues on the top surface of bP and provides a simple method
for accessing the properties of the pristine material.
We transfer few-layer bP crystals (bulk bP crystals were
purchased from smart-elements GmbH) onto hBN exfoliated
on SiO2/Si using a process described elsewhere.15 Prior to the
transfer, the exfoliated hBN crystals are annealed at 600 K in
Ar þ H2 gas (9:1) and then scanned under atomic force microscopy to verify the smoothness of the surface. Electrical contacts to the few-layer bP are fabricated using electron beam
lithography and thermal evaporation of Ti/Au electrodes (5/
80 nm). A schematic of the FET structure and an optical image
of the studied devices are shown in Figures 1(a) and 1(b). Due
to the low optical contrast on hBN, we also verify the bP crystal using Raman spectroscopy after device fabrication (Figure
1(c)). Typical conductance versus back gate voltage, Vbg, for a
10 nm thick as-fabricated device at RT is shown in Figure
1(d). For the as-fabricated samples, we observe p-type transport with a conductance of up to 60 lS at Vbg ¼ 60 V and a
significant hysteresis of about DVbg 60 V (DVbg is taken at
the gate voltage at which the conductance is 1 lS).
In-situ vacuum annealing of the samples at 400 K for 1 h
drastically changes the transport behavior of the bP/hBN
106, 083505-1
C 2015 AIP Publishing LLC
V
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Doganov et al.
Appl. Phys. Lett. 106, 083505 (2015)
FIG. 1. (a) Device schematic of the
bP/hBN FET. (b) Optical image of two
devices with bP thickness of 10 nm
and 8 nm on top of 20 nm hBN. The
few-layer bP is outlined with black
dashed lines. The scale bar is 10 lm.
(c) Raman spectrum of the studied bP/
hBN device and the corresponding bP
Raman peaks. (d) Conductance versus
gate voltage Vbg for an as-fabricated
(red line) and in situ annealed (blue
line) bP channel of thickness 10 nm at
room temperature. The arrows indicate
the gate sweep direction.
FETs. As shown in Figure 1(b), after annealing, the p-doping
of the sample is reduced to near zero, the n-type transport for
positive Vbg is greatly enhanced, and the hysteresis is
reduced by a factor of two DVbg 30 V. Similar effect of in
situ vacuum annealing is also observed in bP samples on
SiO2. To avoid effects arising from the residual gate sweep
hysteresis at RT, further device characterization is performed
at low temperature.
In Figure 2(a) we show the four-probe conductance versus gate voltage at T ¼ 190 K for the 10 nm thick bP/hBN
sample from Figure 1(d). The absence of hysteresis centers
the trans-conductance curve at Vbg ¼ 0 V and results in symmetric threshold voltage Vth ¼ 622 V for both p-type and ntype conduction. In Figure 2(b) we plot dr/dn to demonstrate
that linear conduction regime can be reached for both holes
and electrons. Here, r is the conductivity, and n is an approximation for the carrier density obtained from the capacitance
of the gate, n ¼ VbgCg, where Cg ¼ 6.8 1010 cm2/V is the
capacitance per unit area for the 300 nm SiO2 þ 20 nm
hBN back gate. From the linear conduction regime, dr/dn
¼ const., we extract a field effect mobility of
lFE ¼ 189 cm2V1s1 for holes (at nh ¼ 4.8 1012 cm2) and
lFE ¼ 106 cm2V1s1 for electrons (at ne ¼ 3.4 1012 cm2).
In Figures 2(b) and 2(c) we plot the Isd-Vsd curves at 190 K.
We observe linear, ohmic contacts for both the electron and
the hole conduction sides. Contrary to earlier reports,6,16 this
indicates that Ti/Au contacts can be used to access both the
valance and conduction band, suggesting that the dominant
p-type behavior of the as-fabricated bP samples is not solely
due to a higher Schottky barrier for electron injection.
FIG. 2. (a) Four-point conductance
versus gate voltage at T ¼ 190 K for
the same sample from Fig. 1(d). (b)
Derivative of the conductivity r(Vbg)
showing linear conduction regime at
both negative and positive Vbg. (c) I-V
characteristic on the hole conduction
side for Vbg from 40 V to 60. (d)
I-V characteristic on the electron conduction side for Vbg from þ40 V to
þ60 V.
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Doganov et al.
Recent theoretical and experimental studies have shown that
metastable oxygen adsorbed on the surface of few-layer bP
leads to p-doping upon exposure to air.17,18 It is likely that
these adsorbates are removed during the in situ vacuum
annealing, leading to reduction in p-doping observed here.
We now turn to the temperature dependence of the
measured electron and hole field effect mobilities shown
in Figure 3(a). We observe an increase of mobility with
decreasing temperature from around 170 cm2V1s1 at
300 K to 210 cm2V1s1 at 100 K. Below 100 K, the mobility of our bP/hBN FETs shows almost no dependence on
temperature. In the region above 100 K, we find that the mobility follows lh / T0.25 for holes and approximately
le / T0.5 for electrons. Consequently, the observed temperature dependence in our ultrathin bP FETs is markedly
different than in bulk bP, where l / T3/2 behavior, characteristic of acoustic phonon scattering, has been reported for
both n-type and p-type doped samples in the temperature
region between 30 K and 200 K.19,20
The shape of the temperature dependence of the measured mobility in Figure 3(a) indicates that the transport in
our ultrathin bP/hBN FETs is dominated by CI scattering,
similar to the case of monolayer MoS2 field-effect devices.21–24 To explore the effect of impurities on the mobility
of ultrathin bP, we use the theoretical model presented in
Refs. 18 and 21 to calculate the CI-limited mobility. The
model takes into account scattering from a Coulomb potential within an infinitely thin two-dimensional electron gas
FIG. 3. (a) Temperature dependence of the field effect mobility extracted
from the linear conduction regime for holes (red squares) and electrons (blue
circles). The black dashed lines are fits at temperatures above 100 K showing
approximate dependence lh/ T0.25 for the hole mobility and le/ T0.5 for
the electron mobility. The colored dashed lines are a guide to the eye. (b)
Numerical CIlimited mobility obtained from Eq. (1) with
n ¼ 8 1012 cm2 and Ni ¼ 1013 cm2 for different substrate dielectrics: bP/
SiO2 (red), bP/hBN (green), and hBN/bP/hBN (blue). The open symbols are
the measured hole mobility values from (a). The inset shows a schematic of
the theoretical model.
Appl. Phys. Lett. 106, 083505 (2015)
sandwiched between two dielectrics (see the inset of Figure
3(b)). The dielectric constants of the two surrounding dielectrics, as well as the thickness of the top dielectric can be
varied, thereby allowing modeling of different substrates. The
characteristic temperature dependence of the mobility arises
from the temperature dependant screening of the charged impurity, expressed through the static charge polarizability of
the electron gas. For transport along one direction in an anisotropic semiconductor, we can write for the mobility
ð
1
mef f 2
1
l¼
f ðEÞ 1 f ðEÞ CðEÞ E dE: (1)
2
p n h kB T mx
e
0
Here, n is the carrier density, meff ¼ (mxmy)1/2 is the effective carrier mass in the plane, f ðEÞ is the Fermi-Dirac distribution, and mx and my are the carrier masses along the two
anisotropic directions of bP. From measurement of
the angle-dependence of the intensity ratio Ag2/Ag1 in the
polarization-resolved Raman spectrum, we determine the
orientation of the exfoliated ultrathin bP crystals10 and fabricate the FETs so that transport is along the direction with
lighter mass, mx. The electron momentum relaxation rate for
CI scattering, CðEÞ in Eq. (1), is given by
ð
Ni
d2 k jujkk0 j j2 ð1 cos hkk0 ÞdðEk Ek0 Þ; (2)
CðEÞ ¼
2ph
where Ni is the CI density, hkk0 is the scattering angle
between k and k0 states, and ujkk0 j is the screened scattering
potential as defined in Ref. 18. The dielectric constants and
the thickness of the top oxide enter the expression for the
mobility through ujkk0 j . Following Ref. 21, we calculate
ujkk0 j assuming isotropic static polarizability of the electron
gas with effective mass meff ¼ (mxmy)1/2.We use the effective masses measured in p-doped bulk bP, mx ¼ 0.076 m0
and my ¼ 0.648 m0.19 In Figure 3(b), we show numerical
results for mobility versus temperature assuming n ¼ 8
1012 cm2 and Ni ¼ 1.1 1013 cm2 with which we obtain
theoretical mobility values close to the measured ones. The
different exponents for holes and electrons observed in the
experimental data in Figure 3(a) are due to the different
effective masses of the two charge carrier types, and
also due to the different carrier densities. The field-effect
mobility can only be reliably extracted in the region
dr/dn ¼ const., which from Figure 2(b) is Vbg 70 V for
holes and Vbg 50 V for electrons. An extensive study of the
CI limited mobility in monolayer phosphorene has shown
that the exponent c ¼ d log l=d log T can vary from 0 to
1.2 depending on the carrier density.24 We note that the
carrier density n ¼ 8 1012 cm2 used to obtain quantitative
agreement with the experimental hole mobility in
Figure 3(b) is 70% higher than the density expected from
the capacitive model, n ¼ CgVbg ¼ 4.76 1012 cm2 at
Vbg ¼ 70 V. This small discrepancy is likely due to the
simplicity of the used CI model which assumes isotropic
polarizability and fully neglects surface-optical phonon
scattering between the substrate and the semiconductor.
Discrepancies can also arise from difference in the value
for the effective masses, mx and my, between bulk and
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Doganov et al.
few-layer bP. Despite these shortcomings, we find that overall, the CI model gives excellent qualitative and fairly good
quantitative agreement with the experimental data. In Figure
3(b), we further show numerical mobility results for ultrathin
bP on SiO2 and for an encapsulated hBN/bP/hBN device
obtained by only changing the dielectric constants and keeping the charge density and impurity density the same. Not
surprisingly, we observe that the hBN substrate leads to a
higher mobility than SiO2, due to the higher dielectric constant of hBN and the resulting stronger damping of the
Coulomb scattering potential (caused by the dielectric mismatch between the semiconductor and the substrate21,22).
Similarly, the full encapsulation with hBN would lead to
additional enhancement of the CI-limited mobility indicating
that the hBN/bP/hBN device structure can be utilized to
reduce the effect of charged impurities at low temperatures.
To further clarify the effect played by the hBN substrate,
we also studied annealed ultrathin bP devices on SiO2. In
order to avoid differences arising from sample-to-sample
variation, we fabricated devices from a single ultrathin bP
part of which is lying on hBN and part on SiO2 (see the inset
of Figure 4). Similar to the few-layer bP devices on hBN, we
observe that in-situ annealing at 400 K for 1 h in vacuum
leads to significant reduction in the p-doping and the RT hysteresis. In Figure 4, we show Isd-Vg curves for annealed devices on SiO2 and on hBN. Further to the above discussed
enhancement in mobility, in the bP/hBN device, we also
observe an enhanced subthreshold swing, S ¼ dVbg =d log Isd .
For the bP/hBN FET, we obtain a steeper slope of
S 1.5 V/dec, compared to S 2.1 V/dec on SiO2. The
steeper subthreshold behavior on hBN can be attributed to
the superior quality of the bP/substrate interface, as has been
argued for MoS2 devices on hBN.14 The large values of S in
comparison to the ideal limit of 60 mV/dec at RT are due to
the thick, 300 nm, back gate and the resulting low gate
capacitance Cg.
In conclusion, we have studied the charge transport
properties of ultrathin bP supported on an hBN substrate.
Upon annealing, we observe intrinsic semiconductor behavior with no p-doping, reduced hysteresis at RT, and welldefined electron transport which allows us to reach the linear
conduction regime for n-type transport and study the temperature dependence of both the hole and electron field effect
mobilities. The linear Isd-Vsd characteristics at positive gate
FIG. 4. Source-drain current, Isd, versus gate voltage, Vbg, at fixed
Vsd ¼ 0.1 V for in-situ annealed few-layer bP on hBN (red curve) and on
SiO2 (blue curve). The black dashed lines show the extracted subthreshold
slope S. The inset shows the device geometry.
Appl. Phys. Lett. 106, 083505 (2015)
voltages indicate that Ti/Au electrodes can be used to access
both the valance and conduction band of few-layer black
phosphorus. We model the measured temperature dependence of the mobility with a CI scattering model and conclude
that impurities are the main limitation to the measured mobilities in our bP/hBN FETs. Since the CI limited mobility
scales with 1/Ni, improved crystal quality should allow for
higher mobility values closer to those measured in bulk bP.25
Full encapsulation with high-k dielectrics, and in particular,
with hBN, can also be utilized to enhance the CI limited
mobility at low temperatures. Finally, we find that the in-situ
annealing in vacuum at 400 K also reduces the p-doping and
RT hysteresis of ultrathin bP on SiO2, which is likely due to
removal of oxidation and fabrication residues from the top
surface of the few-layer crystal.
This work was supported by the Singapore National
Research Foundation Fellowship award (RF2008-07-R-144000-245-281), the NRF-CRP award (R-144-000-295-281),
and the Singapore Millennium Foundation-NUS Research
Horizons award (R-144-001-271-592; R-144-001-271-646).
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