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2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1
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COMMUNICATION
Sivacarendran Balendhran,* Junkai Deng, Jian Zhen Ou, Sumeet Walia, James Scott,
Jianshi Tang, Kang L. Wang, Matthew R. Field, Salvy Russo, Serge Zhuiykov,
Michael S. Strano, Nikhil Medhekar,* Sharath Sriram, Madhu Bhaskaran,*
and Kourosh Kalantar-zadeh*
Enhanced Charge Carrier Mobility in Two-Dimensional
High Dielectric Molybdenum Oxide
S. Balendhran, J. Z. Ou, S. Walia, Prof. J. Scott,
Dr. S. Sriram, Dr. M. Bhaskaran, Prof. K. Kalantar-zadeh
MicroNanoElectronics and Sensor Technology
Research Group and Functional Materials and
Microsystems Research Group
School of Electrical and Computer Engineering
RMIT University
Melbourne, Victoria, Australia
E-mail: shiva.balendhran@rmit.edu.au;
madhu.bhaskaran@rmit.edu.au; kourosh.kalantar@rmit.edu.au
Dr. J. Deng, Dr. N. Medhekar
Department of Materials Engineering
Monash University
Clayton, Victoria, Australia
E-mail: nikhil.medhekar@monash.edu
DOI: 10.1002/adma.201203346
In atomically thin two-dimensional (2D) materials, free charges
have quantized energy levels in one spatial dimension, while
they are mobile in the other two.
[1]
The interest in such 2D
materials increased in the late 1970s and early 1980s driven by a
large amount of experimental outcomes from the development
of molecular beam epitaxy (MBE) equipment for the deposi-
tion of high quality thin films of III-V semiconductors.
[2]
This
resulted in the realization of high electron mobility transistors,
with thin films of different bandgaps that can operate at very
high frequencies. In such devices, the conduction band energy
of at least one of the films is forced under the Fermi level at the
junction, creating a narrow quantum well, and the quantization
of free charges.
[3]
Such structures can offer electron mobilities
larger than 10
6
cm
2
V
-1
s
-1
(at near zero Kelvin temperatures),
which is significantly larger than that of silicon.
[4]
However, the
high cost of the rare-earth materials, lack of compatibility, and
technological difficulties for creating III-V structures hinders
their widespread adaptation.
The emergence of graphene has revived the interest in
2D materials, due to its many favorable properties including
enhanced electron mobilities that exceed 10
5
cm
2
V
-1
s
-1
.
[5]
However, graphene lacks the semiconducting characteristics
of prevalent materials, such as silicon with their natural energy
bandgap.
[5,6]
Even though there have been advancements in
introducing a bandgap to graphene,
[7]
the required complex
synthesis processes always resulted in significant loss in the
much desired carrier mobilities.
[8]
It is suggested that alterna-
tive layered materials such as semiconducting transition metal
dichalcogenides (e.g., molybdenum disulfide–MoS
2
– one of
the most researched in this group) would solve the issue of
introducing a bandgap, but still results in relatively low carrier
mobilities.
[9,10]
Recent reports on atomically thin MoS
2
indicate
possible approaches to increase its mobility by introducing a
high dielectric top-gate material (HfO
2
) that reduces Coulomb
scattering to attain values smaller than 220 cm
2
V
-1
s
-1
, which
is still less than values sought after.
[10]
In this work, we propose that 2D semiconducting metal
oxides with high dielectric constant (high- κ) offer a solution for
obtaining high electron mobility.
[11–13]
Advantageously, the elec-
tronic properties, in particular the bandgap, of such 2D metal
oxides can be largely manipulated using well-known chemical
and physical approaches.
[13]
Such manipulations, which impose
their effects on the 2D environment, categorize these materials
as excellent templates for achieving the optimum quantum
parameters required for target applications. The charge mobility
in a thin layer is calculated using μ =
e
m
∗
〈τ 〉 , in which e is the
point charge, τ is the transport relaxation rate of momentum in
the plane, and m
∗
is the electron effective mass. Considering
Born approximation, the transport relaxation time is calculated
using:
[2]
1
τ ( E
k
)
=
2π
kz
μ
+∞
−∞
N
(μ)
i
(z)
V
(μ)
k−kz
(z)
2
× 1 − cos θ
kkz
)
× δ E (k) − E (k
z
)
)
(1)
J. Tang, Prof. K. L. Wang
Device Research Laboratory
Department of Electrical Engineering
University of California, Los Angeles, California, USA
Dr. M. R. Field, Prof. S. Russo
School of Applied Sciences
RMIT University, Melbourne
Victoria, Australia
Dr. S. Zhuiykov
Materials Science and Engineering Division
CSIRO, Highett, Victoria, Australia
Prof. M. S. Strano
Department of Chemical Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts, USA
Adv. Mater. 2012,
DOI: 10.1002/adma.201203346