Semiconducting Carbon Nanotube Schottky Diode
and Integrated Circuit Applications
Yongwoo Lee1, Bongsik Choi1, Jinsu Yoon1, Jinhee Park1, Yeamin Kim1, Han Bin Yoo1, Jun Tae Jang1, Geumho Ahn1,
Hye Ri Yu1, Hyo-Jin Kim1, Dae Hwan Kim1, Dong Myong Kim1, Sungho Kim2*, and Sung-Jin Choi1* 1School of Electrical Engineering, Kookmin University, Seoul, Korea, email: [email protected]
2Department of Electrical Engineering, Sejong University, Seoul, Korea, email: [email protected]
Abstract—A new type of a Schottky diode based on a 99%
semiconducting carbon nanotube (CNT) percolated network is
demonstrated. The fabricated CNT Schottky diode shows
excellent rectifying characteristics which are significantly
modulated by the embedded control gate bias. In addition, we
present integrated circuit applications of digital logic circuits
(OR and AND gates) and an analog circuit (half-wave rectifier)
with the fabricated CNT Schottky diode. The circuits show an
accurate logic function and excellent rectification. Thus, we
believe that our results represent an important step toward
realizing circuits based on CNTs.
I. INTRODUCTION
Carbon nanotubes (CNTs) have been studied for a wide
range of reasons due to their excellent electrical, thermal,
chemical, and mechanical properties [1, 2]. Specifically, CNT
percolated network films based on highly purified
semiconducting CNTs separated by solution processes have
been considered as candidates for use as channel materials in
emerging semiconductor devices [3, 4]. A diode constitutes an
important basic building block in modern micro- and nano-
electronics. However, it is difficult to fabricate a pn junction
diode using CNTs because CNTs initially only exhibit the p-
type characteristics in an ambient environment [5]. Thus, there
have been many efforts to transform the p-type into the n-type
through chemical and electrostatic doping to realize CNT pn
junction diodes [6, 7], but stability issues have remained
challenging. Therefore, a Schottky diode based on CNTs is
promising, but the coexistence of both metallic and
semiconducting CNTs during the synthesis of CNTs hinders the
stable operation and yield of the diodes.
Here, we demonstrate a new type of a Schottky diode and
its applications to digital and analog integrated circuits, which
is based on highly purified, pre-separated solution-processed 99%
semiconducting CNTs. The fabricated CNT Schottky diode
forms Schottky and ohmic contacts using asymmetric
molybdenum (Mo) cathode and palladium (Pd) anode
electrodes, respectively. In addition, a highly p-doped silicon
substrate is utilized as an embedded control gate to modulate
and enhance the rectification ratio of the diodes further. The
proposed CNT Schottky diode exhibited nearly ideal diode
characteristics, yielding a high rectification ratio, i.e., a high
forward/reverse diode current ratio. Furthermore, highly
purified, pre-separated 99% semiconducting CNTs enable the
realization of a high device yield and high stability. Benefiting
Fig. 1. Process flows of the new type of the CNT Schottky diode: (I) Preparation of the starting silicon wafer; (II) Functionalization of the SiO2 surface with PLL;
(III) Semiconducting CNT deposition for network formation; (IV) Anode (or
cathode) deposition for the formation of metal-semiconductor contact; (V) CNT etching using O2 plasma for channel definition; (VI) Cathode (or anode)
deposition.
from these advantages, CNT Schottky diodes were used to
construct logic (OR and AND) and analog (half-wave rectifier)
integrated circuits.
II. DEVICE FABRICATION
Fig. 1a illustrates the details of the fabrication processes of
the new type of the CNT Schottky diode with the embedded
control gate. A highly p-doped silicon substrate served as a
control gate with thermally grown 50-nm-thick silicon dioxide
(SiO2). A biocompatible poly-L-lysine (PLL) solution was
dropped onto the SiO2 surface to functionalize the substrate by
introducing an amine-terminated adhesion layer for the
efficient deposition of the CNT percolated network. The
substrate was subsequently immersed in a commercially
available 0.01 mg/mL CNT solution with 99% semiconducting
purity, followed by rinsing with deionized (DI) water and
isopropanol alcohol. After the deposition of the CNTs, to form
the anode electrode, a Pd layer was deposited using an e-beam
evaporator. It is well known that Pd can form an ohmic contact
with semiconducting CNTs [8, 9]. An additional
photolithography step with O2 plasma was then performed to
remove any possible leakage through the unwanted CNTs to the
outside of the device. Finally, a Mo layer serving as a cathode
electrode in the diode was also deposited by means of e-beam
evaporation. The fabricated CNT Schottky diode was a back-
to-back configuration with a shared cathode.
O
Si
NH2
OH
OSi
NH2
O
NH2
Si OH
Si
NH2
O
OH
O
Si
NH2
OH
O
Si
O
Si
NH2
SiSi
NH2
OO
OHO
OH
NH2
OH
OH
Ⅰ. Substrate preparation
Ⅵ. Cathode (or anode) deposition
Ⅴ. Channel definition Ⅳ. Anode (or cathode) deposition
Ⅱ. PLL solution drop Ⅲ. CNT deposition
Fig. 2. (a) Electrical characteristics of the CNT Schottky diode with the Pd
(anode) and Mo (cathode) electrodes. (b) Comparison of the on-state current (ION) and off-state current (IOFF), and (c) log(ION/IOFF) according to different VG
values in the fabricated CNT Schottky diode.
It is also important to note that the fabricated CNT Schottky
diode was configured for an OR logic gate. However, the AND
logic gate reverses the positions of the anode and cathode
electrodes; in other words, only the order of the deposition of
the electrodes (Pd and Mo) was changed.
III. RESULTS AND DISCUSSION
A. Electrical Characteristics of CNT Schottky diodes
The electrical characteristics (IC-VC; cathode current-
cathode voltage) of the fabricated CNT Schottky diodes were
measured at room temperature in an ambient state. The IC-VC
characteristics of the diodes exhibited typical rectification
behavior (Fig. 2a). As the negative control gate voltage (VG)
was increased, IC improved significantly, showing an ideality
factor (η) approaching 1.71. Although the contact between the
semiconducting CNTs and the Pd electrode initially showed
ohmic behavior, more majority carriers (i.e., holes) could be
injected at the interface between the semiconducting CNTs and
the Pd electrode due to the further reduced effective Schottky
barrier height at a VG value of -10 V, resulting in an improved
forward current (IC). However, when VG increased in the
positive direction, the potential barrier in addition to the
intrinsic Schottky barrier height between the semiconducting
CNTs and the Pd electrode correspondingly increased, thus
preventing majority carrier holes from being injected from the
contact between the semiconducting CNTs and the Pd electrode,
resulting in a loss of the diode characteristics. Fig. 2b shows a
simplified histogram of the on-state current (ION) defined at VC
= –5 V and off-state current (IOFF) defined at VC = +1 V for
different VG values of –10 V, 0 V, and +10 V. In particular, the
fabricated diode at a VG of –10V exhibits a high rectification
ratio of approximately 105, as shown in Fig 2c. This indicates
that the embedded control gate can effectively modulate the
diode characteristics, facilitating the determination and further
enhancement of the logic and analog integrated circuit
functions.
B. Integrated circuits application
In fact, diodes can be used as building blocks to construct
various circuits for digital and analog applications, although
diode-based integrated circuits are rarely realized on CNTs
owing mainly to the low yield and poor stability of CNT devices.
Benefiting from the simple fabrication process proposed in this
work, we can fabricate the CNT Schottky diodes with high
yield and stability level, indeed providing the possibility of
constructing diode-based integrated circuits (Figs. 3 and 4).
Initially, as representative fundamental logic gates, OR and
AND gates were devised on two fabricated CNT Schottky
diodes and one external load resistance (RL = 1 GΩ), as shown
in Fig. 3. Two logic gates have a similar structure, with only the
locations of the electrodes (i.e., anode and cathode) changed.
The input voltages (VIN) were in the form of continuous square
waves with a peak-to-peak (VPP) value of 1 V, and the output
voltage (VOUT) was measured. Both logic gates clearly
presented the correct logic functions, an outcome attributed to
the high yield and stability of our CNT Schottky diodes.
Furthermore, when more negative VG values were applied, the
output responses showed more correct and accurate logic
functions due to the aforementioned improved diode
performances.
Diodes can also be implemented in digital logic gates, and
they are widely used in analog circuits, especially rectifier
circuits. The half-wave rectifier, an analog circuit application,
was demonstrated with two CNT Schottky diodes [10], as
shown in Fig. 4. The implemented half-wave rectifier
Fig. 3. Sequential measurement results of OR and AND gates. The logic gates
were measured with the output voltage (VOUT) for different VG values of –10 V, 0V, and +10 V.
-5 -3 -1 1
10-11
10-9
10-7
10-5
1.71
lI C (
A)l
VG
= +10 V
VG
= -10 V
VC (V)
(a)
(b) (c)
-10 0 1010-12
10-10
10-8
10-6
10-4
lI C
(A
)l
VG (V)
ION
IOFF
-10 0 10
0
2
4
6
Lo
g (
I ON/I
OF
F)
VG (V)
-5 -3 -1 10
4
8
-IC (A
)
VC (V)
LDiode
= 500 nm
VG
= -10 V
VG = +2 V step
Pd Mo
VG
0
1
VG = -10 V
0
1
0
1
0
1
VG = 0 V
1 2 3 4 50
1
VG = +10 V
Time (s)
0
1
VG = -10 V
0
1V
G = +0 V
1 2 3 4 50
1
Time (s)
VG = +10 V
VO
UT
(V)
VA
(V)
VO
UT
(V)
VO
UT
(V)
VA VOUT
RLVOUT
RL
VDD
OR gate AND gateV
OU
T(V
)V
OU
T(V
)V
OU
T(V
)
VB
VA
VB
VB
(V)
0 1 0 1 0 1
0 1 0 0 1 0
VA
(V)
VB
(V)
0
1
0
1
0 1 0 1 0 1
0 1 0 0 1 0
VGVG
Fig. 4. Sequential measurements of a half-wave rectifier circuit for different VG values of –10 V, 0V, and +10 V.
had an AC input signal with sinusoidal waves (VPP value of 1
V), and the output signals were observed by varying the VG
values. When VG was increased negatively, we observed that
the output responses became correct with almost no loss of the
peak voltages. In this case, the efficiency (ηRE) of our half-wave
rectifier was close to the ideal half-wave rectifier value (40.6%)
[11, 12]. Therefore, our results could potentially be promoted
to other semiconducting nanomaterials such as nanowires and
two-dimensional materials, providing ideas and building blocks
for electronic applications based on nanoscale materials.
IV. CONCLUSION
We demonstrated a new type of Schottky diode with high
yield and high stability level based on highly purified, pre-
separated 99% semiconducting CNTs. The fabricated CNT
Schottky diodes provided an excellent rectification ratio, which
was improved even more given the use of an embedded control
gate bias. In addition, we built integrated circuits in the form of
digital logic and analog circuits using the fabricated CNT
Schottky diodes. As excellent output responses were achieved
from the circuits we believe that the proposed diode will be an
important step toward the realization of circuits based on CNTs.
ACKNOWLEDGMENT
This work was supported by the National Research
Foundation (NRF) of Korea under Grants 2016R1A2B4011366
and 2016R1A5A1012966 and partially supported by the Future
Semiconductor Device Technology Development Program
(Grant 10067739) funded by MOTIE (Ministry of Trade,
Industry & Energy) and KSRC (Korea Semiconductor
Research Consortium).
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0
1
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0
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VG = -10 V
-1
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VG = +10 V
Time (ms)0
VO
UT
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