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: sjchoiee@kookmin.ac.kr

2Department of Electrical Engineering, Sejong University, Seoul, Korea, email: sungho85kim@sejong.ac.kr

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

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VG = -10 V

-1

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5040302010

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