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INVESTIGATION ON CNT BASED TRANSISTOR WITH COMPARATIVE ANALYSIS OF ZnO & Al2O3 AS GATE DIELECTRICS

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 01, January 2019, pp. 1581–1587, Article ID: IJMET_10_01_161
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=1
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
Scopus Indexed
INVESTIGATION ON CNT BASED TRANSISTOR
WITH COMPARATIVE ANALYSIS OF ZnO &
Al2O3 AS GATE DIELECTRICS
S.K. Suresh Babu
Centre for Research in Nanotechnology, Department of Nanoscience,
Karunya Institute of Technology and Sciences, India
VLSI Laboratory, Department of Electronics and Communication Engineering,
Karunya Institute of Technology and Sciences, India
Naveen Kumar
Centre for Research in Nanotechnology, Department of Nanoscience,
Karunya Institute of Technology and Sciences, India
D. Jackuline Moni
VLSI Laboratory, Department of Electronics and Communication Engineering,
Karunya Institute of Technology and Sciences, India
ABSTRACT
The present study investigates on the single walled carbon nano tube (CNT) based
transistor and with comparative analysis of Zinc Oxide (ZnO) and Aluminium Oxide
(Al2O3) as gate dielectric films. Structural properties of these gate dielectric films
were analysed and reported. Array network of single walled carbon nanotube
(SWCNT) was formed as a channel in field effect transistor (FET) by drop casting
method. At room temperature gate dielectric (ZnO and Al2O3) films were deposited by
using pulsed laser deposition. Various electrical parameters were analysed for the
fabricated SWCNT/Al2O3 and SWCNT/ZnO FET’s. The achieved threshold voltage
(VTH), ION and IOFF are 0.6 Volts, -2.88mA and -2.19mA was observed for CNT/Al2O3
FET and 0.2Volts, -2.19mA and -4.99mA was observed for CNT/ZnO FET
respectively.
Keywords: Carbon Nano Tube, Zinc Oxide, Aluminium Oxide, Drop casting, Pulsed
laser deposition, Field effect transistor
Cite this Article: S.K. Suresh Babu, Naveen Kumar, D. Jackuline Moni, Investigation
on CNT Based Transistor with Comparative Analysis of ZnO & Al2O3 as Gate
Dielectrics, International Journal of Mechanical Engineering and Technology 10(1),
2019, pp. 1581–1587.
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S.K. Suresh Babu, Naveen Kumar, D. Jackuline Moni
1. INTRODUCTION
In recent research, many researchers have shown attention towards carbon nanotube (CNT)
based electronic devices due to its very good electrical and mechanical properties[1-3]. In this
paper, CNT based transistor with two high K dielectric materials such as ZnO and Al2O3 are
fabricated as gate dielectric of FET and their DC characteristics are compared [4-7]. The highk material shows good impact on the replacement of SiO2 used in the conventional transistor
[8-9]. Single walled carbon nanotube is widely used as a channel, since it has remarkable
properties like electrical, thermal conductivity, and good mechanical properties [10-11].
Single walled CNT is more suitable for the channel of thin film FET [12].
ZnO thin film acts as a good dielectric when it is deposited at room temperature
(Amorphous) [13]. The amorphous Al2O3 has high dielectric properties, low permittivity to
impurities and high thermal conduction [14]. Pulsed laser deposition is another good
technology tool used for the deposition of ZnO and Al2O3 as gate dielectric films in the FET
[15-16]. The atom from the target (Zno / Al2O3) losses and collided with the high gas
molecule (oxygen) and nucleation takes place, which results in deposition of ZnO / Al2O3 thin
film over the substrate. The source, drain and gate were deposited using gold by sputtering.
2. EXPERIMENTAL DETAILS:
The used single walled CNT is less than of 90nm length and 99.9% of pure ZnO and
Al2O3was obtained from Sigma-Aldrich, USA. ZnO and Al2O3 are been made as pellet using
pelletizer to use it as a target in PLD system to make films over the substrate. The gate
dielectric materials ZnO and Al2O3 are deposited as a thin film at room temperature [13-14].
(a)
(b)
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(c)
Figure 1 (a) XRD pattern of Al2O3 thin film deposited using PLD at room temperature
(b) XRD pattern of ZnO thin film deposited using PLD at room temperature
(c) XRD pattern of CNT film prepared by drop casting method
The amorphous nature of these films were confirmed with the obtained XRD results
shown in figure 1(a),1(b) for XRD pattern of Al2O3 and ZnO thin films respectively. Figure
1(c) shows that XRD pattern of SWCNT and Dimethylformamide (DMF) of film shows the
crystalline structure. The peak was observed at 28, 47, 56 and hkl value was (002), (001),
(400) respectively.
Figure 2 (a) SEM image of Al2O3 thin film
Figure 2(b) SEM image of ZnO thin film
Al2O3 thin film deposited at room temperature using PLD shows uniformity deposition
and cluster formation was observed in SEM image figure 2(a). From these results, it’s clear
that the film formation is all over the substrate. Figure 2 (b) shows the SEM image of ZnO
thin film. It confirms the uniform deposition and spherical formational growth.
2.1. Device Fabrication
The gold was used as a back gate for the CNT/Al2O3 and CNT/ZnO fabricated FET’s. ~50nm
thickness of gold was deposited on a glass substrate by DC sputtering. ZnO (8.5) and
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Al2O3(10) have good dielectric constant than the conventional gate dielectric constant of SiO2
in FET. Approximately 45nm was deposited over the gate layer by Pulsed Laser Deposition
with the following conditions: 3000 pulse of laser energy with vacuum condition of 9.6x10-6
Torr of base pressure under the oxygen environment.
The Single walled Carbon Nanotube is used as a channel for the proposed fabrication of
FET. These SWCNT are dispersed with dimethyformamide by 1:1 ratio and it has been
deposited as channel by drop casting method on the glass substrate. It is one of the simplest
and cost effective deposition methods of the channel.
Figure 3(a) and (b) show the schematic structure of fabricating used fabricating
CNT/Al2O3 and CNT/ZnO FET. For both the devices all the fabrication steps are the same
except the gate dielectric. Gate, Source and drain are deposited by gold by DC sputtering
under the vacuum condition. It is deposited at the thickness of ~45 nm.
(a)
(b)
Figure 3 Fabricated structure of carbon nanotube thin film FET with
(a) CNT/ Al2O3 film and (b) CNT/ZnO film configuration
2.2. Electrical Analysis of Fabricated FET
Figure 4 Current Voltage characteristics of CNT film
The resistance of the channel (CNT film) of the FET was observed after depositing source
and drain which is shown in figure 4. This graph helps us to understand the channel was
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resistive and allow the electron to pass after the threshold voltage. This also proves that there
is a interlink of carbon nano tubes between the source and drain.
(a)
(b)
Figure 5 (a) Transfer characteristics of Al2O3/SWCNT FET
(b) Transfer characteristics of ZnO/SWCNT FET
The threshold voltage of the fabricated FET’s was calculated from the transfer
characteristics. To obrain the threshold the the square root of drain current was considered
with the varying gate voltage (VGS) and with constant drain voltage (VDS). For CNT/Al2O3
FET the threshold voltage was observed as 0.6 volts and CNT/ZnO FET as 0.2 Volts as
shown in figure 5 and 6 respectively. From this results the ZnO/CNT has low threshold
voltage than the CNT/Al2O3 FET.
Figure 6 Logarithmic scale of drain current (ID) Vs gate source voltage: (a) CNT/Al2O3 FET
(b) CNT/ZnO FET
The on current and off current of the fabricated devices were observed from the
logarithmic scale of drain current to the gate voltage. There is good ION and IOFF current was
observed for the Al2O3/CNT FET than the ZnO/CNT FET. The electrical parameters of the
Al2O3/CNT FET and ZnO/CNT FET were tabulated in Table 1.
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Table 1 Compartive electrical results of Al2O3/CNT FET and ZnO/CNT FET
Threshold Voltage (VTH)
On current (ION)
Off current (IOFF)
Al2O3
ZnO
Al2O3
ZnO
Al2O3
ZnO
0.6V
0.2V
-2.88 mA
-2.19 mA
-6 mA
-4.99 mA
From the results obtained it is clear that the CNT/ZnO FET over performed in on current
(ION) and threshold voltage (VTH), but in the case of off current (IOFF) CNT/ l2O3 FET was
observed good response than the CNT/ZnO FET. This is due to high dielectric constant of
Al2O3 than the ZnO. From the observed results CNT/ZnO FET shows better response in case
of gate dielectric than the CNT/ Al2O3.
3. CONCLUSIONS
The XRD pattern of ZnO and Al2O3 thin films obtained confirms that the films are in
amorphous, which has better dielectric property than the other crystalline films. From SEM
image the uniformity coating was observed for films. CNT/ZnO and CNT/ Al2O3 FET’s were
been fabricated by various techniques like drop casting, pulsed laser deposition and
sputtering. The comparative electrical studies of these FET were carried out and reported as
threshold voltage (VTH), ION and IOFF are 0.6 Volts, -2.88mA and -2.19mA was observed for
CNT/Al2O3 FET and 0.2Volts, -2.19mA and -4.99mA was observed for CNT/ZnOT FET.
Among these results, the CNT/ZnO FET shows better response than the CNT/ Al2O3 FET in
terms of high leakage current.
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Dielectrics
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