九州大学大学院総合理工学府報告第 41 巻 第 2 号 12-16 頁 令和 2 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 41, No. 2, pp. 12-16, FEB, 2020

Improvements of Electrical Characteristics ofSingle-Crystalline ZnO Nanowire Field-EffectTransistors via Self-Assembled Monolayer Modification
Hao ZENG*1 Tsunaki TAKAHASHI*2,3† Takuro HOSOMI*2,3Kazuki NAGASHIMA*2,3 Masaki KANAI*2 Guozhu ZHANG*2 andTakeshi YANAGIDA*2†
E-mail of corresponding authors: takahashi.t@cm.kyushu-u.ac.jp,yanagida@cm.kyushu-u.ac.jp†(Received December 20, 2019, accepted December 27, 2019)
We have developed a rational method to improve electrical characteristics of oxide nanowire
field-effect transistors (FET) performance using self-assembled monolayer (SAM) modification.
Octadecylphosphonic acid (ODPA) was utilized as the SAM because of its chemical reaction withoxide surface which removes -OH groups from the channel surface of FETs. The long alkyl chain ofODPA SAM is also effective to prevent adsorption of H2O molecules on oxide nanowire surfaces. Byutilizing ODPA SAM modification, the drain current versus gate voltage characteristics of back-gatehydrothermally grown ZnO nanowire FETs are improved: reduction of hysteresis in vacuum
condition and improved switching characteristics in atmospheric condition. Present results could be
useful to achieve stable characteristics of oxide-based nanowire FETs.
Key words: ZnO, Nanowire, self-assembled monolayer, field-effect transistors, octadecylphosphonicacid, surface modification, adsorption, passivation

1.

Introduction

Nanowire field-effect transistors (FETs)
have attracted a growing interest due to theirideal electrostatics for transistor operations
and ultra-low power consumption.1) Among
various kinds of semiconducting nanowires,
metal oxide nanowires have been intensively
studied due to their unique electrical andopticalproperties.2)
However,
actualperformance of oxide nanowire FETs is limited
mainly due to adsorption of ambient H2O
and/or O2 on their channel surface. The
absorption of these molecules on the FETchannel surface causes the large hysteresis
which slow down switching speed and decline
the on/off ratio.3) Solving these issues is crucial
to achieve applications based on thefunctionalities of oxide nanowire FETs.
Kimetal.
havereportedthat
one-dimensional nanotube FETs show large
hysteresis characteristics in their transfercurves.3) H2O molecules are considered as thecause of the hysteresis characteristics which*1 Department of Molecular and Material Sciences,
Graduate Student*2 Institute for Materials Chemistry and Engineering*3 JST, PRESTO

observed in passivation-free nanotube FETs.3)
By removing the surrounding H2O via vacuum
annealing or PMMA passivation, thehysteresis of nanotube FETs can be eliminated.
Similar results have been also reported byusing Ga+ ion beam treatment, laser
annealing, and inorganic passivation.4)-6)
Recently, Lim et al. utilized SAM modification
on various oxide nanowire FETs to achieve
stable device performances.7) However, thesestudies mainly focused on achieving thestability of transfer curves of FETs, and themechanism of the hysteresis characteristics ofnanowire FETs has not been fully understood.
In this study, we demonstrate theimprovement in electrical characteristics ofhydrothermally grown ZnO nanowire FETs via
ODPA modification. By utilizing ODPA SAM
modification, the drain current versus gate
voltage characteristics (transfer curve) ofback-gatehydrothermallygrownZnO
nanowire FETs are improved: reduction ofhysteresis in vacuum condition and improved
switching characteristics in atmosphericcondition. The impacts of O2 and H2O onelectrical characteristics are also discussed.

132.

Improvement of Electrical Characteristics of Single-Cystalline ZnO Nanowire Field-Effect Transistors

Experimental

2.1 ZnO Nanowire GrowthZnO nanowires were grown by hydrothermal
method on a Si(100) substrate. A 5 nm Ti
buffer layer and a 100 nm ZnO thin film weresequentially deposited on the Si substrateusing a conventional sputtering technique.
The substrate was dipped into 200 mL ofaqueous solution containing 25 mM zincnitrate hexahydrate (Zn(NO3)2·6H2O, Wako,99.0%),25mM
hexamethylenetetramine(HMTA, Wako, 99.0%), and 2.0 g branched
polyethylenimine (PEI, Mn = 1800, Aldrich, 50
wt. % in H2O). The solution was kept at 95 °C
for 24 h to grow the ZnO nanowires, followedby rinsing with water.
2.2 Device Fabrication
The grown ZnO nanowires were first treatedby ultrasonic dispersion in isopropanol anddropped on a 100 nm SiO2/p-type Si substrate.
Electrode patterning was performed using
electron-beamlithographyandlift-offtechniques.8) Ti/Pt of 20/100 nm was depositedusing radiofrequency (RF) sputtering. The
distance between the source and drainelectrodes was typically 3-5 m. All devices
were annealed in air at 200 °C for 10 min toremove surface adsorbed water.
2.3 Self-Assembled Monolayer Modification

Fig.1 (a) Schematic of single nanowire FETs structure. (b)Field Emission SEM image of ZnO nanowire FETs

ODPA solution was first prepared with theconcentration ranges from 0.01 mM to 10 mM
in toluene at 60 °C, both ZnO nanowire array
substrates and single nanowire devices wereimmersed and kept in the solution for 1 h.
ODPA was self-assembled onto the surface ofZnO nanowires. Then the substrates wererinsed twice in toluene and dried by dry airblow.
2.4 Electrical Characterizations
All electrical characteristics were measured
using a semiconductor parameter analyzer(Keithley 4200 SCS) with a probe station invarious conditions from vacuum to ambientpressure at room temperature.

3. Results & Discussion
3.1 Device Structures
Fig.1(a) shows a schematic of a typicalback-gate FETs, where a 100 nm SiO2 layer isused as gate dielectric layer and a heavily
doped Si substrate are served as a back gate.
Fig.1(b) shows the SEM image of fabricatedZnO nanowire FET. The channel length is 2
m and the diameter of ZnO nanowire is 100nm.
3.2 Surface Modification on ZnO nanowires
Fig.2(a) shows the SEM image of ZnO
nanowires growth by hydrothermal method.
The average diameter of nanowire is about 100
nm. Fig.2(b) shows the relationship between
the concentration of ODPA solution andcontact angle of water on the ODPA-modified
ZnO nanowires. At the lower concentration,
the ZnO nanowires show hydrophilic
properties
becausethesurfaceofhydrothermally grown ZnO nanowires has-OH groups which have high affinity to H2O.
By increasing the concentration of ODPA in
the modification solution, the contact angle
increases due to the hydrophobic long carbon
chain of ODPA. Since the contact angle
saturates at 1 mM ODPA modification, thisconcentration was used for further single
nanowire FET modifications. Fig.2(c) shows
Fourier transform infrared spectroscopy(FTIR) data of ODPA-modified ZnO nanowires.
Due to the long carbon chain of ODPA (C18),
both -CH2 symmetric vibration at 2850 cm-1
and its antisymmetric vibration at 2918 cm-1
are clearly observed and the absorbance of
these vibrations is stronger than -CH3
antisymmetric vibration at 2958 cm-1. These令和元年度

九州大学大学院総合理工学報告第 41 巻第2号

14Fig.3 Drain current versus gate voltage characteristicsof the single ZnO nanowire FETs with andwithout ODPA modification. The inset shows thelinear scale graph of the characteristics.

Fig.2(a) SEM image of hydrothermally grown ZnOnanowires. (b) Relationship between ODPA
concentration in modification solution and contact
angle. The inset shows an image of the contact
angle obtained at 0.1 mM ODPA modification. (c)FTIR data of ODPA modified ZnO nanowires. Inset
shows schematic of ODPA SAM on ZnO nanowires.

results indicate that ODPA is successfully
modified on ZnO nanowires.
3.3 Role of -OH groups on FET CharacteristicsDue to oxygen vacancies in oxide materials,ZnO
nanowiresaregenerally
n-typesemiconductor. Therefore, ZnO nanowire FETsturn on by applying positive gate voltage. Fig.3
shows the drain current versus gate voltage
characteristics (transfer curve) in vacuum(<10-4 Pa). For the as-fabricated FETs without
ODPA modification, hysteresis characteristics
are observed. For the ODPA-modified FETs, on
the other hand, the hysteresis characteristics
are suppressed.
The obtained results in Fig.3 seem to bedifferent from the previous works. In thenanotube FETs, it has been reported thathysteresis characteristics can be suppressedby removing surrounding H2O.3) The
as-fabricated devices were thermally annealed
and measured under vacuum chamber where
H2O effect on electrical characteristics can besuppressed. Therefore, the observed hysteresis
characteristics of as-fabricated FETs indicatethat surrounding H2O might not be the only
cause of the degradation of FET performances.
For the ODPA-modified FETs, the anchor
group of ODPA plays an important role toreduce the hysteresis characteristics. Wang etal. have shown that the surface ofhydrothermally grown ZnO nanowires havehydroxyl (O(OH)) species.5) These hydroxyl
groups can trap carriers (electron) in thenanowire channels and degrade electricalcharacteristics. The charge/discharge behavior
ofelectron
traps
causeshysteresis
characteristics during voltage sweeping.9)
ODPA can remove the surface -OH groups by
the chemical reaction during the modification
process, which is effective to reduce the H2O
adsorption on the nanowires. Moreover, thelong alkyl chain of ODPA shows good

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Improvement of Electrical Characteristics of Single-Cystalline ZnO Nanowire Field-Effect Transistorshydrophobic behavior which prevents H2O
molecules from touching oxide nanowire
surface. Due to the dual effects, electricalcharacteristics of ODPA-modified FETs are
greatly improved in terms of less hysteresis
characteristics and higher ON/OFF ratio of thedrain current.
3.4 Role of O2, H2O on FET characteristics
Fig.4 shows drain current versus gate
voltage characteristics of as-fabricated andODPA-modified FETs under varied airpressure. As shown in Fig.4(a), theas-fabricated FET without ODPA modificationshows a large degradation in ON/OFF ratio of
the transfer curve when air pressure increases
from 10 Pa to 105 Pa, indicating air (O2, H2O)
has strong impacts on the electrical properties,
such as carrier density and/or mobility, in ZnO
nanowires. In addition to the ON/OFF ratio,
the slope of transfer curve, which generally
correspond to carrier mobility, degrades as airpressure increases. As shown in Fig.4(b), on
the other hand, the ODPA-modified FETshows much better transfer curves than that of
the as-fabricated FET even at the higher airpressure. The good ON/OFF ratio in thetransfer curve of the ODPA-modified FET
indicates that SAM modification is useful toachieve stable oxide nanowire transistor
operation in air.
As discussed above, ODPA prevents H2O
adsorption on ZnO nanowire surface.
Therefore, the differences in the transfer
curves of the as-fabricated and ODPA-modified
FETs can be understood through impacts ofH2O on electrical characteristics. Since theslope of the transfer curve degrades at higher
air pressure only for the as-fabricated FET(Fig.4(a)), H2O adsorption decreases carrier
mobility through an increase in carrier
scattering at nanowire surface.
Although the slope of transfer curve is stable
forODPA-modifiedFETs,
hysteresis
characteristics still remain and enhanced at
higher air pressure (Fig.4(b)). The hysteresis
characteristics are considered to originate inO2 adsorption on ZnO nanowire surface
through ODPA SAM. Dipole moment induced
by absorbed O2 is a possible model of thehysteresis.10) Further investigations such as
surface analysis and calculations of electronic
structures are needed to clarify the origin of
the hysteresis.

4. Summary and ConclusionWe demonstrated the improvements inelectrical characteristics of hydrothermallygrown ZnO nanowire FETs via ODPA
modification. By utilizing ODPA SAM
modification, the transfer curve characteristicsof ZnO nanowire FETs were greatly improved:
reduction of hysteresis in vacuum condition
and improved switching characteristics inatmospheric condition. The impacts of -OH, O2,and H2O on electrical characteristics of ZnO
FETs were separately discussed based on theresults of the atmosphere-controlled electricalcharacterizations. It is clarified that H2O andO2 degrade ON/OFF ratio and hysteresis
characteristics of the transfer curve of FETs,respectively. Present results could be useful toachieve nanowire-based FETs made fromvarious oxide materials.
Acknowledgments

Fig.4

Drain current versus gate voltage characteristics
for (a) as-grown and (b) ODPA-modified ZnO
nanowire FETs at varied air pressure.

This work was supported by KAKENHI(grant number: JP17H04927, JP18H01831,
JP18H05243, JP19J12232, and JP18KK0112).
This work was performed under theCooperative Research Program of “Network令和元年度

九州大学大学院総合理工学報告

Joint Research Center for Materials andDevices” and the MEXT Project of “Integrated
Research Consortium on Chemical Sciences”.
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