Nano Res

1

Modulating the threshold voltage of oxide nanowire

field-effect transistors by Ga+ ion beam

Wenqing Li1, Lei Liao1, Xiangheng Xiao1(), Xinyue Zhao1, Zhigao Dai1, Shishang Guo1, Wei Wu1, Ying

Shi1, Jinxia Xu1, Feng Ren1, Changzhong Jiang1

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-014-0529-5

http://www.thenanoresearch.com on June 30, 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0529-5

Nano Res.

Modulating the threshold voltage of oxide nanowire

field-effect transistors by Ga+ ion beam

Wenqing Li1, Lei Liao1, Xiangheng Xiao1,*, Xinyue

Zhao1, Zhigao Dai1, Shishang Guo1, Wei Wu1, Ying Shi1,

Jinxia Xu1, Feng Ren1, Changzhong Jiang1

1 Wuhan University, China

After Ga+ ion irradiation, oxide nanowire field effect transistors

show good performance, including threshold voltage shift to

negative gate voltage direction and the carrier mobility

enhancement.

2 Nano Res.

Modulating the threshold voltage of oxide nanowire

field-effect transistors by Ga+ ion beam

Wenqing Li1, Lei Liao1, Xiangheng Xiao1(), Xinyue Zhao1, Zhigao Dai1, Shishang Guo1, Wei Wu1, Ying

Shi1, Jinxia Xu1, Feng Ren1, Changzhong Jiang1

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

nanowire,

field effect transistor,

ion irradiation,

threshold voltage

ABSTRACT

In this paper, we report a method to change the threshold voltage of SnO2 and

In2O3 nanowire transistors by Ga+ ion irradiation. Unlike the scenarios in earlier

reports, the threshold voltage of SnO2 and In2O3 nanowire FETs shift to negative

gate voltage direction after Ga+ ion irradiation. Smaller threshold voltage,

achieved by Ga+ ion irradiation, is required for high-performance and

low-voltage operation. The threshold voltage shift can be attributed to the

degradation of surface defects caused by Ga+ ion irradiation. After irradiation,

the current on/off ratio declines slightly, but still close to ~ 106. The results

indicate that Ga+ ion beam irradiation plays a vital role in improving the

performance of oxide nanowire FETs.

1 Introduction

One dimension oxide semiconductors have

attracted great attention due to their excellent

electrical and optical properties. These oxide

semiconductors have always been used to fabricate

field effect transistors (FETs), sensing devices,

memory devices, and solar cells [1-5]. Nanowire

field effect transistors, as a fundamental element of

nanoelectronic device, are particularly important.

To date, researchers have focused on increasing the

performance of devices by means of surface

modification, doping, irradiation-induced

modification, and band structure engineering [6-11].

In particular, ion beam irradiation can tailor the

electronic, magnetic, and optical properties of

nanomaterials [12-18]. Thus, ion beam irradiation is

a potential tool for improving the performance of

Nano Research

DOI (automatically inserted by the publisher)

Address correspondence to Xiangheng Xiao, xxh@whu.edu.cn

Research Article

2 Nano Res.

devices. Recently, some efforts have been made to

elaborate the influence of irradiation on nanowire

FETs, such as the proton-irradiation-mediated ZnO

nanowire FETs [19], and the Ge nanowires

irradiated by Ga+ ion beam [20]. In this study,

stannic oxide (SnO2) nanowire FETs and indium

oxide (In2O3) nanowire FETs were fabricated, and

then Ga+ ion irradiation was performed. By

characterizing two types of nanowire FETs, the gate

threshold voltage of nanowire FETs was observed to

shift to the negative voltage direction. The reason

for this phenomenon can be ascribed to the

decreasing chemisorbed oxygen and water

molecules caused by Ga+ ion irradiation. To date,

getting controllable operation voltage is also a

challenge for nanoscale FETs [21-22]. In this work,

taking control of the gate threshold voltage of

nanowire FETs was realized by Ga+ ion irradiation,

which has the potential application in reducing

operation voltage and changing the model of FETs

from enhanced model to depleted model.

2 Experimental

2.1 Nanowire synthesis

SnO2 and In2O3 nanowires were synthesized by

chemical vapor deposition (CVD) through a

vapor-liquid-solid (VLS) growth mechanism. SnO2

(or In2O3) powder and graphite powder with the

weight ratio of 10:1 was loaded in a quartz boat.

The quartz boat was placed at the center of a

tube-type furnace, and a silicon substrate coated

with 1 nm thick Au catalyst was placed

downstream of the source material. The samples

were annealed at 1000 ℃ for 30 min in a constant

flow of gas (argon/oxygen = 100:1, flow rate = 50

sccm). By the end of the experiment, the pressure of

the furnace was kept at 38 torr. Surface morphology

images of SnO2 and In2O3 nanowires were acquired

by SEM (FEI Sirion FEG). The Raman spectra of

nanowires were detected through the micro-Raman

system (LabRAM HR800).

2.2 Devices fabrication and characterization

After growth, the nanowires were transferred onto

a highly doped silicon substrate with 100 nm thick

SiO2. The Si/SiO2 substrate was spin-coated with

MMA and PMMA, followed by electron beam

lithography (JEOL 6510 with NPGS), and then

metal evaporation and lift-off processes were used

to complete the fabrication of the source and drain

Cr/Au (15/50nm) electrodes. A global back-gate

field effect transistor (FET) was fabricated by these

processes. The electrical measurements of the FETs

were carried out with a TTPX Probe Station (Lake

Shore) and semiconductor parameter analyzer

(Agilent 4155C).

3 Results and discussion

Figure 1(a) shows a scanning electron microscopy

(SEM) image of representative SnO2 nanowires

synthesized by the VLS-CVD. The diameter of SnO2

nanowires ranges from 60 to 200 nm, and these

nanowires have a length up to several tens of

micrometers. Figure 1(b) shows a typical Raman

spectrum of SnO2 nanowires growth on silicon

substrate; the excitation source was a 488 nm laser.

Two fundamental Raman peaks at 634 and 776 cm-1

correspond to the A1g and B2g vibration modes of the

rutile SnO2 structure, respectively [23]. A weak

Raman peak at 479 cm-1 in the Fig. 1(b) is the other

fundamental Raman peak, in agreement with the Eg

vibration modes of the rutile SnO2 structure [23].

According to the Raman spectrum, we assure the

SnO2 nanowires are in a tetragonal rutile phase.

Beside these fundamental Raman peaks, ther e are

three weak peaks at 504, 544, and 700 cm-1, which is

always present in nanoscale materials [24-25].

Figure 1(c) shows a SEM image of In2O3 nanowires

synthesized by CVD. These In2O3 nanowires were

grown with diameters of 40-100 nm and lengths up

to several tens of micrometers. Figure 1(d) depicts a

typical Raman spectrum of In2O3 nanowires. Raman

peaks at 109, 132, 306, 368, 495 and 630 cm-1 belong

3 Nano Res.

to the vibrational modes of bbc-In2O3 [26-27]. The

Raman spectrum indicates that the In2O3 nanowires

synthesized by CVD are in a cubic phase. A Raman

peak at 520.7 cm-1 in Fig. 1(d) is the signature of

silicon substrate.

Figure 1 SEM images of representative (a) SnO2 nanowires and (c) In2O3 nanowires synthesized by CVD. Scale bar, 5 μm.

Room-temperature Raman spectra of (b) SnO2 nanowires and (d) In2O3 nanowires.

Figure 2 (a) Schematic illustrating Ga+ irradiation of a nanowire FET. (b) A SEM image of a SnO2 nanowire FET. Scale bar, 1 μm

Figure 2 shows the schematic illustration of a

SnO2 nanowire FET irradiated by Ga+ ion beam and

a SEM image of a SnO2 nanowire FET. A focus ion

beam (FIB) system, including focused ion beam and

electron beam, was used to perform the Ga+ ion

irradiation process for SnO2 and In2O3 nanowires.

The position of nanowire device could be located by

the electron beam, and the focused ion beam was

used to perform the Ga+ ion irradiation process. The

exposure area and the exposure time were used to

control the irradiation dosage. Normally, the

irradiation dosage was confirmed by D = I·t/(A·e),

where I is the beam current of the focused ion beam,

t is the exposure time, A is the exposure area, and e

4 Nano Res.

is the charge element. A constant beam current of 1.1

pA was used in all the irradiation processes; and an

exposure area of 6.8×10-5 cm2 (corresponding to the

scanning magnification of 5000×) was used for

irradiation. Figure 3(a)(b)(c) shows the transfer

characteristic curves of SnO2 nanowire FETs before

and after Ga+ ion irradiation with the dosages of

5×1011 ions/cm2, 1×1012 ions/cm2, and 3×1012 ions/cm2,

Figure 3 (a-c) The transfer characteristics of the SnO2 nanowire FETs before and after Ga+ treatment measured at Vds= 1 V. (d) The

Vth of SnO2 nanowire FETs versus Ga+ ion irradiation dosage.

respectively. With relatively low dosages, 5×1011 and

1×1012 ions/cm2, the threshold voltage (Vth) of

nanowire devices was observed to shift to negative

voltage after irradiation. When the irradiation

dosage was raised to 3×1012 ions/cm2, the current

on/off ratio of SnO2 nanowires FETs declined

sharply, resulting from the seriously destruction of

SnO2 nanowires caused by Ga+ ion beam. These

results will be discussed in detail later. The carrier

concentration, n, and the carrier mobility, μn, can be

calculated with the followed formulas,

where ε is the relative dielectric constant of SiO2 (~

3.9), r is the radius (~ 50 nm) of SnO2 nanowires, h is

the thickness (100 nm) of the SiO2 layer, L is the gate

channel length (equal to the distance between

source and drain electrodes, which is about ~ 3 μm),

and gm is the transconductance (dId/dVg). The

threshold voltage of nanowire FETs was calculated

by linear extrapolation. The threshold voltage of

SnO2 nanowire FETs shifted from ~ 11 V to ~ -2 V

and ~ -3 V after Ga+ ion irradiation with the dosages

of 5×1011 ions/cm2 and 1×1012 ions/cm2, respectively.

Figure 3(d) shows the Vth of nanowire FETs before

and after Ga+ ion irradiation. As the surface state

has a great influence on the properties of nanowires,

the devices fabricated with these nanowires

behaved with slightly different performance. For

this reason, it is also necessary to research the

change in performance of the same nanowire after

)1(....................)/2ln(

)(2

2

0

rher

VVn

thgs

),2.........(..........2

)/2ln(

0 d

mn

V

rhLg

5 Nano Res.

irradiation. All the nanowire FETs can be positioned

with Au markers fabricated by

UV-photolithography, so it is possible to research

the change in performance of the same nanowire

FET before and after ion irradiation. Figure 3(a)(b)(c)

shows the transfer characteristics of three SnO2

nanowire FETs before and after irradiation with

different dosages. The carrier mobility of

unirradiated SnO2 nanowires, estimated by Eq. (2),

is close to ~ 14 cm2/V·s; after irradiation, the carrier

mobility barely changes, and the carrier

concentration (at Vgs = 20 V), estimated by Eq. (1),

changes from ~ 1.2×1018 to ~ 2.3×1018 cm-3. The

subthreshold slopes of devices change from ~ 770

mV/dec to ~ 2330 and 4350 mV/dec after irradiation

with the dosages of 5×1011 and 1×1012 ions/cm2,

respectively. The on/off ratio of the device also

declines slightly, but is still closes to 106. The

observed threshold voltage shift, which is caused by

irradiation, can be explained by the milling effect of

Ga+ ion beam on SnO2 nanowires. The Ga+ ion beam

can mill SnO2 nanowire and clear the surface of

SnO2 nanowires, and the surface state density of

SnO2 nanowires decreased after Ga+ ion beam

irradiation. The decreasing surface state density of

SnO2 nanowires may cause the threshold voltage

shift of SnO2 nanowire FETs. With the increase of

irradiation dosage, the subthreshold slope of device

was degraded more severely. This phenomenon can

be explained by the introduced defects in the

nanowire caused by irradiation. The defects in the

nanowire, caused by irradiation, may trap the

electrons; the trapping electrons will hinder the

depletion of the transistor channel.

In2O3 is another important oxide semiconductor;

it is widely applied in transistors, electro-optic

devices, and chemical/biosensors. Ga+ ion irradiated

In2O3 nanowire FETs have also been discussed.

Figure 4(a)(b)(c)(d) shows a representative transfer

characteristic curves of four In2O3 nanowire FETs

before and after Ga+ ion beam treatment. Figure 4(e)

Figure 4 (a-d)Representative transfer characteristic curves of In2O3 nanowire FETs before and after Ga+ ion beam treatment measured

at Vds = 1 V. (e) The Vth of In2O3 nanowire FETs versus Ga+ ion irradiation dosage.

shows the threshold voltage of In2O3 nanowire FETs

versus Ga+ ion irradiation dosage. Similar to SnO2

nanowire FETs, the threshold voltage of In2O3

nanowire FETs also shift from ~ 10 V to ~ 1 V after

irradiation. The threshold voltage shift is attributed

to the decrease of the surface state density caused

by Ga+ ion beam irradiation. The carrier mobility of

unirradiated In2O3 nanowire FETs, estimated by Eq.

6 Nano Res.

(2), is close to ~ 56 cm2/V·s. With the dosage of

2×1011 ions/cm2, the carrier mobility of In2O3

nanowire FETs changes from ~ 56 cm2/V·s to ~ 68

cm2/V·s; the subthreshold slope is close to ~ 470

mV/dec and barely changes. In the case of

irradiation with the dosage of 5×1011 ions/cm2, the

carrier mobility slightly enhanced; and the

subthreshold slope changes from ~ 370 mV/dec to ~

1250 mV/dec. But after irradiation with a dosage of

1×1012 ions/cm2, the carrier mobility declines from ~

56 cm2/V·s to ~ 32 cm2/V·s; and the subthreshold

slope augment, obviously. The subthreshold slope

degradation also ascribe to the introduced defects

in the channel caused by the irradiation. The carrier

concentration of In2O3 nanowire FETs (at Vgs = 20 V)

changes from ~ 3.47×1018 cm-3 to ~ 6.58 ×1018 cm-3

after Ga+ ion irradiation.

Owing to the presence of intrinsic defects

(oxygen vacancies) during the growth process,

oxide semiconductors, such as ZnO, SnO2, and

In2O3, are always manifested as n-type

semiconductor. The surface defects of nanowires

can act as adsorption sites. After the growth process,

as-growth nanowires will be exposed in air,

inevitably. In this situation, the surface defects will

absorb gas molecules (O2 and H2O). These

chemisorbed oxygen and water molecules will trap

the free electrons from the nanowire to form O2- and

OH-, resulting in the depletion of surface electron

states and consequently decreasing the

concentration of surface electron carriers [28-29].

Afterwards, the decline of surface Fermi energy

level, caused by the depletion of surface electron

states, will result in upward energy band bending

[30-31]. Figure 5(a) shows a schematic

cross-sectional view of a nanowire on a Si/SO2

substrate; the surface depletion area, caused by

chemisorbed O2- and OH-, is marked in blue color,

and the conductive area is marked in red color. Figure 5(b) shows a schematic energy band diagram

of a nanowire surface potential. EC is the conduction

band energy level, EV is the valence band energy

level, EF is the Fermi energy level, φs is the energy

of band bending, and Wd is the width of the surface

depletion area.

Figure 5 (a) A schematic cross-sectional view of a nanowire. (b) A schematic energy band diagram of a nanowire surface potential. (c)

HRTEM image of an as-grown In2O3 nanowire, and corresponding FFT pattern in the inset, illustrating the formation of cubic phase

7 Nano Res.

with the zone-axis lying in direction. Scale bar, 2 nm. (d) HRTEM image of an In2O3 nanowire after irradiation. The surface

degenerating layer was milled by Ga+ ion beam. The inset shows the FFT pattern illustrating the formation of cubic phase with the

zone-axis lying in direction. Scale bar, 2 nm.

In this paper, we mainly discuss the influence of ion

beam on the electrical properties of nanowire. After

Ga+ ion beam irradiation, we observed the gate

threshold voltage of nanowire FETs shifted to

negative voltage direction. The reason for the

phenomenon may be the decrease of surface state

density caused by the Ga+ ion beam irradiation.

Generally, the Ga+ ion beam can clean and mill the

surface of nanowire during Ga+ ion irradiation.

Figure 5 (c)(d) show high resolution transmission

electron microscopy (HRTEM) images of In2O3

nanowires before and after Ga+ ion irradiation. The

fast Fourier transform (FFT) patterns of the original

TEM inset figure5 (c)(d) show the In2O3 nanowires are in a cubic phase, this result is consistent with

the result of Raman spectrum. Obviously, the Ga+

ion beam milled the nanowire surface, and the

inside high-quality crystal become the new surface

of the nanowire (detail in Fig. 5 (c)(d)). The surface

degenerating layer (about 1 nm) and the absorbed

oxygen molecules can easily be cleared by Ga+ ion

beam. In Fig. 5(d), the crystal structure inside

nanowire has less destroyed by Ga+ ion beam with

low irradiation dosage. As stated earlier, the oxide

semiconductor nanowire always absorbs oxygen

and water molecules to form O2- and OH-. After Ga+

ion beam irradiation, the surface chemisorbed O2-

and OH- decreased, resulting in the decrease of

surface state density. The decrease of surface state

density may reduce the energy of band bending

and narrowing in the surface depletion area; and

then the nanowire FETs can be the strong inversion

state with a low gate voltage. So, the gate threshold

voltage of nanowire FETs shift to negative voltage

direction after ion irradiation. Fan et al. [29]

reported that the gate threshold voltage of

nanowire FET shifted with the change of oxygen

concentration. Hong et al. [32] reported the

distinction in gate threshold voltage between

smooth nanowire FETs and corrugated nanowire

FETs. Our observations also coincide with the

results of the above-mentioned reports. Figure 3

and Fig. 4 show that the gate threshold voltage of

nanowire FET has an obvious shift. This

observation indicates that the electrical properties

of SnO2 and In2O3 nanowires are both improved by

Ga+ ion beam. But, with high irradiation dosage,

more defects caused by Ga+ ion irradiation, will

result in a sharp decrease of the current on/off ratio

of SnO2 and In2O3 nanowire FETs. After Ga+ ion

irradiation with a larger dosage, the nanowires

reveal a metallic-like behavior. We conclude that the

irradiation caused large sum of defects in the

nanowire, and these defects, such as oxygen

vacancies and interstitial atoms, greatly increase the

carrier concentration. The sharply enhanced carrier

concentration leads to that the nanowires reveal a

metallic-like behavior and the carriers in the

channel can not be fully depleted by the gate

voltage.

4 Conclusions

In summary, we have shown that the electrical

properties of SnO2 and In2O3 nanowires can be

improved by Ga+ ion irradiation. After Ga+ ion

irradiation, the gate threshold voltage of SnO2 and

In2O3 nanowires FETs shifts to negative voltage

direction. The reason can be attributed to the

reduction of chemisorbed oxygen molecules caused

by Ga+ ion irradiation. After irradiation, the current

on/off ratio of nanowire FETs declined, but with

low irradiation dosages, the on/off ratio of

nanowire FETs is still close to 106. The results

indicated that Ga+ ion irradiation has the potential

for application in enhancing device performance,

including changing the model of FETs from

enhanced model to depleted model, and surface

treatment of FETs before encapsulation.

]142[

]121[

8 Nano Res.

Acknowledgements

The author thanks the NSFC (51171132, U1260102,

51201115, 11305056, 51371131 and 11375134), NCET

(12-0418), China Postdoctoral Science Foundation

(2014M550406), Hubei Provincial Natural Science

Foundation (2011CDB270, 2012FFA042), Jiangsu

Provincial Natural Science Foundation, Wuhan

Planning Project of Science and Technology

(2014010101010019), the Fundamental Research

Funds for the Central Universities and experimental

technology project of Wuhan University.

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