This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 12415–12419 12415

Transverse piezoelectric field-effect transistor based on single

ZnO nanobelts

Ya Yang, Junjie Qi, Wen Guo, Yousong Gu, Yunhua Huang and Yue Zhang*

Received 2nd May 2010, Accepted 2nd August 2010

DOI: 10.1039/c0cp00420k

A transverse piezoelectric field-effect transistor (TP-FET) based

on single ZnO nanobelts has been fabricated on a metallic

graphite substrate in an atomic force microscope (AFM). The

source-to-drain current of the TP-FET was found to decrease

with increasing loading force under a positive bias due to the

carrier-trapping effect and the creation of a charge-depletion

zone. This TP-FET can be applied as a force/pressure sensor for

measuring nanoNewton forces ranged from 0 to 700 nN.

Due to the unique electronic, optical, mechanical, and

piezoelectric properties of ZnO nanowires/nanobelts, they

have been used to fabricate field-effect transistors,1–3 ultraviolet

lasers,4 light-emitting diodes,5 and mechanical devices.6 Most

importantly, by the utilizing coupled semiconducting-

piezoelectric properties of ZnO, nanogenerators and piezo-

electric diodes based on ZnO nanowires/nanobelts have been

demonstrated.7,8 Recently, by applying a force to create a

piezoelectric field as the gate of an FET, a longitudinal

piezoelectric FET (LP-FET) based on single ZnO nanowires

was fabricated.9 This new type of FET completely changes the

conventional FET structure and may have potential applica-

tions in nanoelectromechanical devices. However, the

practical applications of LP-FETs are greatly limited, because

the detecting force range is only from 0 to 20 nN due to large

deformation of ZnO nanowires, and each ZnO nanowire can

only be used to fabricate one LP-FET device. The appearance

of fractures in ZnO nanowires due longitudinal deformation

also hinders the applications of these LP-FET devices.10,11 The

TP-FET described here is different from an LP-FET as it is

based on single nanobelts lying down on metal surfaces with

the current flowing perpendicularly to the main axis. The

source and drain electrodes are fixed on the surface and at

the bottom of a single nanobelt, respectively. Investigation of

TP-FETs is very important because the carrier path through

nanobelts may be only several tens of nanometres and these

devices may have a higher sensitivity and a larger detecting

range to the applied nanoNewton forces as compared with that

of LP-FETs. Moreover, since the source electrode arrays can be

fabricated on the surface of single ZnO nanobelts they may

have potential applications in the multiplexed detection devices.

However, there has been no report of TP-FETs based on

single ZnO nanowires/nanobelts. As large numbers of

electrical and mechanical nanodevices are fabricated,1–3,7–9

their electrical and mechanical safety become more and more

important. Although mechanical and electrical failures in ZnO

nanowires/nanobelts have been observed,10–12 there are few

reports about the failure of piezoelectric devices based on

single ZnO nanowires/nanobelts. These investigations could

provide important safety guidance for the use of piezoelectric

nanodevices.

In this communication, a TP-FET based on single ZnO

nanobelts on a metallic graphite substrate is constructed by

using a conductive AFM tip. The electrical transport of the

devices under different loading forces is investigated in detail

and the mechanism of the devices is discussed. Electrical

failure of the fabricated devices was observed when the

loading forces were larger than 900 nN, suggesting that the

fabricated devices can work safely under a loading force of

0 to 700 nN.

The ZnO nanobelts were synthesized on a silicon substrate

by a simple chemical vapor deposition method, following the

procedure described by Pan et al.13 The as-grown nanobelts

were characterized by scanning electron microscopy (SEM)

(JSM 6490, Japan) and high-resolution transmission electron

microscopy (HRTEM) (JEOL-2010, Japan). To facilitate the

manipulation of single ZnO nanobelts under the optical

microscope, we chose a large-size nanobelt with a width of

about 10 mm, as shown in Fig. 1(a) and thickness 150 nm, as

shown in Fig. 1(c) and (d). The nanobelts have a wurtzite

structure and grow along the [0001] direction with the �(2�1�10)

top/bottom surfaces [Fig. 1(b)]. The single ZnO nanobelt was

then transferred to a metallic graphite substrate.

The transverse electrical transport properties of single ZnO

nanobelts under the different loading forces were examined

using a standard conductive AFM (Nanoscope IIIa,

Multimode) with PtIr-coated tips (force constants: 0.2 N m�1

and 1 N m�1) at room temperature. The experimental setup

begins with single ZnO nanobelts being placed on a metallic

graphite substrate, which acts as the bottom electrode. The

ZnO nanobelt is then located by the AFM in contact mode.

Once a nanobelt is selected, the conductive AFM tip is used as

the top electrode and the current then flows through the AFM

tip, the single nanobelt, and the metallic graphite substrate. The

schematic diagram of the measurements is shown in Fig. 1(e).

Fig. 2(a) shows the room-temperature photoluminescence

(PL) spectra recorded from the ZnO nanobelt [Fig. 1(c)] taken

by using the 325 nm line of a He–Cd laser as the excitation

source. The strong peak centered at 376 nm corresponds to the

near-band-edge UV emission peak of ZnO. The green emission

peak can not be observed, indicating that the ZnO nanobelt

has a high-quality crystalline structure with a low concentration

of oxygen deficiencies.14 Fig. 2(b) presents a series of consecu-

tively recorded I–V curves on the ZnO nanobelt in Fig. 1(c)9,24

under the different loading forces by using an AFM tip (force

constant: 0.2 N m�1), showing an obvious Schottky rectifying

State Key Laboratory for Advanced Metals and Materials, School ofMaterials Science and Engineering, University of Science andTechnology Beijing, Beijing 100083, P. R. China.E-mail: yuezhang@ustb.edu.cn

COMMUNICATION www.rsc.org/pccp | Physical Chemistry Chemical Physics

Publ

ishe

d on

27

Aug

ust 2

010.

Dow

nloa

ded

by S

ungk

yunk

wan

Uni

vers

ity o

n 03

/04/

2014

07:

50:2

5.

View Article Online / Journal Homepage / Table of Contents for this issue

12416 Phys. Chem. Chem. Phys., 2010, 12, 12415–12419 This journal is c the Owner Societies 2010

behavior. It is interesting to see that the positive current was

found to decrease as the loading force increases. The pheno-

menon was further confirmed by continuously measuring the

electrical transport properties at tens of spots along the

nanobelt under different loading forces. The corresponding

energy-band diagram of the PtIr tip/ZnO nanobelt/metallic

graphite under thermal equilibrium is shown in Fig. 2(c). Since

the work function of graphite is about 4.4 eV and the electron

affinity of ZnO is 4.5 eV,15,16 the contact between ZnO and the

graphite substrate is Ohmic. Since the work function of the

PtIr tip is 5.5 eV,17 the contact between the ZnO nanobelt and

the PtIr tip should be Schottky, which is consistent with the

Schottky rectifying behavior in Fig. 2(b).

To understand the phenomenon observed in Fig. 2(b), the

forward branch of each I–V curve was analyzed by using

the thermionic emission theory.18 According to this theory, the

current for the positive voltages can be given by

I ¼ A�AT2 exp � qfB

KBT

� �exp

qVD

nKBT� 1

� �� �ð1Þ

where A is the effective area of the diode, T is the absolute

temperature, q is the electronic charge, fB is the Schottky

barrier height (SBH) of the junction, VD is the applied voltage

dropped on the Schottky junction, n is the ideality factor, KB is

the Boltzman constant, and A* is the Richardson constant,

which for ZnO is 32 A cm�2 K�2. Usually, a simple plot of

ln I vs. V is used to extract the ideality factor of Schottky

diodes. q/nKBT can be obtained from the slope of the linear

region of such a curve. Under a loading force of 20 nN, the

ideality factor was determined to be about 7.4 by using the

ln I vs. V (o1 V) plot in Fig. 3(a). The ideality factor is

consistent with the reported value (7–9) for Schottky nano-

contacts between AFM tips and ZnO nanorods,19 which

suggests that there is no air gap between the metallic graphite

substrate and the ZnO nanobelt. However, as the loading

forces increase, the ln I vs. V plots are clearly nonlinear even if

the voltage range is from 0 V to 1 V, which is due to the

presence of non-negligible series resistance of ZnO nanobelt in

the diodes.20,21 Therefore, when VD >3KBT/q, eqn (1) can be

revised as

I ¼ A�AT2 exp � qfB

KBT

� �exp

q

nKBTðV � IRÞ

� �ð2Þ

where R is the resistance of ZnO nanobelt, V is the

total voltage drop across the series combination of the

diode and ZnO nanobelt resistor.21 By differentiating eqn (2)

with respect to J (J = I/A), the resistance R of the

Fig. 1 (a) SEM image of the ZnO nanobelts grown on a silicon

substrate. (b) A low-magnification TEM image of a single ZnO

nanobelt. The inset is the corresponding selected-area electron diffrac-

tion (SEAD) pattern of the nanobelt. (c) AFM image of a single ZnO

nanobelt. (d) The corresponding line profile across the ZnO nanobelt.

(e) Schematic diagram of I–V measurements of the fabricated device.

Fig. 2 (a) PL spectrum of a single ZnO nanobelt. (b) Serials of the

representative I–V curves under different loading forces from 20 to

180 nN. (c) Energy-band diagram of the PtIr tip/ZnO nanobelt/

metallic graphite under thermal equilibrium.

Publ

ishe

d on

27

Aug

ust 2

010.

Dow

nloa

ded

by S

ungk

yunk

wan

Uni

vers

ity o

n 03

/04/

2014

07:

50:2

5.

View Article Online

This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 12415–12419 12417

ZnO nanobelts can be extracted from the slope of the plot of

d(V)/d(ln J) vs. J,

dðVÞdðln JÞ ¼ RAJ þ nKBT=q ð3Þ

The d(V)/d(ln J) vs. J plots of the Schottky diode under the

different loading forces produce regular linear curves as shown

in Fig. 3(b), suggesting that this method can be used to deduce

the resistances of ZnO nanobelts under the different loading

forces.21 Because the width of the nanobelt is much larger than

the tip radius, the tip–sample contact can be modeled as a

sphere indenting a flat surface according to the Derjaguin–

Muller–Toporov (DMT) model.22,23 The contact area A can

be given by

A = p[(R/E)(F + |Fadh|)]2/3 (4)

where R is the tip radius (20 nm), E is the effective Young’s

modulus of the tip, F is the loading force, and Fadh is the

tip–sample adhesion force which is experimentally determined.

The obtained resistances of ZnO nanobelts under the different

loading forces are shown in Fig. 3(c), indicating that the

resistance of ZnO nanobelts increases with increasing

loading force.

We now examine the possible mechanisms that are respon-

sible for the increase in resistance of ZnO nanobelts. Usually,

when ZnO is subjected to strain, the change in resistance is

associated with the piezoresistance and piezoelectric effects.9,24

Scrymgeour et al. reported that the low piezoelectric response

corresponds to the low resistivity in ZnO nanorods.25 Since the

high doping concentration or the large numbers of oxygen

vacancies in ZnO nanowires/nanobelts usually result in a low

resistivity, the piezoelectric effect should be negligible in

these nanomaterials. The resistance in single Sb-doped ZnO

nanobelts was found to decrease with increasing loading force

along the [2�1�10] direction, which is attributed to the

piezoresistance effect.24 Moreover, the transverse I–V charac-

teristics of single ZnO nanoleaves with large numbers of

oxygen vacancies show that the current through the nanoleaf

increases as the loading force increases, which is also related to

the piezoresistance effect (the strain-induced change of

bandgap in ZnO).26 However, these results are completely

contrary to the observed phenomenon in this study, indicating

that the increase in resistance of ZnO nanobelts can not

be attributed to the piezoresistance effect. In fact, the PL

spectrum in Fig. 2(a) shows that ZnO nanobelts have a

high-quality crystalline structure with a low concentration of

oxygen deficiencies, suggesting that the high piezoelectric

response exists in the nanobelts due to the high resistivity.

The increase in resistance of ZnO nanobelts is therefore

associated with the piezoelectric effect.

The carrier-trapping effect and creation of a change-

depletion zone induced by the piezoelectric effect have been

used to explain the mechanism of LP-FETs.9 Although the

deformations of the nanobelt in this study are completely

different from that in LP-FETs, we consider that a similar

piezoelectric electric field also exists in the nanobelts. Fig. 4(a)

Fig. 3 (a) Plots of ln I vs. V under different loading forces. (b) Fitting

the plots of d(V)/d(ln J) vs. J of the fabricated device under the

different loading forces. The solid curves represent fitting of the

experimental data points. (c) A plot showing the relationship between

the resistance of ZnO nanobelts and the loading force.

Fig. 4 (a) Structural model of the wurtzite ZnO nanobelt. (b)

Tetrahedral coordination between Zn and O. (c) Distortion of the

tetrahedral unit under the compressive strains along the [2�1�10]

direction, showing the displacement of the center of negative charge

from that of the positive charge. (d) The creation of a piezoelectric

electric field (EPZ) in the ZnO nanobelt due to the deformation.

Publ

ishe

d on

27

Aug

ust 2

010.

Dow

nloa

ded

by S

ungk

yunk

wan

Uni

vers

ity o

n 03

/04/

2014

07:

50:2

5.

View Article Online

12418 Phys. Chem. Chem. Phys., 2010, 12, 12415–12419 This journal is c the Owner Societies 2010

shows the structural model of wurtzite ZnO nanobelts in this

study. When no force is applied to the tetrahedral unit of ZnO,

the positive and negative charge centers are completely

identical, as shown in Fig. 4(b). When the ZnO nanobelt is

mechanically deformed by the AFM tip along the [2�1�10]

direction, the positive and negative charge centers are

displaced with respect to each other [Fig. 4(c)], resulting in a

piezoelectric charge-induced polarization P along the [0001]

direction of the ZnO nanobelt. It is noticed that these piezo-

electric ionic charges cannot move freely. A piezoelectric

potential drop of DV = V+P � V�P p P�d is created along

the [0001] direction [Fig. 4(d)], where d is the contact diameter

between the AFM tip and the nanobelt. When the current

flows through the nanobelt from the AFM tip to the graphite

substrate, the electrons can be trapped in the piezoelectric

electric field (EPZ = DV/d) induced by the potential drop, as

shown in Fig. 4(d). As EPZ increases with the increasing

loading force, more electrons can be trapped by EPZ, resulting

in the decrease in positive current (the increase in resistance).

This situation is similar to the case where a gate voltage is

applied across the width of a ZnO nanobelt in a typical

nanobelt field-effect transistor.27 DV increases as the loading

force increases and affects the transverse electric transport in

the nanobelt, which can be regarded as the gate voltage in the

TP-FET. The free electrons are repulsed away by the negative

potential (V�P ) and a charge-depletion zone forms. As the

loading force increases, the depletion region grows larger,

resulting in a decrease of conducting channels. Both the

carrier-trapping effect and the change of depletion region are

responsible for the decrease in positive current (the increase in

resistance).

Since the loading forces are used to create a piezoelectric

electric field as the gate of LP-FET devices, it is necessary to

acquire knowledge about the safe working range of the

loading forces before these devices can be applied. To obtain

the maximal safe loading force of the fabricated device, we

measured the I–V characteristics of the device under the

different loading forces by using an AFM tip with a force

constant of 1 N m�1, as shown in Fig. 5. The results show that

the positive current decreases with increasing loading force

when the loading forces are lower than 700 nN. If the loading

forces are larger than 900 nN, both the reverse current and

positive current dramatically increase with the increasing

loading force, suggesting that the piezoelectric effect may be

badly weakened under these large loading forces. Our results

suggest that these fabricated TP-FET devices based on single

ZnO nanobelts can be used to detect the nanoNewton forces

ranging from 0 to 700 nN.

In our experiment, the PtIr tip was not seriously damaged

under the large loading forces up to 1300 nN since the PtIr tip

could still be used to scan the morphology of the nanobelt and

measure the electrical transport properties after large loading

forces were removed. To understand the origin of the failure of

TP-FET devices under large loading forces (>900 nN), the

enhanced tunneling effect and the possibility of a strain-

induced structural phase transition were investigated. The

enhanced tunneling effect has been observed in single ZnO

nanoleaves with a large donor impurity density under large

loading forces due to the decrease of the Schottky barrier

height.26 Although the enhanced tunneling effect can also be

used to explain the increase of reverse current with increasing

loading force, the abrupt increase of the positive current can

not be well understood. A strain-induced structural phase

transition may lead to a sudden change in the I–V character-

istics. It has been reported that a phase transition can be

observed at a pressure of about 9 GPa in ZnO crystals.28 In

this study, the pressure can be up to 10.9 GPa under a loading

force of 900 nN, which is larger than the reported phase

transition pressure. Therefore, a structural phase transition

could appear in the ZnO nanobelt when the loading forces are

larger than 900 nN. For the zinc blende phase ZnO, the

pressure can create local dipole moments, but for each dipole

moment created, there also exists a symmetrical moment in the

crystal, which can cancel the effect at the scale of a couple of

atoms. Thus the zinc blende phase ZnO doesn’t exhibit any

piezoelectric effect. The disappearance of the piezoelectric

effect due to a phase transition could be responsible for the

increase in positive current under the large loading forces.

Moreover, since the bandgap of zinc blende phase ZnO

(3.1 eV) is smaller than that of wurtzite ZnO (3.37 eV),29 the

decrease in bandgap will induce a decrease of the

Schottky barrier height between the ZnO nanobelt and

the AFM tip, which can greatly enhance the reverse tunneling

current.26

In summary, we have directly observed a decrease of the

transverse current though single ZnO nanobelts with

compressed deformations induced by an AFM tip under a

positive bias. The TP-FET composed of a ZnO nanobelt

bridging an Ohmic contact and a Schottky contact has been

demonstrated, in which the source-to-drain current is

controlled by the compression of the nanobelt. The mecha-

nism of the device is attributed to the carrier-trapping effect

and the creation of a charge-depletion zone due to the piezo-

electric effect. Failure of the fabricated devices was observed

when the loading forces are larger than 900 nN.

This work was supported by the National Basic Research

Program of China (No. 2007CB936201), the Major Project of

International Cooperation and Exchanges (No. 50620120439,

2006DFB51000), the State Key lab for Advanced Metals

and Materials (No. 2008ZD-09, No. 2008Z-08), and the

Fundamental Research Funds for the Central Universities.

Fig. 5 I–V characteristics of the fabricated TP-FET device under

different loading forces.

Publ

ishe

d on

27

Aug

ust 2

010.

Dow

nloa

ded

by S

ungk

yunk

wan

Uni

vers

ity o

n 03

/04/

2014

07:

50:2

5.

View Article Online

This journal is c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 12415–12419 12419

References

1 J. Goldberger, D. J. Sirbuly, M. Law and P. D. Yang, J. Phys.Chem. B, 2005, 109, 9.

2 Z. Y. Fan and J. G. Lu, Appl. Phys. Lett., 2005, 86, 032111.3 Y. Cheng, P. Xiong, L. Fields, J. P. Zheng, R. S. Yang andZ. L. Wang, Appl. Phys. Lett., 2006, 89, 093114.

4 M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber,R. Russo and P. Yang, Science, 2001, 292, 1897.

5 W. I. Park and G. C. Yi, Adv. Mater., 2004, 16, 87.6 B. A. Buchine, W. L. Hughes, F. L. Degertekin and Z. L. Wang,Nano Lett., 2006, 6, 1155.

7 Z. L. Wang and J. H. Song, Science, 2006, 312, 242.8 Y. Yang, J. J. Qi, Q. L. Liao, H. F. Li, Y. S. Wang, L. D. Tang andY. Zhang, Nanotechnology, 2009, 20, 125201.

9 X. D. Wang, J. Zhou, J. H. Song, J. Liu, N. S. Xu and Z. L. Wang,Nano Lett., 2006, 6, 2768.

10 R. Agrawal, B. Peng and H. D. Espinosa, Nano Lett., 2009, 9,4177.

11 B. M. Wen, J. E. Sader and J. J. Boland, Phys. Rev. Lett., 2008,101, 175502.

12 Y. Yang, Y. Zhang, J. J. Qi, Q. L. Liao, L. D. Tang andY. S. Wang, J. Appl. Phys., 2009, 105, 084319.

13 Z. W. Pan, Z. R. Dai and Z. L. Wang, Science, 2001, 291,1947.

14 Q. H. Li, Q. Wan, Y. X. Liang and T. H. Wang, Appl. Phys. Lett.,2004, 84, 4556.

15 H. Ago, T. Kugler, F. Cacialli, W. R. Salaneck, M. S. P. Shaffer,A. H. Windle and R. H. Friend, J. Phys. Chem. B, 1999, 103, 8116.

16 S. Hasegawa, S. Nishida, T. Yamashita and H. Asahi, J. Ceram.Process. Res., 2005, 6, 245.

17 J. W. G. Wildoer, C. J. P. M. Harmans and H. van Kempen, Phys.Rev. B: Condens. Matter, 1997, 55, R16013.

18 S. M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981.19 W. I. Park, G. C. Yi, J.-W. Kim and S.-M. Park, Appl. Phys. Lett.,

2003, 82, 4358.20 M. W. Allen, S. M. Durbin and J. B. Metson, Appl. Phys. Lett.,

2007, 91, 053512.21 H. Lu, R. Zhang, X. Q. Xiu, Z. L. Xie and Y. D. Zheng, Appl.

Phys. Lett., 2007, 91, 172113.22 B. V. Derjaguin, V. M. Muller and Y. P. J. Toporov, J. Colloid

Interface Sci., 1975, 53, 314.23 V. M. Muller, B. V. Derjaguin and Y. P. Toporov, Colloids Surf.,

1983, 7, 251.24 Y. Yang, J. J. Qi, Y. Zhang, Q. L. Liao, L. D. Tang and Z. Qin,

Appl. Phys. Lett., 2008, 92, 183117.25 D. A. Scrymgeour and J. W. P. Hsu, Nano Lett., 2008, 8, 2204.26 Y. Yang, Q. L. Liao, J. J. Qi, W. Guo and Y. Zhang, Phys. Chem.

Chem. Phys., 2010, 12, 552.27 M. S. Arnold, P. Avouris, Z. W. Pan and Z. L. Wang, J. Phys.

Chem. B, 2003, 107, 659.28 C. H. Bates, W. B. White and R. Roy, Science, 1962, 137, 993.29 S. M. Zhou, H. C. Gong, B. Zhang, Z. L. Du, X. T. Zhang and

S. X. Wu, Nanotechnology, 2008, 19, 175303.

Publ

ishe

d on

27

Aug

ust 2

010.

Dow

nloa

ded

by S

ungk

yunk

wan

Uni

vers

ity o

n 03

/04/

2014

07:

50:2

5.

View Article Online