Journal of Non-Crystalline Solids 338–340 (2004) 806–809

www.elsevier.com/locate/jnoncrysol

High field-effect mobility zinc oxide thin film transistorsproduced at room temperature

E. Fortunato *, A. Pimentel, L. Pereira, A. Goncalves, G. Lavareda,H. Aguas, I. Ferreira, C.N. Carvalho, R. Martins

Department of Materials Science/CENIMAT, Faculty of Sciences and Technology, New University of Lisbon and CEMOP-UNINOVA,

Campus da Caparica, 2829-516 Caparica, Portugal

Abstract

In this paper we present the first results of thin film transistors produced completely at room temperature using ZnO as the active

channel and silicon oxynitride as the gate dielectric. The ZnO-based thin film transistors (ZnO-TFT) present an average optical

transmission (including the glass substrate) of 84% in the visible part of the spectrum. The ZnO-TFT operates in the enhancement

mode with a threshold voltage of 1.8 V. A field effect mobility of 70 cm2/V s, a gate voltage swing of 0.68 V/decade and an on-off

ratio of 5 · 105 were obtained. The combination of transparency, high field-effect mobility and room temperature processing makes

the ZnO-TFT very promising for the next generation of invisible and flexible electronics.

2004 Elsevier B.V. All rights reserved.

PACS: 85.30.T; 73.61.G; 78.66.H; 72.80.Ey; 68.55

1. Introduction

Transparent electronics are nowadays an emerging

technology for the next generation of optoelectronic

devices [1]. The fundamental device that enables the

realization of transparent circuits is a transparent

transistor. The only possibility to perform transparenttransistors is by using oxide semiconductors. Oxide

semiconductors are very interesting materials because

they combine simultaneously high/low conductivity

with high visual transparency and have been widely

used in a variety of applications (e.g. antistatic coat-

ings, touch display panels, solar cells, flat panel

displays, heaters, defrosters, optical coatings, among

others) for more than a half-century. Transparentoxide semiconductor-based transistors have recently

been proposed [2–5], using as active channel intrinsic

zinc oxide (ZnO). These transistors present an on-to-off

ratio of about 106 and relative low channel mobilities

* Corresponding author. Tel.: +351-21 294 8562; fax: +351-21 294

8558.

E-mail address: elvira.fortunato@fct.unl.pt (E. Fortunato).

0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2004.03.096

between 1 and 3 cm2/V s. The main advantage of using

ZnO deals with the fact that it is possible to growth at/

near room temperature high quality polycrystalline zinc

oxide, which is a particular advantage for electronic

drivers, where the response speed is of major impor-

tance. Besides that, since ZnO is a wide band gap

material (3.2 eV), it is transparent in the visible regionof the spectra and therefore, also less light sensitive.

Besides these works Nomura et al. [6] also recently

proposed a transparent transistor using as active

channel a single crystal of InGaO3(ZnO)5. The most

impressive aspect of this transistor is the high channel

mobility of 80 cm2/V s, mainly due to the absence of

structural defects and to a low carrier concentration. In

this work we report the first results concerning thefabrication and characterization of a high field-effect

mobility ZnO-thin film transistor (ZnO-TFT) deposited

at room temperature by rf magnetron sputtering where

the gate dielectric is based on silicon oxynitride and the

drain and source are based on highly conductive gal-

lium doped zinc oxide (GZO). Moreover, the process-

ing technology used to fabricate this device is relatively

simple and it is compatible with inexpensive plastic/flexible substrate technology.

Fig. 1. Schematic diagram (cross-section and top view) of the ZnO-

TFT device structure. The channel layer (ZnO) and the dielectric layer

(SiOxNy) are 150 and 200 nm in thickness, respectively. The gate

(ITO), source and drain (GZO) are 100 nm in thickness. The channel

length and width are 40 and 200 lm, respectively.

40 60 80 100 120 140 160 18010-310-210-1100101102103104105106107108109

1010

Res

istiv

ity (Ω

cm)

rf power (W)

Fig. 2. Dependence of the electrical resistivity on the rf power.

E. Fortunato et al. / Journal of Non-Crystalline Solids 338–340 (2004) 806–809 807

2. Experimental details

2.1. ZnO semiconductor

In order to optimize the electrical, structural and

optical properties of the ZnO film to be used as active

channel in ZnO-TFT, we have varied the deposition

conditions of undoped ZnO by changing the rf power.

The ZnO films were deposited onto soda lime glasssubstrates by rf (13.56 MHz) magnetron sputtering

using a ceramic oxide target ZnO from Super Conductor

Materials, Inc. with a purity of 99.99%. The sputtering

was carried out under room temperature and the argon

deposition pressure was 0.15 Pa. We have avoided the

utilization of oxygen during the sputtering in order to

minimize the defects at the channel-insulator interface,

as well as to be compatible with the lift-off technique.The distance between the substrate and the target was 10

cm and the rf power was changed between 50 and 175

W. The deposition rate was varied between 15 and 30

nm/min. The film thickness was measured using a sur-

face profilometer (Dektak 3 from Sloan Tech.). The

surface morphology was analysed using a field effect

Scanning Electron Microscope (FE-SEM, S-1400 Hit-

achi). The electrical resistivity (q) was measured as afunction of temperature in the range of 300–500 K using

aluminium coplanar electrodes configuration.

X-ray diffraction measurements were performed

using Cu-Ka radiation (Rigaku DMAX III-C diffrac-

tometer). The optical transmittance measurements were

performed with a Shimadzu UV/VIS 3100 PC double

beam spectrophotometer in the wavelength from 300 to

2500 nm.

2.2. ZnO-based TFT

After the optimization of the ZnO as a semiconduc-

tor, several attempts were made concerning the dielec-

tric. Since one of the objectives of this work was to

produce TFT at room temperature, we start to evapo-

rate silicon dioxide (SiO2) by electron gun. The resultsobtained led to TFTs with a very high leakage current

due to the damage of the SiO2 interface during the

deposition of the ZnO. Similar results have been ob-

served by Masuda et al. [2]. In order to decrease the

leakage current more effectively, a silicon oxynitride

(SiOxNy) layer instead of the SiO2 layer was deposited

by rf magnetron sputtering at room temperature at two

consecutive steps. After we deposit the optimized ZnOsemiconductor layer followed by the deposition of the

drain and source (patterned by the lift-off technique)

based on high conductive GZO [7]. Fig. 1 shows a

schematic cross-section and a top view of the ZnO-TFT

produced, using only oxide materials and soda lime glass

as substrate. The thickness of each layer is indicated in

the legend of the figure. The ZnO-TFT had a channel

with of 200 lm and a channel length of 40 lm(W =L ¼ 5).

The electrical characteristics of the TFTs were mea-

sured using a semiconductor parameter analyser (HP

4145B).

3. Results and discussion

3.1. Characteristics of the ZnO films

Fig. 2 shows the dependence of the electrical resis-

tivity as a function of rf power (P ). The highest resis-tivity (108 X cm) was obtained for P ¼ 100 W. For Paround 100 W the films became close to stoichiometry

with low structural defects and consequently higher

resistivity. As we decrease or increase the rf power a

808 E. Fortunato et al. / Journal of Non-Crystalline Solids 338–340 (2004) 806–809

deviation from stoichiometry is obtained with a decreaseon the electrical resistivity. These results are also cor-

roborated by the optical measurements. The visible

transmission for films with high resistivity is around

90% while for films with lower resistivity this value de-

creases to 70% (see inset in Fig. 3). Fig. 3 shows the dark

conductivity as a function of absolute temperature for

two ZnO films produced at 50 and 100 W, respectively.

The data show that the films are thermally activated,presenting the film deposited at 100 W an oxide/semi-

conductor like behavior, while for the one deposited at

50 W exhibits a semi metallic behavior. These results

achieved can be correlated with the existence of deep

and shallow localized defects where the first one domi-

nate entire a wide range of temperatures for films pro-

duced at 50 W and both mechanisms are present in the

films produced at 100 W. The activation energies ob-tained are indicated inside the figure. Fig. 4 shows the

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.410-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

0.07 eV

500 1000 1500 2000 25000

20

40

60

80

100P = 100 W

Tra

nsm

itta

nce

(%)

Wavelength (nm)

P = 50 W

0.13 eV

0.16 eV

p = 100 W

Con

duct

ivity

(Ωcm

)-1

1000/T (K-1)

p = 50 W

1.04 eV

Fig. 3. Conductivity as a function of the reciprocal of absolute tem-

perature. The inset shows the optical transmittance as a function of

wavelength for the same samples. The thickness of the films is 120 nm.

20 25 30 35 40 45 500.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Inte

nsity

(a.u

.)

2θ (degree)

(002) ZnOThickness: 280 nmAr pressure: 0.15 Parf power: 100 W RT deposition

Fig. 4. Typical X-ray diffractogram for a ZnO film produced at 100 W.

X-ray diffraction pattern for ZnO film produced atP ¼ 100 W. For all the films produced only the

ZnO(0 0 2) peak at 2h 34 is observed, revealing that

the films are polycrystalline with a hexagonal structure

and a preferred orientation with the c-axis perpendicularto the substrate.

3.2. Characteristics of the ZnO TFTs

Fig. 5 shows the source-to-drain (IDS) current as a

function of the gate voltage (VGS). It is observed that the

ZnO-TFT has an n-channel, since electrons are gener-

ated by the positive VGS. A high IDS (1 mA) is obtained

for VGS ¼ 10 V at a VDS ¼ 10 V (on-state condition for a-

Si:H TFTs in LCD). Besides that it is clear observed a

nice pinch-off and current saturation indicating that this

ZnO-TFT is in accordance with the standard theory offield-effect transistors. The field-effect mobility (lFE) andthe threshold voltage (VTH) were calculated by fitting a

straight line to the plot of the square root of IDS vs. VGS,

according to the expression (saturation region)

IDS ¼CilFEW

2L

VGSð VTHÞ2 for VDS > VGS VTH;

ð1Þwhere Ci is the capacitance per unit area of the gate

insulator. The obtained lFE is 70 cm2/V s and the VTHis 1.8 V, showing that the ZnO-TFT operates in the

enhancement mode. Enhancement mode is preferable todepletion mode since it is not necessary to apply a gate

voltage to switch off the transistor, because the circuit

designer is simpler and the power dissipation is lower [1].

The high value of lFE deals with the high quality (im-

proved crystallinity and low oxygen vacancies and/or Zn

interstitials working as donors) presented by the ZnO

layer as well as the good channel–insulator interface

obtained. The off-current is very low, on the order of109 A, and the on-to-off ratio is 5 · 105. The gate

-4 -2 0 2 4 6 8 10 1210-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

Gate voltage, VGS (V)

Cur

rent

, ID

S (A)

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

Cur

rent

, IG

S (A)

VDS = 10 V

Fig. 5. Transfer characteristics and gate leakage current (IGS) for a

ZnO-TFT with a width-to-length ratio of 5:1 for VDS ¼ 10 V. The on-

to-off ratio is 5 · 105. The gate leakage current is orders of magnitude

lower than the drain current.

Fig. 6. Optical transmission spectra for the entire ZnO-TFT structure.

The inset shows a photograph of a 5 cm· 5 cm glass substrate with the

ZnO-TFT structures, placed on back text, showing that the ZnO-TFT

is fully transparent to visible light.

Fig. 7. Cross-section SEM image of the ZnO-TFT structure between

source and drain.

E. Fortunato et al. / Journal of Non-Crystalline Solids 338–340 (2004) 806–809 809

voltage swing, S defined as the voltage required to in-

crease the drain current by a factor of 10, is given by [8]

S ¼ dVGS

dðlog IDSÞð2Þ

and was 0.68 V/decade for the ZnO-TFT under analysis.

The S is given by the maximum slope in the transfercurve (Fig. 5). The exposure to ambient light has no

effect on the current–voltage characteristics, which is an

advantage in electronic drivers for displays. Fig. 6 shows

the optical transmittance spectrum through the entire

ZnO-TFT in the wavelength range between 300 and

2500 nm (including the glass substrate with 1.1 mm

thickness). The average optical transmission in the vis-

ible part of the spectrum is 84% while for 550 nm(maximum sensitivity for the human eye) is 88%, which

indicates that transmission losses do due to the ZnO-

TFTs are negligible (4%). The figure inset shows a 5

cm · 5 cm glass substrate with ZnO-TFTs, throughwhich the underlying text is visible. Fig. 7 shows a SEM

cross-sectional view of the ZnO-TFT. There one can see

clearly a highly compact structure showing the densely

columnar growth for polycrystalline films deposited by

rf magnetron sputtering.

4. Conclusions

We succeed the fabrication of transparent bottom-

gate-type ZnO-based TFTs produced by rf magnetron

sputtering at room temperature. The ZnO-TFT present

very good electrical performances, namely a high field

effect mobility of 70 cm2/V s, an on-to-off ratio 5 · 105,a gate voltage swing of 0.68 V/decade and a threshold

voltage of about 1.8 V. The optical transmittance in the

visible part of the spectrum is in average 84%. The use of

a silicon oxynitride dielectric layer proves to form a

good channel–insulator interface.

Concerning future work, we want to improve the

stability of the devices, namely the stresses occurring

during the measurement and we want to move to plasticsubstrates, since all the technological processes are

compatible.

Acknowledgements

The authors would like to acknowledge A. Lopes for

the SEM analysis. This work was supported by the

‘Fundac~ao para a Ciencia e a Tecnologia’ through

Pluriannual Contracts with CENIMAT and by the

projects: POCTI/1999/ESE/35578, POCTI/1999/CTM/

35440 and POCTI/2001/CTM/38924.

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