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Applied Surface Science 253 (2007) 4317–4321

Implanted ZnO thin films: Microstructure, electrical

and electronic properties

J. Lee a, J. Metson b,*, P.J. Evans c, R. Kinsey d, D. Bhattacharyya a

a Department of Mechanical Engineering, University of Auckland, Private Bag 92019, Auckland, New Zealandb Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand

c Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australiad Department of Electrical and Computer Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand

Received 10 July 2006; received in revised form 19 September 2006; accepted 19 September 2006

Available online 23 October 2006

Abstract

Magnetron sputtered polycrystalline ZnO thin films were implanted using Al, Ag, Sn, Sb and codoped with TiN in order to improve the

conductivity and to attempt to achieve p-type behaviour. Structural and electrical properties of the implanted ZnO thin films were examined with X-

ray diffractometry (XRD), scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM) and

conductivity measurements. Depth profiles of the implanted elements varied with the implant species. Implantation causes a partial amorphisation

of the crystalline structure and decreases the effective grain size of the films. One of the findings is the improvement, as a consequence of

implantation, in the conductivity of initially poorly conductive samples. Heavy doping may help for the conversion of conduction type of ZnO thin

films. Annealing in vacuum mitigated structural damage and stress caused by implantation, and improved the conductivity of the implanted ZnO

thin films.

# 2006 Elsevier B.V. All rights reserved.

PACS : 78.66.Hf; 61.72.Vv; 68.55

Keywords: ZnO; Ion implantation; Microstructure; Electrical and electronic properties

1. Introduction

Zinc oxide (ZnO) is a wide band gap (�3.4 eV)

semiconductor material attracting attention for application in

varistors, high power electronics, surface acoustic wave

devices, piezoelectric transducers, gas-sensors and as a window

material for display and solar cells [1]. It is also a candidate for

solid state blue to ultra-violet (UV) optoelectronics, including

lasers. In addition, there are important applications in high

density data storage systems [2], solid-state lighting, secure

communications and bio-detection [3]. Extensive efforts have

been directed into the fabrication of p-type ZnO, aiming at the

realization of UV-light emitting diodes based on the p–n

homojunction in ZnO. However, many of these approaches

suffer from problems of instability and reproducibility of the

material produced [1].

* Corresponding author. Tel.: +64 9 373 7599x83877; fax: +64 9 373 7925.

E-mail address: j.metson@auckland.ac.nz (J. Metson).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.09.033

Ion implantation is an important and controllable technique

for introducing conduction carriers into IV and III–V group

semiconductors, including selective-area doping, electrical and

optical isolation, dry etching, and ion slicing. A wide range of

implant conditions such as ion mass, dose (1012–

1018 atoms cm�2), and implant temperature affect the forma-

tion of lattice interstitials, vacancies and planar defects in a

crystal such as GaN [4,5]. Alivov et al. [1] have attempted to

convert n-type ZnO to p-type by N+ ion implantation of Ga-

doped films, and observed an increase in the resistivity from

about 10�3 to 105 V cm. Miyakawa et al. [5] have reported hole

doping by N+ implantation into n-type Ga-doped ZnO

(ZnO:Ga) thin films in an attempt to realize p-type ZnO films.

However, conductive type conversion (n! p) was not

observed irrespective of implantation conditions. Hartmann

et al. [6] reported that the resistivity of ZnO films was increased

by the implantation of copper. The electrical properties of ZnO

are related closely to the composition and the microstructure,

which affect its energy gap, carrier concentration and porosity

J. Lee et al. / Applied Surface Science 253 (2007) 4317–43214318

Table 1

Deposition conditions of ZnO thin films prepared by magnetron sputtering

No. Ptotal

(mTorr)

Power to

target

Bias/

VDC

Deposition

time

Thickness

(nm)

1 20.0/Ar 250W/RF �50.0 40 min �294

2 20.0/Ar 125W/RF 0 4.0 h �422

[7,8]. There are very few reported studies on the microstructure

and properties of implanted ZnO thin films. We have therefore

studied implantation effects using different dopants on

microstructure, electrical and electronic properties of ZnO

thin films. The dopants, Al, Ag, Sn, Sb, and TiN, were selected

for this initial investigation to determine if these species can

impart p-type properties to ZnO films.

2. Experiment

The ZnO thin films were deposited by radio frequency (r.f.)

magnetron sputtering (Table 1). Working pressure in the

chamber was 2 Pa. The sputtering target was ZnO (99.9%).

Glass microscope slides were used as substrates and were

rotated during deposition.

Low energy ion-implantation was carried out using a metal

vapor vacuum arc (MEVVA) ion source. The base pressure of

Fig. 1. The depth profiles of Al, Ag and Sn implanted ZnO thin films.

the implanter was 2 � 10�4 Pa. The extraction potential was

40 kV and the ion beam current 40 mA. The implantation dose

was�1.0 � 1016 atoms cm�2. Al, Ag, Sn, Sb and TiN cathodes

were used. After implantation some of the implanted ZnO thin

films were annealed under vacuum (�10�5 mbar) at 300 8C for

1 h.

The electrical resistivity of the films was measured by

standard four-probe techniques. Scanning electron microscopy

(SEM, Philips XL-30S with analytical system), X-ray

diffractometry (XRD, Bruker D8) and atomic force microscopy

(AFM, Digital Instruments NanoScope IIIa) were used to

characterize the microstructure of the films. Hall coefficients of

the samples were measured at room temperature by an

automated d.c. Hall measurement system using the Van Der

Pauw technique. The depth profiles were measured using

dynamic secondary ion mass spectrometry (Cameca IMS-5f).

The laser source used in the photoluminescence experiment

was the fourth harmonic of a Q-Switched Nd:YAG laser

(266 nm, 0.45 mW, pulse width 3 ns).

3. Results and discussion

3.1. Microstructural study of implanted ZnO thin films

Depth profiles of Ag, Al and Sn implanted in ZnO films are

shown in Fig. 1. Both depth and distribution of the implant

depend on the implant species. At the low implantation

extraction potential of 40 kV, Al, Ag and Sn concentration

depth profiles have peaks at a depth of 25, 17 and 10 nm,

respectively. The heavier dopants as expected, penetrate a

shorter distance into the films, consistent with other reports [4].

Fig. 2 shows the XRD patterns of Ag and Sn implanted ZnO

thin films. XRD measurements indicate that the films were still

crystalline with a preferred (0 0 2) orientation. However, the as-

implanted samples have smaller 0 0 2 peak area (Fig. 2), 0 0 2

Fig. 2. XRD spectra of unimplanted and implanted ZnO thin films.

J. Lee et al. / Applied Surface Science 253 (2007) 4317–4321 4319

Table 2

XRD data of implanted samples

Implanted elements Al Sn Sb Unimplanted

ZnO films

d (nm) 0.26107 0.26094 0.26104 0.26076

FWHM 0.411 0.407 0.378 0.267

Change +0.144 +0.14 +0.111

Of FWHM (%) 53.9 52.4 29.4

peak shifts and a much larger (up to 54%) FWHM compared to

an unimplanted ZnO thin film (Table 2). These results indicated

the crystalline structure was partly amphophized and grain sizes

of the implanted ZnO thin films become smaller according to

the Scherrer formula [9]. AFM surface images of the implanted

samples before and after implantation are shown in Fig. 3. In

Fig. 3. AFM images of unimplanted a

Fig. 4. SEM micrographs of unimplanted and implanted

the Ag implanted samples, the presence of a modified grain

structure at the surface was observed. Typical surface and cross-

sectional SEM micrographs of the implanted ZnO films are

shown in Fig. 4. Some disruption and defects of the columnar

structure are observed, and grain size becomes smaller, in

agreement with the observed broadening in the XRD patterns

after implantation [10].

3.2. Resistivity of implanted ZnO thin films

The resistivity of the implanted ZnO films is shown in

Table 3. The conductivity of poorly conductive samples is

generally improved as a consequence of implantation, for

example the sample two group. However, implantation often

decreases the conductivity of samples initially with high

conductivity, for example the sample 1 group. The group 1 Al

nd Ag implanted ZnO thin films.

ZnO thin films: (a) top view and (b) cross-section.

J. Lee et al. / Applied Surface Science 253 (2007) 4317–43214320

Table 3

Resistivity of ZnO samples before and after implantation

Samples (group) Implant Resistivity (V cm)

Before After implantation

1 Ag 0.01 0.048

Al 0.012

Sn 0.033

Sb 0.027

2 Sb 92.8 1.07

TiN 0.12

Fig. 5. Luminance spectra of Sb implanted ZnO thin films.

Table 5

XRD data of unannealed and annealed (vacuum, 300 8C) implanted ZnO thin

films

Samples Before annealing After annealing

d spacing (nm) FWHM d spacing (nm) FWHM

#1 0.26098 0.338 0.26078 0.294

#2 0.26107 0.411 0.26063 0.387

#3 0.26112 0.290 0.26077 0.273

and group 2 TiN doped ZnO thin films (Table 3) show better

conductivity compared to the other dopant in the same series of

samples. The Ag implanted ZnO thin film (0.048 V cm) has

greater resistivity than an Al implanted one (0.012 V cm). The

shorter range of the heavier Ag (Fig. 1) may cause more damage

to the crystalline structure in a shallower zone than the lighter

Al (Fig. 4(a)) [6]. Kucheyev et al. [4] reported that ion mass

affects not only the level of implantation-produced lattice

disorder but also the main features of the damage buildup

behaviour in GaN.

3.3. Annealing and Hall measurements

Hall measurements were performed on some of the vacuum

annealed ZnO films. The results of these measurements are

shown in Table 4. After implantation and vacuum annealing,

the resistivity of the TiN implanted (codoping Ti and N) ZnO

thin films is 0.0739 V cm, however the resistivity of the

annealed unimplanted sample is 24.4 V cm. This may be

related to the combination of electron-hole pairs introduced by

this codoping strategy. Although the mobility of implanted

samples decreases to a certain extent after implantation

(irradiation-induced degradation of the carrier mobility [1]),

the carrier concentration of these implanted and annealed films

has been significantly enhanced because of implanted dopants

introducing more carriers, interstitials and a lower energy gap.

For example, the carrier concentration of a TiN implanted and

annealed ZnO thin film reaches 1019 cm�3. The group V

elements Sb and N [11] are potential p-type dopants and TiN

[12] appears to be promising in this regard. However, after

annealing these films still showed n-type conductivity,

suggesting that they may be not doped sufficiently to cause

n- to p-type transfer of the films. Therefore, heavy doping is

demanded for the conversion of conduction type from n- to p-

type without the generation of oxygen vacancies by ion

Table 4

Hall measurements of implanted and annealed ZnO thin films

Samples Resistivity

(V cm)

Carrier

type

Mobility

(cm2/V s)

Carrier

concentration (cm�3)

Sb implanted 0.218 � 0.001 n-Type 10.4 � 0.3 (2.7 � 0.2) � 1018

TiN implanted (73.9 � 0.02)

� 10�3

n-Type 2.4 � 0.3 (3.6 � 0.5) � 1019

As deposited 24.4 � 0.04 n-Type 33.8 � 1.0 (5.6 � 1.0) � 1015

bombardment [5]. For example, Sb doped ZnO thin films on Si

after a dose of about 4 � 1016 ions/cm2 shows some p-type

tendency. The acceptor peak may be located at 3.305 eV

(375.17 nm). Other peaks are assigned as a donor peak

(369.0 nm, 3.36 eV) and a donor–acceptor peak (DAP) peak

(380 nm, 3.26 eV) [11,13,14] (Fig. 5). The peak energy of the

PL spectrum is the photon energy of luminescence. The related

acceptor and donor energy levels are measured from

conduction bands.

Annealing may also cause diffusion of dopants and change

the composition of the ZnO films. The resistivity of the Al

implanted ZnO thin film decreases from 0.012 to 0.0065 V cm

after vacuum annealing. The decrease in resistivity may be

attributed to the enhancement of oxygen deficiency or zinc

excess in the films, which could be exagerated during the

vacuum annealing process, owing to the increased mobility and

outgassing of oxygen in the film [15]. The energy dispersive X-

ray (EDX) analysis result shows that Zn content of Sb

implanted ZnO thin films increases from 54.6 to 56.4 at.% after

annealing. Table 5 shows XRD results before and after

annealing of implanted ZnO thin films. After annealing the d

spacing of the implanted ZnO thin films becomes smaller

presumably due to the reduction in stress of the implanted films

[8]. At the same time the FWHM becomes smaller. The larger

grain sizes of implanted ZnO thin films after vacuum annealing

may also have improved the conductivity of implanted ZnO

thin films.

4. Conclusions

Ion implantation was employed to implant ZnO thin films

with Al, Ag, Sb, Sn and TiN. XRD measurements indicated that

J. Lee et al. / Applied Surface Science 253 (2007) 4317–4321 4321

the implanted ZnO films are partly amorphous and grain sizes

of the implanted ZnO thin films become smaller compared to

the unimplanted samples. Improvement in conductivity is

observed in the implantation of poorly conductive samples.

However, deterioration in conductivity is seen as a consequence

of implantation in samples that were initially highly

conductive. Annealing in vacuum may enhance the Zn excess,

reduce the stress and increase the grain sizes in the implanted

ZnO thin films. After annealing these implanted films still show

n-type behaviour with a significant increase of carrier

concentrations in line with their conductivity improvement.

Heavy doping may help for the conversion of conduction type

in ZnO thin films.

Acknowledgements

J. Lee would like to thank New Zealand Tertiary Education

Commission (Bright Future Doctoral Scholarship) and the

Australian Institute of Nuclear Science and Engineering

(Postgraduate Award). We acknowledge the assistance of Dr.

Z. Li, Prof. W. Gao, Prof. U. Pal, D. Button, Yong Qiu, Dr. G.

Xiong, Prof. K.B. Ucer and C. Hobbis, and support from the

Research Centre for Surface and Material Science, The

University of Auckland.

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