Influence of Substrate Temperature and Post-annealing Treatment on the Microstructure and Electric Properties of ZnO:Al Thin Films

Deposited by Sputtering

C.B. Garcia1,a, E. Ariza2,3,b, C.J. Tavares1,c

1Centro de Física, Universidade do Minho, Campus de Azurém 4800-058, Guimarães, Portugal

2Centre for Mechanical and Materials Technologies (CT2M),Universidade do Minho, Campus de Azurém 4800-058, Guimarães, Portugal

3SEMAT/UM, Universidade do Minho, Campus de Azurém 4800-058, Guimarães, Portugal

a cibeli@fisica.uminho.pt, b edith@dem.uminho.pt, c ctavares@fisica.uminho.pt

Keywords: ZnO:Al, Sputtering, EBSD, TCO

Abstract. In this work, it is reported the characterization of the microstructure and electric

properties of ZnO:Al thin films produced by magnetron sputtering. An AZOY sputtering target (98

wt% ZnO + 2 wt% Al2O3) was used as source material. The microstructure, optical and electrical

properties of ZnO:Al thin films were investigated and correlated with substrate deposition

temperature and post-annealing temperature. It is demonstrated that the microstructural, electrical

and optical properties of the as-deposited thin films are dependent on the substrate temperature. The

crystalline texture of ZnO:Al was improved with temperature deposition as shown in the EBSD

analysis and X-ray diffraction. ZnO:Al thin film deposited at 250 ºC exhibited very good electrical

conductivity, as high as 200 S.cm

-1 with an activation energy of 5.4 meV. As substrate temperature

or heat treatment temperature is increased there is an apparent blue-shift on the absorption edge of

the transmittance spectra, which can be explained by the Burnstein-Moss effect.

Introduction

ZnO:Al thin films can be synthesized as a transparent conducting oxide (TCO), presenting two

essential properties for photovoltaic and optoelectronic applications. These properties are high

transmittance in the UV, VIS and NIR ranges and good electrical conductivity [1-2]. Zinc Oxide is

a wide band-gap compound semiconductor (3.2 eV) [3], which has been used in numerous

photovoltaic and optoelectronics applications [4]. The semiconductor behavior of this material

depends strongly on the crystal structure, microstructure, chemical composition and processing

parameters. Recently, several authors have investigated the influence of sputtering parameters on

the optical and electrical properties of ZnO:Al thin films deposited at room temperature (RT)

[5,6,7]. According to J.H. Shi et al., and in the case of bi-layer structures, the low-temperature

deposition was found to be necessary in order to avoid the degradation of the interface by

interdiffusion in the cells [6]. Other authors have been studying the effect of temperature substrate

on the microstructural, electrical and optical properties of ZnO:Al deposited with substrate heating

[8,9,10]. In this work, ZnO:Al thin films were deposited onto glass substrates by pulsed magnetron

sputtering, at RT and at 250ºC. The films deposited at RT were subjected to post-annealing

treatments at 250ºC and 500ºC during 1 hour. The effect of substrate temperature and heat

treatment on the microstructural, electrical and optical properties of the thin films is discussed.

Experimental procedure

The Al doped ZnO films were deposited by d.c. pulsed reactive magnetron sputtering at RT and

also at 250 ºC on glass substrates (76mm x 26mm and 0.95 / 1.05 mm thickness – ISO8037). Prior

to the deposition, the substrates were cleaned with isopropyl alcohol in an ultrasonic water bath for

15 minutes (RT), and then dried by compressed air. A circular ZnO:Al (2 wt% of Al2O3) target

(AZOY - GfE Metalle und Materialien GmbH) was used and the distance between the target and the

Materials Science Forum Vols. 730-732 (2013) pp 215-220Online available since 2012/Nov/12 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.730-732.215

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substrate holder was fixed at 75 mm. A base pressure of the order of ~ 10-6

Torr was attained in the

deposition chamber before deposition. Argon (Ar) gas was used as working gas, with a 50 sccm

flow rate. The current density was set to 5 mA/cm2 with a unipolar d.c. pulsed frequency of 140

kHz and a 55% duty-cycle. The substrate was negatively biased by a bipolar d.c. pulsed power

supply, using -50 V at 100 kHz. Typical deposition time was 60 minutes, which originated a film

thickness of ~3 µm, verified by scanning electron microscopy observation of the fracture cross-

section area of ZnO:Al films. The samples deposited at RT were subjected to a post-heat treatment,

in order to compare the influence on optical and electrical properties of as-deposited and thermally-

treated films. Heat treatments were performed under vacuum in a heat-furnace with a base pressure

of 4×10-6

Torr. The heating ramp was 8 ºC/min up to 500ºC, where it stabilized for 60 minutes. The

subsequent cooling occurred during 9 hours inside the chamber under vacuum. The surface

morphology and crystallographic texture of the ZnO:Al thin films were investigated using a field-

emission gun scanning electron microscope (FEG-SEM), NanoSEM – FEI Nova 200, equipped

with electron backscattering diffraction (EBSD). EBSD acquisitions were carried out using a 15 nm

of step size. For the x-ray diffraction (XRD) experiments, a Bruker AXS D8 Discover

diffractometer with Cu Kα radiation was used for the crystal structure analysis in the θ-2θ

geometry. Electrical dark conductivity measurements as a function of temperature (20-95 ºC) were

carried out on both the as deposited and heat-treated ZnO:Al films. Prior to this, aluminum

electrical contacts were thermally evaporated on the film surface. The electrical current intensity

was registered using a Keithley 6487 picoammeter, which was also used to supply voltage. The

optical transmittance measurements were carried out with a Shimadzu UV-2501PC

spectrophotometer in the 200-900 nm range.

Results and discussion

The EBSD inverse pole figures (IPF) topographical maps of as-deposited and heat-treated films are

given in Figs.1a–d. Fig.1e shows the stereographic triangle with a color coded map.

Fig.1 – EBSD IPF surface maps of ZnO:Al thin films deposited at (a) RT, (b) 250ºC, RT and post-heat

treated at (c) 250ºC and (d) 500ºC. (e) Stereographic triangle.

E

C D

A B

216 Advanced Materials Forum VI

Fig.1a–b illustrates very well the dependence of the surface crystalline texture on the substrate

temperature during deposition. The texture coefficient in the ZnO films is influenced by the

deposition temperature and increases along the <0 0 2> direction (Fig.1b). This observation is in

agreement with the XRD findings in Fig.2, which will be demonstrated in this paper. According to

M. Ohring, [11], the increase of substrate temperature promotes adatom mobility (energetic gain);

permitting long distance migration and leading subsequently to a more favorable energetically

stable lattice occupation. Due to this fact, a stronger crystalline texture in the thin film structure is

attained [12]. There is no apparent texture in the samples deposited at RT (Fig.1a), resulting from

the lack of adatom mobility during film deposition. As seen from Fig.1c-d) the post-heat treatment

alone can improve the crystalline texture; however, this texture is further enhanced on the films that

were subjected to a higher temperature during deposition.

The XRD patterns for as-deposited ZnO:Al films at 250ºC, as-deposited at RT and after heat

treatments at 250ºC and 500ºC for RT-deposited films, are shown in Fig.2. The reference peaks for

the ZnO, obtained from the International Centre for Diffraction Data database (ICDD card nr. 01-

089-0510), are also included as a label on the top of each diffraction peak, which correspond to a

hexagonal structure (P63mc); no other crystalline phase is identified. All films evidence strong

diffraction peaks indexed to (002) lattice planes; indicating an orientated film growth with

crystallographic c-axes perpendicular to the substrate surface. It is noted that the intensities of the

diffraction peaks are more pronounced for the sample deposited at a substrate temperature of 250

°C. The ZnO:Al thin film deposited at RT revealed a polycrystalline nature, with randomly oriented

crystallites. These patterns show a clear change in the grain growth direction relating with substrate

temperature. This effect is accompanied by a morphological change as observed in the SEM surface

micrographs (Fig. 3). As the substrate temperature increases to 250ºC, there is a concomitant grain

size increase, correlated with a denser atomic packing (Fig. 3b).

25 30 35 40 45 50 55 60 65 70 75

as-deposited at 250ºC

Inte

nsi

ty (

arbit

rary

unit

s)

2θ (degrees)

(004)

(002

)

Fig.2 – XRD patterns for as-deposited ZnO:Al films at 250ºC, as-deposited at RT and after heat treatments at

250ºC and 500ºC for RT-deposited films.

25 30 35 40 45 50 55 60 65 70 75

heat-treated at 250ºC

RT

heat-treated at 500ºC

Inte

nsi

ty (

arbit

rary

un

its)

2θ (degrees)

(00

2)

(10

1)

(10

2)

(10

3)

(112

)

Materials Science Forum Vols. 730-732 217

Fig.3 – SEM surface micrographs of ZnO:Al films deposited at (a) RT, (b) 250ºC, RT and heat-treated at

(c)250ºC and (d) 500ºC.

Summarizing, substrate temperature has great influence on the film microstructure. In order to

analyze the influence of substrate temperature on the film electrical properties, dark conductivity

tests were performed. Figure 4 shows the dark conductivity of the as-deposited and heat-treated

ZnO:Al films. The dark conductivity of the ZnO:Al films strongly depend on the substrate

temperature. A maximum value for the electrical dark conductivity of 200 S⋅cm-1

was measured at

RT for the as-deposited film at 250ºC, with corresponding activation energy of 5.4meV. This value

is much higher than that for the as-deposited film at RT, 26 S⋅cm-1

, with an activation energy of

17.3 meV. The improvement in conductivity was mainly attributed to the enhancement of

crystallinity; poor crystallinity induces pores and defects on the film microstructure, which behaves

as traps for free electrons and barriers for electron transport in the film volume [9].

2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.510

100

5.4 meV

12.3 meV

7.5 meV

as-deposited at 250ºC RT

Heat-treated at 250ºC Heat-treated at 500ºC

σd

ark

(S.c

m-1)

1000/T (K-1)

17.3 meV

Fig.4 – Evolution of the dark conductivity as a function of the temperature for ZnO:Al films deposited at

250ºC, RT and heat-treated at 250ºC and at 500ºC.

218 Advanced Materials Forum VI

The optical transmittance spectra, in the wavelength range of 200 – 900 nm, for (a) ZnO:Al

deposited at 250ºC, (b) ZnO:Al heat-treated at 500ºC (deposited at RT), (c) ZnO:Al heat-treated at

250ºC (deposited at RT) and (d) ZnO:Al deposited at RT are given in Fig. 5. As either substrate

temperature or heat treatment temperature is increased, the absorption edge of ZnO:Al films shifts

to the shorter wavelength region (blue-shift). This shift can be explained by the Burstein- Moss

effect and is related to carrier concentration. [13]. An increase in carrier concentration with

increasing deposition temperatures implies that more Al atoms diffuse into the ZnO layer and

partially replace the Zn2+

sites [ 8,14].

200 300 400 500 600 700 800 9000

20

40

60

80

100

Tra

nsm

itta

nce

(%

)

Wavelength (nm)

ab

d

c

Fig.5 – Optical transmittance spectra for ZnO:Al films deposited at (a) 250ºC, (b) RT and heat-treated at

500ºC, (c) RT and heat-treated at 250ºC and (d) RT.

Conclusion

The authors have investigated the influence of substrate temperature and subsequent heat treatment

on the microstructural, electrical and optical properties of the films deposited by d.c. pulsed reactive

magnetron sputtering onto glass substrates. Through EBSD IPF surface maps it was possible to

observe that the texture coefficient in the ZnO films is influenced by the deposition temperature.

The improvement of the electrical properties was observed for films deposited at 250ºC with highly

textured (200) atomic planes in the growth direction. The maximum dark electrical conductivity and

activation energy at RT obtained in ZnO:Al thin films was obtained for the as-deposited at 250ºC,

having values of 200 S.cm

-1 and 5.4meV, respectively. As either substrate temperature or heat

treatment temperature is increased it is observed a blue-shift on the absorption edge of the

transmittance spectra; this is due to the Burnstein-Moss effect and is related with a semiconductor

band-gap widening.

Acknowledgements

The authors kindly acknowledge the financial support from the Portuguese Foundation for Science

and Technology (FCT) scientific program for the National Network of Electron Microscopy

(RNME) EDE/1511/RME/2005.

Materials Science Forum Vols. 730-732 219

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220 Advanced Materials Forum VI

Advanced Materials Forum VI 10.4028/www.scientific.net/MSF.730-732 Influence of Substrate Temperature and Post-Annealing Treatment on the Microstructure and Electric

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