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«ABSTRACT Transparent conducting Al-doped ZnO (AZO) thin films were deposited on soda-lime glass substrates by DC magnetron sputtering with a novel ...»

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Pin-Chuan Yao1, Shih-Tse Hang2 and Menq-Jiun Wu2

1 Department of Materials Science and Engineering, Da-Yeh University, Changhua 51591, Taiwan

22 Department of Mechatronics Engineering, National Changhua University of Education, Taiwan

E-mail: pcyao@mail.dyu.edu.tw; mwu@cc.ncue.edu.tw

ICETI 2012-A1017_SCI No. 13-CSME-18, E.I.C. Accession 3476 ABSTRACT Transparent conducting Al-doped ZnO (AZO) thin films were deposited on soda-lime glass substrates by DC magnetron sputtering with a novel sintered ceramic target, AZOY® that contains a small amount of Y2 O3 in addition to Al2 O3 and ZnO. The effect of substrate temperature (Ts ) on the structural, electrical and optical properties of the prepared AZO films was evaluated extensively. By elevating Ts, both the electrical conductivity and optical transmittance in the Vis/NIR region of the film could be effectively improved. The substrate heating is closely related to the crystallinity and the surface morphology of the deposited films. It is noteworthy that by employing this target material, high quality AZO thin films could be deposited with a simple and cost effective DC magnetron sputtering system.

Keywords: Al-doped ZnO; DC magnetron sputtering; hydrogen.




RÉSUMÉ Un conducteur transparent de couches minces de ZnO dopé aluminium (AZO) a été déposé sur un substrat de verre sodo-calcique par pulvérisation de magnétron DC avec une cible néo-céramique frittée AZOY® qui contient une faible quantité de Y2 O3 en plus de Al2 O3 et de ZnO. L’effet de la température du substrat (Ts ) sur les propriétés structurelle, électrique et optique sur les couches minces AZO fut évalué exhaustivement.

Par l’élévation de la Ts, la transmission de la conductivité électrique et optique dans la région Vis/NIR de la couche peut-être efficacement améliorée. Le traitement thermique du substrat est étroitement lié à la cristallinité et la morphologie de la surface de la couche déposée. On peut noter que par l’emploi de ce matériau cible, une mince couche AZO peut être déposée par un système simple et moins coûteux de pulvérisation de magnétron DC.

Mots-clés : ZnO dopé aluminium ; pulvérisation de magnétron DC ; hydrogène.

Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013

1. INTRODUCTION Transparent conducting oxide (TCO) films are prerequisites for most photovoltaic applications and flatpanel displays (FPDs) which have been subjected to a large number of investigations for decades [1]. Practically, indium-tin-oxide films (ITOs) are the most widely used TCO materials industrially. Nevertheless, the increasing price and limited supply of indium has promoted the developing of other TCO alternatives. One of the promising substitutes are the Al and Ga doped ZnO (abbreviated as AZO and GZO, respectively) [2]. Zinc oxide is a direct, wide band-gap (Eg =∼ 3.37 eV) semiconductor which has many applications in optoelectronics, electronics, spintronics and sensor materials [3, 4]. Group III-doped ZnO film is transparent and electrical conductive with extraordinary properties such as non-toxic, less expensive, resistant to defects and hydrogen plasma [5]. Moreover, AZO and GZO could be deposited at lower temperature by magnetron sputtering [6, 7]. It has been demonstrated that by DC magnetron sputtering, the Al-doped polycrystalline ZnO films with very low resistivity of 2.7 × 10−4 Ω·cm could be realized at Ts = 250◦ C [10].

For a pilot scale deposition, Szyszka [7] demonstrated an AZO film with resistivity of 3.0 × 10−4 Ω·cm by mid-frequency reactive magnetron sputtering over the borosilicate glass substrates at 200◦ C. RF sputtering is necessary when dielectric compounds such as sintered ZnO/Al2 O3 disks are sputtered [3]. However, since about 1995, the conductive sintered ceramics has been realized due to reducing annealing of the ceramic or due to incorporation of special dopants on a large scale [8]. Accordingly, the major advantages of AZOY® target are twofold: (1) by applying ceramic target, it is much easier to achieve stable deposition conditions and to avoid the Al enrichment problem for those deposited by reactive magnetron sputtering at high temperature; and (2) a sophisticated RF sputtering is not necessary, only a simpler DC sputtering system is affordable by employing this target owing to the conductive nature of the AZOY® ceramics [3].

The ceramic AZOY® target contains a small amount of Y2 O3 in addition to ZnO/Al2 O3 (2%) and has proven to be especially suited for pulsed DC magnetron sputtering. The as-deposited AZO film is transparent and has good electrical conductivity as compared to those deposited by traditional ZnO/Al2 O3 ceramic targets [8]. The doping effect of yttrium may be ascribed to the resemblance in ionic radius of Y3+ (1.06Å) to that of Zn2+ (0.83Å) [9], while the improved optical transmittance is correlated to either the doped yttrium or the increased hydrogen content as proved by Ruske et al. [8] and Kaur et al. [10]. Accordingly, the deposition condition has close relationship to the film quality [11]. In this study, effects of substrate temperatures on the structural, electrical and optical properties of AZO films deposited by DC magnetron sputtering using AZOY® target was extensively investigated.

2. EXPERIMENTIAL The soda-lime glass substrates were cleaned ultrasonically in an acetone and methanol bath, respectively, for 10 min then blown dry with nitrogen before admitted to the sputtering chamber which was pumped down to

6.6 × 10−4 Pa using a cryogenic pump. Before admitting the sputtering gas (Ar/H2 =95:5 in vol., 45 sccm), the AZOY® target was pre-sputtered by argon plasma for 5 minutes under 0.13 Pa working pressure. The power density was set to 4 W/cm2 with discharge power 2700 W while the working distance between the substrate and target was 5.5 cm.

The substrate temperatures (Ts ), ranging from 85◦ C (without intentionally heating) to 500◦ C, were regulated by a PID controller within an error range of ±5◦ C. The deposition rate and film thickness was monitored by a surface profiler (Kosaka, ET3000). Detailed information about surface characterization had been described elsewhere [11] and not shown here for simplicity.

304 Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 Fig. 1. (a) XRD patterns of AZO films; (b) and (c) the derived characteristic parameters as a function of substrate temperature (erroer bars: experimental error for crystal size calculation: D/D = 3%).


3.1. Structural Properties The intensity of XRD patterns shown in Fig. 1a, which has been normalized to the film thickness [12], reveals that all films are polycrystalline with a c-axis ZnO (002) preferential orientation perpendicular to the substrate surface. Even at low Ts, the deposited film was still endowed with good crystallinity. Kluth et al.

[13] developed an empirical model based on the growth model for sputtered metals to describe systematic changes of RF-sputtered AZO films from ceramic targets. Based on Kluth’s model, the films deposited at Ts = 85◦ C belonged to the type A morphology (Zone 1) with predominantly oriented c-axis parallel to the substrate normal. As the Ts increased, the surface structure had shifted to the more compact type B and C morphology (Zone 2) with a c-axis, although highly oriented, exhibiting a small inclination with respect to the substrate normal [8]. As Ts elevated to 500◦ C, the energetic atoms arriving at the substrate surface facilitate the surface migration where the surface free energy of the most densely packed (002) planes has been reduced effectively. Accordingly, the crystallinity of the films was effectively improved [14].

The residual stress could be characterized indirectly by the diffraction angles shift [15, 16]. A closer look at the peak positions in Fig. 1b revealed that there is a 0.10–0.22◦ positive shift as compared to pristine ZnO powder (2θ = 34.42◦, JCPDS#36-1451) for all the deposited films. This deviation confirmed the existence of residual stress vertical to c-axis of the deposited films [1, 15, 17]. However, the structure of AZO films did not vary significantly at high Ts. The lattice parameters along the c-axis, as calculated by Bragg equation, were decreased from 5.19Å without substrate heating to 5.17Å at Ts = 30◦ C. The compressed lattice constant, as compared to pristine ZnO powder (5.21Å), could be attributed to the replacement of larger Zn2+ (ionic radii, 72 pm) by the smaller Al3+ ions (53 pm).

The c-axis were expected to shrink by 1.1–1.8% for Al-doped ZnO ([Al] = 3–5 wt.%) [18]. Since the thermal expansion coefficient, α(T ) of soda lime glass (8.6 × 10−6 /◦ C at 300◦ C) is far larger than those of hexagonal ZnO crystals (at 25◦ C, α and α⊥ were 2.92 and 4.75 × 10−6 /◦ C, respectively) [11], less compressive or more tensile thermal stress would be induced upon cooling from the Ts. Moreover, owing to the large mismatch in α(T), the deposited AZO films on soda lime glass underwent more residual stress than those deposited over other substrates. According to Thornton’s model [12], the increase of FWHM in Fig. 1b is ascribed to the growth at the transition regions which results in larger crystal size at Ts = 400◦ C.

The morphology and microstructures of the deposited films has strong relationship with Ts. The SEM micrographs in Figs. 2a–e revealed that at Ts = 85◦ C, the morphology of the film resembled to those reported by Ruske et al. [8]. The columnar structure became larger at Ts = 200◦ C (Fig. 2f) while at elevated Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 Fig. 2. The SEM photographs of AZO films deposited at various Ts : (a) no heating, (b) 200◦ C, (c) 300◦ C, (d) 400◦ C, (e) 500◦ C, (f) cross-sectional view of (b).

Fig. 3. AFM image of AZO films deposited at Ts : (a) no heating, (b) 200◦ C, (c) 300◦ C, (d) 400◦ C, (e) 500◦ C and (f) the RMS roughness of the films at various Ts.

temperature (Ts 300◦ C), the uniformly distributed columnar structures were slightly curved owing to the transition of surface structure according to the modified structure zone model developed by Kluth et al. [13].

This process might induce major grain growth and, consequently, result in more compact structures with less surface roughness as shown in Figs. 3a–e. The dependence of RMS roughness on Ts also showed roughly similar tendency, as shown in Fig. 3f. There is only one exception occurring at Ts = 200◦ C where the RMS roughness reaches its maximum value, 4.9 nm. This could be explained by the predominantly columnar (textured) growth at Ts = 200◦ C, as shown in Fig. 2f. In general, the regular and dense film structures rendered the films with more compact and smooth morphology.

306 Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 Fig. 4. XPS spectra of AZO films: (a) wide scan spectra, Ts = 400◦ C; (b) Zn2p spectra; (c) ZnLMM spectra, and (d) Al2p spectra.

The X-ray photoelectron spectrum (XPS) reveals valuable information of elemental compositions and bonding state for each constituent element of the deposited films. A wide scan spectrum of AZO film deposited at Ts = 400◦ C is depicted in Fig. 4a where all peaks are identified by the photoelectron lines (Zn2p, Zn3s, Zn3p, Zn3d ; O1s ) and Auger lines (ZnLMM; OKLL), respectively. The very weak C1s peak is attributed to the surface contaminated samples from air. In light of photoelectron lines, all the peaks of Zn2p in Fig. 4b are symmetric which correlate to the covalently ZnO structure. Nor apparent peak shifting or intensity variation could be observed by substrate heating. In contrast, the asymmetric Auger signals, Zn L3 M45 M45 in Fig. 4c comprise a higher energy peak (∼497 eV) which are ascribed to the bonding of Zn with oxygen in the ZnO lattice and, in contrast, a shoulder peak (∼494 eV) which corresponds to the existence of Zn interstitials in the films that are one of the major sources of free electrons in doped ZnO films [15, 19, 20]. Similarly, there is no Ts dependence on the intensity of Auger signals. The binding energy (BE) of Al2p, as depicted in Fig. 4d, is around 73.2 eV whose value lies between that of Al2 O3 film (74.8 eV) and metallic aluminum (72.7 eV). Consequently, it could be predicted that Al atoms substitute for Zn atoms in Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 Fig. 5. (a) The chemical composition of the films; (b) the XPS spectra of O1s deposited at various Ts.

AZO films which could act as Al donors [20]. Minami et al. [12] showed that, in the resultant AZO films deposited by RF magnetron sputtering with sintered ceramic target, the aluminum was always introduced in the form of Al3+ and that the dopant content in the film was irrelevant to Ts below 400◦ C, which is just the case in our study.

Figure 5a shows the chemical composition of the films deposited at various Ts as detected by XPS. It could be observed that the atomic ratio of Y/Zn monotonically decreased with the substrate temperature.

A possible scheme is that vulnerable zinc atoms are more reactive at higher Ts under which the deposition rates for zinc atoms will be improved considerably as compared to those for the more stable yttrium atoms in the gas phase. In our study, the atomic ratio of Al/Zn ranged from 1.6 to 3.2%. Islam et al. [21] had reported that the optimal electrical properties of Al-doped ZnO films might occur at some specific ratio of Al/Zn 2 at.% which were coincident with our observation in that the film with best electrical conductivity occurs at Ts =400◦ C with Al/Zn = 1.9 at.%.

The atomic ratio of O/Zn was somewhat different to those published elsewhere in that the films deposited by AZOY® target are stoichiometric with O/Zn ranging from 75 to 84%. In contrast, the atomic ratio of O/Zn declined to around 24 at.% as reported by Lin et al. [19]. The increase of O/Zn suggested a decrease in oxygen vacancies while an improvement in chemical stability [3]. The chemisorbed oxygen plays an important role in the electrical conductivity. By absorbing electrons from the conduction band at the film surface, the chemisorbed oxygen lead to a decline in carrier concentration and widening of the depletion layer [22]. In Fig. 5b, the O1s peak is decomposed to two components which are designated as OI (531.3 eV) and OII (532.2 eV), respectively. The low BE peak (OI ) was originated from oxygen in ZnO while the high BE component (OII ) was attributed to oxygen impurities other than lattice oxygen, such as physically adsorbed or chemisorbed oxygen, hydroxides within the surface layer [14, 19, 23].

3.2. Electrical Properties As referred elsewhere [24], the conductivity of sputtered columnar polycrystalline zinc oxide thin films is correlated to the ionized adsorbates on crystallite surfaces by controlling the intercrystallite depletion barriers. Consequently, it is desirable to admit H2 -containing atmosphere to the sputter chamber for reducing the oxygen impurities. Figure 7 shows the dependence of the electrical properties of the AZO films on Ts.

The resistivity decreases linearly to a minimum value of 4.62 × 10−4 Ω·cm at Ts = 400◦ C.

308 Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 Fig. 6. Electrical properties of AZO films deposited at various Ts.

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