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The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets
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The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets Ming-Wei Wu a,∗ , Pang-Hsin Lai a , Chia-Hong Hong b , Fang-Cheng Chou c a
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, ROC b Department of Materials Science and Engineering, National Formosa University, Huwei, Yunlin 63201, Taiwan, ROC c Center for Condensed Matter and Science, National Taiwan University, Taipei 10617, Taiwan, ROC Received 9 April 2014; received in revised form 5 May 2014; accepted 6 May 2014
Abstract The properties of sputtering targets have recently been found to affect the performances of sputtered films and the sputtering process. To develop high-quality GZO ceramic targets, the influences of Ga2 O3 content and sintering temperature on the sintering behavior, microstructure, and electrical properties of GZO ceramic targets were studied. The results showed that the increase in Ga2 O3 content from 3 wt% (GZO-3Ga) and 5 wt% (GZO-5Ga) not only inhibited the densification but retarded grain growth. During sintering, ZnGa2 O4 phase formed before 800 ◦ C, and Zn9 Ga2 O12 phase was found after sintering at 1000 ◦ C. Moreover, after sintering at 1200 ◦ C, the number of Zn9 Ga2 O12 precipitates increased at the expense of ZnGa2 O4 and ZnGa2 O4 disappearing completely. The relative density, grain size, and resistivity of GZO-3Ga sintered at 1400 ◦ C in air were 99.3%, 3.3 m, and 2.8 × 10−3 cm, respectively. These properties of GZO ceramics are comparable to properties reported in the literature for AZO sintered in air. © 2014 Elsevier Ltd. All rights reserved. Keywords: GZO; Sintering; Microstructure-final; Electrical properties; Sputtering target
1. Introduction Transparent conductive oxide (TCO) films have been extensively used as the transparent conductive electrode in the optoelectronic and related fields, in applications such as liquid crystal displays, solar cells, and touch panels. To date, tin-doped indium oxide (ITO) film is the predominant TCO film due to its low resistivity (<10−3 cm) and high visible transmittance (>80%).1,2 However, efforts to develop alternatives to ITO have been intense because the cost of ITO film is relatively high. Currently, gallium-doped zinc oxide (GZO) and aluminum-doped zinc oxide (AZO) films are regarded as potential TCO materials due to their versatility.3–13 TCO films can be produced by magnetron sputtering, a multi-functional and versatile method,
∗
Corresponding author. Tel.: +886 2 2771 2171 ext. 2721; fax: +886 2 27317185. E-mail addresses: [email protected], [email protected] (M.-W. Wu).
because of its high deposition rate, feasibility of large-area coating, low deposition temperature, and high quality of the film.11–13 A key material in the production of TCO film is the sputtering target. To optimize the performances of TCO films, most researchers have studied the influences of sputtering parameters on the properties of films, including the type of sputter power,7,13 sputter power,5,12 working pressure,5 base pressure,10 substrate temperature,4–8,11 atmosphere,12 film thickness,3 and post annealing treatment.7,9 Unfortunately, only a few studies have attempted to clarify how the TCO target affects the sputtering process and the properties of various films.14–18 Moreover, the correlations between target performance and film properties, and the information about the properties and processes of sputtering targets, have not been well established. These underlying issues inhibit attempts to optimize sputtered films. Recently, the importance of the sputtering target has been gradually revealed. The sintered density,18 grain size,19 electrical properties,14,15 stoichiometry,16,20,21 and microstructural uniformity17 of the sputtering targets are now known to affect
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significantly not only the properties of various thin films but also the sputtering process. Minami et al.14 studied the influences of the electrical properties of nine commercial AZO targets on the properties of AZO films and indicated that a target with lower resistivity can be used to achieve a higher deposition rate, lower arcing counts, and a more uniform film having lower resistivity. Huang et al.15 investigated the effects of sintering parameters on the characteristics of AZO targets and sputtered films. They also showed that an AZO target with lower resistivity can be used to produce films with better electrical properties and uniformity. These two studies clearly indicated that the electrical properties of AZO targets play an important role in determining film performance. Moreover, several works have investigated the fabrications and properties of the various sputtering targets, particularly those of AZO ceramic targets.15,22–33 Unfortunately, the studies concerning GZO ceramic targets have been rare. Liu et al.34 investigated the effects of Ga2 O3 content (0–3 wt%) on the properties of GZO ceramic targets. Wiff et al.35 and Jung et al.36 concentrated their studies on the Hall mobilities and thermoelectric properties of GZO ceramics, respectively. Before the correlations between GZO targets and films are identified, the process and characteristics of the GZO target must first be examined. The purpose of this study was thus to investigate the influences of Ga2 O3 content (3 and 5 wt%) and sintering temperature (1200–1500 ◦ C) on the sintering behavior, microstructure, and electrical properties of GZO ceramic targets. 2. Experimental procedure In the literature, the compositions of GZO films3,4,6–9 range from 3 to 5.56 wt%. Thus, the compositions of GZO ceramic targets (ZnO:Ga2 O3 ) investigated in this study were 97:3 and 95:5 (wt%). The designations of these two GZO ceramics were GZO-3Ga and GZO-5Ga. ZnO and Ga2 O3 powders with median particle sizes of 0.4 and 0.1 m, respectively, were used in this study. A commercial ammonium polyacrylate of 0.2 wt% and a polyacrylic emulsion of 0.5 wt% were used as the dispersant and binder, respectively. The concentrations of the dispersant and the binder were based on the weight of the dry powder. To prepare GZO powder slurry, the dispersant of 0.2 wt% (2 g) was first added into distilled water (416 g), and the ZnO (970 g for GZO-3Ga; 950 g for GZO-5Ga) and Ga2 O3 (30 g for GZO-3Ga; 50 g for GZO-5Ga) powders were subsequently added into the aqueous solution. The solid contents of these two GZO slurries were 30 vol%, which corresponded to 70.6 wt%. The slurry was ball milled with 2 mm-diameter ZrO2 grinding balls for 3 h by a commercial ball mill (MUBM-340, Sun-Great Technology Co., Taiwan, ROC) with a rotation speed of 300 rpm. The ball to powder ratio (BPR) was only 3:1 because the role of the ZrO2 grinding balls was not to refine the particle size of the ZnO and Ga2 O3 powders but to help homogenize the ZnO and Ga2 O3 powders in the slurry. Afterwards, the binder (5 g) was added into the slurry and the slurry was then ball milled for one extra hour. The as-milled slurry was then spray-dried in hot air using a spray dryer (L-8, Ohkawara Kakohki Co., Yokohama, Japan).
The inlet and outlet temperatures for spray drying were 140 and 90 ◦ C, respectively. The sizes of GZO spray-dried granules ranged from 20 to 40 m. The GZO granules (2.3 g) were compacted by dry pressing at a pressure of 150 MPa into green compact disks that were 13 mm in diameter and 5 mm thick. The green densities of GZO-3Ga and GZO-5Ga compacts were 3.43 and 3.44 g/cm3 , respectively. The green compacts were heated at 5 ◦ C/min to 600 ◦ C and held for 30 minutes to remove the organic additives. After 600 ◦ C debinding, the densities of GZO-3Ga and GZO-5Ga ceramics were 3.48 and 3.50 g/cm3 , respectively. The debound specimens were then heated at 10 ◦ C/min to the sintering temperatures (1200, 1300, 1400, and 1500 ◦ C) for 3 h of sintering, followed by furnace cooling. The atmospheres for debinding and sintering were both air. The sintered densities of the specimens sintered at various temperatures were measured using Archimedes’ method in distilled water. The weight loss after sintering was recorded to understand the evaporation phenomenon at different sintering temperatures. The influences of Ga2 O3 concentration and sintering temperature on weight loss were also further evaluated using thermogravimetric analysis (TGA, STA 449 F3, NETZSCH, Selb, Germany). The TGA specimens were heated at 10 ◦ C/min to 1500 ◦ C for 1 h of sintering in air. For microstructure examination, the specimens were ground, polished, and then thermally etched at 1050 ◦ C for 1 h. The etched specimens were examined under field-emission SEM (LEO-1530, Zeiss, Oberhochem, Germany), and the average grain sizes were calculated according to quantitative metallography.37 The crystal structure of the GZO ceramics were examined by X-ray diffractometer (D8, Brüker, Karlsruhe, Germany) with Cu K␣ radiation. An electron probe microanalyzer (EPMA, JXA-8200SX, JEOL, Tokyo, Japan) was also used to determine the elemental distributions of Ga and Zn in the GZO ceramics. To clarify the effects of the Ga2 O3 content and sintering temperature on the resistivity of GZO ceramics, the specimens sintered at various temperatures were analyzed at room temperature by four-point probe method using a source meter (2400, Keithely Instruments Inc., Cleveland, OH). 3. Results and discussion 3.1. Microstructure and grain size Figs. 1 and 2 show the microstructures of GZO-3Ga and GZO-5Ga ceramics that were sintered at various temperatures, respectively. The result indicates that the densification of GZO3Ga was obvious after sintering at 1300 ◦ C. In contrast, the porosity of GZO-5Ga was still quite high, even after sintering at 1400 ◦ C. The average grain sizes of two GZO ceramics as a function of the sintering temperatures are shown in Fig. 3. The results demonstrate that the grain sizes of GZO-3Ga and GZO-5Ga ceramics increased from 0.6 to 8.4 m and from 0.5 to 3.1 m, respectively, when the sintering temperature was increased from 1200 to 1500 ◦ C. The findings on the microstructure and grain size show that increasing the Ga2 O3 content from 3 to 5 wt% inhibited both the densification and the grain growth. Moreover, the small amounts of precipitates were mostly located
Please cite this article in press as: Wu M-W, et al. The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.022
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Fig. 1. The microstructures of GZO-3Ga ceramics sintered at (a) 1200 ◦ C, (b) 1300 ◦ C, (c) 1400 ◦ C, and (d) 1500 ◦ C.
Fig. 2. The microstructures of GZO-5Ga ceramics sintered at (a) 1200 ◦ C, (b) 1300 ◦ C, (c) 1400 ◦ C, and (d) 1500 ◦ C.
Please cite this article in press as: Wu M-W, et al. The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.022
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Fig. 3. The average grain sizes of two GZO ceramics as a function of sintering temperature.
at the pores, as indicated by white arrows in Figs. 1 and 2, in the two GZO ceramics. The sizes of these precipitates were on the sub-micrometer scale, and these precipitates aggregated to a cluster with a size of 2–5 m. The precipitates were not as easily observed in the less dense ceramics, such as GZO-3Ga sintered at 1200 ◦ C and GZO-5Ga sintered below 1400 ◦ C, due to the presence of large numbers of pores.
deficient in Zn than the ZnO matrix. Jang et al.38 examined the solubility limit of Ga in GZO ceramics and found the maximum solubility of Ga was only about 0.5 mol%. The crystal structures of the precipitates in GZO ceramics have been studied previously.32,34–36,38 Jung et al.36 and Jang et al.38 found that the precipitate in the GZO ceramics was ZnGa2 O4 . Liu et al.32 also indicated that the addition of 0.6 at% Ga to AZO ceramic led to the formation of ZnGa2 O4 . Moreover, Liu et al.34 and Wiff et al.35 indicated that the precipitate in GZO ceramics was Zn9 Ga2 O12 . From the above results, it is clear that the crystal structure of the precipitates in the GZO ceramics is still undetermined. To further identify the crystal structure of the precipitates in the GZO ceramics investigated in this study, XRD was used to analyze GZO-3Ga ceramics sintered at various sintering temperatures, as shown in Fig. 5(a). After sintering at 800 ◦ C, ZnGa2 O4 was detected by XRD. When the sintering temperature was increased to 1000 ◦ C, the amount of ZnGa2 O4 did not obviously change. However, minor amounts of Zn9 Ga2 O12 were also found in the GZO ceramics. After sintering at 1200 ◦ C sintering, more Zn9 Ga2 O12 formed at the expense of ZnGa2 O4 phase and ZnGa2 O4 disappearing completely, as shown in Fig. 5(a). The EPMA quantitative analyses also supported the formation of Zn9 Ga2 O12 after sintering at 1200 ◦ C. Furthermore, GZO5Ga ceramics sintered at 1500 ◦ C exhibited higher amounts of Zn9 Ga2 O12 phase than GZO-3Ga ceramics, as demonstrated in Fig. 5(b). 3.3. Densification versus evaporation
3.2. Elemental distribution and crystal structure To understand the elemental distribution in the GZO ceramics after sintering, GZO-5Ga sintered at 1500 ◦ C was examined by EPMA, as shown in Fig. 4. The Ga and Zn mapping clearly indicated that Ga and Zn atoms were homogeneous in the ZnO matrix. However, the precipitates were richer in Ga and more
Based on quantitative metallography of the microstructures sintered at 1500 ◦ C, the percentages of Zn9 Ga2 O12 phase in the GZO-3Ga and GZO-5Ga ceramics were 2 and 7 vol%, respectively. The theoretical densities of ZnO and Zn9 Ga2 O12 are 5.61 g/cm3 (JCPDS 79-0208) and 5.69 g/cm3 (JCPDS 500448), respectively. Thus, in this study, the theoretical densities
Fig. 4. The elemental mappings of GZO-5 ceramic sintered at 1500 ◦ C (a) microstructure, (b) Ga mapping, and (c) Zn mapping.
Please cite this article in press as: Wu M-W, et al. The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.022
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Fig. 5. The XRD results of (a) GZO-3Ga ceramics sintered at 800, 1000, and 1200 ◦ C, (b) two GZO ceramics sintered at 1500 ◦ C.
of GZO-3Ga and GZO-5Ga were assumed to be 5.61 and 5.62 g/cm3 , respectively. Fig. 6 shows the sintered densities of two GZO ceramics after sintering between 1200 and 1500 ◦ C for 3 h. The sintered densities of GZO-3Ga and GZO-5Ga ceramics sintered at 1200 ◦ C were similar, only about 85.7% and 83.6%, respectively. However, after sintering at 1300 ◦ C, the densification of GZO-3Ga was obviously activated and the sintered density increased to 98.8%. When the sintering temperature was increased further, the sintered density of GZO-3Ga peaked at 99.3% at 1400 ◦ C and then declined to 96.8% at 1500 ◦ C. Moreover, the densification behavior of GZO-5Ga was different from that of GZO-3Ga, as demonstrated in Fig. 6. The sintered densities of GZO-5Ga ceramics were gradually improved by increasing the sintering temperature from 1200 to 1500 ◦ C, peaking at a density of 94.3% after sintering at 1500 ◦ C. These results on the sintered densities were consistent with those on the microstructures, as can be seen from comparing Figs. 1, 2, and 6. Due to the high vapor pressure of ZnO, evaporation and weight loss occur easily during high-temperature sintering
of ZnO-based ceramics.23,27,32 To clarify the evaporation phenomena of the two GZO ceramics, the weight losses as a function of sintering temperature were plotted, as shown in Fig. 7. The results indicated that the weight losses of GZO-3Ga were higher than those of GZO-5Ga, implying that the addition of Ga2 O3 inhibited evaporation. The difference between the weight losses of the two GZO ceramics was about 0.3–0.5 wt% after sintering at 1200, 1300, and 1400 ◦ C. However, when the sintering temperature was increased to 1500 ◦ C, the difference between the weight losses increased to 1.2 wt%. TGA was also used to further identify the difference in the evaporation behaviors of the two GZO ceramics. Fig. 8 shows the TGA curves of two GZO ceramics heated from 100 to 1500 ◦ C and then held at 1500 ◦ C for 1 h. As indicated by the curves, the debinding of organic additives, including the dispersant and the binder, was completed at 400 ◦ C. It should be noted that the weight losses of GZO-3Ga were slightly higher than those of GZO-5Ga between 400 and 1500 ◦ C, consistent with the finding in Fig. 7. After sintering at 1500 ◦ C for 1 h, the
Fig. 6. The relative densities of two GZO ceramics as a function of sintering temperature.
Fig. 7. The weight losses of two GZO ceramics as a function of sintering temperature.
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Fig. 8. The TGA curves of two GZO ceramics heated from 100 to then held at 1500 ◦ C for 1 h.
1500 ◦ C
and
difference between the weight losses of the two GZO ceramics was increased. Thus, the addition of Ga2 O3 reduced the evaporation of ZnO during sintering. Zhang et al.26 and Liu et al.32 also found that the addition of 0.2 wt% Ga and 0.6 at% Ga, respectively, in AZO ceramics can reduce the loss of mass during sintering. The sintered densities of ZnO-based ceramics are controlled by both the densification and evaporation. Evaporation can lead to the formation of pores and thus inhibit the densification of ZnO-based ceramics, including GZO ceramics. Figs. 7 and 8 clearly demonstrate that the weight losses of GZO-3Ga sintered at various temperatures were higher than those of GZO-5Ga. However, the sintered densities of GZO-3Ga were always superior to those of GZO-5Ga, irrespective of the sintering temperature. This contradiction can be attributed to the discrepancy in the amounts of Zn9 Ga2 O12 precipitate that formed in the two GZO ceramics during sintering. ZnO and Ga2 O3 will react to Zn9 Ga2 O12 at temperatures higher than 1000 ◦ C, as shown in Fig. 5, and this reaction could be accompanied by a change in volume and the formation of pores. Most of the Zn9 Ga2 O12 precipitates were located in the pores, as demonstrated in Figs. 1 and 2. Thus, the formation of Zn9 Ga2 O12 obviously reduced the densification and sintered density of the GZO-5Ga ceramic. Liu et al.34 also indicated that the relative densities of GZO ceramics sintered at 1300 ◦ C fell from 99.2% to 96.7% when Ga2 O3 content was increased from 1 to 3 wt%. On the other hand, the grain sizes of GZO-5Ga ceramics sintered at various temperatures were finer than those of GZO-3Ga. This phenomenon can be attributed to the dragging effect of both the Zn9 Ga2 O12 precipitates and pores. 3.4. Resistivity The resistivities of two GZO ceramics sintered at temperatures of 1300–1500 ◦ C are presented in Fig. 9. The resistivities
Fig. 9. The resistivities of two GZO ceramics as a function of sintering temperature.
of GZO-3Ga and GZO-5Ga sintered at 1200 ◦ C were higher than 108 cm and thus could not be measured by the source meter used in this study. Fig. 9 shows that the resistivities of GZO3Ga ceramics sintered at various temperatures were superior to those of GZO-5Ga. The electrical properties of GZO-3Ga were improved only slightly by increasing the sintering temperatures from 1300 to 1500 ◦ C. However, in GZO-5Ga, resistivity was significantly decreased by increasing the sintering temperature from 1300 to 1500 ◦ C. The resistivities of GZO-3Ga and GZO5Ga sintered at 1500 ◦ C were 2.1 × 10−3 and 2.6 × 10−3 cm, respectively. Fig. 9 clearly indicates that the resistivities of GZO-3Ga and GZO-5Ga were similar after sintering at 1400 or 1500 ◦ C, though the relative densities of GZO-3Ga were higher than those of GZO-5Ga. Wu et al.22 examined the electrical properties of AZO ceramic targets and showed that the Hall mobility was much improved, from 0.2 to 23.9 cm2 /Vs, when the relative density rose from 82.2% to 93.7%, indicating that interconnected pores significantly impaired the Hall mobility and the resistivity of AZO ceramic targets. Moreover, when relative density increased from 93.7% to 98.8%, the Hall mobility rose from 23.9 to only 67.3 cm2 /Vs. These findings show that after interconnected pores have been transformed into isolated ones, the increase in sintered density cannot obviously improve the Hall mobilities and the resistivities of GZO and AZO ceramic targets. Table 1 further compares the processes and characteristics of GZO ceramic targets with those of AZO targets, including the sintering method, sintering parameters, relative density, grain size, and resistivity.14,15,22,24,26,28,29,31,33–36,38 The properties of the two GZO ceramics investigated in this study are also included for comparison. As can be seen in Table 1, the relative density and the resistivity of GZO-3Ga sintered at 1500 ◦ C were similar to those of GZO ceramics reported in the literature.34–36,38 Furthermore, the relative densities of AZO ceramic targets sintered in air can be higher than 98%. In general, the grain sizes and resistivities of AZO ceramic targets sintered in air range from 2 to 5 m and from 1 × 10−3 to 4 × 10−3 cm, respectively.
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Table 1 Comparison of the processes and various properties of GZO and AZO ceramic targets.14,15,22,24,26,28,29,31,33–36,38 Material
Sintering method
Sintering temp./time (◦ C/h)
GZO [this study] (3 wt% Ga2 O3 ) GZO [this study] (5 wt% Ga2 O3 ) GZO34 (3 wt% Ga2 O3 ) GZO34 (1 wt% Ga2 O3 ) GZO35 (1 at% Ga) GZO36 (0.5 at% Ga) GZO38 (0.5 mol% Ga) GZO14 (5.45 at% Ga) AZO15 AZO22 AZO24 AZO26 AZO28 AZO29 AZO31 AZO33 AZO14
CS in air
1400 ◦ C/3 h
CS in air
1500 ◦ C/3 h
CS in air
1300 ◦ C/–
CS in air
1300 ◦ C/–
CS in air
1400 ◦ C/10 h
SPS
1000 ◦ C/5 min
CS in air
1300 ◦ C/10 h
–
–
>97.1
CS in air CS in air CS in air CS in air CS in air CS in air CS in air HP –
1400 ◦ C/4 h 1500 ◦ C/1 h 1500 ◦ C/3 h 1450 ◦ C/10 h 1300 ◦ C/8 h 1300 ◦ C/2 h 1300 ◦ C/8 h 1100 ◦ C/10 h –
98.6 98.8 99.8 99.46 99.3 99.9 98.9 99.0 >97.1.
Relative density (%)
Grain size (m)
Resistivity ( cm)
Note
99.3
3.3
2.8 × 10−3
–
94.3
3.1
2.6 × 10−3
–
–
–
–
99.2
–
–
–
96
–
∼1.4 × 10−3
–
∼95
–
∼1 × 10−3
–
–
1
–
4.58 × 10−4
CA
8.85 × 10−4 1.65 × 10−3 0.5 2.14 × 10−3 1.15 × 10−3 3.86 × 10−3 2.1 × 10−3 3.0 × 10−3 4.58 × 10−4 ∼4.81 × 10−3
– – – – – NP NP – CA
∼97
∼2.7 4.4 – – – 2.6 5.7 – –
Note: CS, conventional sintering; SPS, spark plasma sintering; HP, hot pressing; CA, commercially available; NP, nano powder.
Minami et al.14 used AZO and GZO targets with resistivities as low as 4.58 × 10−4 cm for sputtering and achieved uniform films having low resistivity. Unfortunately, the sputtering targets used by Minami et al. were commercially available, so the detailed process history cannot be found. In this study, the relative density, grain size, and resistivity of GZO-3Ga ceramics sintered in air were close to those of AZO, as shown in Table 1. This comparison indicates that the various properties of GZO ceramics investigated in this study were comparable to those of AZO ceramics reported in the literature. 4. Conclusions To clarify the sintering behavior, microstructure, and electrical properties of GZO ceramic targets, GZO with 3 and 5 wt% Ga2 O3 were produced. The effects of Ga2 O3 content and sintering temperature on the sintered density, grain size, weight loss, crystal structure of the precipitate, and resistivity of GZO ceramic were investigated. The findings can be summarized as follows. 1. The increase in Ga2 O3 content from 3 and 5 wt% in GZO ceramics not only inhibits densification but also retards grain growth. The highest densities of GZO-3Ga and GZO-5Ga ceramics achieved were 99.3% and 94.3%, respectively. 2. After sintering at 800 ◦ C, ZnGa2 O4 can be observed in GZO ceramics. When the sintering temperature is increased to
1000 ◦ C, minor Zn9 Ga2 O12 can also be found. After sintering at 1200 ◦ C, more Zn9 Ga2 O12 precipitates are formed at the expense of ZnGa2 O4 and ZnGa2 O4 disappearing completely. The final microstructure of GZO ceramics is composed of ZnO and Zn9 Ga2 O12 . 3. The sintered densities of GZO-3Ga are always superior to those of GZO-5Ga, irrespective of the sintering temperature, though the weight losses of GZO-3Ga are higher than those of GZO-5Ga. This contradiction can be attributed to the discrepancy in the amount of Zn9 Ga2 O12 precipitate that forms between these two GZO ceramics during sintering. 4. The resistivities of GZO-3Ga and GZO-5Ga sintered at 1500 ◦ C were 2.1 × 10−3 and 2.6 × 10−3 cm, respectively. When interconnected pores were transformed into isolated ones, the increase in sintered density did not obviously improve the resistivity of GZO ceramic. 5. In this study, the best properties were exhibited by GZO-3Ga sintered at 1400 ◦ C in air. The relative density, grain size, and resistivity were 99.3%, 3.3 m, and 2.8 × 10−3 cm, respectively. These properties of GZO ceramics are comparable to those of AZO reported in the literature. Acknowledgement The authors thank the National Science Council of the Republic of China for their support of this project under contract number NSC 99-2218-E-150-046.
Please cite this article in press as: Wu M-W, et al. The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.022
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References 1. Minami T. Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications. Thin Solid Films 2008;516:1314–21. 2. Fang G, Li D, Yao BL. Fabrication vacuum annealing of transparent conductive AZO thin films prepared by DC magnetron sputtering. Vacuum 2003;68:363–72. 3. Akazawa H. Double layer structures of transparent conductive oxide suitable for solar cells: Ga-doped ZnO on undoped ZnO. Thin Solid Films 2012;526:195–200. 4. Park SH, Park JB, Song PK. Characteristics of Al-doped, Ga-doped and In-doped zinc-oxide films as transparent conducting electrodes in organic light-emitting diodes. Curr Appl Phys 2010;10:S488–90. 5. Bie X, Lu J, Wang Y, Gong L, Ma Q, Ye Z. Optimization of parameters for deposition of Ga-doped ZnO films by DC reactive magnetron sputtering using Taguchi method. Appl Surf Sci 2011;257:6125–8. 6. Nagamoto K, Kato K, Naganawa S, Kondo T, Sato Y, Makino H. Yamamoto N, and Yamamoto T. Structural, electrical and bending properties of transparent conductive Ga-doped ZnO films on polymer substrates. Thin Solid Films 2011;520:1411–5. 7. Nomoto JI, Konagai M, Okada K, Ito T, Miyata T, Minami T. Comparative study of resistivity characteristics between transparent conducting AZO and GZO thin films for use at high temperatures. Thin Solid Films 2010;518:2937–40. 8. Lee DK, Bang J, Park M, Lee JH, Yang H. Organic acid-based wet etching behaviors of Ga-doped ZnO films sputter-deposited at different substrate temperatures. Thin Solid Films 2010;518:4046–51. 9. Yen WT, Lin YC, Yao PC, Ke JH, Chen YL. Effect of post-annealing on the optoelectronic properties of ZnO:Ga films prepared by pulsed direct current magnetron sputtering. Thin Solid Films 2010;518:3882–5. 10. Kim DK, Kim HB. Dependence of the properties of sputter deposited Al-doped ZnO thin films on base pressure. J Alloys Compd 2012;522: 69–73. 11. Zhang XB, Pei ZL, Gong J, Sun C. Investigation on the electrical properties and inhomogeneous distribution of ZnO:Al thin films prepared by DC magnetron sputtering at low deposition temperature. J Appl Phys 2007;101:014910. 12. Yang W, Liu Z, Peng DL, Zhang F, Huang H, Xie Y, et al. Roomtemperature deposition of transparent conducting Al-doped ZnO. Appl Surf Sci 2009;255:5669–73. 13. Cebulla R, Wendt R, Ellmer K. Al-doped zinc oxide films deposited by stimultaneous RF and DC excitation of a magnetron plasma: relationships between plasma parameters and structural and electrical film properties. J Appl Phys 1998;83:1087–95. 14. Minami T, Oda JI, Nomoto JI, Miyata T. Effect of target properties on transparent conducting impurity-doped ZnO thin films deposited by DC magnetron sputtering. Thin Solid Films 2010;519: 385–90. 15. Huang HS, Tung HC, Chiu CH, Hong IT, Chen RZ, Chang JT, et al. Highly conductive alumina-added ZnO ceramic target prepared by reduction sintering and its effects on the properties of deposited thin films by direct current magnetron sputtering. Thin Solid Films 2010;518:6071–5. 16. Yamada N, Hitosugi T, Kasai J, Hoang NLH, Nakao D, Hirose Y, et al. Transparent conducting Nb-doped anatase TiO2 (TNO) thin films sputtered from various oxide targets. Thin Solid Films 2010;518:3101–4. 17. Nakashima K, Kumahara Y. Effect of tin oxide dispersion on nodule formation in ITO sputtering. Vacuum 2002;66:221–6.
18. Omata T, Kita M, Okada H, Otsuka-Yao-Matsuo S, Ono N, Ikawa H. Characterization of indium–tin oxide sputtering targets showing various densities of nodule formation. Thin Solid Films 2006;503:22–8. 19. Ying KL, Hsieh TE, Hsieh YF. Colloidal dispersion of nano-scale ZnO powders using amphibious and anionic polyelectrolytes. Ceram Int 2009;35:1165–71. 20. Neves N, Barros R, Antunes E, Ferreira I, Calado J, Fortunato E, et al. Sintering behavior of nano- and micro-sized ZnO powder targets for rf magnetron sputtering applications. J Am Ceram Soc 2012;95:204–10. 21. Kubart T, Jensen J, Nyberg T, Liljeholm L, Depla D, Berg S. Influence of the target composition on reactively sputtered titanium oxide films. Vacuum 2009;83:1295–8. 22. Wu MW, Liu DS, Su YH. The densification, microstructure, and electrical properties of aluminum-doped zinc oxide sputtering target for transparent conductive oxide film. J Eur Ceram Soc 2012;32:3265–75. 23. Wu MW. Two-step sintering of aluminum-doped zinc oxide sputtering target by using a submicrometer zinc oxide powder. Ceram Int 2012;38:6229–34. 24. Liu CP, Jeng GR. Properties of aluminum doped zinc oxide materials and sputtering thin films. J Alloys Compd 2009;468:343–9. 25. Sun YH, Xiong WH, Li CH, Yuan L. Effect of dispersant concentration on preparation of an ultrahigh density ZnO-Al2 O3 target by slip casting. J Am Ceram Soc 2009;92:2168–71. 26. Zhang J, Zhang W, Zhao E, Jacques HJ. Study of high-density AZO ceramic target. Mater Sci Semicond Process 2011;14:189–92. 27. Hwang B, Paek YK, Yang SH, Lim S, Seo WS, Oh KS. J Alloys Compd 2011;509:7478–83. 28. Wei T, Zhang Y, Yang Y, Tan R, Cui P, Song W. Effects of ZnAl2 O4 segregation in high temperature sintered Al-doped ZnO sputtering target on optical and electrical properties of deposited thin films. Surf Coat Technol 2013;221:201–6. 29. Neves N, Barros R, Antunes E, Calado J, Fortunato E, Martins R, et al. Aluminum doped zinc oxide sputtering targets obtained from nanostructured powders: processing and application. J Eur Ceram Soc 2012;32:4381–91. 30. Lee SP, Hwang B, Paek YK, Chung TJ, Yang SH, Lim S, et al. Determination of the initial heat treatment temperature in the two-step sintering of xAl–ZnO (x = 1, 2, and 3 wt.%). J Eur Ceram Soc 2013;33:131–7. 31. Yang Y, Lan P, Wang M, Wei T, Tan R, Song W. Nearly full-dense and finegrained AZO:Y ceramics sintered from the corresponding nanoparticles. Nanoscale Res Lett 2012;7:481. 32. Liu J, Zhang W, Song D, Ma Q, Zhang L, Zhang H, et al. Investigation of aluminum-gallium co-doped zinc oxide targets for sputtering thin film and photovoltaic application. J Alloys Compd 2013;575:174–82. 33. Wang XM, Bai X, Duan HY, Shi ZX, Sun J, Lu SG, et al. Preparation of Al-doped ZnO sputter target by hot pressing. Trans Nonferrous Met Soc China 2011;21:1550–6. 34. Liu C, Cai X, Zhang Z, Gao L, Xiao C, Shang F, et al. Effect of gallium doping on the properties of GZO target. Adv Mater Res 2013;77:9–780, 363–67. 35. Wiff JP, Kinemuchi Y, Watari K. Hall mobilities of Al- and Ga-doped ZnO polycrystals. Mater Lett 2009;63:2470–2. 36. Jung KH, Lee KH, Seo WS, Choi SM. An enhancement of a thermoelectric power factor in a Ga-doped ZnO system: a chemical compression by enlarged Ga solubility. Appl Phys Lett 2012;100:253902. 37. Mendelson MI. Average grain size in polycrystalline ceramics. J Am Ceram Soc 1969;52:443–6. 38. Jang MS, Ryu MK, Yoon MH, Lee SH, Kim HK, Onodera A, et al. A study on the Raman spectra of Al-doped and Ga-doped ZnO ceramics. Curr Appl Phys 2009;9:651–7.
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The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets Ming-Wei Wu a,∗ , Pang-Hsin Lai a , Chia-Hong Hong b , Fang-Cheng Chou c a
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, ROC b Department of Materials Science and Engineering, National Formosa University, Huwei, Yunlin 63201, Taiwan, ROC c Center for Condensed Matter and Science, National Taiwan University, Taipei 10617, Taiwan, ROC Received 9 April 2014; received in revised form 5 May 2014; accepted 6 May 2014
Abstract The properties of sputtering targets have recently been found to affect the performances of sputtered films and the sputtering process. To develop high-quality GZO ceramic targets, the influences of Ga2 O3 content and sintering temperature on the sintering behavior, microstructure, and electrical properties of GZO ceramic targets were studied. The results showed that the increase in Ga2 O3 content from 3 wt% (GZO-3Ga) and 5 wt% (GZO-5Ga) not only inhibited the densification but retarded grain growth. During sintering, ZnGa2 O4 phase formed before 800 ◦ C, and Zn9 Ga2 O12 phase was found after sintering at 1000 ◦ C. Moreover, after sintering at 1200 ◦ C, the number of Zn9 Ga2 O12 precipitates increased at the expense of ZnGa2 O4 and ZnGa2 O4 disappearing completely. The relative density, grain size, and resistivity of GZO-3Ga sintered at 1400 ◦ C in air were 99.3%, 3.3 m, and 2.8 × 10−3 cm, respectively. These properties of GZO ceramics are comparable to properties reported in the literature for AZO sintered in air. © 2014 Elsevier Ltd. All rights reserved. Keywords: GZO; Sintering; Microstructure-final; Electrical properties; Sputtering target
1. Introduction Transparent conductive oxide (TCO) films have been extensively used as the transparent conductive electrode in the optoelectronic and related fields, in applications such as liquid crystal displays, solar cells, and touch panels. To date, tin-doped indium oxide (ITO) film is the predominant TCO film due to its low resistivity (<10−3 cm) and high visible transmittance (>80%).1,2 However, efforts to develop alternatives to ITO have been intense because the cost of ITO film is relatively high. Currently, gallium-doped zinc oxide (GZO) and aluminum-doped zinc oxide (AZO) films are regarded as potential TCO materials due to their versatility.3–13 TCO films can be produced by magnetron sputtering, a multi-functional and versatile method,
∗
Corresponding author. Tel.: +886 2 2771 2171 ext. 2721; fax: +886 2 27317185. E-mail addresses: [email protected], [email protected] (M.-W. Wu).
because of its high deposition rate, feasibility of large-area coating, low deposition temperature, and high quality of the film.11–13 A key material in the production of TCO film is the sputtering target. To optimize the performances of TCO films, most researchers have studied the influences of sputtering parameters on the properties of films, including the type of sputter power,7,13 sputter power,5,12 working pressure,5 base pressure,10 substrate temperature,4–8,11 atmosphere,12 film thickness,3 and post annealing treatment.7,9 Unfortunately, only a few studies have attempted to clarify how the TCO target affects the sputtering process and the properties of various films.14–18 Moreover, the correlations between target performance and film properties, and the information about the properties and processes of sputtering targets, have not been well established. These underlying issues inhibit attempts to optimize sputtered films. Recently, the importance of the sputtering target has been gradually revealed. The sintered density,18 grain size,19 electrical properties,14,15 stoichiometry,16,20,21 and microstructural uniformity17 of the sputtering targets are now known to affect
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significantly not only the properties of various thin films but also the sputtering process. Minami et al.14 studied the influences of the electrical properties of nine commercial AZO targets on the properties of AZO films and indicated that a target with lower resistivity can be used to achieve a higher deposition rate, lower arcing counts, and a more uniform film having lower resistivity. Huang et al.15 investigated the effects of sintering parameters on the characteristics of AZO targets and sputtered films. They also showed that an AZO target with lower resistivity can be used to produce films with better electrical properties and uniformity. These two studies clearly indicated that the electrical properties of AZO targets play an important role in determining film performance. Moreover, several works have investigated the fabrications and properties of the various sputtering targets, particularly those of AZO ceramic targets.15,22–33 Unfortunately, the studies concerning GZO ceramic targets have been rare. Liu et al.34 investigated the effects of Ga2 O3 content (0–3 wt%) on the properties of GZO ceramic targets. Wiff et al.35 and Jung et al.36 concentrated their studies on the Hall mobilities and thermoelectric properties of GZO ceramics, respectively. Before the correlations between GZO targets and films are identified, the process and characteristics of the GZO target must first be examined. The purpose of this study was thus to investigate the influences of Ga2 O3 content (3 and 5 wt%) and sintering temperature (1200–1500 ◦ C) on the sintering behavior, microstructure, and electrical properties of GZO ceramic targets. 2. Experimental procedure In the literature, the compositions of GZO films3,4,6–9 range from 3 to 5.56 wt%. Thus, the compositions of GZO ceramic targets (ZnO:Ga2 O3 ) investigated in this study were 97:3 and 95:5 (wt%). The designations of these two GZO ceramics were GZO-3Ga and GZO-5Ga. ZnO and Ga2 O3 powders with median particle sizes of 0.4 and 0.1 m, respectively, were used in this study. A commercial ammonium polyacrylate of 0.2 wt% and a polyacrylic emulsion of 0.5 wt% were used as the dispersant and binder, respectively. The concentrations of the dispersant and the binder were based on the weight of the dry powder. To prepare GZO powder slurry, the dispersant of 0.2 wt% (2 g) was first added into distilled water (416 g), and the ZnO (970 g for GZO-3Ga; 950 g for GZO-5Ga) and Ga2 O3 (30 g for GZO-3Ga; 50 g for GZO-5Ga) powders were subsequently added into the aqueous solution. The solid contents of these two GZO slurries were 30 vol%, which corresponded to 70.6 wt%. The slurry was ball milled with 2 mm-diameter ZrO2 grinding balls for 3 h by a commercial ball mill (MUBM-340, Sun-Great Technology Co., Taiwan, ROC) with a rotation speed of 300 rpm. The ball to powder ratio (BPR) was only 3:1 because the role of the ZrO2 grinding balls was not to refine the particle size of the ZnO and Ga2 O3 powders but to help homogenize the ZnO and Ga2 O3 powders in the slurry. Afterwards, the binder (5 g) was added into the slurry and the slurry was then ball milled for one extra hour. The as-milled slurry was then spray-dried in hot air using a spray dryer (L-8, Ohkawara Kakohki Co., Yokohama, Japan).
The inlet and outlet temperatures for spray drying were 140 and 90 ◦ C, respectively. The sizes of GZO spray-dried granules ranged from 20 to 40 m. The GZO granules (2.3 g) were compacted by dry pressing at a pressure of 150 MPa into green compact disks that were 13 mm in diameter and 5 mm thick. The green densities of GZO-3Ga and GZO-5Ga compacts were 3.43 and 3.44 g/cm3 , respectively. The green compacts were heated at 5 ◦ C/min to 600 ◦ C and held for 30 minutes to remove the organic additives. After 600 ◦ C debinding, the densities of GZO-3Ga and GZO-5Ga ceramics were 3.48 and 3.50 g/cm3 , respectively. The debound specimens were then heated at 10 ◦ C/min to the sintering temperatures (1200, 1300, 1400, and 1500 ◦ C) for 3 h of sintering, followed by furnace cooling. The atmospheres for debinding and sintering were both air. The sintered densities of the specimens sintered at various temperatures were measured using Archimedes’ method in distilled water. The weight loss after sintering was recorded to understand the evaporation phenomenon at different sintering temperatures. The influences of Ga2 O3 concentration and sintering temperature on weight loss were also further evaluated using thermogravimetric analysis (TGA, STA 449 F3, NETZSCH, Selb, Germany). The TGA specimens were heated at 10 ◦ C/min to 1500 ◦ C for 1 h of sintering in air. For microstructure examination, the specimens were ground, polished, and then thermally etched at 1050 ◦ C for 1 h. The etched specimens were examined under field-emission SEM (LEO-1530, Zeiss, Oberhochem, Germany), and the average grain sizes were calculated according to quantitative metallography.37 The crystal structure of the GZO ceramics were examined by X-ray diffractometer (D8, Brüker, Karlsruhe, Germany) with Cu K␣ radiation. An electron probe microanalyzer (EPMA, JXA-8200SX, JEOL, Tokyo, Japan) was also used to determine the elemental distributions of Ga and Zn in the GZO ceramics. To clarify the effects of the Ga2 O3 content and sintering temperature on the resistivity of GZO ceramics, the specimens sintered at various temperatures were analyzed at room temperature by four-point probe method using a source meter (2400, Keithely Instruments Inc., Cleveland, OH). 3. Results and discussion 3.1. Microstructure and grain size Figs. 1 and 2 show the microstructures of GZO-3Ga and GZO-5Ga ceramics that were sintered at various temperatures, respectively. The result indicates that the densification of GZO3Ga was obvious after sintering at 1300 ◦ C. In contrast, the porosity of GZO-5Ga was still quite high, even after sintering at 1400 ◦ C. The average grain sizes of two GZO ceramics as a function of the sintering temperatures are shown in Fig. 3. The results demonstrate that the grain sizes of GZO-3Ga and GZO-5Ga ceramics increased from 0.6 to 8.4 m and from 0.5 to 3.1 m, respectively, when the sintering temperature was increased from 1200 to 1500 ◦ C. The findings on the microstructure and grain size show that increasing the Ga2 O3 content from 3 to 5 wt% inhibited both the densification and the grain growth. Moreover, the small amounts of precipitates were mostly located
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Fig. 1. The microstructures of GZO-3Ga ceramics sintered at (a) 1200 ◦ C, (b) 1300 ◦ C, (c) 1400 ◦ C, and (d) 1500 ◦ C.
Fig. 2. The microstructures of GZO-5Ga ceramics sintered at (a) 1200 ◦ C, (b) 1300 ◦ C, (c) 1400 ◦ C, and (d) 1500 ◦ C.
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Fig. 3. The average grain sizes of two GZO ceramics as a function of sintering temperature.
at the pores, as indicated by white arrows in Figs. 1 and 2, in the two GZO ceramics. The sizes of these precipitates were on the sub-micrometer scale, and these precipitates aggregated to a cluster with a size of 2–5 m. The precipitates were not as easily observed in the less dense ceramics, such as GZO-3Ga sintered at 1200 ◦ C and GZO-5Ga sintered below 1400 ◦ C, due to the presence of large numbers of pores.
deficient in Zn than the ZnO matrix. Jang et al.38 examined the solubility limit of Ga in GZO ceramics and found the maximum solubility of Ga was only about 0.5 mol%. The crystal structures of the precipitates in GZO ceramics have been studied previously.32,34–36,38 Jung et al.36 and Jang et al.38 found that the precipitate in the GZO ceramics was ZnGa2 O4 . Liu et al.32 also indicated that the addition of 0.6 at% Ga to AZO ceramic led to the formation of ZnGa2 O4 . Moreover, Liu et al.34 and Wiff et al.35 indicated that the precipitate in GZO ceramics was Zn9 Ga2 O12 . From the above results, it is clear that the crystal structure of the precipitates in the GZO ceramics is still undetermined. To further identify the crystal structure of the precipitates in the GZO ceramics investigated in this study, XRD was used to analyze GZO-3Ga ceramics sintered at various sintering temperatures, as shown in Fig. 5(a). After sintering at 800 ◦ C, ZnGa2 O4 was detected by XRD. When the sintering temperature was increased to 1000 ◦ C, the amount of ZnGa2 O4 did not obviously change. However, minor amounts of Zn9 Ga2 O12 were also found in the GZO ceramics. After sintering at 1200 ◦ C sintering, more Zn9 Ga2 O12 formed at the expense of ZnGa2 O4 phase and ZnGa2 O4 disappearing completely, as shown in Fig. 5(a). The EPMA quantitative analyses also supported the formation of Zn9 Ga2 O12 after sintering at 1200 ◦ C. Furthermore, GZO5Ga ceramics sintered at 1500 ◦ C exhibited higher amounts of Zn9 Ga2 O12 phase than GZO-3Ga ceramics, as demonstrated in Fig. 5(b). 3.3. Densification versus evaporation
3.2. Elemental distribution and crystal structure To understand the elemental distribution in the GZO ceramics after sintering, GZO-5Ga sintered at 1500 ◦ C was examined by EPMA, as shown in Fig. 4. The Ga and Zn mapping clearly indicated that Ga and Zn atoms were homogeneous in the ZnO matrix. However, the precipitates were richer in Ga and more
Based on quantitative metallography of the microstructures sintered at 1500 ◦ C, the percentages of Zn9 Ga2 O12 phase in the GZO-3Ga and GZO-5Ga ceramics were 2 and 7 vol%, respectively. The theoretical densities of ZnO and Zn9 Ga2 O12 are 5.61 g/cm3 (JCPDS 79-0208) and 5.69 g/cm3 (JCPDS 500448), respectively. Thus, in this study, the theoretical densities
Fig. 4. The elemental mappings of GZO-5 ceramic sintered at 1500 ◦ C (a) microstructure, (b) Ga mapping, and (c) Zn mapping.
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Fig. 5. The XRD results of (a) GZO-3Ga ceramics sintered at 800, 1000, and 1200 ◦ C, (b) two GZO ceramics sintered at 1500 ◦ C.
of GZO-3Ga and GZO-5Ga were assumed to be 5.61 and 5.62 g/cm3 , respectively. Fig. 6 shows the sintered densities of two GZO ceramics after sintering between 1200 and 1500 ◦ C for 3 h. The sintered densities of GZO-3Ga and GZO-5Ga ceramics sintered at 1200 ◦ C were similar, only about 85.7% and 83.6%, respectively. However, after sintering at 1300 ◦ C, the densification of GZO-3Ga was obviously activated and the sintered density increased to 98.8%. When the sintering temperature was increased further, the sintered density of GZO-3Ga peaked at 99.3% at 1400 ◦ C and then declined to 96.8% at 1500 ◦ C. Moreover, the densification behavior of GZO-5Ga was different from that of GZO-3Ga, as demonstrated in Fig. 6. The sintered densities of GZO-5Ga ceramics were gradually improved by increasing the sintering temperature from 1200 to 1500 ◦ C, peaking at a density of 94.3% after sintering at 1500 ◦ C. These results on the sintered densities were consistent with those on the microstructures, as can be seen from comparing Figs. 1, 2, and 6. Due to the high vapor pressure of ZnO, evaporation and weight loss occur easily during high-temperature sintering
of ZnO-based ceramics.23,27,32 To clarify the evaporation phenomena of the two GZO ceramics, the weight losses as a function of sintering temperature were plotted, as shown in Fig. 7. The results indicated that the weight losses of GZO-3Ga were higher than those of GZO-5Ga, implying that the addition of Ga2 O3 inhibited evaporation. The difference between the weight losses of the two GZO ceramics was about 0.3–0.5 wt% after sintering at 1200, 1300, and 1400 ◦ C. However, when the sintering temperature was increased to 1500 ◦ C, the difference between the weight losses increased to 1.2 wt%. TGA was also used to further identify the difference in the evaporation behaviors of the two GZO ceramics. Fig. 8 shows the TGA curves of two GZO ceramics heated from 100 to 1500 ◦ C and then held at 1500 ◦ C for 1 h. As indicated by the curves, the debinding of organic additives, including the dispersant and the binder, was completed at 400 ◦ C. It should be noted that the weight losses of GZO-3Ga were slightly higher than those of GZO-5Ga between 400 and 1500 ◦ C, consistent with the finding in Fig. 7. After sintering at 1500 ◦ C for 1 h, the
Fig. 6. The relative densities of two GZO ceramics as a function of sintering temperature.
Fig. 7. The weight losses of two GZO ceramics as a function of sintering temperature.
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Fig. 8. The TGA curves of two GZO ceramics heated from 100 to then held at 1500 ◦ C for 1 h.
1500 ◦ C
and
difference between the weight losses of the two GZO ceramics was increased. Thus, the addition of Ga2 O3 reduced the evaporation of ZnO during sintering. Zhang et al.26 and Liu et al.32 also found that the addition of 0.2 wt% Ga and 0.6 at% Ga, respectively, in AZO ceramics can reduce the loss of mass during sintering. The sintered densities of ZnO-based ceramics are controlled by both the densification and evaporation. Evaporation can lead to the formation of pores and thus inhibit the densification of ZnO-based ceramics, including GZO ceramics. Figs. 7 and 8 clearly demonstrate that the weight losses of GZO-3Ga sintered at various temperatures were higher than those of GZO-5Ga. However, the sintered densities of GZO-3Ga were always superior to those of GZO-5Ga, irrespective of the sintering temperature. This contradiction can be attributed to the discrepancy in the amounts of Zn9 Ga2 O12 precipitate that formed in the two GZO ceramics during sintering. ZnO and Ga2 O3 will react to Zn9 Ga2 O12 at temperatures higher than 1000 ◦ C, as shown in Fig. 5, and this reaction could be accompanied by a change in volume and the formation of pores. Most of the Zn9 Ga2 O12 precipitates were located in the pores, as demonstrated in Figs. 1 and 2. Thus, the formation of Zn9 Ga2 O12 obviously reduced the densification and sintered density of the GZO-5Ga ceramic. Liu et al.34 also indicated that the relative densities of GZO ceramics sintered at 1300 ◦ C fell from 99.2% to 96.7% when Ga2 O3 content was increased from 1 to 3 wt%. On the other hand, the grain sizes of GZO-5Ga ceramics sintered at various temperatures were finer than those of GZO-3Ga. This phenomenon can be attributed to the dragging effect of both the Zn9 Ga2 O12 precipitates and pores. 3.4. Resistivity The resistivities of two GZO ceramics sintered at temperatures of 1300–1500 ◦ C are presented in Fig. 9. The resistivities
Fig. 9. The resistivities of two GZO ceramics as a function of sintering temperature.
of GZO-3Ga and GZO-5Ga sintered at 1200 ◦ C were higher than 108 cm and thus could not be measured by the source meter used in this study. Fig. 9 shows that the resistivities of GZO3Ga ceramics sintered at various temperatures were superior to those of GZO-5Ga. The electrical properties of GZO-3Ga were improved only slightly by increasing the sintering temperatures from 1300 to 1500 ◦ C. However, in GZO-5Ga, resistivity was significantly decreased by increasing the sintering temperature from 1300 to 1500 ◦ C. The resistivities of GZO-3Ga and GZO5Ga sintered at 1500 ◦ C were 2.1 × 10−3 and 2.6 × 10−3 cm, respectively. Fig. 9 clearly indicates that the resistivities of GZO-3Ga and GZO-5Ga were similar after sintering at 1400 or 1500 ◦ C, though the relative densities of GZO-3Ga were higher than those of GZO-5Ga. Wu et al.22 examined the electrical properties of AZO ceramic targets and showed that the Hall mobility was much improved, from 0.2 to 23.9 cm2 /Vs, when the relative density rose from 82.2% to 93.7%, indicating that interconnected pores significantly impaired the Hall mobility and the resistivity of AZO ceramic targets. Moreover, when relative density increased from 93.7% to 98.8%, the Hall mobility rose from 23.9 to only 67.3 cm2 /Vs. These findings show that after interconnected pores have been transformed into isolated ones, the increase in sintered density cannot obviously improve the Hall mobilities and the resistivities of GZO and AZO ceramic targets. Table 1 further compares the processes and characteristics of GZO ceramic targets with those of AZO targets, including the sintering method, sintering parameters, relative density, grain size, and resistivity.14,15,22,24,26,28,29,31,33–36,38 The properties of the two GZO ceramics investigated in this study are also included for comparison. As can be seen in Table 1, the relative density and the resistivity of GZO-3Ga sintered at 1500 ◦ C were similar to those of GZO ceramics reported in the literature.34–36,38 Furthermore, the relative densities of AZO ceramic targets sintered in air can be higher than 98%. In general, the grain sizes and resistivities of AZO ceramic targets sintered in air range from 2 to 5 m and from 1 × 10−3 to 4 × 10−3 cm, respectively.
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Table 1 Comparison of the processes and various properties of GZO and AZO ceramic targets.14,15,22,24,26,28,29,31,33–36,38 Material
Sintering method
Sintering temp./time (◦ C/h)
GZO [this study] (3 wt% Ga2 O3 ) GZO [this study] (5 wt% Ga2 O3 ) GZO34 (3 wt% Ga2 O3 ) GZO34 (1 wt% Ga2 O3 ) GZO35 (1 at% Ga) GZO36 (0.5 at% Ga) GZO38 (0.5 mol% Ga) GZO14 (5.45 at% Ga) AZO15 AZO22 AZO24 AZO26 AZO28 AZO29 AZO31 AZO33 AZO14
CS in air
1400 ◦ C/3 h
CS in air
1500 ◦ C/3 h
CS in air
1300 ◦ C/–
CS in air
1300 ◦ C/–
CS in air
1400 ◦ C/10 h
SPS
1000 ◦ C/5 min
CS in air
1300 ◦ C/10 h
–
–
>97.1
CS in air CS in air CS in air CS in air CS in air CS in air CS in air HP –
1400 ◦ C/4 h 1500 ◦ C/1 h 1500 ◦ C/3 h 1450 ◦ C/10 h 1300 ◦ C/8 h 1300 ◦ C/2 h 1300 ◦ C/8 h 1100 ◦ C/10 h –
98.6 98.8 99.8 99.46 99.3 99.9 98.9 99.0 >97.1.
Relative density (%)
Grain size (m)
Resistivity ( cm)
Note
99.3
3.3
2.8 × 10−3
–
94.3
3.1
2.6 × 10−3
–
–
–
–
99.2
–
–
–
96
–
∼1.4 × 10−3
–
∼95
–
∼1 × 10−3
–
–
1
–
4.58 × 10−4
CA
8.85 × 10−4 1.65 × 10−3 0.5 2.14 × 10−3 1.15 × 10−3 3.86 × 10−3 2.1 × 10−3 3.0 × 10−3 4.58 × 10−4 ∼4.81 × 10−3
– – – – – NP NP – CA
∼97
∼2.7 4.4 – – – 2.6 5.7 – –
Note: CS, conventional sintering; SPS, spark plasma sintering; HP, hot pressing; CA, commercially available; NP, nano powder.
Minami et al.14 used AZO and GZO targets with resistivities as low as 4.58 × 10−4 cm for sputtering and achieved uniform films having low resistivity. Unfortunately, the sputtering targets used by Minami et al. were commercially available, so the detailed process history cannot be found. In this study, the relative density, grain size, and resistivity of GZO-3Ga ceramics sintered in air were close to those of AZO, as shown in Table 1. This comparison indicates that the various properties of GZO ceramics investigated in this study were comparable to those of AZO ceramics reported in the literature. 4. Conclusions To clarify the sintering behavior, microstructure, and electrical properties of GZO ceramic targets, GZO with 3 and 5 wt% Ga2 O3 were produced. The effects of Ga2 O3 content and sintering temperature on the sintered density, grain size, weight loss, crystal structure of the precipitate, and resistivity of GZO ceramic were investigated. The findings can be summarized as follows. 1. The increase in Ga2 O3 content from 3 and 5 wt% in GZO ceramics not only inhibits densification but also retards grain growth. The highest densities of GZO-3Ga and GZO-5Ga ceramics achieved were 99.3% and 94.3%, respectively. 2. After sintering at 800 ◦ C, ZnGa2 O4 can be observed in GZO ceramics. When the sintering temperature is increased to
1000 ◦ C, minor Zn9 Ga2 O12 can also be found. After sintering at 1200 ◦ C, more Zn9 Ga2 O12 precipitates are formed at the expense of ZnGa2 O4 and ZnGa2 O4 disappearing completely. The final microstructure of GZO ceramics is composed of ZnO and Zn9 Ga2 O12 . 3. The sintered densities of GZO-3Ga are always superior to those of GZO-5Ga, irrespective of the sintering temperature, though the weight losses of GZO-3Ga are higher than those of GZO-5Ga. This contradiction can be attributed to the discrepancy in the amount of Zn9 Ga2 O12 precipitate that forms between these two GZO ceramics during sintering. 4. The resistivities of GZO-3Ga and GZO-5Ga sintered at 1500 ◦ C were 2.1 × 10−3 and 2.6 × 10−3 cm, respectively. When interconnected pores were transformed into isolated ones, the increase in sintered density did not obviously improve the resistivity of GZO ceramic. 5. In this study, the best properties were exhibited by GZO-3Ga sintered at 1400 ◦ C in air. The relative density, grain size, and resistivity were 99.3%, 3.3 m, and 2.8 × 10−3 cm, respectively. These properties of GZO ceramics are comparable to those of AZO reported in the literature. Acknowledgement The authors thank the National Science Council of the Republic of China for their support of this project under contract number NSC 99-2218-E-150-046.
Please cite this article in press as: Wu M-W, et al. The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.022
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ARTICLE IN PRESS M.-W. Wu et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
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Please cite this article in press as: Wu M-W, et al. The sintering behavior, microstructure, and electrical properties of gallium-doped zinc oxide ceramic targets. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.05.022