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Structural variation of thin films deposited from Zn3In2O6 target by RF-sputtering
Pergamon
Materials Research Bulletin 36 (2001) 1075–1082
Structural variation of thin films deposited from Zn3In2O6 target by RF-sputtering T. Ushiroa, D. Tsujia, A. Fukushimaa, T. Morigaa, I. Nakabayashia,*, K. Murayamab, K. Tominagab a
Department of Chemical Science and Technology, Faculty of Engineering, Tokushima University, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan b Department of Electric and Electronic Engineering, Faculty of Engineering, Tokushima University, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan (Refereed) Received 11 September 2000; accepted 15 November 2000
Abstract We investigated chemical composition, crystal structure and electrical and optical properties of rf-sputtered thin films deposited from Zn3In2O6 polycrystalline target at different substrate temperatures. Continuous structural variation from Zn3In2O6 to In2O3 through Zn2In2O5 was observed on the deposited films with increasing the substrate temperature. ICP analysis of the films showed the ratio of zinc to indium decreased with increasing the temperature, in a good accordance with the structural variation. Conductivity of the film gradually increased in the temperature range where Zn3In2O6 changed to Zn2In2O5 but abruptly decreased in the range where In2O3 phase appeared. Optical band gap increased when the film was mainly composed of In2O3. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; B. Sputtering; C. X-ray diffraction; D. Semiconductivity
1. Introduction Tin-doped indium oxide (ITO) is a representative material of the transparent and conductive oxides (TCOs). ITO films have both high electrical conductivity and high optical
* Corresponding author. Tel.: ⫹81-88-656-7422; fax: ⫹81-88-655-7025. E-mail address: [email protected]. (I. Nakabayashi). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 5 8 3 - 9
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transparency in the visible region, so that they are widely used as transparent electrodes for flat panel displays or solar cells. However indium, which is the main component of ITO film, is so rare and so expensive that alternative materials with higher conductivity and better transparency than ITO are desired [1–7]. So we focused on pseudobinary oxides of zincindium-oxide system. We clarified that nine homologous compounds ZnkIn2Ok⫹3 (k ⫽ 3, 4, 5, 6, 7, 9, 11, 13 and 15) exist for the bulk system and that conductivity increased as k-number decreased [8]. Although Zn2In2O5 has been reported to be formed at ⬎1550°C [9], we could not prepare Zn2In2O5. Minami et al. reported as high as 2900 (⍀cm)⫺1 for Zn2In2O5 films sputtered from polycrystalline targets containing 10 – 60 at.% zinc [10]. In this article, we report characterization of crystal structure and electrical and optical properties of sputtered films deposited from Zn3In2O6 polycrystalline target, which possesses the highest conductivity among the bulk homologous compounds, at various substrate temperatures.
2. Experimental 2.1. Target preparation The polycrystalline target of Zn3In2O6 was synthesized from In2O3 (purity 99.9%, Kanto Chemical Co.) and ZnO (purity 99%) powders by solid state reaction. These starting powders were weighed amounts of the stoichiometric proportion (In:Zn ⫽ 2:3 on cation basis) and mixed in an alumina mortar. The mixed powder was calcined at 900°C for 12 h in air and then reground. Pellets pressed from this powder were heated at 1400°C for 24 h in air then cooled down to room temperature in a furnace. Each pellet was surrounded by powder of the same composition to inhibit vaporization of ZnO during firing. The resultant powder as reground again and filled tightly into the target holder with a diameter of 70 mm. 2.2. Film deposition The Zn3In2O6 films were prepared by a conventional rf-magnetron sputtering method. A corning #7059 glass substrate was located at the distance of 40 mm above the Zn3In2O6 target. After the target was presputtered for about 20min, the deposition of a film was performed with rf-power of 100 W in an Ar atmosphere at a pressure of 1.0 Pa for 2 h. Substrate temperatures were varied from room temperature (RT) to 380°C. 2.3. Characterization The chemical composition (In:Zn ratio) of the films was estimated by Inductively Coupled Plasma (ICP) analysis (Shimazu ICPS-5000). Samples were dissolved in 1N HCl solution and diluted to an appropriate concentration. The crystal structure was confirmed by X-ray diffraction technique (RIGAKU RINT-2500) using CuK␣ radiation (40 kV, 100 mA, -2 scan). Film thickness was measured using a conventional surface roughness detector (DEKTAK 3030). The electrical conductivity was measured by a four-point probe, and Hall
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Table 1 Metal ratio of ZnO-In2O3 films estimated by ICP analysis Substrate temperature (°C)
In:Zn metal ratio
320 290 260 230 RT
2.0:0.44 2.0:1.1 2.0:1.4 2.0:1.7 2.0:2.7
Fig. 1. X-ray diffraction patterns for ZnO-In2O3 films deposited at different temperatures.
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Fig. 2. Magnified X-ray diffraction patterns of Fig. 1. Peaks are indexed as (hkl).
mobility was evaluated by the van der Pauw method. The applied current and magnetic field were 6 A and 475 G, respectively, for all the Hall measurements. The optical band gap was estimated using the transmission spectrum in the wavelength range from 300 to 800 nm with a spectrophotometer (JASCO V-550).
3. Results and discussion Table 1 represents the metal ratio (In:Zn) of the films determined by ICP analysis. The ratios are normalized in such a way that In ⫽ 2. The ratio of indium and zinc was 2.0:2.7 for the film deposited at RT, it shows the film was almost stoichiometric. But the ratio of zinc
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Fig. 3. Film thickness as a function of substrate temperature.
to indium decreased with increasing the substrate temperature. The ratio of them was 2.0:0.4 for the film deposited at 320°C. This indicates that zinc would be hardly deposited at higher substrate temperature. Fig. 1 shows X-ray diffraction patterns of ZnkIn2Ok⫹3 films deposited at various substrate temperatures. All the films deposited were well crystallized and it seems that the films were composed of the homologous compounds with c-axis orientation from RT up to 290°C. But above 320°C, all the peaks could be assigned to the bixbyite-type In2O3. Fig. 2 shows magnified X-ray diffraction patterns of Fig. 1. As the substrate temperature increased, the (0012) and the (0015) peaks assigned to Zn3In2O6 at RT shifted gradually to the lower angle side, whereas the (0018) peak of the compound shifted to the higher angle side. The (0012), (0015) and (0018) of Zn3In2O6 at RT seem to approach to the (006), (008) and (0010) of Zn2 In2O5 at 290°C, respectively. At the substrate temperature between RT and 260°C, each peak lies at the intermediate position of the corresponding peaks. Some minor peaks assigned to In2O3 as well as those assigned to Zn2In2O5 could be observed at 290°C. This structural variation is attributable to reduction of zinc content, which is in a good agreement with the results of ICP analysis. Zn3In2O6 are reported to be a line compound for the bulk samples [8]. Thin-film work enabled continuous structural variation from Zn3In2O6 to Zn2In2O5 by controlling the substrate temperature. Fig. 3 shows the film thickness as a function of substrate temperature. The thickness of film prepared at substrate temperature of RT was about 2200 nm, but the film thickness prepared at 380°C was about 1500 nm. As the substrate temperature increased, the film thickness decreased. This phenomenon would be related with the fact that zinc content of the film decreased with increasing the substrate temperature. Fig. 4 shows conductivity , Hall mobility and free carrier concentration n of the films
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Fig. 4. Conductivity (F), carrier concentration (E) and Hall mobility (■) as a function of substrate temperature.
as a function of substrate temperature. All the films exhibited n-type semiconductor. Conductivity and carrier concentration gradually increased in the temperature range from RT to 260°C. The conductivity reached about 1900 (⍀cm)⫺1 for the Zn2In2O5 film deposited at 260°C. This tendency is applicable to our previous speculation based on the bulk studies [8]. However, no sooner had In2O3 phase appeared in the film than conductivity and carrier concentration decreased. The films deposited at 320°C and 380°C showed the similar XRD pattern as the In2O3 film. As for electrical properties, free carrier concentrations agreed well between our films and the zinc-free bixbyite-type In2O3 film reported in the literature [3], though Hall mobilities showed about one-third as low as that in the literature. Conductivity, Hall mobility and free carrier concentration remained unchanged in the region where the bixbyite-type In2O3 phase appeared, regardless the difference of zinc content in the film. These trends are in good agreement with the results reported in our previous paper [11].
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Fig. 5. Optical transmission spectra for ZnO-In2O3 films deposited at different temperatures.
Comparing this film work with our previous bulk studies [8], all the Zn3In2O6-Zn2In2O5 films are superior to the bulk samples insofar as conductivity is concerned. Hall measurements indicate that this is due to a high carrier concentration. The carrier concentration in the film samples is one or two orders of magnitude higher than that in the bulk samples. As the sputtering gas used was pure-Ar, the films would have a lot of oxygen vacancies, which produce electron donors. Fig. 5 shows optical transmission spectra for films deposited at different temperature. Transmittance in visible region decreased gradually though film thickness decreased up to 290°C, and then turned to increase remarkably. The decrease in transmittance results from the increase of the carrier concentration. Optical band gap observed for the films deposited below 290°C was estimated to be 3.2 eV, whereas that for the films above 320°C was to be 3.4 eV. The films deposited below 290°C were mainly composed of solid solution between Zn3In2O6 and Zn2In2O5 and those deposited above 320°C were composed of In2O3. Band gap of In2O3 is reported to be 3.75 eV [3], but observed band gap of the films deposited at 320°C and 380°C is smaller than this value. On the other hand, band gap of ZnO is reported to 3.28 eV. Though the film deposited at 320°C showed the bixbyite-type In2O3 phase, the band gap lay the intermediate value between In2O3 and ZnO. Existence of zinc in the film would reduce the band gap. To be concluded these results suggest that the substrate temperature strongly affects the phase deposited, resulting in the variations of electrical and optical properties.
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4. Conclusion The ZnO-In2O3 system films have been prepared by rf-magnetron sputtering. The films deposited at different substrate temperatures from RT to 380°C using Zn3In2O6 polycrystalline target. We investigated them and have got conclusions as follows. 1. As the substrate temperature increases, the ratio of zinc to indium in the film decreases. In accordance with this reduction of zinc content, the phase observed varied from Zn3In2O6 to In2O3 through Zn2In2O5 continuously. 2. Variation of electrical and optical properties corresponds to the variation of phases appeared in the film. 3. The highest conductivity of 1900 (⍀cm)⫺1 was obtained the film deposited at 260°C. The film was composed of Zn2In2O5. References [1] R. Wang, L.H. King, A.W. Sleight, J. Mater. Res. 11 (1996) 1659. [2] N. Ueda, T. Omata, N. Hikuma, K. Ueda, H. Mizoguchi, T. Hashimoto, H. Kawazoe, Appl. Phys. Lett. 61 (1992) 1954. [3] T. Minami, S. Tanaka, T. Kakumu, S. Takata, Jpn. J. Appl. Phys. 34 (1995) 971. [4] R.J. Cave, J.M. Phillips, L. Kwo, G.A. Tomas, R.B. van Dover, S.A. Carter, J.J. Krajewski, W.F. Peck, Jr., J.H. Marshall, D.H. Rapkine, Appl. Phys. Lett. 64 (1994) 2071. [5] T. Minami, Y. Takeda, T. Kakumu, S. Takata, I. Fukuda, J. Vac. Sci. Technol. A(15) 3 (1997) 958. [6] T. Minami, Y. Takeda, S. Takata, T. Kakumu, Thin Solid Films 308 (1997) 13. [7] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939. [8] T. Moriga, D.D. Edwards, T.O. Mason, G.B. Palmer, K.R. Poeppelmeier, J.L. Schindler, C.R. Kannewurt, I. Nakabayashi, J. Am. Ceram. Soc. 81 (5) (1998) 1310. [9] V.H. Kasper, Z. Anorg. Allg. Chem. 349 (3– 4) (1967) 113. [10] T. Minami, T. Kakumu, S. Tanaka, J. Vac. Sci. Technol. A14 (3) (1996) 1704. [11] T. Moriga, T. Okamoto, K. Hiruta, A. Fujiwara, K. Tominaga, I. Nakabayashi, J Solid State Chem. 155 (2) (2000) 312.
Materials Research Bulletin 36 (2001) 1075–1082
Structural variation of thin films deposited from Zn3In2O6 target by RF-sputtering T. Ushiroa, D. Tsujia, A. Fukushimaa, T. Morigaa, I. Nakabayashia,*, K. Murayamab, K. Tominagab a
Department of Chemical Science and Technology, Faculty of Engineering, Tokushima University, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan b Department of Electric and Electronic Engineering, Faculty of Engineering, Tokushima University, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan (Refereed) Received 11 September 2000; accepted 15 November 2000
Abstract We investigated chemical composition, crystal structure and electrical and optical properties of rf-sputtered thin films deposited from Zn3In2O6 polycrystalline target at different substrate temperatures. Continuous structural variation from Zn3In2O6 to In2O3 through Zn2In2O5 was observed on the deposited films with increasing the substrate temperature. ICP analysis of the films showed the ratio of zinc to indium decreased with increasing the temperature, in a good accordance with the structural variation. Conductivity of the film gradually increased in the temperature range where Zn3In2O6 changed to Zn2In2O5 but abruptly decreased in the range where In2O3 phase appeared. Optical band gap increased when the film was mainly composed of In2O3. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; B. Sputtering; C. X-ray diffraction; D. Semiconductivity
1. Introduction Tin-doped indium oxide (ITO) is a representative material of the transparent and conductive oxides (TCOs). ITO films have both high electrical conductivity and high optical
* Corresponding author. Tel.: ⫹81-88-656-7422; fax: ⫹81-88-655-7025. E-mail address: [email protected]. (I. Nakabayashi). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 5 8 3 - 9
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transparency in the visible region, so that they are widely used as transparent electrodes for flat panel displays or solar cells. However indium, which is the main component of ITO film, is so rare and so expensive that alternative materials with higher conductivity and better transparency than ITO are desired [1–7]. So we focused on pseudobinary oxides of zincindium-oxide system. We clarified that nine homologous compounds ZnkIn2Ok⫹3 (k ⫽ 3, 4, 5, 6, 7, 9, 11, 13 and 15) exist for the bulk system and that conductivity increased as k-number decreased [8]. Although Zn2In2O5 has been reported to be formed at ⬎1550°C [9], we could not prepare Zn2In2O5. Minami et al. reported as high as 2900 (⍀cm)⫺1 for Zn2In2O5 films sputtered from polycrystalline targets containing 10 – 60 at.% zinc [10]. In this article, we report characterization of crystal structure and electrical and optical properties of sputtered films deposited from Zn3In2O6 polycrystalline target, which possesses the highest conductivity among the bulk homologous compounds, at various substrate temperatures.
2. Experimental 2.1. Target preparation The polycrystalline target of Zn3In2O6 was synthesized from In2O3 (purity 99.9%, Kanto Chemical Co.) and ZnO (purity 99%) powders by solid state reaction. These starting powders were weighed amounts of the stoichiometric proportion (In:Zn ⫽ 2:3 on cation basis) and mixed in an alumina mortar. The mixed powder was calcined at 900°C for 12 h in air and then reground. Pellets pressed from this powder were heated at 1400°C for 24 h in air then cooled down to room temperature in a furnace. Each pellet was surrounded by powder of the same composition to inhibit vaporization of ZnO during firing. The resultant powder as reground again and filled tightly into the target holder with a diameter of 70 mm. 2.2. Film deposition The Zn3In2O6 films were prepared by a conventional rf-magnetron sputtering method. A corning #7059 glass substrate was located at the distance of 40 mm above the Zn3In2O6 target. After the target was presputtered for about 20min, the deposition of a film was performed with rf-power of 100 W in an Ar atmosphere at a pressure of 1.0 Pa for 2 h. Substrate temperatures were varied from room temperature (RT) to 380°C. 2.3. Characterization The chemical composition (In:Zn ratio) of the films was estimated by Inductively Coupled Plasma (ICP) analysis (Shimazu ICPS-5000). Samples were dissolved in 1N HCl solution and diluted to an appropriate concentration. The crystal structure was confirmed by X-ray diffraction technique (RIGAKU RINT-2500) using CuK␣ radiation (40 kV, 100 mA, -2 scan). Film thickness was measured using a conventional surface roughness detector (DEKTAK 3030). The electrical conductivity was measured by a four-point probe, and Hall
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Table 1 Metal ratio of ZnO-In2O3 films estimated by ICP analysis Substrate temperature (°C)
In:Zn metal ratio
320 290 260 230 RT
2.0:0.44 2.0:1.1 2.0:1.4 2.0:1.7 2.0:2.7
Fig. 1. X-ray diffraction patterns for ZnO-In2O3 films deposited at different temperatures.
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Fig. 2. Magnified X-ray diffraction patterns of Fig. 1. Peaks are indexed as (hkl).
mobility was evaluated by the van der Pauw method. The applied current and magnetic field were 6 A and 475 G, respectively, for all the Hall measurements. The optical band gap was estimated using the transmission spectrum in the wavelength range from 300 to 800 nm with a spectrophotometer (JASCO V-550).
3. Results and discussion Table 1 represents the metal ratio (In:Zn) of the films determined by ICP analysis. The ratios are normalized in such a way that In ⫽ 2. The ratio of indium and zinc was 2.0:2.7 for the film deposited at RT, it shows the film was almost stoichiometric. But the ratio of zinc
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Fig. 3. Film thickness as a function of substrate temperature.
to indium decreased with increasing the substrate temperature. The ratio of them was 2.0:0.4 for the film deposited at 320°C. This indicates that zinc would be hardly deposited at higher substrate temperature. Fig. 1 shows X-ray diffraction patterns of ZnkIn2Ok⫹3 films deposited at various substrate temperatures. All the films deposited were well crystallized and it seems that the films were composed of the homologous compounds with c-axis orientation from RT up to 290°C. But above 320°C, all the peaks could be assigned to the bixbyite-type In2O3. Fig. 2 shows magnified X-ray diffraction patterns of Fig. 1. As the substrate temperature increased, the (0012) and the (0015) peaks assigned to Zn3In2O6 at RT shifted gradually to the lower angle side, whereas the (0018) peak of the compound shifted to the higher angle side. The (0012), (0015) and (0018) of Zn3In2O6 at RT seem to approach to the (006), (008) and (0010) of Zn2 In2O5 at 290°C, respectively. At the substrate temperature between RT and 260°C, each peak lies at the intermediate position of the corresponding peaks. Some minor peaks assigned to In2O3 as well as those assigned to Zn2In2O5 could be observed at 290°C. This structural variation is attributable to reduction of zinc content, which is in a good agreement with the results of ICP analysis. Zn3In2O6 are reported to be a line compound for the bulk samples [8]. Thin-film work enabled continuous structural variation from Zn3In2O6 to Zn2In2O5 by controlling the substrate temperature. Fig. 3 shows the film thickness as a function of substrate temperature. The thickness of film prepared at substrate temperature of RT was about 2200 nm, but the film thickness prepared at 380°C was about 1500 nm. As the substrate temperature increased, the film thickness decreased. This phenomenon would be related with the fact that zinc content of the film decreased with increasing the substrate temperature. Fig. 4 shows conductivity , Hall mobility and free carrier concentration n of the films
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Fig. 4. Conductivity (F), carrier concentration (E) and Hall mobility (■) as a function of substrate temperature.
as a function of substrate temperature. All the films exhibited n-type semiconductor. Conductivity and carrier concentration gradually increased in the temperature range from RT to 260°C. The conductivity reached about 1900 (⍀cm)⫺1 for the Zn2In2O5 film deposited at 260°C. This tendency is applicable to our previous speculation based on the bulk studies [8]. However, no sooner had In2O3 phase appeared in the film than conductivity and carrier concentration decreased. The films deposited at 320°C and 380°C showed the similar XRD pattern as the In2O3 film. As for electrical properties, free carrier concentrations agreed well between our films and the zinc-free bixbyite-type In2O3 film reported in the literature [3], though Hall mobilities showed about one-third as low as that in the literature. Conductivity, Hall mobility and free carrier concentration remained unchanged in the region where the bixbyite-type In2O3 phase appeared, regardless the difference of zinc content in the film. These trends are in good agreement with the results reported in our previous paper [11].
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Fig. 5. Optical transmission spectra for ZnO-In2O3 films deposited at different temperatures.
Comparing this film work with our previous bulk studies [8], all the Zn3In2O6-Zn2In2O5 films are superior to the bulk samples insofar as conductivity is concerned. Hall measurements indicate that this is due to a high carrier concentration. The carrier concentration in the film samples is one or two orders of magnitude higher than that in the bulk samples. As the sputtering gas used was pure-Ar, the films would have a lot of oxygen vacancies, which produce electron donors. Fig. 5 shows optical transmission spectra for films deposited at different temperature. Transmittance in visible region decreased gradually though film thickness decreased up to 290°C, and then turned to increase remarkably. The decrease in transmittance results from the increase of the carrier concentration. Optical band gap observed for the films deposited below 290°C was estimated to be 3.2 eV, whereas that for the films above 320°C was to be 3.4 eV. The films deposited below 290°C were mainly composed of solid solution between Zn3In2O6 and Zn2In2O5 and those deposited above 320°C were composed of In2O3. Band gap of In2O3 is reported to be 3.75 eV [3], but observed band gap of the films deposited at 320°C and 380°C is smaller than this value. On the other hand, band gap of ZnO is reported to 3.28 eV. Though the film deposited at 320°C showed the bixbyite-type In2O3 phase, the band gap lay the intermediate value between In2O3 and ZnO. Existence of zinc in the film would reduce the band gap. To be concluded these results suggest that the substrate temperature strongly affects the phase deposited, resulting in the variations of electrical and optical properties.
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4. Conclusion The ZnO-In2O3 system films have been prepared by rf-magnetron sputtering. The films deposited at different substrate temperatures from RT to 380°C using Zn3In2O6 polycrystalline target. We investigated them and have got conclusions as follows. 1. As the substrate temperature increases, the ratio of zinc to indium in the film decreases. In accordance with this reduction of zinc content, the phase observed varied from Zn3In2O6 to In2O3 through Zn2In2O5 continuously. 2. Variation of electrical and optical properties corresponds to the variation of phases appeared in the film. 3. The highest conductivity of 1900 (⍀cm)⫺1 was obtained the film deposited at 260°C. The film was composed of Zn2In2O5. References [1] R. Wang, L.H. King, A.W. Sleight, J. Mater. Res. 11 (1996) 1659. [2] N. Ueda, T. Omata, N. Hikuma, K. Ueda, H. Mizoguchi, T. Hashimoto, H. Kawazoe, Appl. Phys. Lett. 61 (1992) 1954. [3] T. Minami, S. Tanaka, T. Kakumu, S. Takata, Jpn. J. Appl. Phys. 34 (1995) 971. [4] R.J. Cave, J.M. Phillips, L. Kwo, G.A. Tomas, R.B. van Dover, S.A. Carter, J.J. Krajewski, W.F. Peck, Jr., J.H. Marshall, D.H. Rapkine, Appl. Phys. Lett. 64 (1994) 2071. [5] T. Minami, Y. Takeda, T. Kakumu, S. Takata, I. Fukuda, J. Vac. Sci. Technol. A(15) 3 (1997) 958. [6] T. Minami, Y. Takeda, S. Takata, T. Kakumu, Thin Solid Films 308 (1997) 13. [7] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939. [8] T. Moriga, D.D. Edwards, T.O. Mason, G.B. Palmer, K.R. Poeppelmeier, J.L. Schindler, C.R. Kannewurt, I. Nakabayashi, J. Am. Ceram. Soc. 81 (5) (1998) 1310. [9] V.H. Kasper, Z. Anorg. Allg. Chem. 349 (3– 4) (1967) 113. [10] T. Minami, T. Kakumu, S. Tanaka, J. Vac. Sci. Technol. A14 (3) (1996) 1704. [11] T. Moriga, T. Okamoto, K. Hiruta, A. Fujiwara, K. Tominaga, I. Nakabayashi, J Solid State Chem. 155 (2) (2000) 312.