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Growth of the Bi2O3 thin films under atmospheric pressure by means of halide CVD
Journal of Physics and Chemistry of Solids 65 (2004) 1349–1352 www.elsevier.com/locate/jpcs
Growth of the Bi2O3 thin films under atmospheric pressure by means of halide CVD T. Takeyamaa,*, N. Takahashib, T. Nakamurab, S. Itoc b
a Graduate School of Science and Technology, Shizuoka University, 3-5-2 Johoku, Hamamatsu 432-8561, Japan Department of Materials Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan c Research Center, Asahi Glass Co., Ltd, 1150 Hazawa-cho, Yokohama 221-8755, Japan
Received 2 September 2003; revised 15 March 2004; accepted 26 March 2004 Available online 10 May 2004
Abstract Films of Bi2O3 were grown on glass substrate under atmospheric pressure by means of halide chemical vapour deposition (AP-HCVD) using BiI3 and O2 as the starting materials. In the XRD diffractogram of the film a strong diffraction peak appears at 27.918 assigned to the (111) diffraction of the d-Bi2O3 with cubic structure. X-ray pole figure suggested that the k111l direction of the film is perpendicular to the substrate surface, while the k110l axis directs towards all directions parallel to the substrate surface. It is for the first that d-Bi2O3 film was prepared on glass substrate. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Thin films
1. Introduction Bismuth oxide (Bi2O3) thin films are interesting materials with significantly wide band gap, high refractive index and dielectric permittivity [1]. Therefore, Bi2O3 has been used for optical coatings, Schottky barrier solar cell and metal/insulator/semiconductor (MIS) capacitors [2 – 4]. Also, there are a lot of reports regarding their preparation methods, structural characteristics and optical properties. As for the thin films of polycrystalline bismuth oxide, Agasiev et al. prepared by a thermal evaporation method, followed by annealing in air [5]. George et al., Misho et al., G. Bandoli et al. and Switzer, et al. prepared them with activated reactive evaporation [6], chemical spray pyrolysis [7], metal organic chemical vapour deposition [8] and electrodeposition [9], respectively. However, few studies on preparation of Bi2O3 film with single crystalline form have been reported so far, as far as we aware. This is the motivation of this research to develop a new method to prepare single crystal bismuth oxide thin films. * Corresponding author. Address: Department of Materials Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan. Tel./fax: þ81-53-478-1197. E-mail address: [email protected] (T. Takeyama). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.03.006
As for the preparation of metal oxide thin films, a simple CVD method, which is called as ‘an atmospheric pressure halide chemical vapor deposition’ (AP-HCVD), is applicable, and we have already succeeded in the epitaxially growth of high quality ZnO films on sapphire substrate and ZrO2 nanofilms on silicon substrate using the AP-HCVD [10 –12]. In this paper, we reported the results of the application of the AP-HCVD to the preparation of Bi2O3 onto borosilicate glass substrate.
2. Experimental Films of Bi2O3 were grown on a glass substrate in a vertical quartz reactor under atmospheric pressure by means of the AP-HCVD. The set-up used in the present study is illustrated in Fig. 1. The temperature of the furnace and heaters were monitored by Pt – Rh thermocouples and regulated by solid state controllers. The source materials used were BiI3 of 99.9% purity, and O2 of 99.995% purity. The glass substrate was degreased by successive cleaning in acetone and deionized water, followed by etching with a mixed solution of H3PO4 –H2SO4 (1:3) at 433 K for 10 min before being dried in a stream of dry N2. Afterwards, the substrate was placed on a quartz susceptor in the reactor.
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Fig. 1. Schematic diagram of AP-HCVD and temperature profile used in this work. Table 1 Typical Growth Conditions Substrate
Borosilicate glass
Bil3 source temperature Bil3 partial pressure O2 partial pressure Carrier gas Total flow rate Growth temperature
483 K 8.0 Pa 3.0 £ 104 Pa N2 2.67 £ 1025 m3 s21 1073 K
BiI3 was evaporated from a source boat at a temperature of 483 K, and supplied to the growth zone. Purified N2 was used as a carrier gas. Input partial pressures of BiI3 and O2 were varied by changing the flow rate of the carrier gas. Typical experimental conditions are summarized in Table 1. The deposited films were examined by high-resolution X-ray diffraction measurements (HR-XRD, ATX-G, Rigaku Co.). High resolution X-ray diffraction measurements (HRXRD) using a CuKa1 of 0.15428 nm at 50 kV/300 mA were carried out using a Rigaku ATX-G Diffractometer in order to examine the crystal structure and the growth direction of the deposited films. Measurements of the out-of-plane XRD pattern (separate scan of 2u=v and v in the horizontal plane) and out-of-plane rocking curve ð2u fixed v scan) were carried out as well using the same diffractomenter. The film thickness and surface morphology were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) using a JEOL JSM 5500LV microscope and a Shimadzu SPM-9500J2 microscope, respectively.
3. Results and discussion Fig. 2 shows XRD u-2u scan patterns of the as-grown Bi2O3 film deposited on a glass substrate at temperatures of
Fig. 2. XRD u-2u scan patterns of the as-grown Bi2O3 film deposited on a glass substrate as a function of various temperatures, (a) deposited at 1073 K, (b) deposited at 873 K and (c) deposited at 723 K.
1073 (a) 873 (b) and 723 K (c). The growth rate was about 1.00 mm/h independent of the temperature. This implies that at 1073 K the deposited film consists of d-Bi2O3 with cubic structure, while at 873 K tetragonal BiOI is formed. Below 723 K the films were amorphous films. In the XRD pattern of the d-Bi2O3 film at 1073 K it is worth noting that the (111) diffraction appearing 27.918 is very strong compared with other diffraction lines (Fig. 2(a)), implying that when the d-Bi2O3 is deposited onto the substrate, the k111l direction is preferred. The lattice constant was calculated to be a ¼ 0:5539 nm utilizing the observed (111) diffraction, which is slightly larger than a reported value of 0.5525 nm for the single d-Bi2O3 crystal [13]. This may be due to the contamination of a trace amount of iodine, which is constituent of source materials. Also, a full-width at half-maximum (FWHM) value of about 0.368 is 10% smaller than those deposited by electrodeposition [9]. Therefore, it is apparent that the AP-HCVD using BiI3 was a suitable technique for Bi2O3 thin film. Fig. 3 shows the X-ray pole figure of the as-deposited film at 1073 K. The data was obtained by setting the v and 2u values corresponding to cubic (111) and (220) Bragg diffraction, respectively. In Fig. 3(a), there are a spot at c ¼ 908 arising from the (111) diffraction and a circle at c ¼ 19:58 surrounding the spot due to those equivalent to
T. Takeyama et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1349–1352
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Fig. 5. Surface SEM micrograph of a Bi2O3 film grown from the reactions of BiI3 and O2 for 30 min. at 1073 K.
Fig. 3. X-ray pole figure of Bi2O3 film at 1073 K on a glass substrate. (a) (111) and (b) (220).
k111l diffraction. This implies that the k111l direction is preferably grown during the deposition. In the Fig. 3(b), the circles are observed around c ¼ 55 and 08 arising from the k220l diffraction. This means that there is no specific
Fig. 4. AFM image of a Bi2O3 film deposited at 1073 K on a glass substrate.
direction for the k110l axis of d-Bi2O3. Namely, the k110l axis is directed towards all the direction perpendicular to the k111l axis. Taking into account of a series of X-ray diffraction (outof-plane XRD, 2u fixed v scan and X-ray pole figure) examined, the thin film deposited at 1073 K consists of a single phase of d-Bi2O3 with cubic structure. There is no contamination of monoclinic a-Bi2O3 because no a-Bi2O3 peak fitted to the diffraction profile of JCPDS (41-1449) was detected. This is also supported by the fact that if the deposited thin film consists of b-Bi2O3, the pole figure fixed at the (220) diffraction would not show circles at around c ¼ 55 and 08 (Fig. 3(b)). For these reasons it is safely concluded that the film grown at 1073 K hardly contains tetragonal b-Bi2O3. The AFM and SEM images of the d-Bi2O3 film deposited at 1073 K are shown in Figs. 4 and 5, respectively. From the AFM image the mean square roughness for a 5 £ 5 mm2 of the d-Bi2O3 film was estimated to be 2.023 nm, suggesting that the surface is flat and smooth. Also, there is no clacks found at the surface of the d-Bi2O3 film by the SEM observation. Fig. 6 shows the optical transmittance in the visibleinfrared region of d-Bi2O3 film deposited at 1073 K.
Fig. 6. Optical transmittance spectrum in the visible-infrared region of the d-Bi2O3 film deposited at 1073 K on a glass substrate.
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The observed spectrum shows almost 100% transparent in the infrared region, and there is any absorption edge in the visible region. This implies that there is an absorption edge in the UV region and it is still under investigation. Monoclinic a-Bi2O3 and tetragonal b-phase have a band gap of 2.85 and 2.58 eV at 300 K, respectively, [1]. Therefore, the thin film deposited in this study hardly contains monoclinic a-phase and tetragonal b-phase. This agrees well with the XRD investigation mentioned above.
4. Conclusions It is for the first time that in the preparation of a single phase d-Bi2O3 film is deposited on a glass substrate by the AP-HCVD using BiI3 and O2 in a hot-wall reactor. X-ray diffraction and X-ray pole figure analyses confirm that at 1073 K d-Bi2O3 film with a cubic structure, which is transparent in the infrared and visible regions, is formed. As a result, it is concluded that the AP-HCVD is an excellent method for preparing high-quality d-Bi2O3 films.
References [1] L. Leontie, M. Caraman, M. Alexe, C. Harnagea, Surface Science 480 (2002) 507 –510. [2] P.B. Clapham, Br. J. Appl. Phys. 18 (1967) 363. [3] T.A. Raju, A.A. Talwai, J. Appl. Phys. 52 (1981) 4877. [4] E.Y. Wang, K.A. Pandelisev, J. Appl. Phys. 52 (1981) 4818. [5] A.A. Agasiev, A.Kh. Zeinally, S.J. Alekperov, Ya.Yu. Guseinov, Mater. Res. Bull. 21 (1986) 765. [6] J. George, B. Pradeep, K.S. Joseph, Thin Solid Films 148 (1987) 181. [7] R.H. Misho, W.A. Murad, Z.A.R. Salmin, Solar Energy Mater. 21 (1991) 347. [8] G. Bandoli, D. Barreca, E. Brescacin, G.A. Rizzi, E. Tondello, Chem. Vap. Depos. 2 (1996) 238. [9] J.A. Switzer, M.G. Shumsky, E.W. bohannan, Science 284 (1999) 293. [10] K. Kaiya, K. Omichi, N. Takahashi, T. Nakamura, S. Okamoto, H. Yamamoto, Thin Solid Films 409 (2002) 16. [11] N. Takahashi, M. Hosogi, T. Nakamura, Y. Momose, S. Nonaka, H. Yagi, Y. Shinriki, K. Tamanuki, J. Mater. Chem. 12 (2002) 719. [12] N. Takahashi, M. Makino, T. Nakamura, H. Yamamoto, Chem. Mater. 14 (2002) 3622–3624. [13] JCPDS File NO. 27–0052 (1990).
Growth of the Bi2O3 thin films under atmospheric pressure by means of halide CVD T. Takeyamaa,*, N. Takahashib, T. Nakamurab, S. Itoc b
a Graduate School of Science and Technology, Shizuoka University, 3-5-2 Johoku, Hamamatsu 432-8561, Japan Department of Materials Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan c Research Center, Asahi Glass Co., Ltd, 1150 Hazawa-cho, Yokohama 221-8755, Japan
Received 2 September 2003; revised 15 March 2004; accepted 26 March 2004 Available online 10 May 2004
Abstract Films of Bi2O3 were grown on glass substrate under atmospheric pressure by means of halide chemical vapour deposition (AP-HCVD) using BiI3 and O2 as the starting materials. In the XRD diffractogram of the film a strong diffraction peak appears at 27.918 assigned to the (111) diffraction of the d-Bi2O3 with cubic structure. X-ray pole figure suggested that the k111l direction of the film is perpendicular to the substrate surface, while the k110l axis directs towards all directions parallel to the substrate surface. It is for the first that d-Bi2O3 film was prepared on glass substrate. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Thin films
1. Introduction Bismuth oxide (Bi2O3) thin films are interesting materials with significantly wide band gap, high refractive index and dielectric permittivity [1]. Therefore, Bi2O3 has been used for optical coatings, Schottky barrier solar cell and metal/insulator/semiconductor (MIS) capacitors [2 – 4]. Also, there are a lot of reports regarding their preparation methods, structural characteristics and optical properties. As for the thin films of polycrystalline bismuth oxide, Agasiev et al. prepared by a thermal evaporation method, followed by annealing in air [5]. George et al., Misho et al., G. Bandoli et al. and Switzer, et al. prepared them with activated reactive evaporation [6], chemical spray pyrolysis [7], metal organic chemical vapour deposition [8] and electrodeposition [9], respectively. However, few studies on preparation of Bi2O3 film with single crystalline form have been reported so far, as far as we aware. This is the motivation of this research to develop a new method to prepare single crystal bismuth oxide thin films. * Corresponding author. Address: Department of Materials Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan. Tel./fax: þ81-53-478-1197. E-mail address: [email protected] (T. Takeyama). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.03.006
As for the preparation of metal oxide thin films, a simple CVD method, which is called as ‘an atmospheric pressure halide chemical vapor deposition’ (AP-HCVD), is applicable, and we have already succeeded in the epitaxially growth of high quality ZnO films on sapphire substrate and ZrO2 nanofilms on silicon substrate using the AP-HCVD [10 –12]. In this paper, we reported the results of the application of the AP-HCVD to the preparation of Bi2O3 onto borosilicate glass substrate.
2. Experimental Films of Bi2O3 were grown on a glass substrate in a vertical quartz reactor under atmospheric pressure by means of the AP-HCVD. The set-up used in the present study is illustrated in Fig. 1. The temperature of the furnace and heaters were monitored by Pt – Rh thermocouples and regulated by solid state controllers. The source materials used were BiI3 of 99.9% purity, and O2 of 99.995% purity. The glass substrate was degreased by successive cleaning in acetone and deionized water, followed by etching with a mixed solution of H3PO4 –H2SO4 (1:3) at 433 K for 10 min before being dried in a stream of dry N2. Afterwards, the substrate was placed on a quartz susceptor in the reactor.
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T. Takeyama et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1349–1352
Fig. 1. Schematic diagram of AP-HCVD and temperature profile used in this work. Table 1 Typical Growth Conditions Substrate
Borosilicate glass
Bil3 source temperature Bil3 partial pressure O2 partial pressure Carrier gas Total flow rate Growth temperature
483 K 8.0 Pa 3.0 £ 104 Pa N2 2.67 £ 1025 m3 s21 1073 K
BiI3 was evaporated from a source boat at a temperature of 483 K, and supplied to the growth zone. Purified N2 was used as a carrier gas. Input partial pressures of BiI3 and O2 were varied by changing the flow rate of the carrier gas. Typical experimental conditions are summarized in Table 1. The deposited films were examined by high-resolution X-ray diffraction measurements (HR-XRD, ATX-G, Rigaku Co.). High resolution X-ray diffraction measurements (HRXRD) using a CuKa1 of 0.15428 nm at 50 kV/300 mA were carried out using a Rigaku ATX-G Diffractometer in order to examine the crystal structure and the growth direction of the deposited films. Measurements of the out-of-plane XRD pattern (separate scan of 2u=v and v in the horizontal plane) and out-of-plane rocking curve ð2u fixed v scan) were carried out as well using the same diffractomenter. The film thickness and surface morphology were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) using a JEOL JSM 5500LV microscope and a Shimadzu SPM-9500J2 microscope, respectively.
3. Results and discussion Fig. 2 shows XRD u-2u scan patterns of the as-grown Bi2O3 film deposited on a glass substrate at temperatures of
Fig. 2. XRD u-2u scan patterns of the as-grown Bi2O3 film deposited on a glass substrate as a function of various temperatures, (a) deposited at 1073 K, (b) deposited at 873 K and (c) deposited at 723 K.
1073 (a) 873 (b) and 723 K (c). The growth rate was about 1.00 mm/h independent of the temperature. This implies that at 1073 K the deposited film consists of d-Bi2O3 with cubic structure, while at 873 K tetragonal BiOI is formed. Below 723 K the films were amorphous films. In the XRD pattern of the d-Bi2O3 film at 1073 K it is worth noting that the (111) diffraction appearing 27.918 is very strong compared with other diffraction lines (Fig. 2(a)), implying that when the d-Bi2O3 is deposited onto the substrate, the k111l direction is preferred. The lattice constant was calculated to be a ¼ 0:5539 nm utilizing the observed (111) diffraction, which is slightly larger than a reported value of 0.5525 nm for the single d-Bi2O3 crystal [13]. This may be due to the contamination of a trace amount of iodine, which is constituent of source materials. Also, a full-width at half-maximum (FWHM) value of about 0.368 is 10% smaller than those deposited by electrodeposition [9]. Therefore, it is apparent that the AP-HCVD using BiI3 was a suitable technique for Bi2O3 thin film. Fig. 3 shows the X-ray pole figure of the as-deposited film at 1073 K. The data was obtained by setting the v and 2u values corresponding to cubic (111) and (220) Bragg diffraction, respectively. In Fig. 3(a), there are a spot at c ¼ 908 arising from the (111) diffraction and a circle at c ¼ 19:58 surrounding the spot due to those equivalent to
T. Takeyama et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1349–1352
1351
Fig. 5. Surface SEM micrograph of a Bi2O3 film grown from the reactions of BiI3 and O2 for 30 min. at 1073 K.
Fig. 3. X-ray pole figure of Bi2O3 film at 1073 K on a glass substrate. (a) (111) and (b) (220).
k111l diffraction. This implies that the k111l direction is preferably grown during the deposition. In the Fig. 3(b), the circles are observed around c ¼ 55 and 08 arising from the k220l diffraction. This means that there is no specific
Fig. 4. AFM image of a Bi2O3 film deposited at 1073 K on a glass substrate.
direction for the k110l axis of d-Bi2O3. Namely, the k110l axis is directed towards all the direction perpendicular to the k111l axis. Taking into account of a series of X-ray diffraction (outof-plane XRD, 2u fixed v scan and X-ray pole figure) examined, the thin film deposited at 1073 K consists of a single phase of d-Bi2O3 with cubic structure. There is no contamination of monoclinic a-Bi2O3 because no a-Bi2O3 peak fitted to the diffraction profile of JCPDS (41-1449) was detected. This is also supported by the fact that if the deposited thin film consists of b-Bi2O3, the pole figure fixed at the (220) diffraction would not show circles at around c ¼ 55 and 08 (Fig. 3(b)). For these reasons it is safely concluded that the film grown at 1073 K hardly contains tetragonal b-Bi2O3. The AFM and SEM images of the d-Bi2O3 film deposited at 1073 K are shown in Figs. 4 and 5, respectively. From the AFM image the mean square roughness for a 5 £ 5 mm2 of the d-Bi2O3 film was estimated to be 2.023 nm, suggesting that the surface is flat and smooth. Also, there is no clacks found at the surface of the d-Bi2O3 film by the SEM observation. Fig. 6 shows the optical transmittance in the visibleinfrared region of d-Bi2O3 film deposited at 1073 K.
Fig. 6. Optical transmittance spectrum in the visible-infrared region of the d-Bi2O3 film deposited at 1073 K on a glass substrate.
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T. Takeyama et al. / Journal of Physics and Chemistry of Solids 65 (2004) 1349–1352
The observed spectrum shows almost 100% transparent in the infrared region, and there is any absorption edge in the visible region. This implies that there is an absorption edge in the UV region and it is still under investigation. Monoclinic a-Bi2O3 and tetragonal b-phase have a band gap of 2.85 and 2.58 eV at 300 K, respectively, [1]. Therefore, the thin film deposited in this study hardly contains monoclinic a-phase and tetragonal b-phase. This agrees well with the XRD investigation mentioned above.
4. Conclusions It is for the first time that in the preparation of a single phase d-Bi2O3 film is deposited on a glass substrate by the AP-HCVD using BiI3 and O2 in a hot-wall reactor. X-ray diffraction and X-ray pole figure analyses confirm that at 1073 K d-Bi2O3 film with a cubic structure, which is transparent in the infrared and visible regions, is formed. As a result, it is concluded that the AP-HCVD is an excellent method for preparing high-quality d-Bi2O3 films.
References [1] L. Leontie, M. Caraman, M. Alexe, C. Harnagea, Surface Science 480 (2002) 507 –510. [2] P.B. Clapham, Br. J. Appl. Phys. 18 (1967) 363. [3] T.A. Raju, A.A. Talwai, J. Appl. Phys. 52 (1981) 4877. [4] E.Y. Wang, K.A. Pandelisev, J. Appl. Phys. 52 (1981) 4818. [5] A.A. Agasiev, A.Kh. Zeinally, S.J. Alekperov, Ya.Yu. Guseinov, Mater. Res. Bull. 21 (1986) 765. [6] J. George, B. Pradeep, K.S. Joseph, Thin Solid Films 148 (1987) 181. [7] R.H. Misho, W.A. Murad, Z.A.R. Salmin, Solar Energy Mater. 21 (1991) 347. [8] G. Bandoli, D. Barreca, E. Brescacin, G.A. Rizzi, E. Tondello, Chem. Vap. Depos. 2 (1996) 238. [9] J.A. Switzer, M.G. Shumsky, E.W. bohannan, Science 284 (1999) 293. [10] K. Kaiya, K. Omichi, N. Takahashi, T. Nakamura, S. Okamoto, H. Yamamoto, Thin Solid Films 409 (2002) 16. [11] N. Takahashi, M. Hosogi, T. Nakamura, Y. Momose, S. Nonaka, H. Yagi, Y. Shinriki, K. Tamanuki, J. Mater. Chem. 12 (2002) 719. [12] N. Takahashi, M. Makino, T. Nakamura, H. Yamamoto, Chem. Mater. 14 (2002) 3622–3624. [13] JCPDS File NO. 27–0052 (1990).