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Structural evolution during crystallization of β-BaB2O4 thin films fabricated by chemical solution deposition technique
January 2003
Materials Letters 57 (2003) 1056 – 1061 www.elsevier.com/locate/matlet
Structural evolution during crystallization of h-BaB2O4 thin films fabricated by chemical solution deposition technique Takeshi Kobayashi*, Ryo Ogawa, Makoto Kuwabara Department of Materials Science, Kuwabara Laboratory (Building no. 4 of Faculty of Engineering Room 405), University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan Received 15 April 2002; accepted 23 May 2002
Abstract Structural evolution during crystallization of (00l)-oriented h-BaB2O4 thin films fabricated by chemical solution deposition technique has been studied. The films were characterized by X-ray diffractometry (XRD), infrared spectroscopy (IR) and scanning electron microscopy (SEM). The results indicate that h-BaB2O4 thin films crystallize in the temperature range 500 – 550 jC, and the crystallinity, the degree of orientation and the crystallite size of the films turned out to increase in the temperature range 550 – 600 jC. The crystallinity and the degree of (00l) preferred orientation further increase in the temperature range 600 – 650 jC, while the crystallite size does not increase further. The crystallization behavior of h-BaB2O4 thin films is discussed in relation to the structural evolution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: h-BaB2O4; Thin films; Crystal growth; Grain boundaries; Microstructure; Chemical solution deposition; Orientation
1. Introduction Beta barium borate (h-BaB2O4, referred to as hBBO) is a nonlinear-optical material developed by Chen et al. [1]. Single crystal of h-BBO is applied in laser systems as a UV light source using frequency doubling of laser light [2] and an optical parametric oscillation [3]. h-BBO thin films, on the other hand, are attractive materials to be used in various optical devices such as frequency converters, waveguides and switches in * Corresponding author. Tel.: +81-3-5841-8656; fax: +81-35841-7127. E-mail address: [email protected] (T. Kobayashi).
compact optical systems working especially in the UV range. There have been some attempts to fabricate h-BBO thin films by a pulsed laser deposition [4], a metal – organic chemical vapor deposition [5] and a chemical solution deposition [6]. The chemical solution deposition technique has an advantage in composition control since the processing can be performed under the atmospheric pressure. Moreover, this technique also has an advantage that allows homogeneous addition of a trace amount of rare-earth elements such as europium and lanthanum. By the addition of such elements, h-BBO thin films can be applied to luminescent and k-electron devices. In this point of view, single-phase h-BBO thin films with (00l) preferred orientation on Si(100) and SiO2 substrates were fabricated by a chemical solution
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 9 2 4 - 2
T. Kobayashi et al. / Materials Letters 57 (2003) 1056–1061
Fig. 1. Schematic model of h-BBO thin film that we reported previously. Open arrows represent a-axis direction and closed arrows represent c-axis direction, respectively. The crystallite sizes parallel to the substrate surfaces are 0.8 – 1.5 Am and those perpendicular to the substrate surfaces are nearly equal to the film thickness, respectively. For example, the thickness of the five coated films is about 200 nm and the crystallite sizes perpendicular to the substrate surfaces are also 200 nm.
deposition technique [7]. The obtained films turned out to have a mosaic structure that is composed of hBBo crystallites with (00l) preferred orientation. The film structure is schematically shown in Fig. 1. (00l) planes of some of the crystallites were found to tilt a little relative to the substrate surface normal direction, and the in-plane orientation of the crystallites was random. The crystallite sizes parallel to the substrate surfaces are 0.8 –1.5 Am and those perpendicular to the substrate surfaces are nearly equal to the film thickness, respectively. For example, the thickness of the five-coated films is about 200 nm and the crystallite sizes perpendicular to the substrate surfaces are also 200 nm. The crystallite sizes of the present h-BBO thin films are much larger than those of the films reported by Yogo et al. [6]. The larger crystallite size is advantageous to obtain better nonlinear optical properties. Thus, clarifying the structural evolution during crystallization of h-BBO thin films is important to fabricate desired films composed of large crystallites. In this paper, we report mainly the structural evolution during crystallization of h-BBO thin films fabricated by a chemical solution deposition technique.
2. Experimental procedures
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metal sources. The solvents were prepared by mixing acetic acid (AcOH), ethanol (EtOH) and 2-methoxyethanol (2-MOE). Barium acetate was dissolved into the solvents, and then stirred for 12 h. Boric acid was added to the mixture and the solution were stirred for 12 h to obtain transparent precursor solutions. The composition of the precursor solutions was as follows: Ba/B/ AcOH/EtOH/2-MOE = 1:2:40:5 in molar ratio. The precursor solutions were spin-coated on Si(100) substrates covered with about 5 nm thick oxide at a rotation speed of 5000 rpm. The obtained films were dried at 250 jC in air for about 30 min. Coating and drying were repeated five times. The dried films were finally calcined at 450 jC for 3 h to remove organic residues, followed by further heat treatment at varied temperatures (500 – 800 jC) in flowing oxygen gas. The thickness of the dried films is about 270 nm and that of the films fired at 500 –800 jC is about 200 nm, respectively. 2.2. Characterization The crystallization and orientation quality of the films were investigated by X-ray diffractometry (XRD, Mac Science, MXP-18) using Cu Ka radiation. The films were also characterized by infrared spectroscopy (IR, Nihon Bunko). The surface morphology of the films was observed by scanning electron microscopy (SEM, Hitachi, S-5000).
3. Results Fig. 2 shows XRD patterns for the h-BBO thin films fired in the temperature range 500– 800 jC. For the films composed of single-phase h-BBO, the degree of (00l) preferred orientation f calculated by Lotgering’s method [8] is shown in the figure. The degree of (00l) preferred orientation is given as follows: f ¼
I0ð006Þ Ið006Þ P P0 ; P0 ¼ P ; P¼P ; I0ðhklÞ IðhklÞ 1 P0
ð1Þ
2.1. Sample preparation Barium acetate (Ba(OCOCH3)2; purity = 99%) and boric acid (H3BO3; purity = 99.5%) were used as the
where I0(006) is (006) peak intensity and SI0(hkl) is the sum of intensity of all peaks for the powder diffraction data of h-BBO. I(006) is (006) peak
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intensity and SI(hkl) is the sum of the intensities of all peaks for the XRD data of the h-BBO films shown in Fig. 2. Sample F500 (F500 denotes the film fired at 500 jC) was found to be amorphous and the films fired in the temperature range 500– 650 jC were found to be single-phase h-BBO films with (00l) preferred orientation. Sample F650 showed the largest (006) peak intensity and the largest f value ( f = 0.96) as among all of the films examined. On the other hand, samples F700 and F725 turned out to have both a-BBO phase with (2011) preferred orientation and h-BBO phase with (00l) preferred orientation. The films fired at more than 700 jC were found to be single-phase a-BBO thin films with (2011) preferred orientation. (006) rocking curves for the films obtained in the temperature range 550 – 650 jC are shown in Fig. 3. Fig. 2. XRD patterns of the films at 500 – 800 jC. Closed circles denote the peaks derived from h-BBO. Open circles denote the peaks derived from a-BBO. Closed triangles denote the peaks derived from Si substrates.
Fig. 3. (006) rocking curves of the films fired at 550, 600 and 650 jC.
Fig. 4. IR spectra for the films. The absorption peaks at 1400 and 1240 cm 1 can be assigned to stretching vibration of BUO bond in (B3O6)3 anionic groups.
T. Kobayashi et al. / Materials Letters 57 (2003) 1056–1061
Fig. 5. Changes in XRD (006) integrated intensity and IR absorption intensity at 1400 cm 1 with firing temperature.
The width at half-maximum (FWHM) of the rocking curves was 4.6j for sample F600 and 3.4j for sample F650. The rocking curve for sample F550, on the other hand, was too distorted to estimate the FWHM value.
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In Fig. 4, IR spectra for h-BBO thin films fabricated in the present study are shown. Strong absorption peaks were observed at 1400 and 1240 cm 1 for the films fired in the temperature range 550– 725 jC. Both peaks can be assigned to the stretching vibration of BUO bond in (B3O6)3 anionic groups [9]. Fig. 5 shows the integrated intensities of XRD (006) peak and the IR absorption peak intensities at 1400 cm 1 as a function of firing temperature. The infrared absorption peak intensity for sample F650 is almost the same as that for sample F600, while the integrated intensity of XRD (006) peak for sample F650 is about 1.3 times larger than that of sample F600. Surface SEM images for h-BBO thin films fired in the temperature range 500– 650 jC are shown in Fig. 6. Crystallites of 0.2 –0.3 Am in diameter were observed partly on the surface of sample F550. Samples F600 and F650 showed structural morphologies similar to those illustrated in Fig. 1. The crystallite sizes of samples F600 and F650 were found to be 0.8 –1.5 Am. Crystallite boundaries in sample F600 were more distinct than those in sample F650.
Fig. 6. SEM images of the surface of the films fired at 500 – 650 jC.
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4. Discussion From abovementioned results, we propose a model of the structural evolution during crystallization in the present h-BaB2O4 thin films, which is illustrated in Fig. 7. The steps of the structural evolution are explained as follows. (a) In the temperature range 500 – 550 jC, the films crystallize partly. The crystallites grow toward the direction normal to the substrates. (b) In the temperature range 550 –600 jC, the crystallites grow in the in-plane direction. (c) The crystallites cease to grow when the growing crystallites coalesce. (d) In the temperature range 600 – 650 jC, the neighboring crystallites rotate to reduce the strain energy caused by mismatch between neighboring crystallites, resulting in the increase in degree of (00l) preferred orientation of h-BBO films. In the following discussion, we describe each of the step based on the obtained results. 4.1. Initial stage of crystallization The films turned out to crystallize in the temperature range 500– 550 jC as we can see from the XRD patterns and the IR spectra. Furthermore, the films turned out to have the (00l) preferred orientation at the initial stage of crystallization. The lowest interfacial energy of (00l) planes of h-BBO due to the highest
atomic density is responsible for the (00l) preferred orientation of the films. We already discussed this point detail in our previous paper [7]. The IR absorption intensity of sample F550 was about one-half of that of samples F600 and F650 (see Fig. 5). The results suggest that not all of boron elements form (B3O6)3 anionic groups in samples F550. Therefore, the crystallization occurs partly in sample F550, particularly, sample F550 is composed of h-BBO phase and noncrystalline phase. The crystallized parts of sample F550 probably correspond to the crystallites (0.2 – 0.3 Am in diameter) which can be seen in the SEM image. On the other hand, the crystallites of samples F600 and F650 are 0.8 – 1.5 Am in diameter. The results suggest that the crystallites, at the initial stage of crystallization, grow toward normal direction to the substrate rather than in-plane direction. The (006) rocking curve for sample F550 was distorted as shown in Fig. 3. This means that (00l) planes of some of the crystallites tilt largely from the surface of the substrates. (00l) planes of some of the crystallites can tilt more largely than (104) planes, because the angle between these planes is as high as 16.1j [7]. Such crystallites, which have largely tilted (00l) planes, probably showed a (104) peak in the XRD patterns. From the discussion, we have concluded that the films crystallize partly and grow toward the normal direction to the substrates in the temperature range 500 – 550 jC. 4.2. Increase in crystallinity and crystal growth
Fig. 7. Schematic model of structural evolution during crystallization of the present h-BBO films in the temperature range (a) 500 – 550 jC, (b), (c) 550 – 600 jC and (d) 600 – 650 jC.
The IR intensity of samples F600 and F650 at 1400 cm 1 was the highest among all of the films (Fig. 4). The results suggest that the films fully crystallized in temperature range 550 – 600 jC. The crystallite size increased in size (from 0.2– 0.3 to 0.8 – 1.5 Am) with elevated temperature from 550 to 600 jC as we can see in the SEM images. This suggests that the crystallites grow in the in-plane direction in the temperature range 550– 600 jC. The growing crystallites probably cease to grow when they coalesce. Small angle tilt boundaries or twist boundaries are formed at the coalescence area because of the mismatch between (00l) planes of neighboring crystallites. Thus, the coalescence would be responsi-
T. Kobayashi et al. / Materials Letters 57 (2003) 1056–1061
ble for the formation of the crystallite boundaries seen on the surface of samples F600 and F650. The mismatch between (00l) planes of neighboring crystallites becomes very low in some cases. In such a case, small angle tilt boundaries and twist boundaries are not formed, and the neighboring crystallites coalesce into one crystallite. The (006) rocking curve of sample F600 showed is a sharp one, while that of sample F550 is a distorted one (see Fig. 3). This suggests that the largely tilted crystallites are difficult to grow in the in-plane direction. 4.3. Increase in degree of (00l) preferred orientation The IR intensity for sample F650 was almost the same as sample F600 (see Fig. 5). On the other hand, the XRD intensity for sample F650 was about 1.3 times larger than that of sample F600. Furthermore, the degree of (00l) preferred orientation increased from 0.92 to 0.96 with increasing firing temperature ranged from 600 to 650 jC. The crystallite size of sample F650 is almost the same as sample F600, suggesting that the crystallite size did not increase in the temperature range 600– 650 jC. Sample F650, however, showed more distinct boundaries compared to sample F600. The crystallite boundaries can be small angle tilt boundary or twist boundary caused by the mismatch of (00l) planes at boundaries as described in Section 4.2. Such boundaries usually have a strain energy. In order to reduce a strain energy, neighboring crystallites generally rotate to reduce a mismatch at the boundaries leading to formation of boundary grooving [10]. In the present case, such a rotation of crystallites probably occurs. The rotation of the crystallites gives rise to a boundary grooving resulting in the formation of distinct boundaries. As shown in Fig. 3, FWHM of the (006) rocking curve of film F650 (3.4j) was lower than that of film F600 (4.6j). This result is in good agreement with the rotation of the crystallites.
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5. Summary In this paper, we discussed the structural evolution during crystallization of h-BaB2O4 thin films fabricated by chemical solution deposition technique. The structural evolution during crystallization is summarized as follows. (a) In the temperature range 500 –550 jC, the films crystallized partly. The crystallites grow toward the direction normal to the substrates. (b) In the temperature range 550 – 600 jC, the crystallites grow in the in-plane direction. (c) The crystallites cease to grow when the growing crystallites coalesce. (d) In the temperature range 600 – 650 jC, the neighboring crystallites rotate to reduce the strain energy derived from mismatch. As a result, the degree of (00l) preferred orientation of h-BBO film increases.
References [1] C. Chen, B. Wu, A. Jiang, G. You, Sci. Sin., Ser. B 28 (1985) 235. [2] D. Eimerl, L. Davis, S. Velsko, E. Graham, A. Zalkin, Optical, mechanical, and thermal properties of h-BaB2O4, J. Appl. Phys. 62 (1987) 968. [3] M.H. Dunn, M. Ebrahimzadeh, Science 286 (1999) 1513. [4] R.F. Xiao, P.Y.L.C. Ng, G.K. Wong, Appl. Phys. Lett. 67 (1997) 305. [5] D.B. Studebaker, G.T. Stauf, T.H. Baum, T.J. Marks, H. Zhou, G.K. Wong, Appl. Phys. Lett. 70 (1997) 565. [6] T. Yogo, K. Niwa, K. Kikuta, M. Ichida, A. Nakamura, S. Hirano, J. Mater. Chem. 7 (1997) 929. [7] T. Kobayashi, R. Ogawa, K. Miyazawa, M. Kuwabara, J. Mater. Res. 17 (2002) 844. [8] F.K. Lotgering, J. Inorg. Nucl. Chem. 9 (1959) 113. [9] U. Moryc, W.S. Ptak, J. Mol. Struct. 511 – 512 (1999) 241. [10] T.A. Derouin, C.D.E. Lakeman, X.H. Wu, J.S. Speck, F.F. Lange, J. Mater. Res. 12 (1997) 1391.
Materials Letters 57 (2003) 1056 – 1061 www.elsevier.com/locate/matlet
Structural evolution during crystallization of h-BaB2O4 thin films fabricated by chemical solution deposition technique Takeshi Kobayashi*, Ryo Ogawa, Makoto Kuwabara Department of Materials Science, Kuwabara Laboratory (Building no. 4 of Faculty of Engineering Room 405), University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan Received 15 April 2002; accepted 23 May 2002
Abstract Structural evolution during crystallization of (00l)-oriented h-BaB2O4 thin films fabricated by chemical solution deposition technique has been studied. The films were characterized by X-ray diffractometry (XRD), infrared spectroscopy (IR) and scanning electron microscopy (SEM). The results indicate that h-BaB2O4 thin films crystallize in the temperature range 500 – 550 jC, and the crystallinity, the degree of orientation and the crystallite size of the films turned out to increase in the temperature range 550 – 600 jC. The crystallinity and the degree of (00l) preferred orientation further increase in the temperature range 600 – 650 jC, while the crystallite size does not increase further. The crystallization behavior of h-BaB2O4 thin films is discussed in relation to the structural evolution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: h-BaB2O4; Thin films; Crystal growth; Grain boundaries; Microstructure; Chemical solution deposition; Orientation
1. Introduction Beta barium borate (h-BaB2O4, referred to as hBBO) is a nonlinear-optical material developed by Chen et al. [1]. Single crystal of h-BBO is applied in laser systems as a UV light source using frequency doubling of laser light [2] and an optical parametric oscillation [3]. h-BBO thin films, on the other hand, are attractive materials to be used in various optical devices such as frequency converters, waveguides and switches in * Corresponding author. Tel.: +81-3-5841-8656; fax: +81-35841-7127. E-mail address: [email protected] (T. Kobayashi).
compact optical systems working especially in the UV range. There have been some attempts to fabricate h-BBO thin films by a pulsed laser deposition [4], a metal – organic chemical vapor deposition [5] and a chemical solution deposition [6]. The chemical solution deposition technique has an advantage in composition control since the processing can be performed under the atmospheric pressure. Moreover, this technique also has an advantage that allows homogeneous addition of a trace amount of rare-earth elements such as europium and lanthanum. By the addition of such elements, h-BBO thin films can be applied to luminescent and k-electron devices. In this point of view, single-phase h-BBO thin films with (00l) preferred orientation on Si(100) and SiO2 substrates were fabricated by a chemical solution
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 9 2 4 - 2
T. Kobayashi et al. / Materials Letters 57 (2003) 1056–1061
Fig. 1. Schematic model of h-BBO thin film that we reported previously. Open arrows represent a-axis direction and closed arrows represent c-axis direction, respectively. The crystallite sizes parallel to the substrate surfaces are 0.8 – 1.5 Am and those perpendicular to the substrate surfaces are nearly equal to the film thickness, respectively. For example, the thickness of the five coated films is about 200 nm and the crystallite sizes perpendicular to the substrate surfaces are also 200 nm.
deposition technique [7]. The obtained films turned out to have a mosaic structure that is composed of hBBo crystallites with (00l) preferred orientation. The film structure is schematically shown in Fig. 1. (00l) planes of some of the crystallites were found to tilt a little relative to the substrate surface normal direction, and the in-plane orientation of the crystallites was random. The crystallite sizes parallel to the substrate surfaces are 0.8 –1.5 Am and those perpendicular to the substrate surfaces are nearly equal to the film thickness, respectively. For example, the thickness of the five-coated films is about 200 nm and the crystallite sizes perpendicular to the substrate surfaces are also 200 nm. The crystallite sizes of the present h-BBO thin films are much larger than those of the films reported by Yogo et al. [6]. The larger crystallite size is advantageous to obtain better nonlinear optical properties. Thus, clarifying the structural evolution during crystallization of h-BBO thin films is important to fabricate desired films composed of large crystallites. In this paper, we report mainly the structural evolution during crystallization of h-BBO thin films fabricated by a chemical solution deposition technique.
2. Experimental procedures
1057
metal sources. The solvents were prepared by mixing acetic acid (AcOH), ethanol (EtOH) and 2-methoxyethanol (2-MOE). Barium acetate was dissolved into the solvents, and then stirred for 12 h. Boric acid was added to the mixture and the solution were stirred for 12 h to obtain transparent precursor solutions. The composition of the precursor solutions was as follows: Ba/B/ AcOH/EtOH/2-MOE = 1:2:40:5 in molar ratio. The precursor solutions were spin-coated on Si(100) substrates covered with about 5 nm thick oxide at a rotation speed of 5000 rpm. The obtained films were dried at 250 jC in air for about 30 min. Coating and drying were repeated five times. The dried films were finally calcined at 450 jC for 3 h to remove organic residues, followed by further heat treatment at varied temperatures (500 – 800 jC) in flowing oxygen gas. The thickness of the dried films is about 270 nm and that of the films fired at 500 –800 jC is about 200 nm, respectively. 2.2. Characterization The crystallization and orientation quality of the films were investigated by X-ray diffractometry (XRD, Mac Science, MXP-18) using Cu Ka radiation. The films were also characterized by infrared spectroscopy (IR, Nihon Bunko). The surface morphology of the films was observed by scanning electron microscopy (SEM, Hitachi, S-5000).
3. Results Fig. 2 shows XRD patterns for the h-BBO thin films fired in the temperature range 500– 800 jC. For the films composed of single-phase h-BBO, the degree of (00l) preferred orientation f calculated by Lotgering’s method [8] is shown in the figure. The degree of (00l) preferred orientation is given as follows: f ¼
I0ð006Þ Ið006Þ P P0 ; P0 ¼ P ; P¼P ; I0ðhklÞ IðhklÞ 1 P0
ð1Þ
2.1. Sample preparation Barium acetate (Ba(OCOCH3)2; purity = 99%) and boric acid (H3BO3; purity = 99.5%) were used as the
where I0(006) is (006) peak intensity and SI0(hkl) is the sum of intensity of all peaks for the powder diffraction data of h-BBO. I(006) is (006) peak
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intensity and SI(hkl) is the sum of the intensities of all peaks for the XRD data of the h-BBO films shown in Fig. 2. Sample F500 (F500 denotes the film fired at 500 jC) was found to be amorphous and the films fired in the temperature range 500– 650 jC were found to be single-phase h-BBO films with (00l) preferred orientation. Sample F650 showed the largest (006) peak intensity and the largest f value ( f = 0.96) as among all of the films examined. On the other hand, samples F700 and F725 turned out to have both a-BBO phase with (2011) preferred orientation and h-BBO phase with (00l) preferred orientation. The films fired at more than 700 jC were found to be single-phase a-BBO thin films with (2011) preferred orientation. (006) rocking curves for the films obtained in the temperature range 550 – 650 jC are shown in Fig. 3. Fig. 2. XRD patterns of the films at 500 – 800 jC. Closed circles denote the peaks derived from h-BBO. Open circles denote the peaks derived from a-BBO. Closed triangles denote the peaks derived from Si substrates.
Fig. 3. (006) rocking curves of the films fired at 550, 600 and 650 jC.
Fig. 4. IR spectra for the films. The absorption peaks at 1400 and 1240 cm 1 can be assigned to stretching vibration of BUO bond in (B3O6)3 anionic groups.
T. Kobayashi et al. / Materials Letters 57 (2003) 1056–1061
Fig. 5. Changes in XRD (006) integrated intensity and IR absorption intensity at 1400 cm 1 with firing temperature.
The width at half-maximum (FWHM) of the rocking curves was 4.6j for sample F600 and 3.4j for sample F650. The rocking curve for sample F550, on the other hand, was too distorted to estimate the FWHM value.
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In Fig. 4, IR spectra for h-BBO thin films fabricated in the present study are shown. Strong absorption peaks were observed at 1400 and 1240 cm 1 for the films fired in the temperature range 550– 725 jC. Both peaks can be assigned to the stretching vibration of BUO bond in (B3O6)3 anionic groups [9]. Fig. 5 shows the integrated intensities of XRD (006) peak and the IR absorption peak intensities at 1400 cm 1 as a function of firing temperature. The infrared absorption peak intensity for sample F650 is almost the same as that for sample F600, while the integrated intensity of XRD (006) peak for sample F650 is about 1.3 times larger than that of sample F600. Surface SEM images for h-BBO thin films fired in the temperature range 500– 650 jC are shown in Fig. 6. Crystallites of 0.2 –0.3 Am in diameter were observed partly on the surface of sample F550. Samples F600 and F650 showed structural morphologies similar to those illustrated in Fig. 1. The crystallite sizes of samples F600 and F650 were found to be 0.8 –1.5 Am. Crystallite boundaries in sample F600 were more distinct than those in sample F650.
Fig. 6. SEM images of the surface of the films fired at 500 – 650 jC.
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4. Discussion From abovementioned results, we propose a model of the structural evolution during crystallization in the present h-BaB2O4 thin films, which is illustrated in Fig. 7. The steps of the structural evolution are explained as follows. (a) In the temperature range 500 – 550 jC, the films crystallize partly. The crystallites grow toward the direction normal to the substrates. (b) In the temperature range 550 –600 jC, the crystallites grow in the in-plane direction. (c) The crystallites cease to grow when the growing crystallites coalesce. (d) In the temperature range 600 – 650 jC, the neighboring crystallites rotate to reduce the strain energy caused by mismatch between neighboring crystallites, resulting in the increase in degree of (00l) preferred orientation of h-BBO films. In the following discussion, we describe each of the step based on the obtained results. 4.1. Initial stage of crystallization The films turned out to crystallize in the temperature range 500– 550 jC as we can see from the XRD patterns and the IR spectra. Furthermore, the films turned out to have the (00l) preferred orientation at the initial stage of crystallization. The lowest interfacial energy of (00l) planes of h-BBO due to the highest
atomic density is responsible for the (00l) preferred orientation of the films. We already discussed this point detail in our previous paper [7]. The IR absorption intensity of sample F550 was about one-half of that of samples F600 and F650 (see Fig. 5). The results suggest that not all of boron elements form (B3O6)3 anionic groups in samples F550. Therefore, the crystallization occurs partly in sample F550, particularly, sample F550 is composed of h-BBO phase and noncrystalline phase. The crystallized parts of sample F550 probably correspond to the crystallites (0.2 – 0.3 Am in diameter) which can be seen in the SEM image. On the other hand, the crystallites of samples F600 and F650 are 0.8 – 1.5 Am in diameter. The results suggest that the crystallites, at the initial stage of crystallization, grow toward normal direction to the substrate rather than in-plane direction. The (006) rocking curve for sample F550 was distorted as shown in Fig. 3. This means that (00l) planes of some of the crystallites tilt largely from the surface of the substrates. (00l) planes of some of the crystallites can tilt more largely than (104) planes, because the angle between these planes is as high as 16.1j [7]. Such crystallites, which have largely tilted (00l) planes, probably showed a (104) peak in the XRD patterns. From the discussion, we have concluded that the films crystallize partly and grow toward the normal direction to the substrates in the temperature range 500 – 550 jC. 4.2. Increase in crystallinity and crystal growth
Fig. 7. Schematic model of structural evolution during crystallization of the present h-BBO films in the temperature range (a) 500 – 550 jC, (b), (c) 550 – 600 jC and (d) 600 – 650 jC.
The IR intensity of samples F600 and F650 at 1400 cm 1 was the highest among all of the films (Fig. 4). The results suggest that the films fully crystallized in temperature range 550 – 600 jC. The crystallite size increased in size (from 0.2– 0.3 to 0.8 – 1.5 Am) with elevated temperature from 550 to 600 jC as we can see in the SEM images. This suggests that the crystallites grow in the in-plane direction in the temperature range 550– 600 jC. The growing crystallites probably cease to grow when they coalesce. Small angle tilt boundaries or twist boundaries are formed at the coalescence area because of the mismatch between (00l) planes of neighboring crystallites. Thus, the coalescence would be responsi-
T. Kobayashi et al. / Materials Letters 57 (2003) 1056–1061
ble for the formation of the crystallite boundaries seen on the surface of samples F600 and F650. The mismatch between (00l) planes of neighboring crystallites becomes very low in some cases. In such a case, small angle tilt boundaries and twist boundaries are not formed, and the neighboring crystallites coalesce into one crystallite. The (006) rocking curve of sample F600 showed is a sharp one, while that of sample F550 is a distorted one (see Fig. 3). This suggests that the largely tilted crystallites are difficult to grow in the in-plane direction. 4.3. Increase in degree of (00l) preferred orientation The IR intensity for sample F650 was almost the same as sample F600 (see Fig. 5). On the other hand, the XRD intensity for sample F650 was about 1.3 times larger than that of sample F600. Furthermore, the degree of (00l) preferred orientation increased from 0.92 to 0.96 with increasing firing temperature ranged from 600 to 650 jC. The crystallite size of sample F650 is almost the same as sample F600, suggesting that the crystallite size did not increase in the temperature range 600– 650 jC. Sample F650, however, showed more distinct boundaries compared to sample F600. The crystallite boundaries can be small angle tilt boundary or twist boundary caused by the mismatch of (00l) planes at boundaries as described in Section 4.2. Such boundaries usually have a strain energy. In order to reduce a strain energy, neighboring crystallites generally rotate to reduce a mismatch at the boundaries leading to formation of boundary grooving [10]. In the present case, such a rotation of crystallites probably occurs. The rotation of the crystallites gives rise to a boundary grooving resulting in the formation of distinct boundaries. As shown in Fig. 3, FWHM of the (006) rocking curve of film F650 (3.4j) was lower than that of film F600 (4.6j). This result is in good agreement with the rotation of the crystallites.
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5. Summary In this paper, we discussed the structural evolution during crystallization of h-BaB2O4 thin films fabricated by chemical solution deposition technique. The structural evolution during crystallization is summarized as follows. (a) In the temperature range 500 –550 jC, the films crystallized partly. The crystallites grow toward the direction normal to the substrates. (b) In the temperature range 550 – 600 jC, the crystallites grow in the in-plane direction. (c) The crystallites cease to grow when the growing crystallites coalesce. (d) In the temperature range 600 – 650 jC, the neighboring crystallites rotate to reduce the strain energy derived from mismatch. As a result, the degree of (00l) preferred orientation of h-BBO film increases.
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