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Growth of crystalline quartz films with AT-cut plane by means of catalyst-enhanced vapor-phase epitaxy under atmospheric pressure
Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149 www.elsevier.com/locate/jpcs
Growth of crystalline quartz films with AT-cut plane by means of catalyst-enhanced vapor-phase epitaxy under atmospheric pressure Naoyuki Takahashia,*, Takato Nakumuraa, Satoshi Nonakab, Yoshinori Kubob, Yoichi Sinrikib, Katsumi Tamanukib a
Department of Materials Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatus, Shizuoka 432-8561, Japan b Humo Laboratory Company, 5-9-11 Nishiogi-Kita, Suginami-ku, Tokyo 167-0042, Japan Received 7 December 2004; revised 4 February 2005; accepted 9 March 2005
Abstract Crystalline quartz films with an AT-cut plane have been grown by catalyst-enhanced vapor-phase epitaxy, at atmospheric pressure, using two quartz buffer layers on a sapphire (110) substrate. In this method, the first quartz buffer layer was deposited on the sapphire (110) substrate at 773 K. After annealing at 823 K, the second buffer layer was deposited at 723 K. The crystal quartz epitaxial layer was then grown at 843 K. The X-ray full-width-at-half-maximum (FWHM) value of the crystalline quartz film obtained was smaller than that of crystalline quartz prepared using a hydrothermal process. The crystalline quality of the quartz films was dependent on the thickness of the buffer layers. Furthermore, it was found that angle control of the cut plane depended on the film thickness of the second buffer layer. The quartz films grown by vapor phase epitaxy show good oscillation characteristics at room temperature. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Vapour deposition; C. X-ray diffraction; D. Crystal structure; D. Piezoelectricity
1. Introduction Crystalline quartz is a piezoelectric and has been used in a variety of devices as an oscillator [1–3]. The most popular and best-known example is a cellular phone. The conventional route to preparing oscillator plates involves crystal growth of quartz in an autoclave for 30 days using a hydrothermal method [4,5], followed by cutting and polishing of the crystals. There are two problems associated with this method. The first is that the hydrothermal process, which was first developed in the middle of the 19th century, has hardly changed in a long time [4,5]. The process is energyconsuming, and more than 90% of the resulting crystals are wasted during the formation of the oscillator plates. The second problem is that making the plates less than 10 mm in thickness using mechanical polishing is not easy. In a previous paper, we reported that quartz epitaxial films with Z-face (c-axis orientation) were successfully * Corresponding author. Tel./fax: C81 53 478 1197. E-mail address: [email protected] (N. Takahashi).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.03.002
grown on a sapphire (001) substrate by catalyst-enhanced vapor-phase epitaxy under atmospheric pressure [6]. However, AT-cut plates have been widely used as oscillators because their oscillation frequency is independent of temperature. In the present paper, we report the growth of crystalline quartz films with an AT-cut face using a double buffer layer on a sapphire (110) substrate. 2. Experimental details Quartz films were grown by atmospheric pressure vaporphase epitaxy (AP–VPE) using Si(OC2H5)4 and O2 as starting materials, in the presence of gaseous HCl. The purity of Si(OC2H5)4 and O2 used was 99.99 and 99.999%, respectively. The experimental set-up used in the present study has been described previously [6]. The growth of the hexagonal quartz epitaxial layer was carried out in a vertical glass reactor under atmospheric pressure. Optical-grade polished sapphire of surface area 10!10 mm with the (110) orientation (a-face) was used as a substrate. The misorientation was within G0.28. The substrate was degreased by successive cleaning in acetone and de-ionized water, and then chemically etched with a mixed solution of
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N. Takahashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149
on O2
on
on
on
Si(OC2H5)4+ HCl Quartz epitaxial layer
900
Temperature / K
Quartz First buffer layer
Annealing
800
Quartz second buffer layer
700
600
Time Fig. 1. Time chart of the growth process by AP–VPE using the quartz buffer layer.
3. Results and discussion Fig. 2 shows a typical XRD patterns with a qK2q scan of the crystalline quartz film along with its DCXRD u-scan profile. In the qK2q scan profile, an intense diffraction line
appears at 26.638, as shown in Fig. 2(a), assigned to (011) of the crystal quartz crystal with a hexagonal structure. It is seen from Fig. 2(b) that the X-ray rocking curve gives a symmetric narrow line at 2qZ26.63768 with a full-width-athalf-maximum value of 0.0058, which is similar to that of the bulk crystal prepared by hydrothermal synthesis. It is therefore obvious that the quartz film deposited onto the sapphire substrate has reasonably high crystal quality. Fig. 3 shows an X-ray pole figure of the crystalline quartz film. It can be seen from Fig. 3 that the X-ray pole figure with a fixed 2q at the (011) reflection of the quartz has a bright spot centered around 878. If the spot is at the center, the (011) surface was grown perpendicular to the substrate. However, the bright spot is off-center, indicating that quartz film with the [011] direction tilted about 38 off the normal to the sapphire surface (Fig. 3) has been grown. This indicates that the as-grown quartz film has a surface corresponding to the AT-cut plane. It should be noted that AT-cut plates have
(a)
6
4
(b) Intensity
Quartz(011)
8
Intensity / 104 x c.p.s
H3PO4–H2SO4(1:3) at 433 K for 600 s before being dried in a stream of dry nitrogen. Subsequently, the sapphire substrate was placed on a susceptor in the reactor. Si(OC2H5)4 was transported to the reactor using N2 as a carrier gas bubbling through the Si(OC2H5)4 solution which was kept at 343 K. Simultaneously, gaseous HCl as a catalyst was supplied in order to facilitate the decomposition of Si(OC2H5)4. The growth process consists of three steps. Fig. 1 shows the typical quartz growth process using the quartz buffer layers. First, a quartz buffer layer was grown at 773 K, followed by an increase in the temperature to 823 K in order to anneal the layer for 900 s. Subsequently, a second buffer layer was grown at 723 K and annealed at 843 K for 600 s. Finally, the epitaxial quartz film was grown at 843 K for 10,800 s under the following conditions: the partial pressures of Si(OC2H5)4, oxygen and hydrogen chloride were 3.3!102, 3.3!104 and 1.7!102 Pa, respectively. The total flow rate was 1.33!10K5 m3 sK1. The crystalline quality of the quartz films was assessed by X-ray diffraction (XRD) (Rigaku Co., RINT2000) analysis and XRD pole figure (Rigaku Co., ATX–G) analysis. Secondary ion mass spectroscopy (SIMS) profiles were performed using a HITACHI PHI-6650 Q-pole SIMS instrument. The oscillation characteristics of the as-grown quartz film were measured under room temperature using a crystal bulk frequency measurement system (Humo Laboratory Co., MQB-300D). All measurements were performed using films of approximately 100 mm thickness.
2 26.55
26.60 26.65 2θ /ω degree
26.70
0 20
40
60
80
2θ / degree Fig. 2. (a) Typical XRD profile with a qK2q scan of the crystal quartz film along with (b) a DCXRD 2q/u-scan profile.
N. Takahashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149
Quartz film (100 µm) Second buffer Layer First buffer Layer 100 nm Sapphire (110)
600
FWHM of quartz film / s
1147
400
200
FWHM=0.005˚
0 0
50
100
150
200
Thickness of quartz buffer layer / nm Fig. 5. FWHM of the (011) diffraction line for the hexagonal quartz films as a function of the thickness of the second quartz buffer layer on (110) sapphire.
Fig. 3. Pole figure of the as-grown quartz film with 100 mm thickness.
been widely used as an oscillator because their oscillation frequency is independent of temperature. In order to clarify the effects of the buffer layers, the correlation between the buffer layers and crystalline quality of crystal quartz film was investigated. Fig. 4 shows the FWHM of the (011) diffraction line for quartz films grown with an AT-cut plane as a function of the thickness of the first quartz buffer layer on the (110) sapphire substrate. In this experiment, the thickness of the quartz film was 100 mm. As is evident from Fig. 4, the FWHM decreases with increasing thickness of the first buffer layer up to 100 nm, and then gradually increases. A minimum FWHM value of 240 s is obtained for the first buffer layer thickness of approximately 100 nm. Fig. 5 shows the FWHM of the (011) diffraction line for the obtained hexagonal quartz films as a function of the thickness of the second quartz buffer layer on (110) sapphire. In this experiment, the thickness of the first buffer layer and the quartz film were 100 nm and 100 mm, respectively. The FWHM decreases with increasing
thickness of the second buffer layer up to 50 nm and then gradually increases. A minimum FWHM value of 18 s is obtained for the second buffer layer thickness of approximately 50 nm. This value is comparable to that (30 s) of the crystal quartz prepared by hydrothermal synthesis implying that the optimum thickness of the buffer layer is around 50 nm. This means that the crystalline quality of the quartz films is dependent on the thickness of the buffer layer. A tilt from the [011] direction of the quartz films as a function of the thickness of the quartz first buffer layer on sapphire (110) is shown in Fig. 6. The value of the tilt varied from 2 to 48 despite variation in the thickness of the first buffer layer. Fig. 7 shows the tilt from the [011] direction of a quartz film as a function of the thickness of the second buffer layer on sapphire (110). The value of the tilt decreased with increasing thickness of the buffer layer up to 38, and then gradually increased. The tilt value of 38 corresponding to the AT-cut plane is obtained for the second buffer layer thickness of approximately 50 nm. This result implies that control of the AT-cut plane is dependent only on the thickness of the second buffer layer. Fig. 8 shows the SIMS profiles of the quartz film with the AT-cut plane on sapphire (110) substrates. As shown in 10
Quartz film (100 µm) First buffer Layer Sapphire (110)
Tilt from (011) face / degree
FWHM of quartz film / s
2000
1000
8
6
4
2
0 0
50
100
150
200
Thickness of quartz buffer layer / nm Fig. 4. FWHM of the (011) diffraction line for the hexagonal quartz films as a function of the thickness of the first quartz buffer layer on (110) sapphire.
0
0
100
200
Thickness of first buffer layer (nm) Fig. 6. The tilt from the [011] direction of the quartz films as a function of the thickness of the first buffer layer on (110) sapphire.
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N. Takahashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149
5 8
Intensity (decibel)
Tilt from (011) face / degree
10
6
4
2
0
–5
0 0
100
16.5
200
16.7
16.8
16.9
Fig. 8, only signals from Si and O were observed. The chlorine signal was not observed in the SIMS profiles. Therefore, chloride concentration is not higher than 0.1 at%. Generally, quartz prepared by the hydrothermal method contains impurities of Al and Na. Therefore, quartz films grown by catalyst-enhanced vapor-phase epitaxy under atmospheric pressure have excellent crystalline quality. Fig. 9 shows the oscillation characteristics of the asgrown quartz film of 100 mm thickness at room temperature. As can be seen in Fig. 9, an oscillation is observed at 16.4 MHz, which satisfies the empirical relationship formulated for film thickness t and frequency f:
Fig. 9. The oscillation characteristics of the as-grown quartz film of 100 mm thickness at room temperature.
150
Frequency /MHz
Fig. 7. The tilt from the [011] direction of the quartz films as a function of the thickness of the second buffer layer on (110) sapphire.
100
50
0 0
20
40
60
80
100
Thickness of quartz film / µm
ft Z 1:667 !103
(1)
wherein t and f are in mm and MHz, respectively. Also, it was found that the observed maximum oscillation intensity in db at 16.67 MHz is comparable to that of the plates sliced from the quartz crystal prepared by the conventional hydrothermal method. Fig. 10 shows the frequency of the quartz films with the AT-cut plane as a function of the thickness of the quartz film. As is evident from Fig. 10, the value of the frequency increases with decreasing thickness of the quartz film. This 107 Si
106
Concentration / atoms.cc-1
16.6
Frequency (MHz)
Thickness of first buffer layer / nm
O
105
Fig. 10. The frequency of the quartz films with an AT-cut plane as a function of the thickness of the quartz film.
variation in frequency agrees with Eq. (1). It is therefore evident that direct deposition of quartz oscillator films is possible using a simple vapor phase epitaxy with Si(OC2H5)4 as the starting material. Quartz films grown without a buffer layer hardly showed any indication of oscillation under AC bias. As the buffer layers after the two-step deposition showed a very weak XRD diffraction line at 2qZ26.68 assigned to the (011) reflection, epitaxial growth of the quartz is easier onto the buffer layer than directly onto the sapphire surface. It is, therefore, presumed that the key to the successful preparation of oscillating quartz films is the buffer layer deposition prior to epitaxial growth.
104
4. Conclusions 103 102 Al 101 100 0
500
1000
Etching time (s)
1500 (1 µm)
Fig. 8. SIMS profiles of a quartz film with an AT-cut plane on a (110) sapphire substrate.
We have succeeded in the preparation of epitaxially grown crystalline quartz films with an AT-cut plane. XRD and X-ray pole-figure measurements confirmed that the quartz film was a single crystal. It was found that angle control of the cut plane depended on the film thickness of the second buffer layer. Quartz films grown by vapor phase epitaxy show oscillation characteristics at room temperature. In conclusion, catalyst-enhanced vapor-phase epitaxy under atmospheric pressure using double quartz buffer
N. Takahashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149
layers is an excellent method for preparing high quality crystalline quartz with an AT-cut plane.
Acknowledgements This work was supported by the Development of Innovative Technology (No. 14501) in JAPAN MEXT (Ministry of Education, Culture, Sports, Science and Technology).
1149
References [1] C. Sauerbrey, G. Z. Physik 155 (1859) 206. [2] S. Taki, Taikabutsu 39 (1987) 189. [3] U. Gertig, W.C. Heraeus, G.H. Hanau, Chem. Ing. Tech. 60 (1988) 896. [4] C. Schafhautl, Munch. Geol. Anz. 1857; 557. [5] F. Iwasaki, H. Iwasaki, J. Cryst. Growth 237–239 (2002) 820. [6] N. Takahashi, M. Hosogi, T. Nakamura, Y. Momose, S. Nonaka, H. Yagi, Y. Shinriki, K. Tamanuki, J. Mater. Chem. 12 (2002) 719.
Growth of crystalline quartz films with AT-cut plane by means of catalyst-enhanced vapor-phase epitaxy under atmospheric pressure Naoyuki Takahashia,*, Takato Nakumuraa, Satoshi Nonakab, Yoshinori Kubob, Yoichi Sinrikib, Katsumi Tamanukib a
Department of Materials Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatus, Shizuoka 432-8561, Japan b Humo Laboratory Company, 5-9-11 Nishiogi-Kita, Suginami-ku, Tokyo 167-0042, Japan Received 7 December 2004; revised 4 February 2005; accepted 9 March 2005
Abstract Crystalline quartz films with an AT-cut plane have been grown by catalyst-enhanced vapor-phase epitaxy, at atmospheric pressure, using two quartz buffer layers on a sapphire (110) substrate. In this method, the first quartz buffer layer was deposited on the sapphire (110) substrate at 773 K. After annealing at 823 K, the second buffer layer was deposited at 723 K. The crystal quartz epitaxial layer was then grown at 843 K. The X-ray full-width-at-half-maximum (FWHM) value of the crystalline quartz film obtained was smaller than that of crystalline quartz prepared using a hydrothermal process. The crystalline quality of the quartz films was dependent on the thickness of the buffer layers. Furthermore, it was found that angle control of the cut plane depended on the film thickness of the second buffer layer. The quartz films grown by vapor phase epitaxy show good oscillation characteristics at room temperature. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Vapour deposition; C. X-ray diffraction; D. Crystal structure; D. Piezoelectricity
1. Introduction Crystalline quartz is a piezoelectric and has been used in a variety of devices as an oscillator [1–3]. The most popular and best-known example is a cellular phone. The conventional route to preparing oscillator plates involves crystal growth of quartz in an autoclave for 30 days using a hydrothermal method [4,5], followed by cutting and polishing of the crystals. There are two problems associated with this method. The first is that the hydrothermal process, which was first developed in the middle of the 19th century, has hardly changed in a long time [4,5]. The process is energyconsuming, and more than 90% of the resulting crystals are wasted during the formation of the oscillator plates. The second problem is that making the plates less than 10 mm in thickness using mechanical polishing is not easy. In a previous paper, we reported that quartz epitaxial films with Z-face (c-axis orientation) were successfully * Corresponding author. Tel./fax: C81 53 478 1197. E-mail address: [email protected] (N. Takahashi).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.03.002
grown on a sapphire (001) substrate by catalyst-enhanced vapor-phase epitaxy under atmospheric pressure [6]. However, AT-cut plates have been widely used as oscillators because their oscillation frequency is independent of temperature. In the present paper, we report the growth of crystalline quartz films with an AT-cut face using a double buffer layer on a sapphire (110) substrate. 2. Experimental details Quartz films were grown by atmospheric pressure vaporphase epitaxy (AP–VPE) using Si(OC2H5)4 and O2 as starting materials, in the presence of gaseous HCl. The purity of Si(OC2H5)4 and O2 used was 99.99 and 99.999%, respectively. The experimental set-up used in the present study has been described previously [6]. The growth of the hexagonal quartz epitaxial layer was carried out in a vertical glass reactor under atmospheric pressure. Optical-grade polished sapphire of surface area 10!10 mm with the (110) orientation (a-face) was used as a substrate. The misorientation was within G0.28. The substrate was degreased by successive cleaning in acetone and de-ionized water, and then chemically etched with a mixed solution of
1146
N. Takahashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149
on O2
on
on
on
Si(OC2H5)4+ HCl Quartz epitaxial layer
900
Temperature / K
Quartz First buffer layer
Annealing
800
Quartz second buffer layer
700
600
Time Fig. 1. Time chart of the growth process by AP–VPE using the quartz buffer layer.
3. Results and discussion Fig. 2 shows a typical XRD patterns with a qK2q scan of the crystalline quartz film along with its DCXRD u-scan profile. In the qK2q scan profile, an intense diffraction line
appears at 26.638, as shown in Fig. 2(a), assigned to (011) of the crystal quartz crystal with a hexagonal structure. It is seen from Fig. 2(b) that the X-ray rocking curve gives a symmetric narrow line at 2qZ26.63768 with a full-width-athalf-maximum value of 0.0058, which is similar to that of the bulk crystal prepared by hydrothermal synthesis. It is therefore obvious that the quartz film deposited onto the sapphire substrate has reasonably high crystal quality. Fig. 3 shows an X-ray pole figure of the crystalline quartz film. It can be seen from Fig. 3 that the X-ray pole figure with a fixed 2q at the (011) reflection of the quartz has a bright spot centered around 878. If the spot is at the center, the (011) surface was grown perpendicular to the substrate. However, the bright spot is off-center, indicating that quartz film with the [011] direction tilted about 38 off the normal to the sapphire surface (Fig. 3) has been grown. This indicates that the as-grown quartz film has a surface corresponding to the AT-cut plane. It should be noted that AT-cut plates have
(a)
6
4
(b) Intensity
Quartz(011)
8
Intensity / 104 x c.p.s
H3PO4–H2SO4(1:3) at 433 K for 600 s before being dried in a stream of dry nitrogen. Subsequently, the sapphire substrate was placed on a susceptor in the reactor. Si(OC2H5)4 was transported to the reactor using N2 as a carrier gas bubbling through the Si(OC2H5)4 solution which was kept at 343 K. Simultaneously, gaseous HCl as a catalyst was supplied in order to facilitate the decomposition of Si(OC2H5)4. The growth process consists of three steps. Fig. 1 shows the typical quartz growth process using the quartz buffer layers. First, a quartz buffer layer was grown at 773 K, followed by an increase in the temperature to 823 K in order to anneal the layer for 900 s. Subsequently, a second buffer layer was grown at 723 K and annealed at 843 K for 600 s. Finally, the epitaxial quartz film was grown at 843 K for 10,800 s under the following conditions: the partial pressures of Si(OC2H5)4, oxygen and hydrogen chloride were 3.3!102, 3.3!104 and 1.7!102 Pa, respectively. The total flow rate was 1.33!10K5 m3 sK1. The crystalline quality of the quartz films was assessed by X-ray diffraction (XRD) (Rigaku Co., RINT2000) analysis and XRD pole figure (Rigaku Co., ATX–G) analysis. Secondary ion mass spectroscopy (SIMS) profiles were performed using a HITACHI PHI-6650 Q-pole SIMS instrument. The oscillation characteristics of the as-grown quartz film were measured under room temperature using a crystal bulk frequency measurement system (Humo Laboratory Co., MQB-300D). All measurements were performed using films of approximately 100 mm thickness.
2 26.55
26.60 26.65 2θ /ω degree
26.70
0 20
40
60
80
2θ / degree Fig. 2. (a) Typical XRD profile with a qK2q scan of the crystal quartz film along with (b) a DCXRD 2q/u-scan profile.
N. Takahashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149
Quartz film (100 µm) Second buffer Layer First buffer Layer 100 nm Sapphire (110)
600
FWHM of quartz film / s
1147
400
200
FWHM=0.005˚
0 0
50
100
150
200
Thickness of quartz buffer layer / nm Fig. 5. FWHM of the (011) diffraction line for the hexagonal quartz films as a function of the thickness of the second quartz buffer layer on (110) sapphire.
Fig. 3. Pole figure of the as-grown quartz film with 100 mm thickness.
been widely used as an oscillator because their oscillation frequency is independent of temperature. In order to clarify the effects of the buffer layers, the correlation between the buffer layers and crystalline quality of crystal quartz film was investigated. Fig. 4 shows the FWHM of the (011) diffraction line for quartz films grown with an AT-cut plane as a function of the thickness of the first quartz buffer layer on the (110) sapphire substrate. In this experiment, the thickness of the quartz film was 100 mm. As is evident from Fig. 4, the FWHM decreases with increasing thickness of the first buffer layer up to 100 nm, and then gradually increases. A minimum FWHM value of 240 s is obtained for the first buffer layer thickness of approximately 100 nm. Fig. 5 shows the FWHM of the (011) diffraction line for the obtained hexagonal quartz films as a function of the thickness of the second quartz buffer layer on (110) sapphire. In this experiment, the thickness of the first buffer layer and the quartz film were 100 nm and 100 mm, respectively. The FWHM decreases with increasing
thickness of the second buffer layer up to 50 nm and then gradually increases. A minimum FWHM value of 18 s is obtained for the second buffer layer thickness of approximately 50 nm. This value is comparable to that (30 s) of the crystal quartz prepared by hydrothermal synthesis implying that the optimum thickness of the buffer layer is around 50 nm. This means that the crystalline quality of the quartz films is dependent on the thickness of the buffer layer. A tilt from the [011] direction of the quartz films as a function of the thickness of the quartz first buffer layer on sapphire (110) is shown in Fig. 6. The value of the tilt varied from 2 to 48 despite variation in the thickness of the first buffer layer. Fig. 7 shows the tilt from the [011] direction of a quartz film as a function of the thickness of the second buffer layer on sapphire (110). The value of the tilt decreased with increasing thickness of the buffer layer up to 38, and then gradually increased. The tilt value of 38 corresponding to the AT-cut plane is obtained for the second buffer layer thickness of approximately 50 nm. This result implies that control of the AT-cut plane is dependent only on the thickness of the second buffer layer. Fig. 8 shows the SIMS profiles of the quartz film with the AT-cut plane on sapphire (110) substrates. As shown in 10
Quartz film (100 µm) First buffer Layer Sapphire (110)
Tilt from (011) face / degree
FWHM of quartz film / s
2000
1000
8
6
4
2
0 0
50
100
150
200
Thickness of quartz buffer layer / nm Fig. 4. FWHM of the (011) diffraction line for the hexagonal quartz films as a function of the thickness of the first quartz buffer layer on (110) sapphire.
0
0
100
200
Thickness of first buffer layer (nm) Fig. 6. The tilt from the [011] direction of the quartz films as a function of the thickness of the first buffer layer on (110) sapphire.
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N. Takahashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149
5 8
Intensity (decibel)
Tilt from (011) face / degree
10
6
4
2
0
–5
0 0
100
16.5
200
16.7
16.8
16.9
Fig. 8, only signals from Si and O were observed. The chlorine signal was not observed in the SIMS profiles. Therefore, chloride concentration is not higher than 0.1 at%. Generally, quartz prepared by the hydrothermal method contains impurities of Al and Na. Therefore, quartz films grown by catalyst-enhanced vapor-phase epitaxy under atmospheric pressure have excellent crystalline quality. Fig. 9 shows the oscillation characteristics of the asgrown quartz film of 100 mm thickness at room temperature. As can be seen in Fig. 9, an oscillation is observed at 16.4 MHz, which satisfies the empirical relationship formulated for film thickness t and frequency f:
Fig. 9. The oscillation characteristics of the as-grown quartz film of 100 mm thickness at room temperature.
150
Frequency /MHz
Fig. 7. The tilt from the [011] direction of the quartz films as a function of the thickness of the second buffer layer on (110) sapphire.
100
50
0 0
20
40
60
80
100
Thickness of quartz film / µm
ft Z 1:667 !103
(1)
wherein t and f are in mm and MHz, respectively. Also, it was found that the observed maximum oscillation intensity in db at 16.67 MHz is comparable to that of the plates sliced from the quartz crystal prepared by the conventional hydrothermal method. Fig. 10 shows the frequency of the quartz films with the AT-cut plane as a function of the thickness of the quartz film. As is evident from Fig. 10, the value of the frequency increases with decreasing thickness of the quartz film. This 107 Si
106
Concentration / atoms.cc-1
16.6
Frequency (MHz)
Thickness of first buffer layer / nm
O
105
Fig. 10. The frequency of the quartz films with an AT-cut plane as a function of the thickness of the quartz film.
variation in frequency agrees with Eq. (1). It is therefore evident that direct deposition of quartz oscillator films is possible using a simple vapor phase epitaxy with Si(OC2H5)4 as the starting material. Quartz films grown without a buffer layer hardly showed any indication of oscillation under AC bias. As the buffer layers after the two-step deposition showed a very weak XRD diffraction line at 2qZ26.68 assigned to the (011) reflection, epitaxial growth of the quartz is easier onto the buffer layer than directly onto the sapphire surface. It is, therefore, presumed that the key to the successful preparation of oscillating quartz films is the buffer layer deposition prior to epitaxial growth.
104
4. Conclusions 103 102 Al 101 100 0
500
1000
Etching time (s)
1500 (1 µm)
Fig. 8. SIMS profiles of a quartz film with an AT-cut plane on a (110) sapphire substrate.
We have succeeded in the preparation of epitaxially grown crystalline quartz films with an AT-cut plane. XRD and X-ray pole-figure measurements confirmed that the quartz film was a single crystal. It was found that angle control of the cut plane depended on the film thickness of the second buffer layer. Quartz films grown by vapor phase epitaxy show oscillation characteristics at room temperature. In conclusion, catalyst-enhanced vapor-phase epitaxy under atmospheric pressure using double quartz buffer
N. Takahashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1145–1149
layers is an excellent method for preparing high quality crystalline quartz with an AT-cut plane.
Acknowledgements This work was supported by the Development of Innovative Technology (No. 14501) in JAPAN MEXT (Ministry of Education, Culture, Sports, Science and Technology).
1149
References [1] C. Sauerbrey, G. Z. Physik 155 (1859) 206. [2] S. Taki, Taikabutsu 39 (1987) 189. [3] U. Gertig, W.C. Heraeus, G.H. Hanau, Chem. Ing. Tech. 60 (1988) 896. [4] C. Schafhautl, Munch. Geol. Anz. 1857; 557. [5] F. Iwasaki, H. Iwasaki, J. Cryst. Growth 237–239 (2002) 820. [6] N. Takahashi, M. Hosogi, T. Nakamura, Y. Momose, S. Nonaka, H. Yagi, Y. Shinriki, K. Tamanuki, J. Mater. Chem. 12 (2002) 719.