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Growth of highly oriented LiTaO3 thin film on Si with amorphous SiO2 buffer layer by pulsed laser deposition
Materials Letters 61 (2007) 1052 – 1055 www.elsevier.com/locate/matlet
Growth of highly oriented LiTaO3 thin film on Si with amorphous SiO2 buffer layer by pulsed laser deposition Xinchang Wang a,⁎, Yongtao Tian a , Liangliang Cao b , Zhizhen Ye b a
Key Laboratory of Material Physics, and Department of Physics, Zhengzhou University, Zhengzhou 450052, PR China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 12 January 2006; accepted 15 June 2006 Available online 7 July 2006
Abstract High-quality single-phase, c-axis textured LiTaO3 thin films have been deposited on Si(100) substrate with amorphous SiO2 buffer layer for optic waveguide application by pulsed laser deposition under optimized conditions of 30 Pa oxygen pressure and 650 °C. The amorphous SiO2 buffer layer with a thickness of 100 nm was coated on the Si(100) by thermal oxidation at 1000 °C. Li-enriched LiTaO3 ceramic target was used during the deposition. In order to study the influence of oxygen pressure on the orientation, crystallinity and morphology, different oxygen pressures (10 Pa, 20 Pa, 30 Pa and 40 Pa) were used. X-ray diffraction (XRD) results showed that LiTaO3 thin films exhibited highly c-axis orientation under 30 Pa. It was observed by scanning electron microscopy (SEM) that the as-grown film in the optimal conditions was characterized by a dense and homogeneous surface without cracks, and the average grain size was in the order of 25 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: LiTaO3 thin films; Pulsed laser deposition; SiO2/Si substrate; c-axis oriented growth
1. Introduction Due to its large electro-optic and nonlinear coefficients, LiTaO3 is an attractive host material for integrated-optic devices, and in comparison with LiNbO3, the photodamage resistance ability of LiTaO3 is known to be larger than that of LiNbO3 in the visible spectral range [1]. Recently, with the development of integrating optical devices, the growing interests have been placed on the preparation of optical thin film, especially for the optical waveguide. Thin films could possess many potential advantages over the bulk material, including that large refractive index differences achievable with thin films lead to tighter mode confinement and economical substrates are available [2]. Up to now, numerous techniques have been used to deposit LiTaO3 thin films, including r.f. sputtering [3], molecular beam epitaxy (MBE) [4], chemical vapor deposition (CVD) [5], polymeric precursor method [6], sol-gel method [7], and pulsed laser deposition (PLD) [2,8]. Among these techniques, the PLD
⁎ Corresponding author. Tel.: +86 371 67766917. E-mail address: [email protected] (X. Wang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.047
technique exhibits some advantages over the others, including easy preparation of epitaxial films, low substrate temperature, and higher compositional consistency with the target. In the commercial and technological point of view, the preparation of high-quality LiTaO3 thin film on silicon substrate is particularly attractive, because it can be very effective to combine the optical–electronics devices with the micro-electronics and integrate the multifunctional devices. However, there are significant obstacles for integrating the LiTaO3 optical waveguiding films on Si substrate, because of the following reasons: (a) LiTaO3 has different crystal structure with silicon, and the lattice and thermal mismatch between LiTaO3 and silicon wafer is quite large, and (b) it is of great importance to prepare the epitaxial or caxis oriented LiTaO3 thin films. Because in the c-axis oriented LiTaO3 films, optimized optical properties can often be obtained, (c) since the refractive index of Si is higher than LiTaO3, an intermediate oxide layer with a low refractive index is required to form a waveguide structure. For this purpose, an amorphous SiO2 layer with a low refractive index would be an ideal candidate because it can be grown directly by thermal oxidization method. The lattice constants, refractive index, and thermal expansion coefficients of LiTaO3, Si and amorphous SiO2 are given in Table 1.
X. Wang et al. / Materials Letters 61 (2007) 1052–1055 Table 1 Comparison of lattice constants, thermal expansion coefficients and refractive indices for LiTaO3, Si, and amorphous SiO2 Materials
Lattice constant (Å)
Thermal expansion coeficient (at 25 °C)
Refractive index
LiTaO3
aH = 5.154 cH = 13.784 a = 5.431 –
αa = 14 × 10− 6
no = 2.188 ne = 2.183 n = 3.42 n = 1.46
Si Amorphous SiO2
αa = 2.6 × 10− 6 –
In our previous work, we have successfully prepared highly c-axis oriented LiNbO3 optical waveguiding film on SiO2/Si substrate by using the PLD method [9–12]. In this paper, we report the first successful preparation of highly c-axis textured LiTaO3 films on SiO2/Si substrates by the PLD technique without resorting to the use of an electric field. The effect of oxygen pressure on film texture and crystallinity has been systematically investigated. 2. Experimental We have used the similar PLD system as reported previously [9], including a KrF excimer laser (λ = 248 nm, τ = 25 ns, frequency: 5 Hz, energy density: 4 J/cm2). The substrate temperature was kept at 650 °C. The target-to-substrate distance was adjusted to 4 cm. The films were prepared under the oxygen ambience and both the target and the substrate were continuously rotated during deposition. The SiO2 coating (with a thickness of 100 nm) was grown on Si(100) substrate by thermal oxidization under the oxygen ambience in 1000 °C. Due to the high volatility of Li, the Li-deficient phases such as LiTa3O8 were often formed during deposition. Therefore, the Li-enriched LiTaO3 target, which was a sintered polycrystalline bulk ceramic disk prepared from mixed powders of Li2CO3 and Ta2O5 in air at 1250 °C for 4 h with an excess Li content (Li/Ta atomic rate: 1.2:1), was used. The laser deposition generally lasts for about 1 h and then the substrate temperature was cooled slowly to room temperature to avoid possible oxygen deficiency and prevent the generation of cracks in the films. During the deposition, the oxygen pressure was a very important parameter and it would determine the orientation of the as-grown LiTaO3 films to a great extent. We have prepared a series of samples with different oxygen pressures (10 Pa, 20 Pa, 30 Pa and 40 Pa) in order to try to find out the optimal condition. Crystal structure and the orientation of the films were determined by X-ray diffraction (XRD, Bede D1 system). The surface morphology and the thickness of thin films were examined by scanning electron microscope (SEM, Sirion by FEI).
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Fig. 1(a) represents the diffraction of the standard powder sample of LiTaO3 powder, I(hkl), from the JCPDS file: I(012):I(104):I(110):I (006):I(202):I(024):I(116):I(122):I(018) = 100:40:25:4:16:15:20:13:6. In this study, the film grown under 10 Pa oxygen pressure was generally polycrystalline without preferential orientation (as shown in Fig. 1(b)). When the oxygen pressure was amplified from 10 Pa to 30 Pa, the relative intensity of (012) reflection continuously decreased and that of (006) reflection was enhanced, which indicated that more grains in the films were grown with their c-axis orientation (as shown in Fig. 1(b) to (d)). When oxygen pressure was up to 30 Pa, the (006) reflection became the predominant one, which showed that highly caxis oriented LiTaO3 films could be grown on Si substrate with amorphous SiO2 buffer layer at 30 Pa oxygen pressure. The c-axis orientation was nearly equivalent to “z-cut” LiTaO3 in bulk, allowing relatively easy use of the nonlinear and piezoelectric (d33), electro-optic (γ33), permittivity (ε33), and refractive index (n33) coefficient, which are essential for active devices in integrated optics. The measured clattice constant of 13.761 Å is very close to the value of 13.784 Å for bulk LiTaO3, which indicated that the as-grown film had a good crystalline quality. We think that the minor difference was mainly due to the thermal mismatch between the thin film and the substrate so that the films were in compression at the room temperature. When oxygen pressure continued to be enhanced and was up to 40 Pa (as shown in Fig. 1(e)), the intensity of the (006) reflection decreased and the (108) reflection was relatively intensified, indicating that the trend of the caxis orientation of LiTaO3 film changed with the continuous enhancement of oxygen pressure. The influence of oxygen pressure on film orientation can be quantitatively described by means of the orientation degree f. According to Hu et al. [13], the degree of c-axis orientation (f) of LiTaO3 films is given as: f ¼
In ð006Þ−Ion ð006Þ 1−Ion ð006Þ
ð1Þ
Where In is the ratio of the measured intensity of the (006) diffraction peak to that of the sum of the intensities of all diffraction peaks, ΣI (hkl), in the range 2θ = 20°–60° and Ion is the value of In for a randomly oriented film that can be obtained from the standard powder diffraction data cards. For partially c-axis oriented films, f is in the range between 0 and 1. In particular, randomly oriented polycrystalline films and the
3. Results and discussion To study the effect of oxygen pressure on crystallite orientation, LiTaO3 films were grown under substrate temperature of 650 °C in the oxygen pressure range of 10–40 Pa with an interval of 10 Pa. Fig. 1 shows the dependence of the XRD patterns of LiTaO3 films on oxygen pressure. Besides the peaks of LiTaO3, no peaks were observed in any of XRD patterns, which indicated that there weren't Li-deficient phases in the as-grown films and the SiO2 buffer layer was amorphous.
Fig. 1. XRD θ–2θ scans of LiTaO3 thin films deposited on SiO2/Si substrates at different oxygen pressures, (b) 10 Pa, (c) 20 Pa, (d) 30 Pa, (e) 40 Pa, in comparison with the standard powder diffraction data (a).
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X. Wang et al. / Materials Letters 61 (2007) 1052–1055
PDF powders, In(006) = Ion(006), and f = 0, whereas for the completely c-axis oriented films, In(006) = 1, and f = 1. The calculated values are plotted in Fig. 2 versus oxygen pressure. As we can see, f increases gradually as oxygen pressure from 10 to 30 Pa, and f decreases with increasing continually oxygen pressure. These results clearly suggest that oxygen pressure exerts a prominent effect on the crystallinity and the c-axis orientation of LiTaO3 films, and the optimized oxygen pressure was 30 Pa. The effect of oxygen pressure can be explained through the expansion of the plasma. The expansion of the plasma produced by laser ablation in the presence of a buffer gas is known to be dominated by the interaction of the ejected species and the background gas atoms or molecules [14–17]. With increasing background pressure, the kinetic energy of the ejected species will decrease. It is well known that the kinetic energy of the species reaching the substrate should be in a limited and optimum range, since low kinetic energies would prevent the high mobility required during the initial stages of growth whereas too high energies may damage the film. In our experiment, the ejected species reach the substrate with an optimum kinetic energy in oxygen pressure of 30 Pa which can promote the growth of the c-axis oriented films (high f values). For oxygen pressure above 30 Pa, the ejected species slow down significantly and reach the substrate with a very low kinetic energy through a diffusion process. This prevents the growth of the oriented film and leads to low f values. For oxygen pressure below 30 Pa, the ejected species reaching the substrate have high kinetic energy. This may damage as-grown film and even cause re-sputtering [18]. Moreover, the surface morphology is a very important parameter for the applications of LiTaO3 thin film, especially in optical devices. The smooth surface with uniform, compact and small grains was expected. Panels (a) and (b) of Fig. 3 are the SEM micrographs at two magnifications of the LiTaO3 film prepared in the optimal condition mentioned above. The 10,000× data reveal a smooth surface without cracks. Actually, because of the great mismatches of the thermal expansion coefficients between the LiTaO3 films and SiO2/Si substrate, LiTaO3 films often present surface with cracks. In this point, we believe that the problem can be solved through increasing the cooling time as illustrated in Fig. 3(a). A closer examination of the surface (Fig. 3(b)) showed the as-grown film contained a large amount of small grains and sharply defined grain boundaries. The size of the crystalline grains determines how strongly the film material will scatter light. According to the investigation of Lee and Feigelson [19], for thin-film fer-
Fig. 2. Orientation degree f of (006) peak of LiTaO3 films deposited on SiO2/Si substrate at the substrate temperature of 650 °C versus oxygen pressure.
Fig. 3. Scanning electron micrographs of LiTaO3 film deposited on SiO2/Si at the oxygen pressure of 30 Pa for two magnifications.
roelectric waveguide, the scattering losses can mainly result from the rough surface, which can be easily expressed as σ ∼ r0, where σ is the rms roughness, and r0 the average island radius. Consequently, minimizing the grain size or maximizing the nucleation density can effectively reduce surface roughness. The region evaluated in Fig. 3 contained hemispherical grains with relatively homogeneous grain size in the order of 25 nm and also appeared to be very dense. In comparison with 46 nm in the previous report [6], we have obtained the smaller grain size and the larger nucleation density so that it would be very helpful to prepare the LiTaO3 thin film with a smooth surface.
Fig. 4. High-resolution cross-sectional SEM micrograph of the as-grown LiTaO3/SiO2/Si interface region.
X. Wang et al. / Materials Letters 61 (2007) 1052–1055
Meanwhile, since the wavelength of interest for optical communication is in the near infrared region of about 1000 nm [20], we think that the present grain size is suitable for integrated optics application. The high-resolution SEM micrograph of the cross-sectional LiTaO3/ SiO2/Si multilayer prepared in optimal condition is shown in Fig. 4. A sandwich-like structure can be clearly identified. The interfaces of the amorphous SiO2 with the LiTaO3 thin film and silicon wafer are very sharp and smooth. For optical application, the quality of the interface is also a very important factor that can determine the optical propagation in the as-grown thin film. Therefore, the sharp interface as mentioned above is expected to be favorable to reduce the interface scattering of light. The SiO2 buffer layer, which shows the amorphous characteristic demonstrated in the above XRD patterns, and the as-grown LiTaO3 thin films were estimated to be about 100 nm and 200 nm in thickness, respectively. A preliminary study of the thickness of the amorphous SiO2 buffer layer was also undertaken, which indicated that the relatively thick buffer layer was contributed to reduce the diffusion of Si atoms into LiTaO3 thin film from the Si substrate and form the sharp interface so as to confine the optical propagation tightly and prevent mode leakage. Again, the thickness of the LiTaO3 thin film should be also carefully determined. Feigelson [21] found that the relatively thick film could also increase the optical losses that might be an increase in scattering or absorption losses in the thicker film due to the longer growth times or some unprogrammed process changes. Therefore, the refractive index and propagation losses of the as-grown thin film should be investigated and the details of its optical properties will be described in a future paper.
4. Conclusions In summary, highly c-axis oriented LiTaO3 thin films have been deposited on Si(100) substrate with amorphous SiO2 buffer layer under an optimal growth condition by the PLD technique. The results showed that oxygen pressure was the key parameter for c-axis orientation and crystal quality of LiTaO3 films. Highly c-axis oriented LiTaO3 thin film could be deposited at optimized oxygen pressure of 30 Pa. The films exhibited smooth surface, continuous, uniform and small grain size, and the sharply defined interface, which can effectively prevent the scattering of light, and low optical diffraction losses are expected in such film.
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Acknowledgement The National Nature Science Foundation of China under Grant No. 90101009 financially supported this study. References [1] W.B. Spillman, N.A. Sanford, R.A. Soref, Opt. Lett. 8 (1983) 497. [2] J.A. Agostinelli, G.H. Braunstein, T.N. Blanton, Appl. Phys. Lett. 63 (1992) 123. [3] T.N. Blanton, D.K. Chatterjee, Thin Solid Films 256 (1995) 59. [4] F. Gitmans, Z. Sitar, P. Günter, Vacuum 46 (1995) 939. [5] S.A. Kulinich, T. Yamaki, S. Bysakh, H. Yamamoto, K. Mitsuishi, M. Song, K. Terashima, K. Furuya, J. Cryst. Growth 247 (2003) 408. [6] A.H.M. González, A.Z. Simões, M.A. Zaghete, J.A. Varela, Mater. Charact. 50 (2003) 233. [7] S.D. Cheng, Y. Zhou, C.H. Kam, X.Q. Han, W.X. Que, Y.L. Lam, Y.C. Chan, J.T. Oh, W.S. Gan, Mater. Lett. 44 (2000) 125. [8] L.S. Hung, J.A. Agostinelli, J.M. Mir, L.R. Zheng, Appl. Phys. Lett. 61 (24) (1993) 3071. [9] X.C. Wang, Z.Z. Ye, G.B. Wu, L.L. Cao, B.H. Zhao, Mater. Lett. 59 (2005) 2994. [10] X.C. Wang, Z.Z. Ye, J.H. He, J.Y. Huang, B.H. Zhao, Int. J. Mod. Phys. B 16 (28) (2002) 4343. [11] X.C. Wang, J.H. He, J.Y. Huang, B.H. Zhao, Z.Z. Ye, J. Mater. Sci. Lett. 22 (2003) 225. [12] X.C. Wang, Z.Z. Ye, J.H. He, L.L. Cao, B.H. Zhao, Mater. Lett. 58 (2004) 3597. [13] W.S. Hu, Z.G. Liu, D. Feng, J. Appl. Phys. 80 (12) (1996) 7089. [14] P.E. Dyer, A. Issa, P.H. Key, Appl. Surf. Sci. 46 (1990) 89. [15] J.C.S. Kools, J. Appl. Phys. 74 (1993) 6401. [16] W.K.A. Kummuduni, Y. Nakayama, Y. Nakata, T. Okada, M. Maeda, J. Appl. Phys. 74 (1993) 7510. [17] J. Gonzalo, C.N. Afonso, I. Madariaga, J. Appl. Phys. 81 (1997) 951. [18] J.A. Chaos, J. Gonzalo, C.N. Afonso, J. Perrière, M.T. García-González, Appl. Phys., A 72 (2001) 705. [19] S.Y. Lee, R.S. Feigelson, J. Cryst. Growth 186 (1998) 594. [20] S.D. Cheng, C.H. Kam, Y. Zhou, Y.L. Lam, Y.C. Chan, X. Wang, Z. Sun, SPIE 3491 (1998) 462. [21] R.S. Feigelson, J. Cryst. Growth 166 (1996) 1.
Growth of highly oriented LiTaO3 thin film on Si with amorphous SiO2 buffer layer by pulsed laser deposition Xinchang Wang a,⁎, Yongtao Tian a , Liangliang Cao b , Zhizhen Ye b a
Key Laboratory of Material Physics, and Department of Physics, Zhengzhou University, Zhengzhou 450052, PR China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 12 January 2006; accepted 15 June 2006 Available online 7 July 2006
Abstract High-quality single-phase, c-axis textured LiTaO3 thin films have been deposited on Si(100) substrate with amorphous SiO2 buffer layer for optic waveguide application by pulsed laser deposition under optimized conditions of 30 Pa oxygen pressure and 650 °C. The amorphous SiO2 buffer layer with a thickness of 100 nm was coated on the Si(100) by thermal oxidation at 1000 °C. Li-enriched LiTaO3 ceramic target was used during the deposition. In order to study the influence of oxygen pressure on the orientation, crystallinity and morphology, different oxygen pressures (10 Pa, 20 Pa, 30 Pa and 40 Pa) were used. X-ray diffraction (XRD) results showed that LiTaO3 thin films exhibited highly c-axis orientation under 30 Pa. It was observed by scanning electron microscopy (SEM) that the as-grown film in the optimal conditions was characterized by a dense and homogeneous surface without cracks, and the average grain size was in the order of 25 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: LiTaO3 thin films; Pulsed laser deposition; SiO2/Si substrate; c-axis oriented growth
1. Introduction Due to its large electro-optic and nonlinear coefficients, LiTaO3 is an attractive host material for integrated-optic devices, and in comparison with LiNbO3, the photodamage resistance ability of LiTaO3 is known to be larger than that of LiNbO3 in the visible spectral range [1]. Recently, with the development of integrating optical devices, the growing interests have been placed on the preparation of optical thin film, especially for the optical waveguide. Thin films could possess many potential advantages over the bulk material, including that large refractive index differences achievable with thin films lead to tighter mode confinement and economical substrates are available [2]. Up to now, numerous techniques have been used to deposit LiTaO3 thin films, including r.f. sputtering [3], molecular beam epitaxy (MBE) [4], chemical vapor deposition (CVD) [5], polymeric precursor method [6], sol-gel method [7], and pulsed laser deposition (PLD) [2,8]. Among these techniques, the PLD
⁎ Corresponding author. Tel.: +86 371 67766917. E-mail address: [email protected] (X. Wang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.047
technique exhibits some advantages over the others, including easy preparation of epitaxial films, low substrate temperature, and higher compositional consistency with the target. In the commercial and technological point of view, the preparation of high-quality LiTaO3 thin film on silicon substrate is particularly attractive, because it can be very effective to combine the optical–electronics devices with the micro-electronics and integrate the multifunctional devices. However, there are significant obstacles for integrating the LiTaO3 optical waveguiding films on Si substrate, because of the following reasons: (a) LiTaO3 has different crystal structure with silicon, and the lattice and thermal mismatch between LiTaO3 and silicon wafer is quite large, and (b) it is of great importance to prepare the epitaxial or caxis oriented LiTaO3 thin films. Because in the c-axis oriented LiTaO3 films, optimized optical properties can often be obtained, (c) since the refractive index of Si is higher than LiTaO3, an intermediate oxide layer with a low refractive index is required to form a waveguide structure. For this purpose, an amorphous SiO2 layer with a low refractive index would be an ideal candidate because it can be grown directly by thermal oxidization method. The lattice constants, refractive index, and thermal expansion coefficients of LiTaO3, Si and amorphous SiO2 are given in Table 1.
X. Wang et al. / Materials Letters 61 (2007) 1052–1055 Table 1 Comparison of lattice constants, thermal expansion coefficients and refractive indices for LiTaO3, Si, and amorphous SiO2 Materials
Lattice constant (Å)
Thermal expansion coeficient (at 25 °C)
Refractive index
LiTaO3
aH = 5.154 cH = 13.784 a = 5.431 –
αa = 14 × 10− 6
no = 2.188 ne = 2.183 n = 3.42 n = 1.46
Si Amorphous SiO2
αa = 2.6 × 10− 6 –
In our previous work, we have successfully prepared highly c-axis oriented LiNbO3 optical waveguiding film on SiO2/Si substrate by using the PLD method [9–12]. In this paper, we report the first successful preparation of highly c-axis textured LiTaO3 films on SiO2/Si substrates by the PLD technique without resorting to the use of an electric field. The effect of oxygen pressure on film texture and crystallinity has been systematically investigated. 2. Experimental We have used the similar PLD system as reported previously [9], including a KrF excimer laser (λ = 248 nm, τ = 25 ns, frequency: 5 Hz, energy density: 4 J/cm2). The substrate temperature was kept at 650 °C. The target-to-substrate distance was adjusted to 4 cm. The films were prepared under the oxygen ambience and both the target and the substrate were continuously rotated during deposition. The SiO2 coating (with a thickness of 100 nm) was grown on Si(100) substrate by thermal oxidization under the oxygen ambience in 1000 °C. Due to the high volatility of Li, the Li-deficient phases such as LiTa3O8 were often formed during deposition. Therefore, the Li-enriched LiTaO3 target, which was a sintered polycrystalline bulk ceramic disk prepared from mixed powders of Li2CO3 and Ta2O5 in air at 1250 °C for 4 h with an excess Li content (Li/Ta atomic rate: 1.2:1), was used. The laser deposition generally lasts for about 1 h and then the substrate temperature was cooled slowly to room temperature to avoid possible oxygen deficiency and prevent the generation of cracks in the films. During the deposition, the oxygen pressure was a very important parameter and it would determine the orientation of the as-grown LiTaO3 films to a great extent. We have prepared a series of samples with different oxygen pressures (10 Pa, 20 Pa, 30 Pa and 40 Pa) in order to try to find out the optimal condition. Crystal structure and the orientation of the films were determined by X-ray diffraction (XRD, Bede D1 system). The surface morphology and the thickness of thin films were examined by scanning electron microscope (SEM, Sirion by FEI).
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Fig. 1(a) represents the diffraction of the standard powder sample of LiTaO3 powder, I(hkl), from the JCPDS file: I(012):I(104):I(110):I (006):I(202):I(024):I(116):I(122):I(018) = 100:40:25:4:16:15:20:13:6. In this study, the film grown under 10 Pa oxygen pressure was generally polycrystalline without preferential orientation (as shown in Fig. 1(b)). When the oxygen pressure was amplified from 10 Pa to 30 Pa, the relative intensity of (012) reflection continuously decreased and that of (006) reflection was enhanced, which indicated that more grains in the films were grown with their c-axis orientation (as shown in Fig. 1(b) to (d)). When oxygen pressure was up to 30 Pa, the (006) reflection became the predominant one, which showed that highly caxis oriented LiTaO3 films could be grown on Si substrate with amorphous SiO2 buffer layer at 30 Pa oxygen pressure. The c-axis orientation was nearly equivalent to “z-cut” LiTaO3 in bulk, allowing relatively easy use of the nonlinear and piezoelectric (d33), electro-optic (γ33), permittivity (ε33), and refractive index (n33) coefficient, which are essential for active devices in integrated optics. The measured clattice constant of 13.761 Å is very close to the value of 13.784 Å for bulk LiTaO3, which indicated that the as-grown film had a good crystalline quality. We think that the minor difference was mainly due to the thermal mismatch between the thin film and the substrate so that the films were in compression at the room temperature. When oxygen pressure continued to be enhanced and was up to 40 Pa (as shown in Fig. 1(e)), the intensity of the (006) reflection decreased and the (108) reflection was relatively intensified, indicating that the trend of the caxis orientation of LiTaO3 film changed with the continuous enhancement of oxygen pressure. The influence of oxygen pressure on film orientation can be quantitatively described by means of the orientation degree f. According to Hu et al. [13], the degree of c-axis orientation (f) of LiTaO3 films is given as: f ¼
In ð006Þ−Ion ð006Þ 1−Ion ð006Þ
ð1Þ
Where In is the ratio of the measured intensity of the (006) diffraction peak to that of the sum of the intensities of all diffraction peaks, ΣI (hkl), in the range 2θ = 20°–60° and Ion is the value of In for a randomly oriented film that can be obtained from the standard powder diffraction data cards. For partially c-axis oriented films, f is in the range between 0 and 1. In particular, randomly oriented polycrystalline films and the
3. Results and discussion To study the effect of oxygen pressure on crystallite orientation, LiTaO3 films were grown under substrate temperature of 650 °C in the oxygen pressure range of 10–40 Pa with an interval of 10 Pa. Fig. 1 shows the dependence of the XRD patterns of LiTaO3 films on oxygen pressure. Besides the peaks of LiTaO3, no peaks were observed in any of XRD patterns, which indicated that there weren't Li-deficient phases in the as-grown films and the SiO2 buffer layer was amorphous.
Fig. 1. XRD θ–2θ scans of LiTaO3 thin films deposited on SiO2/Si substrates at different oxygen pressures, (b) 10 Pa, (c) 20 Pa, (d) 30 Pa, (e) 40 Pa, in comparison with the standard powder diffraction data (a).
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X. Wang et al. / Materials Letters 61 (2007) 1052–1055
PDF powders, In(006) = Ion(006), and f = 0, whereas for the completely c-axis oriented films, In(006) = 1, and f = 1. The calculated values are plotted in Fig. 2 versus oxygen pressure. As we can see, f increases gradually as oxygen pressure from 10 to 30 Pa, and f decreases with increasing continually oxygen pressure. These results clearly suggest that oxygen pressure exerts a prominent effect on the crystallinity and the c-axis orientation of LiTaO3 films, and the optimized oxygen pressure was 30 Pa. The effect of oxygen pressure can be explained through the expansion of the plasma. The expansion of the plasma produced by laser ablation in the presence of a buffer gas is known to be dominated by the interaction of the ejected species and the background gas atoms or molecules [14–17]. With increasing background pressure, the kinetic energy of the ejected species will decrease. It is well known that the kinetic energy of the species reaching the substrate should be in a limited and optimum range, since low kinetic energies would prevent the high mobility required during the initial stages of growth whereas too high energies may damage the film. In our experiment, the ejected species reach the substrate with an optimum kinetic energy in oxygen pressure of 30 Pa which can promote the growth of the c-axis oriented films (high f values). For oxygen pressure above 30 Pa, the ejected species slow down significantly and reach the substrate with a very low kinetic energy through a diffusion process. This prevents the growth of the oriented film and leads to low f values. For oxygen pressure below 30 Pa, the ejected species reaching the substrate have high kinetic energy. This may damage as-grown film and even cause re-sputtering [18]. Moreover, the surface morphology is a very important parameter for the applications of LiTaO3 thin film, especially in optical devices. The smooth surface with uniform, compact and small grains was expected. Panels (a) and (b) of Fig. 3 are the SEM micrographs at two magnifications of the LiTaO3 film prepared in the optimal condition mentioned above. The 10,000× data reveal a smooth surface without cracks. Actually, because of the great mismatches of the thermal expansion coefficients between the LiTaO3 films and SiO2/Si substrate, LiTaO3 films often present surface with cracks. In this point, we believe that the problem can be solved through increasing the cooling time as illustrated in Fig. 3(a). A closer examination of the surface (Fig. 3(b)) showed the as-grown film contained a large amount of small grains and sharply defined grain boundaries. The size of the crystalline grains determines how strongly the film material will scatter light. According to the investigation of Lee and Feigelson [19], for thin-film fer-
Fig. 2. Orientation degree f of (006) peak of LiTaO3 films deposited on SiO2/Si substrate at the substrate temperature of 650 °C versus oxygen pressure.
Fig. 3. Scanning electron micrographs of LiTaO3 film deposited on SiO2/Si at the oxygen pressure of 30 Pa for two magnifications.
roelectric waveguide, the scattering losses can mainly result from the rough surface, which can be easily expressed as σ ∼ r0, where σ is the rms roughness, and r0 the average island radius. Consequently, minimizing the grain size or maximizing the nucleation density can effectively reduce surface roughness. The region evaluated in Fig. 3 contained hemispherical grains with relatively homogeneous grain size in the order of 25 nm and also appeared to be very dense. In comparison with 46 nm in the previous report [6], we have obtained the smaller grain size and the larger nucleation density so that it would be very helpful to prepare the LiTaO3 thin film with a smooth surface.
Fig. 4. High-resolution cross-sectional SEM micrograph of the as-grown LiTaO3/SiO2/Si interface region.
X. Wang et al. / Materials Letters 61 (2007) 1052–1055
Meanwhile, since the wavelength of interest for optical communication is in the near infrared region of about 1000 nm [20], we think that the present grain size is suitable for integrated optics application. The high-resolution SEM micrograph of the cross-sectional LiTaO3/ SiO2/Si multilayer prepared in optimal condition is shown in Fig. 4. A sandwich-like structure can be clearly identified. The interfaces of the amorphous SiO2 with the LiTaO3 thin film and silicon wafer are very sharp and smooth. For optical application, the quality of the interface is also a very important factor that can determine the optical propagation in the as-grown thin film. Therefore, the sharp interface as mentioned above is expected to be favorable to reduce the interface scattering of light. The SiO2 buffer layer, which shows the amorphous characteristic demonstrated in the above XRD patterns, and the as-grown LiTaO3 thin films were estimated to be about 100 nm and 200 nm in thickness, respectively. A preliminary study of the thickness of the amorphous SiO2 buffer layer was also undertaken, which indicated that the relatively thick buffer layer was contributed to reduce the diffusion of Si atoms into LiTaO3 thin film from the Si substrate and form the sharp interface so as to confine the optical propagation tightly and prevent mode leakage. Again, the thickness of the LiTaO3 thin film should be also carefully determined. Feigelson [21] found that the relatively thick film could also increase the optical losses that might be an increase in scattering or absorption losses in the thicker film due to the longer growth times or some unprogrammed process changes. Therefore, the refractive index and propagation losses of the as-grown thin film should be investigated and the details of its optical properties will be described in a future paper.
4. Conclusions In summary, highly c-axis oriented LiTaO3 thin films have been deposited on Si(100) substrate with amorphous SiO2 buffer layer under an optimal growth condition by the PLD technique. The results showed that oxygen pressure was the key parameter for c-axis orientation and crystal quality of LiTaO3 films. Highly c-axis oriented LiTaO3 thin film could be deposited at optimized oxygen pressure of 30 Pa. The films exhibited smooth surface, continuous, uniform and small grain size, and the sharply defined interface, which can effectively prevent the scattering of light, and low optical diffraction losses are expected in such film.
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