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Growth and optical properties of completely c -axis orientated LiNbO3 films deposited by pulsed laser deposition
Solid State Communications 142 (2007) 694–697 www.elsevier.com/locate/ssc
Growth and optical properties of completely c-axis orientated LiNbO3 films deposited by pulsed laser deposition Xinchang Wang a,∗ , Zhizhen Ye b , Binghui Zhao b a Key Laboratory of Material Physics, School of Physics and Engineering, Zhengzhou University, Zhengzhou 450052, PR China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China
Received 2 February 2007; received in revised form 19 April 2007; accepted 24 April 2007 by J. Fontcuberta Available online 1 May 2007
Abstract Completely c-axis orientated LiNbO3 films have been deposited for waveguiding applications on SiO2 /Si substrates by pulsed laser deposition without the application of external electric field or any other buffer layers. The stoichiometric LiNbO3 ceramic target was used. Drastic influences of the substrate temperature and oxygen pressure on the stoichiometry and c-axis orientation of LiNbO3 films are shown. Completely c-axis oriented and nearly stoichiometric LiNbO3 films can be grown at 600 ◦ C in the oxygen pressure of 30 Pa. The films with column grain have smooth surface, uniform and small grain size, and the sharply defined interface. Favourable optical waveguiding properties of LiNbO3 films are demonstrated by the prism coupler technique. c 2007 Published by Elsevier Ltd
PACS: 77.84.Dy; 67.70.+n; 42.70.-a; 81.15.Fg Keywords: A. Ferroelectric; A. Thin films; C. Orientation; D. Optical properties
1. Introduction Bulk crystal lithium niobate (LiNbO3 ) has a very important application for integrated optical devices based on waveguide structures, but LiNbO3 films offer several potential advantages of technological processing in view of integrating complex microelectronic and optoelectronic hybrid systems. In particular, integration of c-axis orientated LiNbO3 films upon Si wafers with SiO2 coating is commercially important. However, the problem of affinity with the LiNbO3 crystal is that neither the low-index plane of crystalline Si nor amorphous SiO2 is lattice matched with the crystal lattice of LiNbO3 . This is the main reason that little work regarding LiNbO3 film growth on Si and SiO2 surfaces has been reported. C-axis orientated LiNbO3 films on SiO2 /Si have been achieved using pulsed laser deposition (PLD) only by (a) using an Al2 O3 buffer layer [1] or (b) by using an electric field applied normal to the substrate during growth [2]. PLD has
been shown to be very appropriate for the complex oxide films deposition due to its intrinsic feature of easily transferring the target stoichiometry to the substrate. But even with the pulsed laser ablation deposition it has been found to be difficult to get epitaxial LiNbO3 films since Li is highly volatile [3–5]. In order to prevent the appearance of such Li-deficient phase, a Li-enriched targets [6,7] or high oxygen pressures together with single-crystal targets [8–10] are used as an effective means of suppressing the Li-deficient phase. Thus, it is obvious that conditions for producing optical quality LiNbO3 stoichiometric films are not yet well established, and certainly, a lot of work is still to be done on this subject. In this paper we report the preparation of high optical quality LiNbO3 stoichiometric thin films upon SiO2 /Si by the PLD without the application of external electric field or any other buffer layers, and the stoichiometric LiNbO3 ceramic target was used. 2. Experimental details
∗ Corresponding author. Tel.: +86 371 67766917.
E-mail address: [email protected] (X. Wang). c 2007 Published by Elsevier Ltd 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.04.036
A KrF excimer laser operating at a wavelength of 248 nm was applied for pulsed laser deposition. The setup was
X. Wang et al. / Solid State Communications 142 (2007) 694–697
described earlier [11]. The pulse width and the energy density were 25 ns and 3.5 J/cm2 respectively. Polycrystalline LiNbO3 ceramic target was prepared using a sintering mixture of Li2 O and Nb2 O5 powders in air at 1190 ◦ C for 2 h, and the Li/Nb ratios of the target were 1:1. The laser repetition rate was 3 Hz. The substrate of Si(100) wafer was located at the distance of 4 cm from the target. The deposition temperature and oxygen pressure were varied to obtain highquality stoichiometric LiNbO3 films. Before the deposition, the amorphous SiO2 buffer layer was grown on Si(100) by thermal oxidation under ∼105 Pa oxygen pressure and at 1000 ◦ C for 4 h, and the thickness of SiO2 buffer layer was measured by transmission electron microscopy (TEM). The target and the substrate continuously rotate in order to ensure a constant angular distribution of the ejected matter from the target. After deposition, the films were in situ cooled down to room temperature very slowly in the oxygen atmosphere to avoid possible oxygen deficiency and achieve crack-free LiNbO3 films. The crystalline quality and orientation of the films were investigated by X-ray diffraction (XRD). The stoichiometry was investigated by X-ray photoelectron spectroscopy (XPS). The surface morphology of the films was examined by scanning electron microscope (SEM). The crystalline structure and microstructure of the films were measured by TEM. The optical waveguide properties of the as-grown LiNbO3 films were investigated by the prism coupler technique using a 632.8 nm wavelength light source. 3. Results and discussions The orientation and the crystallinity of as-deposited films are examined by XRD using Cu Kα radiation. Fig. 1 shows the XRD θ–2θ patterns of the LiNbO3 thin films deposited on SiO2 /Si substrates at various oxygen pressure and substrate temperature. No reflections from SiO2 layer were observed in Fig. 1, which indicates that the SiO2 buffer layer is amorphous in nature. Fig. 1(a) shows that the (012), (006), (116), and (018) reflections of LiNbO3 appeared under oxygen pressure 10 Pa and 600 ◦ C. Moreover, another peak near LiNbO3 (006) reflection also appears in this pattern. Ghica et al. [12] observed a similar peak, ascribing it to the (602) LiNb3 O8 reflection. When the oxygen pressure is raised to 30 Pa, as shown in Fig. 1(b), the LiNb3 O8 reflection disappears, and only (006) reflection of LiNbO3 is detected. These findings indicate that the need for a high gas pressure to achieve the correct Li content in the films may be related the plasma expansion process. Meanwhile, the experimental results show that the substrate temperature is also a key parameter for depositing high-quality stoichiometric LiNbO3 films. At 450 ◦ C, the as-deposited LiNbO3 film is amorphous. As the temperature becomes higher, the orientation and the crystallinity of LiNbO3 thin film are strongly increased. At 600 ◦ C, completely caxis oriented and good crystalline quality LiNbO3 films are achieved under 30 Pa oxygen gas. The c-axis lattice parameter calculated by using the XRD pattern is 1.3874 nm. The value is almost the same as that of LiNbO3 single crystal, 1.386
695
Fig. 1. XRD patterns of LiNbO3 thin films deposited on SiO2 /Si by PLD at (a) 600 ◦ C, 10 Pa O2 , (b) 600 ◦ C, 30 Pa O2 , (c) 650 ◦ C, 30 Pa O2 .
nm, which indicate that the LiNbO3 film has high-quality crystallinity. However, increasing the substrate temperature to 650 ◦ C, the (006) peak of LiNbO3 intensity decreases, and the (602) peak of LiNb3 O8 is observed. This is due to the reevaporation of the adsorbed Li on the film surface with a rising substrate temperature [13]. The stoichiometry of the film deposited at 600 ◦ C in the oxygen pressure of 30 Pa is further confirmed by means of XPS, with the data plotted in Fig. 2. The atomic ratio of Li:Nb:O is determined as 0.99:1.00:2.95. The atomic percentages of Li, Nb are identical to the nominal stoichiometry of LiNbO3 target materials. The deficiency of oxygen was believed to be due to the Ar+ bombardment of the film surface during the cleaning procedure before XPS measurements. These results clearly indicate that completely c-axis textured and nearly stoichiometric LiNbO3 films can be deposited using stoichiometric LiNbO3 ceramic target by selecting an appropriate deposition temperature and oxygen pressure. The surface morphology of LiNbO3 film is a very important parameter for its applications, especially for integrated-optic devices. The smooth surface with uniform, compact and small grains was expected. Fig. 3(a) and (b) are SEM micrographs at two magnifications of the LiNbO3 film deposited at the temperature of 600 ◦ C and the oxygen pressure of 30 Pa. Fig. 3(a) shows that the surface of LiNbO3 film is mirror-like, crack-free and no large droplets. A closer examination of the surface (Fig. 3(b)) shows the as-grown film contained a large amount of small and uniform grains. The grain size is about 35 nm. When considering potential application in a waveguide, this small grain-size is more suitable than a large one as the surface scattering ratio of a small grain size is lower than that of a large one. As shown in the TEM photograph in Fig. 4, LiNbO3 films have columnar grain structures with their c-axis well aligned throughout the film depth, and the thickness of the amorphous SiO2 layer is about 230 nm. Moreover, the interfaces of the amorphous SiO2 with the LiNbO3 thin film and silicon wafer
696
X. Wang et al. / Solid State Communications 142 (2007) 694–697
Fig. 3. SEM images of LiNbO3 film deposited on SiO2 /Si at 600 ◦ C and 30 Pa for two magnifications.
Fig. 4. Cross-sectional TEM image of LiNbO3 films deposited at 600 ◦ C and 30 Pa.
Fig. 2. XPS spectrum of LiNbO3 films deposited at 600 ◦ C in the oxygen pressure of 30 Pa.
are very clear. Fig. 5 shows a high-resolution cross-sectional TEM image of c-axis LiNbO3 on SiO2 /Si substrate deposited at a 600 ◦ C in the oxygen pressure of 30 Pa and the electron diffraction pattern. The TEM and electron diffraction patterns show that the as-deposited LiNbO3 film has a good crystalline quality and completely c-axis orientation. As shown in TEM image, the interface of the amorphous SiO2 with the LiNbO3 thin film is very sharp and smooth, and there is little atomic diffusion takes place on the interfaces. This sharp interface is expected to be favourable to reduce the interface scattering of light. The refractive indexes and propagation losses for several LiNbO3 films are measured by a prism coupler method and a 632.8 nm He–Ne laser. The refractive indexes of the films are obtained. The results are listed in Table 1. From the table, the n e is found to be between 2.200 and 2.215 and n o between 2.275 and 2.280. These values are very well matched with those for stoichiometric bulk LiNbO3 crystal (n e = 2.20 and n o = 2.29). The optical propagation losses of the TE0 mode for several LiNbO3 films are measured. As shown in Fig. 6, the propagation loss of the film deposited at the substrate
Fig. 5. High-resolution cross-sectional TEM image and the electron diffraction pattern of c-axis LiNbO3 deposited at 600 ◦ C and 30 Pa.
temperature 600 ◦ C and the oxygen pressure 30 Pa is as low as 1.14 dB/cm. It is known that the optical propagation loss can be influenced by film orientation, crystallinity and Li deficient phase (LiNb3 O8 ). From Fig. 1, it can be seen that the films grown under different parameters have different crystallinity, c-axis orientation, and LiNbO3 film deposited at the substrate temperature 600 ◦ C and the oxygen pressure 30 Pa has high quality crystallinity, completely c-axis orientation and nearly stoichiometry. Thus, LiNbO3 film grown under 600 ◦ C and 30 Pa has a minimum propagation loss.
X. Wang et al. / Solid State Communications 142 (2007) 694–697
697
of LiNbO3 films. Completely c-axis oriented and nearly stoichiometric LiNbO3 films can be deposited at 600 ◦ C in the oxygen pressure of 30 Pa. The films with column grain exhibit smooth surface, uniform and small grain size, and the sharply defined interface. The optical indexes of the deposited films matched very closely stoichiometric bulk LiNbO3 crystals, and the optical propagation losses are as low as 1.14 dB/cm at a wavelength of 632.8 nm. Acknowledgements This work was financially supported by the National Nature Science Foundation (NNSF) of China under the grant No. 90101009. References Fig. 6. TE0 mode optical-waveguiding-loss measurement in LiNbO3 films deposited at (a) 600 ◦ C, 10 Pa O2 , (b) 650 ◦ C, 30 Pa O2 , (c) 600 ◦ C, 30 Pa O2 . Table 1 The refractive indexes of LiNbO3 films under various oxygen pressure and substrate temperature Sample
Ts (◦ C)
PO2 (Pa)
no
ne
a b c
600 650 600
10 30 30
2.277 2.280 2.275
2.215 2.212 2.200
4. Conclusion High quality, low loss LiNbO3 films have been grown on SiO2 /Si substrates by the PLD without the application of external electric field or any other buffer layers. The results show that substrate temperature and oxygen pressure are the key parameters for the stoichiometry and c-axis orientation
[1] W.S. Hu, Z.G. Liu, Z.C. Wu, J.M. Liu, X.Y. Chen, D. Feng, Appl. Surf. Sci. 141 (1999) 197. [2] W.S. Hu, Z.G. Liu, Y.Q. Lu, S.N. Zhu, D. Feng, Opt. Lett. 21 (1996) 946. [3] Y. Kakehi, A. Okamoto, Y. Sakurai, Y. Nishikawa, T. Yotsuya, S. Ogawa, Appl. Surf. Sci. 169–170 (2001) 560. [4] J.A. Chaos, J. Gonzalo, C.N. Afonso, J. Perri`ere, M.T. Garc´ıa-Gonz´alez, Appl. Phys. A 72 (2001) 705. [5] I. Tsukada, S. Higuchi, Japan J. Appl. Phys. 43 (2004) 5307. [6] Y. Shibata, K. Kaya, K. Akashi, T. Kawai, S. Kawai, Appl. Phys. Lett. 61 (1992) 1000. [7] S. Yamada, Y. Nishibe, S. Ohtsubo, Y. Yonezawa, A. Morimoto, T. Shimizu, K. Ishida, Y. Masaki, Japan J. Appl. Phys. 41 (2002) L275. [8] D.W. Kim, S.M. Oh, S.H. Lee, S.H. Lee, T.W. Noh, Japan J. Appl. Phys. 37 (1998) 2016. [9] D.H. Lee, B.C. Shin, B.H. Min, Mater. Sci. Eng. B 95 (2002) 137. [10] H.K. Lam, J.Y. Dai, H.L.W. Chan, J. Cryst. Growth 268 (2004) 144. [11] X.C. Wang, Z.Z. Ye, G.B. Wu, L.L. Cao, B.H. Zhao, Mater. Lett. 59 (2005) 2994. [12] D. Ghina, C. Ghina, M. Gartner, V. Nelea, C. Martin, A. Cavaleru, I.N. Mihailescu, Appl. Surf. Sci. 138–139 (1999) 617. [13] S.K. Park, M.S. Baek, S.C. Bae, S.Y. Kwun, K.T. Kim, K.W. Kim, Solid State Commun. 111 (1999) 347.
Growth and optical properties of completely c-axis orientated LiNbO3 films deposited by pulsed laser deposition Xinchang Wang a,∗ , Zhizhen Ye b , Binghui Zhao b a Key Laboratory of Material Physics, School of Physics and Engineering, Zhengzhou University, Zhengzhou 450052, PR China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China
Received 2 February 2007; received in revised form 19 April 2007; accepted 24 April 2007 by J. Fontcuberta Available online 1 May 2007
Abstract Completely c-axis orientated LiNbO3 films have been deposited for waveguiding applications on SiO2 /Si substrates by pulsed laser deposition without the application of external electric field or any other buffer layers. The stoichiometric LiNbO3 ceramic target was used. Drastic influences of the substrate temperature and oxygen pressure on the stoichiometry and c-axis orientation of LiNbO3 films are shown. Completely c-axis oriented and nearly stoichiometric LiNbO3 films can be grown at 600 ◦ C in the oxygen pressure of 30 Pa. The films with column grain have smooth surface, uniform and small grain size, and the sharply defined interface. Favourable optical waveguiding properties of LiNbO3 films are demonstrated by the prism coupler technique. c 2007 Published by Elsevier Ltd
PACS: 77.84.Dy; 67.70.+n; 42.70.-a; 81.15.Fg Keywords: A. Ferroelectric; A. Thin films; C. Orientation; D. Optical properties
1. Introduction Bulk crystal lithium niobate (LiNbO3 ) has a very important application for integrated optical devices based on waveguide structures, but LiNbO3 films offer several potential advantages of technological processing in view of integrating complex microelectronic and optoelectronic hybrid systems. In particular, integration of c-axis orientated LiNbO3 films upon Si wafers with SiO2 coating is commercially important. However, the problem of affinity with the LiNbO3 crystal is that neither the low-index plane of crystalline Si nor amorphous SiO2 is lattice matched with the crystal lattice of LiNbO3 . This is the main reason that little work regarding LiNbO3 film growth on Si and SiO2 surfaces has been reported. C-axis orientated LiNbO3 films on SiO2 /Si have been achieved using pulsed laser deposition (PLD) only by (a) using an Al2 O3 buffer layer [1] or (b) by using an electric field applied normal to the substrate during growth [2]. PLD has
been shown to be very appropriate for the complex oxide films deposition due to its intrinsic feature of easily transferring the target stoichiometry to the substrate. But even with the pulsed laser ablation deposition it has been found to be difficult to get epitaxial LiNbO3 films since Li is highly volatile [3–5]. In order to prevent the appearance of such Li-deficient phase, a Li-enriched targets [6,7] or high oxygen pressures together with single-crystal targets [8–10] are used as an effective means of suppressing the Li-deficient phase. Thus, it is obvious that conditions for producing optical quality LiNbO3 stoichiometric films are not yet well established, and certainly, a lot of work is still to be done on this subject. In this paper we report the preparation of high optical quality LiNbO3 stoichiometric thin films upon SiO2 /Si by the PLD without the application of external electric field or any other buffer layers, and the stoichiometric LiNbO3 ceramic target was used. 2. Experimental details
∗ Corresponding author. Tel.: +86 371 67766917.
E-mail address: [email protected] (X. Wang). c 2007 Published by Elsevier Ltd 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.04.036
A KrF excimer laser operating at a wavelength of 248 nm was applied for pulsed laser deposition. The setup was
X. Wang et al. / Solid State Communications 142 (2007) 694–697
described earlier [11]. The pulse width and the energy density were 25 ns and 3.5 J/cm2 respectively. Polycrystalline LiNbO3 ceramic target was prepared using a sintering mixture of Li2 O and Nb2 O5 powders in air at 1190 ◦ C for 2 h, and the Li/Nb ratios of the target were 1:1. The laser repetition rate was 3 Hz. The substrate of Si(100) wafer was located at the distance of 4 cm from the target. The deposition temperature and oxygen pressure were varied to obtain highquality stoichiometric LiNbO3 films. Before the deposition, the amorphous SiO2 buffer layer was grown on Si(100) by thermal oxidation under ∼105 Pa oxygen pressure and at 1000 ◦ C for 4 h, and the thickness of SiO2 buffer layer was measured by transmission electron microscopy (TEM). The target and the substrate continuously rotate in order to ensure a constant angular distribution of the ejected matter from the target. After deposition, the films were in situ cooled down to room temperature very slowly in the oxygen atmosphere to avoid possible oxygen deficiency and achieve crack-free LiNbO3 films. The crystalline quality and orientation of the films were investigated by X-ray diffraction (XRD). The stoichiometry was investigated by X-ray photoelectron spectroscopy (XPS). The surface morphology of the films was examined by scanning electron microscope (SEM). The crystalline structure and microstructure of the films were measured by TEM. The optical waveguide properties of the as-grown LiNbO3 films were investigated by the prism coupler technique using a 632.8 nm wavelength light source. 3. Results and discussions The orientation and the crystallinity of as-deposited films are examined by XRD using Cu Kα radiation. Fig. 1 shows the XRD θ–2θ patterns of the LiNbO3 thin films deposited on SiO2 /Si substrates at various oxygen pressure and substrate temperature. No reflections from SiO2 layer were observed in Fig. 1, which indicates that the SiO2 buffer layer is amorphous in nature. Fig. 1(a) shows that the (012), (006), (116), and (018) reflections of LiNbO3 appeared under oxygen pressure 10 Pa and 600 ◦ C. Moreover, another peak near LiNbO3 (006) reflection also appears in this pattern. Ghica et al. [12] observed a similar peak, ascribing it to the (602) LiNb3 O8 reflection. When the oxygen pressure is raised to 30 Pa, as shown in Fig. 1(b), the LiNb3 O8 reflection disappears, and only (006) reflection of LiNbO3 is detected. These findings indicate that the need for a high gas pressure to achieve the correct Li content in the films may be related the plasma expansion process. Meanwhile, the experimental results show that the substrate temperature is also a key parameter for depositing high-quality stoichiometric LiNbO3 films. At 450 ◦ C, the as-deposited LiNbO3 film is amorphous. As the temperature becomes higher, the orientation and the crystallinity of LiNbO3 thin film are strongly increased. At 600 ◦ C, completely caxis oriented and good crystalline quality LiNbO3 films are achieved under 30 Pa oxygen gas. The c-axis lattice parameter calculated by using the XRD pattern is 1.3874 nm. The value is almost the same as that of LiNbO3 single crystal, 1.386
695
Fig. 1. XRD patterns of LiNbO3 thin films deposited on SiO2 /Si by PLD at (a) 600 ◦ C, 10 Pa O2 , (b) 600 ◦ C, 30 Pa O2 , (c) 650 ◦ C, 30 Pa O2 .
nm, which indicate that the LiNbO3 film has high-quality crystallinity. However, increasing the substrate temperature to 650 ◦ C, the (006) peak of LiNbO3 intensity decreases, and the (602) peak of LiNb3 O8 is observed. This is due to the reevaporation of the adsorbed Li on the film surface with a rising substrate temperature [13]. The stoichiometry of the film deposited at 600 ◦ C in the oxygen pressure of 30 Pa is further confirmed by means of XPS, with the data plotted in Fig. 2. The atomic ratio of Li:Nb:O is determined as 0.99:1.00:2.95. The atomic percentages of Li, Nb are identical to the nominal stoichiometry of LiNbO3 target materials. The deficiency of oxygen was believed to be due to the Ar+ bombardment of the film surface during the cleaning procedure before XPS measurements. These results clearly indicate that completely c-axis textured and nearly stoichiometric LiNbO3 films can be deposited using stoichiometric LiNbO3 ceramic target by selecting an appropriate deposition temperature and oxygen pressure. The surface morphology of LiNbO3 film is a very important parameter for its applications, especially for integrated-optic devices. The smooth surface with uniform, compact and small grains was expected. Fig. 3(a) and (b) are SEM micrographs at two magnifications of the LiNbO3 film deposited at the temperature of 600 ◦ C and the oxygen pressure of 30 Pa. Fig. 3(a) shows that the surface of LiNbO3 film is mirror-like, crack-free and no large droplets. A closer examination of the surface (Fig. 3(b)) shows the as-grown film contained a large amount of small and uniform grains. The grain size is about 35 nm. When considering potential application in a waveguide, this small grain-size is more suitable than a large one as the surface scattering ratio of a small grain size is lower than that of a large one. As shown in the TEM photograph in Fig. 4, LiNbO3 films have columnar grain structures with their c-axis well aligned throughout the film depth, and the thickness of the amorphous SiO2 layer is about 230 nm. Moreover, the interfaces of the amorphous SiO2 with the LiNbO3 thin film and silicon wafer
696
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Fig. 3. SEM images of LiNbO3 film deposited on SiO2 /Si at 600 ◦ C and 30 Pa for two magnifications.
Fig. 4. Cross-sectional TEM image of LiNbO3 films deposited at 600 ◦ C and 30 Pa.
Fig. 2. XPS spectrum of LiNbO3 films deposited at 600 ◦ C in the oxygen pressure of 30 Pa.
are very clear. Fig. 5 shows a high-resolution cross-sectional TEM image of c-axis LiNbO3 on SiO2 /Si substrate deposited at a 600 ◦ C in the oxygen pressure of 30 Pa and the electron diffraction pattern. The TEM and electron diffraction patterns show that the as-deposited LiNbO3 film has a good crystalline quality and completely c-axis orientation. As shown in TEM image, the interface of the amorphous SiO2 with the LiNbO3 thin film is very sharp and smooth, and there is little atomic diffusion takes place on the interfaces. This sharp interface is expected to be favourable to reduce the interface scattering of light. The refractive indexes and propagation losses for several LiNbO3 films are measured by a prism coupler method and a 632.8 nm He–Ne laser. The refractive indexes of the films are obtained. The results are listed in Table 1. From the table, the n e is found to be between 2.200 and 2.215 and n o between 2.275 and 2.280. These values are very well matched with those for stoichiometric bulk LiNbO3 crystal (n e = 2.20 and n o = 2.29). The optical propagation losses of the TE0 mode for several LiNbO3 films are measured. As shown in Fig. 6, the propagation loss of the film deposited at the substrate
Fig. 5. High-resolution cross-sectional TEM image and the electron diffraction pattern of c-axis LiNbO3 deposited at 600 ◦ C and 30 Pa.
temperature 600 ◦ C and the oxygen pressure 30 Pa is as low as 1.14 dB/cm. It is known that the optical propagation loss can be influenced by film orientation, crystallinity and Li deficient phase (LiNb3 O8 ). From Fig. 1, it can be seen that the films grown under different parameters have different crystallinity, c-axis orientation, and LiNbO3 film deposited at the substrate temperature 600 ◦ C and the oxygen pressure 30 Pa has high quality crystallinity, completely c-axis orientation and nearly stoichiometry. Thus, LiNbO3 film grown under 600 ◦ C and 30 Pa has a minimum propagation loss.
X. Wang et al. / Solid State Communications 142 (2007) 694–697
697
of LiNbO3 films. Completely c-axis oriented and nearly stoichiometric LiNbO3 films can be deposited at 600 ◦ C in the oxygen pressure of 30 Pa. The films with column grain exhibit smooth surface, uniform and small grain size, and the sharply defined interface. The optical indexes of the deposited films matched very closely stoichiometric bulk LiNbO3 crystals, and the optical propagation losses are as low as 1.14 dB/cm at a wavelength of 632.8 nm. Acknowledgements This work was financially supported by the National Nature Science Foundation (NNSF) of China under the grant No. 90101009. References Fig. 6. TE0 mode optical-waveguiding-loss measurement in LiNbO3 films deposited at (a) 600 ◦ C, 10 Pa O2 , (b) 650 ◦ C, 30 Pa O2 , (c) 600 ◦ C, 30 Pa O2 . Table 1 The refractive indexes of LiNbO3 films under various oxygen pressure and substrate temperature Sample
Ts (◦ C)
PO2 (Pa)
no
ne
a b c
600 650 600
10 30 30
2.277 2.280 2.275
2.215 2.212 2.200
4. Conclusion High quality, low loss LiNbO3 films have been grown on SiO2 /Si substrates by the PLD without the application of external electric field or any other buffer layers. The results show that substrate temperature and oxygen pressure are the key parameters for the stoichiometry and c-axis orientation
[1] W.S. Hu, Z.G. Liu, Z.C. Wu, J.M. Liu, X.Y. Chen, D. Feng, Appl. Surf. Sci. 141 (1999) 197. [2] W.S. Hu, Z.G. Liu, Y.Q. Lu, S.N. Zhu, D. Feng, Opt. Lett. 21 (1996) 946. [3] Y. Kakehi, A. Okamoto, Y. Sakurai, Y. Nishikawa, T. Yotsuya, S. Ogawa, Appl. Surf. Sci. 169–170 (2001) 560. [4] J.A. Chaos, J. Gonzalo, C.N. Afonso, J. Perri`ere, M.T. Garc´ıa-Gonz´alez, Appl. Phys. A 72 (2001) 705. [5] I. Tsukada, S. Higuchi, Japan J. Appl. Phys. 43 (2004) 5307. [6] Y. Shibata, K. Kaya, K. Akashi, T. Kawai, S. Kawai, Appl. Phys. Lett. 61 (1992) 1000. [7] S. Yamada, Y. Nishibe, S. Ohtsubo, Y. Yonezawa, A. Morimoto, T. Shimizu, K. Ishida, Y. Masaki, Japan J. Appl. Phys. 41 (2002) L275. [8] D.W. Kim, S.M. Oh, S.H. Lee, S.H. Lee, T.W. Noh, Japan J. Appl. Phys. 37 (1998) 2016. [9] D.H. Lee, B.C. Shin, B.H. Min, Mater. Sci. Eng. B 95 (2002) 137. [10] H.K. Lam, J.Y. Dai, H.L.W. Chan, J. Cryst. Growth 268 (2004) 144. [11] X.C. Wang, Z.Z. Ye, G.B. Wu, L.L. Cao, B.H. Zhao, Mater. Lett. 59 (2005) 2994. [12] D. Ghina, C. Ghina, M. Gartner, V. Nelea, C. Martin, A. Cavaleru, I.N. Mihailescu, Appl. Surf. Sci. 138–139 (1999) 617. [13] S.K. Park, M.S. Baek, S.C. Bae, S.Y. Kwun, K.T. Kim, K.W. Kim, Solid State Commun. 111 (1999) 347.