- Email: [email protected]
Sol–gel waveguide thin film of YBO3: preparation and characterization
Optical Materials 15 (2000) 1±6
www.elsevier.nl/locate/optmat
Sol±gel waveguide thin ®lm of YBO3: preparation and characterization L. Lou a,*, D. Boyer b, G. Bertrand-Chadeyron b, E. Bernstein c, R. Mahiou b, J. Mugnier a a
Laboratoire Physico-Chimie des Mat eriaux Luminescents (CNRS-UMR 5620), Universit e Lyon 1, 43, Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France b Laboratoire des Mat eriaux Inorganiques ESA 6002, Universit e Blaise Pascal et ENSCCF.F, 63177 Aubi ere Cedex, France c D epartment de Physique des Mat eriaux (CNRS-UMR 5586), Universit e Lyon 1, 43, Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 30 June 1999; accepted 22 February 2000
Abstract Pure and Eu3 ions doped YBO3 waveguide thin ®lms were dip-coated for the ®rst time through sol±gel route. The quality of the ®lm depends signi®cantly on the dip-coating conditions, especially on environment humidity. We succeeded to prepare crack free thick ®lms with 16 layers and thickness of about 850 nm. It is found that the coating process obeys a two-step kinetics. The ®lms were characterized by dierent methods, such as m-line spectroscopy, X-ray diraction, electron microscopy and waveguide ¯uorescence spectroscopy (WFS). When the annealing temperature is lower than 600°C the ®lm is amorphous and has good waveguide performance. The attenuation of the propagation we have reached is about 0.5 dB/cm. The ®lm starts to crystallize at 700°C. The waveguide performance of the ®lm decreases dramatically with the annealing temperature above 700°C. The result of WFS also indicates that this new kind of thin ®lm is a good host material for active dopants. Our preliminary results show that the new thin ®lm is of great potential for future application in optoelectronics. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 78.66.w; 68.55.jk; 68.55.nq Keywords: Yttrium orthoborate; Sol±gel thin ®lm; Waveguide
1. Introduction Orthoborates materials are of interest due to their high UV transparency, their non-linear
* Corresponding author. Permanent address: Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, PeopleÕs Republic of China. Tel.: +86-5513601064; fax: +86-551-3631760. E-mail address: [email protected] (L. Lou).
properties and their exceptional optical damage threshold. Many of them, such as yttrium and lanthanide orthoborates, are also good host materials for active luminescent dopants. Dierent methods have been developed to prepare orthoborate powders, mainly solid state reaction, wet process and sol±gel process. From general aspects of the material, its physical and chemical properties, thin ®lms of yttrium and lanthanide orthoborates would be attractive for future applications. But no papers, as far as we know, on thin ®lms of
0925-3467/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 0 ) 0 0 0 1 4 - 8
2
L. Lou et al. / Optical Materials 15 (2000) 1±6
YBO3 and related compounds were reported. In recent years, a new sol±gel method has been developed to produce pure and doped YBO3 powders [1]. It provides a new possible way to prepare thin ®lms of YBO3 . In this paper, we report for the ®rst time, the success in preparing a waveguide thin ®lm of YBO3 through the sol±gel route. Both pure and doped YBO3 waveguide thin ®lms were prepared and characterized by dierent methods. Our preliminary work shows that this kind of ®lms has good mechanical and optical properties and is of great potential for future application in optoelectronics.
2. Film preparation YBO3 sol±gel thin ®lms are prepared using a dip-coating technique. In general, the quality of sol±gel ®lms depends on many parameters of the ®lm deposition process, especially the humidity of the environment, in which the ®lm is dip-coated. Although for many materials the ®lm with reasonable quality can be prepared without special care for the humidity, for rare earth orthoborates, such as YBO3 , LuBO3 , the humidity greatly aects the quality of the coated ®lms. This is similar to the situation of borosilicate [2]. For this reason, a humidity controllable dip-coating system was used (Fig. 1). As shown in the ®gure the dry and clean N2 gas continuously passes through the water and brings the water vapor to the dip-coating chamber or the
Fig. 1. Schematic diagram of humidity controllable dip-coating system.
N2 gas goes directly to the chamber. Which way will be chosen depends on the required humidity and the surrounding atmosphere. The relative humidity in the chamber is monitored by a relative humidity sensor and modulated by adjusting a valve to control the ¯ow of the gas. Dierent kinds of substrates were used, which included ordinary glass slide, pyrex plate, fused silica plate, silica/silicon wafer and silicon wafer. The boron yttrium and boron europium heterometallic alkoxides were prepared from lanthanide salt [1]. The sol is ®ltered using a 0.2-lm ®lter and kept in a Te¯on bath. After carefully cleaning the substrate was dipped into the sol bath for a few seconds. Then it was drawn out smoothly. The lifting speed was around 6±10 cm/min, which was dependent upon the thickness of a single layer we wanted to get. After coating, each layer was dried at 80°C for 15 min and then heat treated at a given temperature in a silica tube with continuous ¯ow of oxygen gas. The thickness of a single layer was between 40 and 60 nm. If the layer is too thick then it will crack even at lower heat treatment temperature. Repeating the same procedure a multicoated thicker ®lm can be prepared. For some applications, for example, thin ®lm scintillator, a thick ®lm is preferable. Up to now, we have succeeded to make a crack free waveguide thin ®lm with 16 layers and the thickness of about 850 nm. In order to get high quality transparent ®lm it is necessary to choose the coating condition properly. We found a few factors, which strongly aect the quality of the ®lm of which the environment humidity plays the most important role. When the humidity is too high the coated ®lm is opalescent and milky, which cannot support propagation modes in the ®lm. If the humidity is too low we will see that the ®lm is clearly divided into two parts with an opaque boundary between them (Fig. 2). In order to avoid these two situations the surrounding humidity for coating these kinds of ®lms should be chosen carefully, which would be called proper humidity. The second factor is the aging time of the sol. For a fresh sol, the proper relative humidity for dip-coating is higher. With the aging of the sol the proper humidity decreases. It is found that the aging process itself depends on the environment
L. Lou et al. / Optical Materials 15 (2000) 1±6
Fig. 2. A typical dip-coated layer showing two-step kinetics.
humidity. If the sol is isolated from the humidity the aging process will slow down. It is also noticed that the evaporation of the solvent and sol polymerization are important factors during the aging process. The sol can be stabilized by adding certain amounts of acetylacetone and hence can be used for longer time. For stabilized sol, the proper humidity is also dierent from the one for non-stabilized sol. Because the aging process of the sol is complicated and not easy to control, in all cases the proper humidity for dip-coating has to be determined experimentally. A prominent feature of YBO3 dip-coating process is a Ôtwo-step kineticsÕ for ®lm formation which appears more clearly at a lower relative humidity. It is noticed that a very thin ®lm forms ®rst on the substrate at the beginning stage of the substrate drawing. At the same time, the liquid shrinks into the center part of the substrate. Up to a certain time a thicker layer starts to form and the liquid stops shrinking further into the center. At the beginning of the thick part of the layer, there is a narrow opaque region. A dip-coated layer typical for two-step kinetics is shown in Fig. 2. The area of the very thin ®rst part of the dipcoated ®lm depends on the humidity and drawing speed. The area increases when the humidity and/ or drawing speed decreases. We proposed a possible model to understand the phenomenon. During the dip-coating two different processes take place at the same time: evaporation of the organic solvent and the polymerization (condensation). The latter is governed by the humidity of the environment. These two
3
processes can be considered independent, and compete with each other. During the ®rst stage of the substrate drawing, these two competing processes have not reached the dynamical equilibrium state. When the humidity is low the polymerization process develops slowly and the liquid is less viscous. Therefore, only a very thin layer remains on the substrate. The excess liquid goes down along the substrate and the viscosity of the liquid increases with time mainly due to evaporation of the solvent. At certain times, these processes reach the balance and the second stage starts. A thick layer starts to form on the substrate. On the other extreme case, the humidity is high and the polymerization is fast. Many gel particles form in the coated layer during the substrate drawing, which causes the ®lm to be opalescent. 3. Characterization of YBO3 thin ®lm Dierent methods have been used to characterize the ®lm and to follow its time evolution. 3.1. m-lines spectroscopy [3] m-lines spectroscopy is an useful method for determining optogeometric parameters of thin ®lms, such as thickness and refractive index. The ®lm of YBO3 is mechanically good for prism coupling. A multicoated ®lm should be used for mlines spectroscopic measurement. It is found that the ®lm with about ®ve layers can support one mode of each polarization (1 transverse electric mode: TE0 and 1 transverse magnetic mode: TM0 ). Although it is possible to determine its refractive index and thickness using m-lines spectroscopy using TE0 and TM0 modes, it will be more accurate for optogeometric analysis to use a sample which can support two or more modes of propagation of each polarization. For this purpose, a ®lm with 16 coatings was prepared, which supports two TE (TE0 and TE1 ) and two TM (TM0 and TM1 ) modes. Under the assumption of a stepwise index pro®le, the m-lines spectroscopic measurement on the 600°C heat-treated waveguide gives an index of 1:592 0:001 at 543.5 nm and a thickness of 850 nm. The result is consistent with
4
L. Lou et al. / Optical Materials 15 (2000) 1±6
the ellipsometry measurements for a single layer sample. A single crack free transparent layer of YBO3 was prepared on silicon wafer and checked by spectroscopic ellipsometry (SOPRA ES 4G). Its refractive index is 1.59 and the thickness of the single layer is between 40 and 60 nm depending on the dip-coating condition. The consistency con®rms the validity of the assumption about the stepwise index pro®le for multicoated YBO3 ®lms. 3.2. Attenuation of the propagation (transmission losses) The scattering-detection method was used for the measurement of transmission losses of a ®lm. A CCD camera took the image of the scattered light intensity distribution along the propagation of a guided wave (He±Ne laser, k 632.8 nm). The digitized image was then analysed to determine the propagation losses. The ®lm with annealing temperature below 600°C is transparent and has a good propagation performance. The best result for propagation attenuation of YBO3 thin ®lms we have obtained was as low as 0:5 0:1 dB/cm. However, when the annealing temperature is higher than 700°C the transmission losses increased dramatically and the propagation length of the laser beam was around 0.5 cm. 3.3. Structure evolution with annealing temperature X-ray diraction, optical microscopy and electron microscopy were used to follow the structure evolution of the ®lm with the annealing temperature. A special sample of YBO3 was dip-coated on a fused silica substrate. It has seven layers. After coating of each layer the sample was dried at 80°C for 15 min and then annealed at 400°C for 15 min. The sample was then annealed at higher temperatures ranging from 500°C to 1000°C for 15 min. The samples with dierent annealing temperatures T were analyzed by X-ray diractometer at room temperature. The typical results are shown in Fig. 3. It can be seen clearly from the ®gure that the ®lm is amorphous when the heat treatment temperature is below 600°C and the sample starts to crystallize at 700°C. The peak which appears at
Fig. 3. X-ray diraction of YBO3 ®lms with dierent annealing temperatures: (a) 600°C; (b) 700°C; (c) 1000°C.
29.5° (2h) corresponds to Y2 O3 which remains at 700°C and disappears after heating at higher temperature. The others peaks agree with the features of X-rays diraction spectrum of the vaterite YBO3 [4] in powdered form. However, due to the layered form of YBO3 in the present case, the peak relative to the diraction by 0 0 L and 1 0 L (with L 2n) reticular plans appear the more intense ones. Such observation can be correlated with the lamellar character of the YBO3 waveguide induced by the weak thickness of the ®lm. A sample of only one layer was prepared on silica subtstrate. It was annealed at 700°C, then peeled o and put on a grid for transmission electron microscopy (TEM) analyses. Conventional (CTEM) and high resolution (HRTEM) observations con®rmed that the ®lm begins to crystallize at 700°C. Most of the sample was found to be still amorphous, or just started to crystallize and form very small crystals. However some larger crystals, as those shown in Fig. 4(a), were also present in this ®lm. Their structure, as con®rmed by electron diraction patterns, correspond to YBO3 (see Fig. 4(b)). This fact demonstrates that waveguide performance degrades dramatically when the annealing temperature exceeds 700°C. The crystal growth with time can be monitored by an optical microscope. When the sample was heated at 700°C we could see some small spots on the ®lm beginning to appear. These small spots grew with the annealing time.
L. Lou et al. / Optical Materials 15 (2000) 1±6
5
Fig. 4. Electron micrographs of a sample annealed at 700°C: (a) CTEM image showing some big crystals (dark zones) of YBO3 ; (b) associated diraction pattern, that can be indexed as hexagonal YBO3 by the three observable diraction ring corresponding to (1 0 0), (1 0 1) and (1 0 2) re¯ections.
3.4. Fluorescence of the doped YBO3 ®lm Eu3 ions are doped into the ®lm. The doping does not change the propagation performance of the ®lm. The ¯uorescence of the sample was measured under the waveguide con®guration (WFS) [5]. Fig. 5 gives the measured emission spectra for samples heat-treated at 500° and 600°C. When the heat treatment temperature is equal to or below 500°C, the spectra were composed of a broad band which is not sensitive to the annealing temperature. Compared with the ¯uorescence of the sol or gel, we can observe that this band is due to the organic residues. For the sample heat-treated at 600°C, the ¯uorescence spectrum changed completely. The ¯uorescence mainly comes from Eu3 ions. The spectrum is dierent from the one of the high temperature annealed powdered sample [6]. The main peak of the spectrum is located at about 16 200 cmÿ1 , which corresponds to the transition 5 D0 ® 7 F2 , exhibiting the feature of a non-central
Fig. 5. WFS of YBO3 thin ®lms annealed at: (a) 500°C; (b) 600°C for 15 min (excitation: argon laser line 514.5 nm, 100 mw).
symmetric site. Such observation is consistent with the amorphous structure of the ®lm. 4. Conclusion 1. Pure and doped YBO3 waveguide thin ®lm have been prepared through the sol±gel route for the ®rst time and its optogeometric parameters have been determined. 2. The coating process of the ®lm is very sensitive to the environment humidity. In order to get a high quality transparent ®lm the proper humidity should be chosen for ®lm coating. 3. The ®lm is amorphous when the annealing temperature is below 600°C and starts to crystallize at 700°C.
6
L. Lou et al. / Optical Materials 15 (2000) 1±6
4. The amorphous ®lm has a good waveguide performance. The transmission losses have reached 0.5 dB/cm. 5. YBO3 and related compounds are good matrices for optically active dopants. It is a promising new kind of thin ®lm materials for future application in optoelectronics and for thin ®lm scintillator. Acknowledgements This work was partly funded by Rh^ one±Alpes region. One of the authors, L. Lou, is grateful to Rh^ one±Alpes region for the ®nancial support. The authors acknowledge many helpful discussions
with Dr. J.C. Plenet, C. Bovier, C. Pedrini and C. Dujardin.
References [1] D. Boyer, G. Betrand-Chadeyron, R. Mahiou, C. Caperaa, J.C. Cousseins, J. Mater. Chem. 9 (1999) 211. [2] M.A. Villegas, M. Aparicio, A. Dur an, J. Non-Cryst. Solids 218 (1997) 146. [3] M. Ulrich, R. Torge, Appl. Opt. 12 (1973) 2901. [4] G. Chadeyron, M. El-Ghozzi, R. Mahiou, A. Arbus, J.C. Cousseins, J. Solid State Chem. 128 (1997) 261. [5] A. Bahtat, M. Bouazaoui, M. Bahtat, J. Mugnier, Opt. Commun. 111 (1994) 55. [6] G. Chadeyron, R. Mahiou, M. El-Ghozzi, A. Arbus, D. Zambon, J.C. Cousseins, J. Lumines. 72±74 (1997) 564.
www.elsevier.nl/locate/optmat
Sol±gel waveguide thin ®lm of YBO3: preparation and characterization L. Lou a,*, D. Boyer b, G. Bertrand-Chadeyron b, E. Bernstein c, R. Mahiou b, J. Mugnier a a
Laboratoire Physico-Chimie des Mat eriaux Luminescents (CNRS-UMR 5620), Universit e Lyon 1, 43, Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France b Laboratoire des Mat eriaux Inorganiques ESA 6002, Universit e Blaise Pascal et ENSCCF.F, 63177 Aubi ere Cedex, France c D epartment de Physique des Mat eriaux (CNRS-UMR 5586), Universit e Lyon 1, 43, Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 30 June 1999; accepted 22 February 2000
Abstract Pure and Eu3 ions doped YBO3 waveguide thin ®lms were dip-coated for the ®rst time through sol±gel route. The quality of the ®lm depends signi®cantly on the dip-coating conditions, especially on environment humidity. We succeeded to prepare crack free thick ®lms with 16 layers and thickness of about 850 nm. It is found that the coating process obeys a two-step kinetics. The ®lms were characterized by dierent methods, such as m-line spectroscopy, X-ray diraction, electron microscopy and waveguide ¯uorescence spectroscopy (WFS). When the annealing temperature is lower than 600°C the ®lm is amorphous and has good waveguide performance. The attenuation of the propagation we have reached is about 0.5 dB/cm. The ®lm starts to crystallize at 700°C. The waveguide performance of the ®lm decreases dramatically with the annealing temperature above 700°C. The result of WFS also indicates that this new kind of thin ®lm is a good host material for active dopants. Our preliminary results show that the new thin ®lm is of great potential for future application in optoelectronics. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 78.66.w; 68.55.jk; 68.55.nq Keywords: Yttrium orthoborate; Sol±gel thin ®lm; Waveguide
1. Introduction Orthoborates materials are of interest due to their high UV transparency, their non-linear
* Corresponding author. Permanent address: Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, PeopleÕs Republic of China. Tel.: +86-5513601064; fax: +86-551-3631760. E-mail address: [email protected] (L. Lou).
properties and their exceptional optical damage threshold. Many of them, such as yttrium and lanthanide orthoborates, are also good host materials for active luminescent dopants. Dierent methods have been developed to prepare orthoborate powders, mainly solid state reaction, wet process and sol±gel process. From general aspects of the material, its physical and chemical properties, thin ®lms of yttrium and lanthanide orthoborates would be attractive for future applications. But no papers, as far as we know, on thin ®lms of
0925-3467/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 0 ) 0 0 0 1 4 - 8
2
L. Lou et al. / Optical Materials 15 (2000) 1±6
YBO3 and related compounds were reported. In recent years, a new sol±gel method has been developed to produce pure and doped YBO3 powders [1]. It provides a new possible way to prepare thin ®lms of YBO3 . In this paper, we report for the ®rst time, the success in preparing a waveguide thin ®lm of YBO3 through the sol±gel route. Both pure and doped YBO3 waveguide thin ®lms were prepared and characterized by dierent methods. Our preliminary work shows that this kind of ®lms has good mechanical and optical properties and is of great potential for future application in optoelectronics.
2. Film preparation YBO3 sol±gel thin ®lms are prepared using a dip-coating technique. In general, the quality of sol±gel ®lms depends on many parameters of the ®lm deposition process, especially the humidity of the environment, in which the ®lm is dip-coated. Although for many materials the ®lm with reasonable quality can be prepared without special care for the humidity, for rare earth orthoborates, such as YBO3 , LuBO3 , the humidity greatly aects the quality of the coated ®lms. This is similar to the situation of borosilicate [2]. For this reason, a humidity controllable dip-coating system was used (Fig. 1). As shown in the ®gure the dry and clean N2 gas continuously passes through the water and brings the water vapor to the dip-coating chamber or the
Fig. 1. Schematic diagram of humidity controllable dip-coating system.
N2 gas goes directly to the chamber. Which way will be chosen depends on the required humidity and the surrounding atmosphere. The relative humidity in the chamber is monitored by a relative humidity sensor and modulated by adjusting a valve to control the ¯ow of the gas. Dierent kinds of substrates were used, which included ordinary glass slide, pyrex plate, fused silica plate, silica/silicon wafer and silicon wafer. The boron yttrium and boron europium heterometallic alkoxides were prepared from lanthanide salt [1]. The sol is ®ltered using a 0.2-lm ®lter and kept in a Te¯on bath. After carefully cleaning the substrate was dipped into the sol bath for a few seconds. Then it was drawn out smoothly. The lifting speed was around 6±10 cm/min, which was dependent upon the thickness of a single layer we wanted to get. After coating, each layer was dried at 80°C for 15 min and then heat treated at a given temperature in a silica tube with continuous ¯ow of oxygen gas. The thickness of a single layer was between 40 and 60 nm. If the layer is too thick then it will crack even at lower heat treatment temperature. Repeating the same procedure a multicoated thicker ®lm can be prepared. For some applications, for example, thin ®lm scintillator, a thick ®lm is preferable. Up to now, we have succeeded to make a crack free waveguide thin ®lm with 16 layers and the thickness of about 850 nm. In order to get high quality transparent ®lm it is necessary to choose the coating condition properly. We found a few factors, which strongly aect the quality of the ®lm of which the environment humidity plays the most important role. When the humidity is too high the coated ®lm is opalescent and milky, which cannot support propagation modes in the ®lm. If the humidity is too low we will see that the ®lm is clearly divided into two parts with an opaque boundary between them (Fig. 2). In order to avoid these two situations the surrounding humidity for coating these kinds of ®lms should be chosen carefully, which would be called proper humidity. The second factor is the aging time of the sol. For a fresh sol, the proper relative humidity for dip-coating is higher. With the aging of the sol the proper humidity decreases. It is found that the aging process itself depends on the environment
L. Lou et al. / Optical Materials 15 (2000) 1±6
Fig. 2. A typical dip-coated layer showing two-step kinetics.
humidity. If the sol is isolated from the humidity the aging process will slow down. It is also noticed that the evaporation of the solvent and sol polymerization are important factors during the aging process. The sol can be stabilized by adding certain amounts of acetylacetone and hence can be used for longer time. For stabilized sol, the proper humidity is also dierent from the one for non-stabilized sol. Because the aging process of the sol is complicated and not easy to control, in all cases the proper humidity for dip-coating has to be determined experimentally. A prominent feature of YBO3 dip-coating process is a Ôtwo-step kineticsÕ for ®lm formation which appears more clearly at a lower relative humidity. It is noticed that a very thin ®lm forms ®rst on the substrate at the beginning stage of the substrate drawing. At the same time, the liquid shrinks into the center part of the substrate. Up to a certain time a thicker layer starts to form and the liquid stops shrinking further into the center. At the beginning of the thick part of the layer, there is a narrow opaque region. A dip-coated layer typical for two-step kinetics is shown in Fig. 2. The area of the very thin ®rst part of the dipcoated ®lm depends on the humidity and drawing speed. The area increases when the humidity and/ or drawing speed decreases. We proposed a possible model to understand the phenomenon. During the dip-coating two different processes take place at the same time: evaporation of the organic solvent and the polymerization (condensation). The latter is governed by the humidity of the environment. These two
3
processes can be considered independent, and compete with each other. During the ®rst stage of the substrate drawing, these two competing processes have not reached the dynamical equilibrium state. When the humidity is low the polymerization process develops slowly and the liquid is less viscous. Therefore, only a very thin layer remains on the substrate. The excess liquid goes down along the substrate and the viscosity of the liquid increases with time mainly due to evaporation of the solvent. At certain times, these processes reach the balance and the second stage starts. A thick layer starts to form on the substrate. On the other extreme case, the humidity is high and the polymerization is fast. Many gel particles form in the coated layer during the substrate drawing, which causes the ®lm to be opalescent. 3. Characterization of YBO3 thin ®lm Dierent methods have been used to characterize the ®lm and to follow its time evolution. 3.1. m-lines spectroscopy [3] m-lines spectroscopy is an useful method for determining optogeometric parameters of thin ®lms, such as thickness and refractive index. The ®lm of YBO3 is mechanically good for prism coupling. A multicoated ®lm should be used for mlines spectroscopic measurement. It is found that the ®lm with about ®ve layers can support one mode of each polarization (1 transverse electric mode: TE0 and 1 transverse magnetic mode: TM0 ). Although it is possible to determine its refractive index and thickness using m-lines spectroscopy using TE0 and TM0 modes, it will be more accurate for optogeometric analysis to use a sample which can support two or more modes of propagation of each polarization. For this purpose, a ®lm with 16 coatings was prepared, which supports two TE (TE0 and TE1 ) and two TM (TM0 and TM1 ) modes. Under the assumption of a stepwise index pro®le, the m-lines spectroscopic measurement on the 600°C heat-treated waveguide gives an index of 1:592 0:001 at 543.5 nm and a thickness of 850 nm. The result is consistent with
4
L. Lou et al. / Optical Materials 15 (2000) 1±6
the ellipsometry measurements for a single layer sample. A single crack free transparent layer of YBO3 was prepared on silicon wafer and checked by spectroscopic ellipsometry (SOPRA ES 4G). Its refractive index is 1.59 and the thickness of the single layer is between 40 and 60 nm depending on the dip-coating condition. The consistency con®rms the validity of the assumption about the stepwise index pro®le for multicoated YBO3 ®lms. 3.2. Attenuation of the propagation (transmission losses) The scattering-detection method was used for the measurement of transmission losses of a ®lm. A CCD camera took the image of the scattered light intensity distribution along the propagation of a guided wave (He±Ne laser, k 632.8 nm). The digitized image was then analysed to determine the propagation losses. The ®lm with annealing temperature below 600°C is transparent and has a good propagation performance. The best result for propagation attenuation of YBO3 thin ®lms we have obtained was as low as 0:5 0:1 dB/cm. However, when the annealing temperature is higher than 700°C the transmission losses increased dramatically and the propagation length of the laser beam was around 0.5 cm. 3.3. Structure evolution with annealing temperature X-ray diraction, optical microscopy and electron microscopy were used to follow the structure evolution of the ®lm with the annealing temperature. A special sample of YBO3 was dip-coated on a fused silica substrate. It has seven layers. After coating of each layer the sample was dried at 80°C for 15 min and then annealed at 400°C for 15 min. The sample was then annealed at higher temperatures ranging from 500°C to 1000°C for 15 min. The samples with dierent annealing temperatures T were analyzed by X-ray diractometer at room temperature. The typical results are shown in Fig. 3. It can be seen clearly from the ®gure that the ®lm is amorphous when the heat treatment temperature is below 600°C and the sample starts to crystallize at 700°C. The peak which appears at
Fig. 3. X-ray diraction of YBO3 ®lms with dierent annealing temperatures: (a) 600°C; (b) 700°C; (c) 1000°C.
29.5° (2h) corresponds to Y2 O3 which remains at 700°C and disappears after heating at higher temperature. The others peaks agree with the features of X-rays diraction spectrum of the vaterite YBO3 [4] in powdered form. However, due to the layered form of YBO3 in the present case, the peak relative to the diraction by 0 0 L and 1 0 L (with L 2n) reticular plans appear the more intense ones. Such observation can be correlated with the lamellar character of the YBO3 waveguide induced by the weak thickness of the ®lm. A sample of only one layer was prepared on silica subtstrate. It was annealed at 700°C, then peeled o and put on a grid for transmission electron microscopy (TEM) analyses. Conventional (CTEM) and high resolution (HRTEM) observations con®rmed that the ®lm begins to crystallize at 700°C. Most of the sample was found to be still amorphous, or just started to crystallize and form very small crystals. However some larger crystals, as those shown in Fig. 4(a), were also present in this ®lm. Their structure, as con®rmed by electron diraction patterns, correspond to YBO3 (see Fig. 4(b)). This fact demonstrates that waveguide performance degrades dramatically when the annealing temperature exceeds 700°C. The crystal growth with time can be monitored by an optical microscope. When the sample was heated at 700°C we could see some small spots on the ®lm beginning to appear. These small spots grew with the annealing time.
L. Lou et al. / Optical Materials 15 (2000) 1±6
5
Fig. 4. Electron micrographs of a sample annealed at 700°C: (a) CTEM image showing some big crystals (dark zones) of YBO3 ; (b) associated diraction pattern, that can be indexed as hexagonal YBO3 by the three observable diraction ring corresponding to (1 0 0), (1 0 1) and (1 0 2) re¯ections.
3.4. Fluorescence of the doped YBO3 ®lm Eu3 ions are doped into the ®lm. The doping does not change the propagation performance of the ®lm. The ¯uorescence of the sample was measured under the waveguide con®guration (WFS) [5]. Fig. 5 gives the measured emission spectra for samples heat-treated at 500° and 600°C. When the heat treatment temperature is equal to or below 500°C, the spectra were composed of a broad band which is not sensitive to the annealing temperature. Compared with the ¯uorescence of the sol or gel, we can observe that this band is due to the organic residues. For the sample heat-treated at 600°C, the ¯uorescence spectrum changed completely. The ¯uorescence mainly comes from Eu3 ions. The spectrum is dierent from the one of the high temperature annealed powdered sample [6]. The main peak of the spectrum is located at about 16 200 cmÿ1 , which corresponds to the transition 5 D0 ® 7 F2 , exhibiting the feature of a non-central
Fig. 5. WFS of YBO3 thin ®lms annealed at: (a) 500°C; (b) 600°C for 15 min (excitation: argon laser line 514.5 nm, 100 mw).
symmetric site. Such observation is consistent with the amorphous structure of the ®lm. 4. Conclusion 1. Pure and doped YBO3 waveguide thin ®lm have been prepared through the sol±gel route for the ®rst time and its optogeometric parameters have been determined. 2. The coating process of the ®lm is very sensitive to the environment humidity. In order to get a high quality transparent ®lm the proper humidity should be chosen for ®lm coating. 3. The ®lm is amorphous when the annealing temperature is below 600°C and starts to crystallize at 700°C.
6
L. Lou et al. / Optical Materials 15 (2000) 1±6
4. The amorphous ®lm has a good waveguide performance. The transmission losses have reached 0.5 dB/cm. 5. YBO3 and related compounds are good matrices for optically active dopants. It is a promising new kind of thin ®lm materials for future application in optoelectronics and for thin ®lm scintillator. Acknowledgements This work was partly funded by Rh^ one±Alpes region. One of the authors, L. Lou, is grateful to Rh^ one±Alpes region for the ®nancial support. The authors acknowledge many helpful discussions
with Dr. J.C. Plenet, C. Bovier, C. Pedrini and C. Dujardin.
References [1] D. Boyer, G. Betrand-Chadeyron, R. Mahiou, C. Caperaa, J.C. Cousseins, J. Mater. Chem. 9 (1999) 211. [2] M.A. Villegas, M. Aparicio, A. Dur an, J. Non-Cryst. Solids 218 (1997) 146. [3] M. Ulrich, R. Torge, Appl. Opt. 12 (1973) 2901. [4] G. Chadeyron, M. El-Ghozzi, R. Mahiou, A. Arbus, J.C. Cousseins, J. Solid State Chem. 128 (1997) 261. [5] A. Bahtat, M. Bouazaoui, M. Bahtat, J. Mugnier, Opt. Commun. 111 (1994) 55. [6] G. Chadeyron, R. Mahiou, M. El-Ghozzi, A. Arbus, D. Zambon, J.C. Cousseins, J. Lumines. 72±74 (1997) 564.