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Thermal evaporation of rare-earth chlorides: application to vapor phase deposition of rare earth-doped fluoride glass waveguides
Journal of Non-Crystalline Solids 276 (2000) 72±77
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Thermal evaporation of rare-earth chlorides: application to vapor phase deposition of rare earth-doped ¯uoride glass waveguides B. Boulard *, S. Coste, Y. Gao, C. Legein, C. Dugopolovski Laboratoire des Fluorures, UPRESA 6010, Facult e des sciences, Universit e du Maine, 72085 Le Mans Cedex 9, France Received 1 March 2000; received in revised form 7 April 2000
Abstract A systematic study of the evaporation rate for single and binary mixtures of rare earth (RE) chlorides (NdCl3 ; ErCl3 ; TmCl3 ; PrCl3 ; YbCl3 ) has been performed. Single RE chlorides follow the physical evaporation law except NdCl3 that could be explained by a possible dimerisation in the vapor state. Erbium- and neodymium-doped planar and channel waveguides have been prepared by dual evaporation of RE chloride and PbF2 ±ZnF2 ±GaF3 ¯uoride glass. Also, planar waveguides are attacked by atmospheric moisture whereas, channel waveguides are stable, suggesting that the deposited glass is more compact in the latter case. The low optical loss (0.4 dB/cm at 633 nm for 1.5 mol% Er-doped channel waveguide) outlines the good quality of these devices. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 42.8.E; 81.15; 68.55.L
1. Introduction Rare earth (RE)-doped ¯uoride glasses are of current interest for optical ampli®ers in the middle infrared region (i.e., 1.3 and 1.55 lm) [1], because of their low phonon energy as compared to oxides. In the last few years, the need for low cost and more compact devices has enhanced searches for planar and channel waveguides that can be prepared by dierent techniques such as sol±gel synthesis and ion exchange [2,3].
* Corresponding author. Tel.: +33-2 43 83 33 70; fax: +33-2 43 83 35 06. E-mail address: [email protected] (B. Boulard).
Physical vapor deposition of PbF2 ±ZnF2 ± GaF3 (PZG) ¯uoride glass has made possible the preparation of step index channel waveguides by the use of lithographic techniques such as `lift-o' [4]. Doping with RE has also been achieved by dual evaporation of RE ¯uoride and PZG glass [5] and optical ampli®cation has been demonstrated at 1.047 lm in NdF3 -doped channel waveguides [6]. Unfortunately, the optical losses were found to be quite high (6±9 dB/cm), probably because of convection currents in the RE containing crucible which is heated at a temperature >1100°C. The doping with RE chlorides may oer some advantages: (i) they are more volatile than the ¯uoride counterpart (the evaporation temperature is
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 2 6 9 - 6
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
750±800°C), at least three orders of magnitude higher, (ii) the radiative transition probabilities should be increased since RE±Cl bonds are more covalent than RE±F bond, (iii) chloride environment of the RE ion should lead to lower phonon energies and thus minimize non-radiative (multiphonon) relaxation of the excited ion [7,8]. One problem is the high sensitivity of chloride to moisture; however, PZG bulk glasses can accept up to 4 mol% chlorine [9]. In the ®rst step, the deposition rate of a single RE chloride has been measured in order to check the ability to control the doping rate with temperature. The experimental results reported so far are compared with thermodynamic data. Mixtures of xPrCl3 ± 100 ÿ xYbCl3 have also been studied since energy transfers are expected in co-doped ¯uoride glasses that increase the emission probability of the praseodymium ion [10]. In the second step, ErCl3 - or NdCl3 -doped PZG glass planar or channel waveguides have been prepared and their stability in atmospheric air are compared. The eect of structure of the deposited glass and RE distribution are discussed.
73
For waveguide fabrication, the RE chloride and PZG mixture were simultaneously evaporated in separate crucibles heated by an RF coil (Fig. 1). The substrate (CaF2 single crystal plates) was placed at the intersection of the two evaporation cones; the PZG crucible was held closer to the substrate so that the deposition rate was about 40 times higher for PZG than for the RE chloride, to achieve low doping rates (0.1% to a few %). The substrate was rotated during the deposition process to assure homogeneous doping and thickness (about 2 lm). In order to improve the stability of the RE chloride-doped ®lm in air, a protecting layer was deposited, simply by stopping the evaporation of the RE chloride; the thickness of the protecting layer was about 1±2 lm. After deposition, the substrate was cleaved along the (1 1 1) plane to allow light injection. Both planar and channel waveguides were prepared. In the latter case, the lift-o technique was used: the ®lm was deposited on the substrate covered with a resist mask made by classical lithographic techniques. The temperature of the substrate was 150°C, unless the resist mask could
2. Experimental conditions High purity RE chloride powders (>99.9%), either anhydrous (NdCl3 , ErCl3 , TmCl3 ) or hydrated (PrCl3 6H2 O, YbCl3 6H2 O) were used as starting materials. Thermal gravimetric analysis (TGA) of the hydrated chloride showed a complete dehydration at 210°C for PrCl3 and 235°C for YbCl3 . The evaporation of the RE chloride was conducted in a vessel connected to a vacuum system with a liquid nitrogen trap, allowing a pressure around 10ÿ4 mbar. The chloride powder (200 mg) was placed in a graphite crucible and heated using an RF coil. A silica substrate (25 mm in diameter) was held 5 cm above the crucible. The evaporation time varied from 10 to 45 min and the substrate was weighed in a glove box before and after evaporation. Prior to evaporation, the chloride powder was heated in the vessel under vacuum at 250°C for 30 min to make sure that the chloride was anhydrous.
Fig. 1. Scheme of the vessel used for dual evaporation of PZG glass and rare-earth chloride.
74
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
not be removed after deposition. More details on the technique are presented in Ref. [4].
3. Results 3.1. Evaporation of RE chlorides The deposition rates r of RECl3 were measured at dierent temperatures in the range 650±850°C. The shapes of the curves were found similar for
Fig. 2. Deposition rate versus temperature for RE chlorides: (a) pure RE chlorides; (b) comparison of pure NdCl3 and 50SrCl2 ±NdCl3 mixture.
PrCl3 , ErCl3 , TmCl3 and YbCl3 in the order of increasing deposition rate (Fig. 2(a)). For NdCl3 , r increased very quickly so that the doping rate could not be controlled easily. For this reason, we studied the evaporation of the SrCl2 ±NdCl3 mixture in order to decrease the evaporation rate of NdCl3 . SrCl2 was chosen because its volatility is negligible compared to NdCl3 and it forms a binary eutectic; the melting point was decreased from 760°C to 620°C for 50NdCl3 ±50SrCl2 [11]. No SrCl2 was actually detected in the deposit. Fig. 2(b) shows the deposition rate of pure NdCl3 and 50NdCl3 ±50SrCl2 : as can be seen, the evaporation rate is dramatically reduced. For any sample, no reduced species (such as RECl2 ) was detected on the X-ray diraction spectra of the deposit. The evaporation of xPrCl3 ± 100 ÿ xYbCl3 was investigated for the preparation of co-doped YbCl3 ±PrCl3 glasses; the study was conducted at 710°C. Since the deposition rates of pure PrCl3 and YbCl3 were quite dierent, non-congruent evaporation was predicted. The compositions of the deposit and the starting chloride mixture are presented in Fig. 3. The experimental curves are compared with the one deduced from Raoult's law
Fig. 3. Comparison between starting charge and deposit composition (in mol%) for binary xPrCl3 ± 100 ÿ xYbCl3 . The full line represents the composition predicted for an ideal solution (Raoult law).
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
75
that depicts the behavior of ideal solution (solid or liquid) xdeposit
xrPrCl3 1 ÿ xrYbCl3
xrPrCl3 rPrCl3 with 5:8 at 710°C: rYbCl3
1
3.2. RE chloride-doped waveguides The planar waveguides were very sensitive to atmospheric moisture whether or not they were protected with an undoped PZG layer. Shortly after the sample was removed from the evaporation vessel, small bubbles appeared at the surface. Sometimes, they enhanced cracks that developed from the middle of the bubble (Fig. 4). Microanalysis with SEM detected oxygen in the bubbles, con®rming hydrolysis. The number of bubbles of hydrolysis increased with the doping rate. This particular behavior has not been observed for PZG bulk glass doped with RECl3 (1 mol%), the surface of which remained unchanged after a few days in contact with air. The results were quite dierent for channel waveguides: no degradation was detected at the surface of the channel. However, hydrolysis was initiated at the PZG ®lm±substrate interface just after the cleavage of the CaF2 for NdCl3 -doped samples: Fig. 5 (right photo) clearly shows the result of hydrolysis for a 0.1 mol% NdCl3 -doped PZG ®lm; nothing happened for ErCl3 -doped samples (Fig. 5±left photo), even at a high doping rate, up to 7 mol%.
Fig. 4. Photograph of the surface of a rare ErCl3 -doped planar waveguide: (1) left: large bubbles; (2) right: cracks developing from the center of the bubbles.
Fig. 5. Optical micrograph of the end face of the waveguide after substrate cleavage: (1) right: 0.1 mol% NdCl3 PZG planar (the same behavior was observed on channel waveguides, this picture was chosen only because of its better quality) waveguide, shortly after the cleavage. `bubbles' at the ®lm-substrate interface are due to hydrolysis; (2) left: 1.5 mol% ErCl3 PZG channel waveguide, 8 lm wide: no degradation is observed even after a few months.
4. Discussion 4.1. Evaporation of RE chlorides According to the theory, the evaporation rate and thus deposition rate r is dependent on the temperature and follows the relationship ln rT 1=2 A ÿ
B ; T
2
where T is in Kelvin, B DH °=R with R 8:315 J Kÿ1 molÿ1 and DH ° is the standard enthalpy of sublimation or vaporization, depending on the temperature range. Fig. 6 shows the experimental curves; all the chlorides, as well as the NdCl3 ±SrCl2 mixture exhibit a linear variation, the slope being quite similar for YbCl3 , PrCl3 , ErCl3 and TmCl3 . Table 1 compares the enthalpy found in the literature [12] and the B values obtained in this work: the ®t was made at T < Tm , the melting point, except for the mixture which was liquid. The results are roughly consistent with Ds H °, though slightly smaller, except for pure NdCl3 ; the slope is found to be about twice as great as the enthalpy of sublimation. If one considers that dimeric entities Nd2 Cl6 are formed in the vapor state, the slope B should actually be equal to 2Ds H =R, close to the measured value. For a binary chloride liquid solution with one volatile component (i.e., 50SrCl2 ±50NdCl3 ), the enthalpy of vaporization of NdCl3 may dier from
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B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
Fig. 6. Linear dependence of evaporation rate for RE chlorides.
into account. Thin ®lms evaporated at a high rate develop high tensile stresses, as shown by birefringence measurement for PZG planar waveguides [13]; the refractive index and thus the density were actually found to be lower than the values for the bulk glass. Calculation using ®nite element techniques of stress concentration in thin ®lms bonded to sti substrate have shown that ®lms with L h, are in general less stressed than ®lms with L h (L being the width and h the thickness of the ®lm); moreover, stress concentrations are localized near the ®lm±substrate interface [14]. The channel waveguides should thus be more compact than the equivalent planar waveguides and so, less sensitive to moisture except at the ®lm±CaF2 interface. One can estimate that the low density allows water absorption that enhances hydrolysis of RE chloride within the ®lm following the reaction:
the enthalpy of vaporization of pure liquid NdCl3 since
RCl3 xH2 O g ! RCl3ÿx OHx xHCl g
Dv Hsolution Dv H °pure ÿ DH NdCl3
The cracks observed at the surface of the ®lms indicate a volume expansion under the protecting layer. In the case of channel waveguide, the compact protecting layer makes water adsorption impossible and thus prevents the hydrolysis of the chloride-doped ®lm. It seems obvious that hydrolysis is much more important when chloride forms aggregates in the ¯uoride matrix. This could be the case for NdCl3 since the thermodynamic study of the evaporation suggests the formation of dimeric entities in the vapor state. Both high tensile stress at the interface and chloride aggregation should be responsible for the hydrolysis of the channel waveguide that is initiated at the ®lm±substrate interface. Luminescence measurement on 1.5% ErCl3 channel waveguide has given a lifetime of nearly 4 ms for 1.5 lm
3
where DH NdCl3 represents the variation of enthalpy upon transfer of one mole of NdCl3 from pure liquid to the solution, also called the partial molar enthalpy of mixing of NdCl3 . From the slope measured for 50SrCl2 ±50NdCl3 , one can see that the solution is non-ideal and exhibits a high endothermic enthalpy of mixing. On the contrary, the mixture of YbCl3 and PrCl3 roughly behaves as an ideal solution. 4.2. RE chloride-doped thin ®lms To study the reason for the dierence of stability for planar and channel waveguides, the compaction of the deposited glass has to be taken
Table 1 Thermodynamic data for RECl3 [12] (m melting, v vaporization) and ®tted B values from Eq. (1) Tm °C Tv °C Dv H (kJ molÿ1 ) Ds H (kJ molÿ1 ) BR (kJ molÿ1 ) *
PrCl3
TmCl3
ErCl3
YbCl3
NdCl3
50SrCl2 ±50NdCl3
786 1905 219 269 21825
821 1487 184 222 19320
776 1497 184 216 194 17
865 ) ) ) 152 41
760 1940 216 267 451 95
620 ) ) ) 92 18
YbCl3 decomposes at 1230°C, before vaporization.
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
emission, which is similar to the value observed in ErF3 -doped bulk samples at the same concentration [15]. This result means that no ErCl3 aggregate was formed in the deposited glass that would give optical quenching. No measurement was possible for NdCl3 channel waveguides because the injection faces were degraded. Finally, the optical losses were estimated by the diusion technique: we obtained 0.4 dB/cm at 633 nm for ErCl3 -doped waveguides. 5. Conclusion In summary, we have demonstrated the feasibility of doping channel waveguides with RE chloride by using physical vapor deposition. It appeared that the sensitivity to atmospheric moisture was related to the density of the deposited ®lm. Thermodynamic study of the evaporation of single chloride suggests that they evaporate as monomeric entities except for NdCl3 . The tendency to form aggregates could thus be responsible for the lower stability of NdCl3 -doped channel waveguides compared to ErCl3 -doped samples. One very good result is that optical loss has been greatly reduced as compared to RE ¯uoride-doped waveguides. However, spectroscopic measurement will be necessary to see whether or not, the introduction of RE ions as chloride actually improves their luminescence properties. Further work on Pr3 -doped and co-doped 3 Pr ±Yb3 channel waveguides are now being undertaken.
77
Acknowledgements This work has been supported by CNETFrance Telecom (contract no. 96-1B). References [1] B. Jacquier, J. Alloys Compounds 225 (1995) 15. [2] J. Ballato, R.E. Riman, E. Snitzer, J. Non-Cryst. Solids 213&214 (1997) 126. [3] E. Josse, J.E. Broquin, G. Fonteneau, R. Rimet, J. Lucas, J. Non-Cryst. Solids 213&214 (1997) 152. [4] Y. Gao, B. Boulard, M. Lemiti, R. Rimet, P. Loeer, H. Poignant, J. Non-Cryst. Solids 256&257 (1999) 183. [5] C. Jacoboni, O. Perrot, B. Boulard, J. Non-Cryst. Solids 184 (1995) 184. [6] E. Lebrasseur, B. Jacquier, M.C. Marco De Lucas, E. Josse, J.L. Adam, G. Fonteneau, Y. Gao, B. Boulard, C. Jacoboni, J.E. Broquin, R. Rimet, J. Alloys Compounds 275±277 (1998) 716. [7] Y. Kawamoto, R. Kanno, R. Yokota, M. Takahashi, J. Solid State Chem. 103 (1993) 334. [8] K. Soga, M. Uo, H. Inoue, A. Makishima, J. Am. Ceram. Soc. 78 (1) (1995) 129. [9] N. Auriault, thesis, Universite du Maine, Le Mans, France, 1986. [10] Y. Ohishi, T. Kanamori, T. Nishi, S. Takahashi, E. Snitzer, IEEE Trans. Photonics Technol. Lett. 3 (11) (1991) 990. [11] G. Vogel, A. Schneider, Inorg. Nucl. Chem. Lett. 8 (1972) 513. [12] T. Moeller, Comprehensive Inorganic Chemistry, Pergamon, Oxford, 1973, p. 86. [13] B. Boulard, L. Guinvarc'h, Phys. Chem. Glasses 38 (3) (1997) 120. [14] J.C. Lambropoulos, S.M. Wan, Mater. Sci. Eng. A 107 (1989) 169. [15] E. Lebrasseur, Y. Gao, B. Boulard, B. Jacquier, in: 25th ECOC, vol. 1, 1999, p. 54.
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Thermal evaporation of rare-earth chlorides: application to vapor phase deposition of rare earth-doped ¯uoride glass waveguides B. Boulard *, S. Coste, Y. Gao, C. Legein, C. Dugopolovski Laboratoire des Fluorures, UPRESA 6010, Facult e des sciences, Universit e du Maine, 72085 Le Mans Cedex 9, France Received 1 March 2000; received in revised form 7 April 2000
Abstract A systematic study of the evaporation rate for single and binary mixtures of rare earth (RE) chlorides (NdCl3 ; ErCl3 ; TmCl3 ; PrCl3 ; YbCl3 ) has been performed. Single RE chlorides follow the physical evaporation law except NdCl3 that could be explained by a possible dimerisation in the vapor state. Erbium- and neodymium-doped planar and channel waveguides have been prepared by dual evaporation of RE chloride and PbF2 ±ZnF2 ±GaF3 ¯uoride glass. Also, planar waveguides are attacked by atmospheric moisture whereas, channel waveguides are stable, suggesting that the deposited glass is more compact in the latter case. The low optical loss (0.4 dB/cm at 633 nm for 1.5 mol% Er-doped channel waveguide) outlines the good quality of these devices. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 42.8.E; 81.15; 68.55.L
1. Introduction Rare earth (RE)-doped ¯uoride glasses are of current interest for optical ampli®ers in the middle infrared region (i.e., 1.3 and 1.55 lm) [1], because of their low phonon energy as compared to oxides. In the last few years, the need for low cost and more compact devices has enhanced searches for planar and channel waveguides that can be prepared by dierent techniques such as sol±gel synthesis and ion exchange [2,3].
* Corresponding author. Tel.: +33-2 43 83 33 70; fax: +33-2 43 83 35 06. E-mail address: [email protected] (B. Boulard).
Physical vapor deposition of PbF2 ±ZnF2 ± GaF3 (PZG) ¯uoride glass has made possible the preparation of step index channel waveguides by the use of lithographic techniques such as `lift-o' [4]. Doping with RE has also been achieved by dual evaporation of RE ¯uoride and PZG glass [5] and optical ampli®cation has been demonstrated at 1.047 lm in NdF3 -doped channel waveguides [6]. Unfortunately, the optical losses were found to be quite high (6±9 dB/cm), probably because of convection currents in the RE containing crucible which is heated at a temperature >1100°C. The doping with RE chlorides may oer some advantages: (i) they are more volatile than the ¯uoride counterpart (the evaporation temperature is
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 2 6 9 - 6
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
750±800°C), at least three orders of magnitude higher, (ii) the radiative transition probabilities should be increased since RE±Cl bonds are more covalent than RE±F bond, (iii) chloride environment of the RE ion should lead to lower phonon energies and thus minimize non-radiative (multiphonon) relaxation of the excited ion [7,8]. One problem is the high sensitivity of chloride to moisture; however, PZG bulk glasses can accept up to 4 mol% chlorine [9]. In the ®rst step, the deposition rate of a single RE chloride has been measured in order to check the ability to control the doping rate with temperature. The experimental results reported so far are compared with thermodynamic data. Mixtures of xPrCl3 ± 100 ÿ xYbCl3 have also been studied since energy transfers are expected in co-doped ¯uoride glasses that increase the emission probability of the praseodymium ion [10]. In the second step, ErCl3 - or NdCl3 -doped PZG glass planar or channel waveguides have been prepared and their stability in atmospheric air are compared. The eect of structure of the deposited glass and RE distribution are discussed.
73
For waveguide fabrication, the RE chloride and PZG mixture were simultaneously evaporated in separate crucibles heated by an RF coil (Fig. 1). The substrate (CaF2 single crystal plates) was placed at the intersection of the two evaporation cones; the PZG crucible was held closer to the substrate so that the deposition rate was about 40 times higher for PZG than for the RE chloride, to achieve low doping rates (0.1% to a few %). The substrate was rotated during the deposition process to assure homogeneous doping and thickness (about 2 lm). In order to improve the stability of the RE chloride-doped ®lm in air, a protecting layer was deposited, simply by stopping the evaporation of the RE chloride; the thickness of the protecting layer was about 1±2 lm. After deposition, the substrate was cleaved along the (1 1 1) plane to allow light injection. Both planar and channel waveguides were prepared. In the latter case, the lift-o technique was used: the ®lm was deposited on the substrate covered with a resist mask made by classical lithographic techniques. The temperature of the substrate was 150°C, unless the resist mask could
2. Experimental conditions High purity RE chloride powders (>99.9%), either anhydrous (NdCl3 , ErCl3 , TmCl3 ) or hydrated (PrCl3 6H2 O, YbCl3 6H2 O) were used as starting materials. Thermal gravimetric analysis (TGA) of the hydrated chloride showed a complete dehydration at 210°C for PrCl3 and 235°C for YbCl3 . The evaporation of the RE chloride was conducted in a vessel connected to a vacuum system with a liquid nitrogen trap, allowing a pressure around 10ÿ4 mbar. The chloride powder (200 mg) was placed in a graphite crucible and heated using an RF coil. A silica substrate (25 mm in diameter) was held 5 cm above the crucible. The evaporation time varied from 10 to 45 min and the substrate was weighed in a glove box before and after evaporation. Prior to evaporation, the chloride powder was heated in the vessel under vacuum at 250°C for 30 min to make sure that the chloride was anhydrous.
Fig. 1. Scheme of the vessel used for dual evaporation of PZG glass and rare-earth chloride.
74
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
not be removed after deposition. More details on the technique are presented in Ref. [4].
3. Results 3.1. Evaporation of RE chlorides The deposition rates r of RECl3 were measured at dierent temperatures in the range 650±850°C. The shapes of the curves were found similar for
Fig. 2. Deposition rate versus temperature for RE chlorides: (a) pure RE chlorides; (b) comparison of pure NdCl3 and 50SrCl2 ±NdCl3 mixture.
PrCl3 , ErCl3 , TmCl3 and YbCl3 in the order of increasing deposition rate (Fig. 2(a)). For NdCl3 , r increased very quickly so that the doping rate could not be controlled easily. For this reason, we studied the evaporation of the SrCl2 ±NdCl3 mixture in order to decrease the evaporation rate of NdCl3 . SrCl2 was chosen because its volatility is negligible compared to NdCl3 and it forms a binary eutectic; the melting point was decreased from 760°C to 620°C for 50NdCl3 ±50SrCl2 [11]. No SrCl2 was actually detected in the deposit. Fig. 2(b) shows the deposition rate of pure NdCl3 and 50NdCl3 ±50SrCl2 : as can be seen, the evaporation rate is dramatically reduced. For any sample, no reduced species (such as RECl2 ) was detected on the X-ray diraction spectra of the deposit. The evaporation of xPrCl3 ± 100 ÿ xYbCl3 was investigated for the preparation of co-doped YbCl3 ±PrCl3 glasses; the study was conducted at 710°C. Since the deposition rates of pure PrCl3 and YbCl3 were quite dierent, non-congruent evaporation was predicted. The compositions of the deposit and the starting chloride mixture are presented in Fig. 3. The experimental curves are compared with the one deduced from Raoult's law
Fig. 3. Comparison between starting charge and deposit composition (in mol%) for binary xPrCl3 ± 100 ÿ xYbCl3 . The full line represents the composition predicted for an ideal solution (Raoult law).
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
75
that depicts the behavior of ideal solution (solid or liquid) xdeposit
xrPrCl3 1 ÿ xrYbCl3
xrPrCl3 rPrCl3 with 5:8 at 710°C: rYbCl3
1
3.2. RE chloride-doped waveguides The planar waveguides were very sensitive to atmospheric moisture whether or not they were protected with an undoped PZG layer. Shortly after the sample was removed from the evaporation vessel, small bubbles appeared at the surface. Sometimes, they enhanced cracks that developed from the middle of the bubble (Fig. 4). Microanalysis with SEM detected oxygen in the bubbles, con®rming hydrolysis. The number of bubbles of hydrolysis increased with the doping rate. This particular behavior has not been observed for PZG bulk glass doped with RECl3 (1 mol%), the surface of which remained unchanged after a few days in contact with air. The results were quite dierent for channel waveguides: no degradation was detected at the surface of the channel. However, hydrolysis was initiated at the PZG ®lm±substrate interface just after the cleavage of the CaF2 for NdCl3 -doped samples: Fig. 5 (right photo) clearly shows the result of hydrolysis for a 0.1 mol% NdCl3 -doped PZG ®lm; nothing happened for ErCl3 -doped samples (Fig. 5±left photo), even at a high doping rate, up to 7 mol%.
Fig. 4. Photograph of the surface of a rare ErCl3 -doped planar waveguide: (1) left: large bubbles; (2) right: cracks developing from the center of the bubbles.
Fig. 5. Optical micrograph of the end face of the waveguide after substrate cleavage: (1) right: 0.1 mol% NdCl3 PZG planar (the same behavior was observed on channel waveguides, this picture was chosen only because of its better quality) waveguide, shortly after the cleavage. `bubbles' at the ®lm-substrate interface are due to hydrolysis; (2) left: 1.5 mol% ErCl3 PZG channel waveguide, 8 lm wide: no degradation is observed even after a few months.
4. Discussion 4.1. Evaporation of RE chlorides According to the theory, the evaporation rate and thus deposition rate r is dependent on the temperature and follows the relationship ln rT 1=2 A ÿ
B ; T
2
where T is in Kelvin, B DH °=R with R 8:315 J Kÿ1 molÿ1 and DH ° is the standard enthalpy of sublimation or vaporization, depending on the temperature range. Fig. 6 shows the experimental curves; all the chlorides, as well as the NdCl3 ±SrCl2 mixture exhibit a linear variation, the slope being quite similar for YbCl3 , PrCl3 , ErCl3 and TmCl3 . Table 1 compares the enthalpy found in the literature [12] and the B values obtained in this work: the ®t was made at T < Tm , the melting point, except for the mixture which was liquid. The results are roughly consistent with Ds H °, though slightly smaller, except for pure NdCl3 ; the slope is found to be about twice as great as the enthalpy of sublimation. If one considers that dimeric entities Nd2 Cl6 are formed in the vapor state, the slope B should actually be equal to 2Ds H =R, close to the measured value. For a binary chloride liquid solution with one volatile component (i.e., 50SrCl2 ±50NdCl3 ), the enthalpy of vaporization of NdCl3 may dier from
76
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
Fig. 6. Linear dependence of evaporation rate for RE chlorides.
into account. Thin ®lms evaporated at a high rate develop high tensile stresses, as shown by birefringence measurement for PZG planar waveguides [13]; the refractive index and thus the density were actually found to be lower than the values for the bulk glass. Calculation using ®nite element techniques of stress concentration in thin ®lms bonded to sti substrate have shown that ®lms with L h, are in general less stressed than ®lms with L h (L being the width and h the thickness of the ®lm); moreover, stress concentrations are localized near the ®lm±substrate interface [14]. The channel waveguides should thus be more compact than the equivalent planar waveguides and so, less sensitive to moisture except at the ®lm±CaF2 interface. One can estimate that the low density allows water absorption that enhances hydrolysis of RE chloride within the ®lm following the reaction:
the enthalpy of vaporization of pure liquid NdCl3 since
RCl3 xH2 O g ! RCl3ÿx OHx xHCl g
Dv Hsolution Dv H °pure ÿ DH NdCl3
The cracks observed at the surface of the ®lms indicate a volume expansion under the protecting layer. In the case of channel waveguide, the compact protecting layer makes water adsorption impossible and thus prevents the hydrolysis of the chloride-doped ®lm. It seems obvious that hydrolysis is much more important when chloride forms aggregates in the ¯uoride matrix. This could be the case for NdCl3 since the thermodynamic study of the evaporation suggests the formation of dimeric entities in the vapor state. Both high tensile stress at the interface and chloride aggregation should be responsible for the hydrolysis of the channel waveguide that is initiated at the ®lm±substrate interface. Luminescence measurement on 1.5% ErCl3 channel waveguide has given a lifetime of nearly 4 ms for 1.5 lm
3
where DH NdCl3 represents the variation of enthalpy upon transfer of one mole of NdCl3 from pure liquid to the solution, also called the partial molar enthalpy of mixing of NdCl3 . From the slope measured for 50SrCl2 ±50NdCl3 , one can see that the solution is non-ideal and exhibits a high endothermic enthalpy of mixing. On the contrary, the mixture of YbCl3 and PrCl3 roughly behaves as an ideal solution. 4.2. RE chloride-doped thin ®lms To study the reason for the dierence of stability for planar and channel waveguides, the compaction of the deposited glass has to be taken
Table 1 Thermodynamic data for RECl3 [12] (m melting, v vaporization) and ®tted B values from Eq. (1) Tm °C Tv °C Dv H (kJ molÿ1 ) Ds H (kJ molÿ1 ) BR (kJ molÿ1 ) *
PrCl3
TmCl3
ErCl3
YbCl3
NdCl3
50SrCl2 ±50NdCl3
786 1905 219 269 21825
821 1487 184 222 19320
776 1497 184 216 194 17
865 ) ) ) 152 41
760 1940 216 267 451 95
620 ) ) ) 92 18
YbCl3 decomposes at 1230°C, before vaporization.
B. Boulard et al. / Journal of Non-Crystalline Solids 276 (2000) 72±77
emission, which is similar to the value observed in ErF3 -doped bulk samples at the same concentration [15]. This result means that no ErCl3 aggregate was formed in the deposited glass that would give optical quenching. No measurement was possible for NdCl3 channel waveguides because the injection faces were degraded. Finally, the optical losses were estimated by the diusion technique: we obtained 0.4 dB/cm at 633 nm for ErCl3 -doped waveguides. 5. Conclusion In summary, we have demonstrated the feasibility of doping channel waveguides with RE chloride by using physical vapor deposition. It appeared that the sensitivity to atmospheric moisture was related to the density of the deposited ®lm. Thermodynamic study of the evaporation of single chloride suggests that they evaporate as monomeric entities except for NdCl3 . The tendency to form aggregates could thus be responsible for the lower stability of NdCl3 -doped channel waveguides compared to ErCl3 -doped samples. One very good result is that optical loss has been greatly reduced as compared to RE ¯uoride-doped waveguides. However, spectroscopic measurement will be necessary to see whether or not, the introduction of RE ions as chloride actually improves their luminescence properties. Further work on Pr3 -doped and co-doped 3 Pr ±Yb3 channel waveguides are now being undertaken.
77
Acknowledgements This work has been supported by CNETFrance Telecom (contract no. 96-1B). References [1] B. Jacquier, J. Alloys Compounds 225 (1995) 15. [2] J. Ballato, R.E. Riman, E. Snitzer, J. Non-Cryst. Solids 213&214 (1997) 126. [3] E. Josse, J.E. Broquin, G. Fonteneau, R. Rimet, J. Lucas, J. Non-Cryst. Solids 213&214 (1997) 152. [4] Y. Gao, B. Boulard, M. Lemiti, R. Rimet, P. Loeer, H. Poignant, J. Non-Cryst. Solids 256&257 (1999) 183. [5] C. Jacoboni, O. Perrot, B. Boulard, J. Non-Cryst. Solids 184 (1995) 184. [6] E. Lebrasseur, B. Jacquier, M.C. Marco De Lucas, E. Josse, J.L. Adam, G. Fonteneau, Y. Gao, B. Boulard, C. Jacoboni, J.E. Broquin, R. Rimet, J. Alloys Compounds 275±277 (1998) 716. [7] Y. Kawamoto, R. Kanno, R. Yokota, M. Takahashi, J. Solid State Chem. 103 (1993) 334. [8] K. Soga, M. Uo, H. Inoue, A. Makishima, J. Am. Ceram. Soc. 78 (1) (1995) 129. [9] N. Auriault, thesis, Universite du Maine, Le Mans, France, 1986. [10] Y. Ohishi, T. Kanamori, T. Nishi, S. Takahashi, E. Snitzer, IEEE Trans. Photonics Technol. Lett. 3 (11) (1991) 990. [11] G. Vogel, A. Schneider, Inorg. Nucl. Chem. Lett. 8 (1972) 513. [12] T. Moeller, Comprehensive Inorganic Chemistry, Pergamon, Oxford, 1973, p. 86. [13] B. Boulard, L. Guinvarc'h, Phys. Chem. Glasses 38 (3) (1997) 120. [14] J.C. Lambropoulos, S.M. Wan, Mater. Sci. Eng. A 107 (1989) 169. [15] E. Lebrasseur, Y. Gao, B. Boulard, B. Jacquier, in: 25th ECOC, vol. 1, 1999, p. 54.