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Characterization of Ge-doped silica films with low optical loss grown by flame hydrolysis deposition
Materials Science and Engineering B107 (2004) 317–320
Characterization of Ge-doped silica films with low optical loss grown by flame hydrolysis deposition Letian Zhang a,∗ , Xin Wang b , Wenfa Xie a , Yong Hou a , Wei Zheng a , Yushu Zhang a a
National Integrated Optoelectronics Laboratory, Jilin University, Jiefang Road 119, Changchun 130023, PR China b College of the Humanities, Jilin University, Qianwei Road 10, Changchun 130012, PR China Accepted 11 December 2003
Abstract Ge-doped silica films have been deposited on Si substrates from SiCl4 , GeCl4 , and H2 /O2 by flame hydrolysis deposition (FHD), and annealed to 1150 ◦ C for 2 h. X-ray photoelectron spectroscopy (XPS) showed that the positions of the peaks correspond to the Si, O, Ge, and C levels for Ge-doped SiO2 film. Furthermore, the optical properties of the samples using Variable Angle Spectroscopic Ellipsometry (VASE) illustrate that the refractive index of the samples increase with the increasing GeCl4 flow ratio which indicates that the amount of germanium incorporated into the films increase. We also contrast the refractive indices of samples annealed to different temperatures. When the GeO2 content to SiO2 is equal to 16.28% (the atom ratio of Ge and Si is 10:90), the optical loss of the film is less than 0.527 dB/cm at 1550 nm. © 2004 Elsevier B.V. All rights reserved. Keywords: Ge-doped silica; Flame hydrolysis deposition; Refractive index; Optical loss
1. Introduction Recent developmental efforts in planar lightwave circuits (PLCs) based on silica waveguides have been directed to the dense wavelength-division multiplexed (DWDM) systems. As a result, the need for innovative optical devices and integrated circuits has increased [1,2]. The choice for optical materials has been general. Silica waveguides have attracted much attention that provides low propagation to fibber coupling losses since the core dimensions and refractive index are easily fixed to match that of silica-based optical fibbers [3,4]. Various processes have been explored for the fabrication of silica waveguides, among them, flame hydrolysis deposition (FHD) [5–9] appears to be one of the promising methods for the fabrication of integrated optical devices in respect of a variety of controllable parameters at low production cost and the glass layer can be loaded with other dopants such as Ge, B, and Ti. Many researches have been carried out in the fabrications of Ge-doped silica films, such as sol–gel, PECVD, magnetron rf-sputtering and VAD, however, few investigations on the characterization of the films. ∗
Corresponding author. Tel.: +86-431-8499017; fax: +86-431-8981524. E-mail address: [email protected] (L. Zhang). 0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.12.003
In this letter, we represent the composition of the FHD deposited Ge-doped silica films and report on the analysis of some experimental data, which is then used to elucidate the influence of processing conditions on the properties of the products. 2. Experimental The Ge-doped silica films were deposited on (1 0 0) Si substrates using flame hydrolysis deposition. In this study, SiCl4 , and GeCl4 mixtures carried by He gas were used as source gas, and introduced into the oxy/hydrogen torch through mass flow controllers, respectively, where they are subsequently hydrolyzed. The temperature of the substrates was kept constant during deposition by water-cooling the substrate holder. The hydrolysis process is described by the following reactions: SiCl4 + 2H2 + O2 = SiO2 + 4HCl GeCl4 + 2H2 + O2 = GeO2 + 4HCl As a consequence, SiO2 –GeO2 glass particles were formed on Si substrate placed on the turntable. The most prominent feature of the Ge-doped silica film is the remarkable dependence of the GeO2 concentration. During
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L. Zhang et al. / Materials Science and Engineering B107 (2004) 317–320
Fig. 1. XPS survey spectrum of the fabricated film annealed to 1150 ◦ C for 2 h (Ge:Si = 10:90).
deposition process, we adjusted the GeCl4 flow rate varying between 6 and 14 sccm, SiCl4 flow rate was kept at 20 sccm, while the value of H2 and O2 keeping constant. After that, the Si wafers with the porous particles were put into electric furnace heating up to different temperatures of 1000, 1050, 1100, and 1150 ◦ C for 2 h for consolidation. The refractive indices of the films and different annealing temperatures were contrasted from Variable Angle Spectroscopic Ellipsometer (VASE), and we obtained the relationship between different GeO2 content and the refractive index at 1550 nm in our experiment. VASE data were acquired at four angles of incidence (55, 60, 65, 70◦ ) over the spectral
range 250–1700 nm. The chemical compositions of the films were analyzed by X-ray photoelectron spectrometer (XPS) on VG ESCALAB MK II in vacuum at a base pressure of 1 × 10−7 Pa with the resolution of 0.2 eV using a Mg K␣ source. We also measured the optical losses of the samples at 1550 nm.
3. Results and discussion Fig. 1 shows the XPS spectrum of the fabricated film that have been annealed to 1150 ◦ C for 2 h from GeCl4 ,
Fig. 2. Comparison of the refractive indices of the films annealed to 1000, 1050, 1100, and 1150 ◦ C for 2 h, respectively.
L. Zhang et al. / Materials Science and Engineering B107 (2004) 317–320
319
Fig. 3. The dependence of the refractive indices at 1550 nm of the films on GeO2 /(GeO2 + SiO2 ) weight percent (SiCi4 : 20 sccm, GeCl4 varies from 0, 6 to 14 sccm).
SiCl4 , and H2 /O2 gas mixtures. We can observe the peaks of Ge, Si, O, and C elements in the spectrum. It indicates that germanium has been incorporated into the matrix of SiO2 . The quantification of peaks of Ge 3d and Si 2p for the 1150 ◦ C-annealed film gives the ratio of Ge:Si as 10:90. Fig. 2 shows the comparison of refractive indices of the films (deposited above) annealed to different temperatures (1000, 1050, 1100, and 1150 ◦ C) on wavelength by VASE. VASE data were acquired at four angles of incidence (55, 60, 65, 70◦ ) over the spectral range 250–1700 nm. As the
wavelength increases, the refractive index of the film decreases. And when the annealing temperature increased, the refractive index also rises. It can be observed that the curve is almost identical at lower temperatures, while is marked secerned at higher temperatures. The melting point of GeO2 is 1115 ◦ C, therefore, the nearer to 1115 ◦ C, the more interfusion with SiO2 , and the higher the refractive index. Whats more, the increase in the film density is the reason for the higher refractive index. The refractive indices of 1000, 1050, 1100, and 1150 ◦ C around 1550 nm are 1.4586, 1.4603, 1.4621, and 1.4639, respectively.
Fig. 4. The dependence of the extinction coefficient of the 1150 ◦ C-annealed films on wavelength.
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With the increase of GeO2 /(GeO2 + SiO2 ) weight percent, the changes in refractive indices at 1550 nm of the 1150 ◦ C-annealed films are shown in Fig. 3. The refractive indices of the samples increase from 1.4513 (pure SiO2 annealed to 1150 ◦ C) to 1.4905 with the increasing GeCl4 flow ratio varied from 0, 6 to 14 sccm, which indicates that the amount of GeO2 incorporated into the films increase. It can be concluded that they are approximately linear correlation. For the 10GeO2 –90SiO2 film, which contains 10 mol% of GeO2 , the refractive index is 0.868% increased. When the flow rate of GeCl4 is equal to 14 sccm, the GeO2 weight percent is 54.21% (Ge:Si = 40:60) measured by XPS, and the refractive index of the film increases to 1.4905. The increase in the refractive indices of the films with the increasing GeO2 weight percent is due to the replacement of Si–O by Ge–O and the higher refractive index of GeO2 . Fig. 4 shows the dependence of the extinction coefficient of the 1150 ◦ C-annealed film (Ge:Si = 10:90) on wavelength (250–1700 nm). It can be observed that the extinction coefficient of the film annealed to 1150 ◦ C for 2 h is small as a whole, and the maximum is 0.00448 at 274 nm. When the wavelength is larger than 1024 nm, the extinction coefficient becomes smaller and smaller and down to zero. It is clear that the value is beyond the range of precision. The extinction coefficient appears to maintain the magnitude of 10−6 in a great measure, and the minimum 1.4961 × 10−6 that can be measured. The propagation loss evaluated from the extinction coefficient is 0.527 dB/cm. This value is small enough to fabricate integrated optical circuits. We can choose pure SiO2 (annealed to 1400 ◦ C for 2 h, n = 1.4564) as buffer layer [9] and Ge-doped SiO2 (described above, n = 1.4639) as core layer for waveguide. The difference between them is n = 0.515%. These films are used for channel waveguide exposed to UV laser light through a phase mask, as introduced elsewhere.
4. Conclusions Glass Ge-doped SiO2 films have been deposited on Si substrates from GeCl4 , SiCl4 , and H2 /O2 using FHD at various GeCl4 flow rate. The XPS survey spectrum of the fabricated film is given to indicate that germanium has been incorporated into the matrix of SiO2 , and the ratio of Ge and Si was estimated from the Ge 3d and Si 2p peaks of the XPS spectrum. We contrasted the change of the refractive index on various annealing temperatures. The concentration of GeO2 in the deposited film was proportional to the refractive index at 1550 nm approximately in our experimental conditions. We also measured the extinction coefficient of the samples and obtained the optical loss. When the GeO2 content to SiO2 is equal to 16.28% (10GeO2 –90SiO2 ), the optical loss of the film is less than 0.527 dB/cm at 1550 nm. These values are fit for the integrated optical circuit. Acknowledgements This work was supported by the Major State Basic Research Program under Grant No. G2000036602.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
M. Kawachi, Optical Quantum Electron. 22 (1990) 391–416. S. Valette, J. Modern Optics 35 (6) (1988) 993–1005. Y.P. Li, C.H. Henry, IEE Proc. Optoelectron. 143 (5) (1996) 263–280. S. Valette, J.P. Jadot, et al., Solid State Technol. 32 (1989) 69–75. M. Kawachi, M. Yasu, T. Edahiro, Electron. Lett. 19 (15) (1983) 583–584. T. Kominato, Y. Ohmori, et al., Electron. Lett. 26 (5) (1990) 327–328. M. Kawachi, M. Yasu, M. Kobayashi, Jpn. J. Appl. Phys. 22 (12) (1983) 1932. N. Takato, M. Yasu, M. Kawachi, Electron. Lett. 22 (6) (1986) 321– 322. L. Zhang, W. Xie, et al., Opt. Mat. 22 (2003) 283–287.
Characterization of Ge-doped silica films with low optical loss grown by flame hydrolysis deposition Letian Zhang a,∗ , Xin Wang b , Wenfa Xie a , Yong Hou a , Wei Zheng a , Yushu Zhang a a
National Integrated Optoelectronics Laboratory, Jilin University, Jiefang Road 119, Changchun 130023, PR China b College of the Humanities, Jilin University, Qianwei Road 10, Changchun 130012, PR China Accepted 11 December 2003
Abstract Ge-doped silica films have been deposited on Si substrates from SiCl4 , GeCl4 , and H2 /O2 by flame hydrolysis deposition (FHD), and annealed to 1150 ◦ C for 2 h. X-ray photoelectron spectroscopy (XPS) showed that the positions of the peaks correspond to the Si, O, Ge, and C levels for Ge-doped SiO2 film. Furthermore, the optical properties of the samples using Variable Angle Spectroscopic Ellipsometry (VASE) illustrate that the refractive index of the samples increase with the increasing GeCl4 flow ratio which indicates that the amount of germanium incorporated into the films increase. We also contrast the refractive indices of samples annealed to different temperatures. When the GeO2 content to SiO2 is equal to 16.28% (the atom ratio of Ge and Si is 10:90), the optical loss of the film is less than 0.527 dB/cm at 1550 nm. © 2004 Elsevier B.V. All rights reserved. Keywords: Ge-doped silica; Flame hydrolysis deposition; Refractive index; Optical loss
1. Introduction Recent developmental efforts in planar lightwave circuits (PLCs) based on silica waveguides have been directed to the dense wavelength-division multiplexed (DWDM) systems. As a result, the need for innovative optical devices and integrated circuits has increased [1,2]. The choice for optical materials has been general. Silica waveguides have attracted much attention that provides low propagation to fibber coupling losses since the core dimensions and refractive index are easily fixed to match that of silica-based optical fibbers [3,4]. Various processes have been explored for the fabrication of silica waveguides, among them, flame hydrolysis deposition (FHD) [5–9] appears to be one of the promising methods for the fabrication of integrated optical devices in respect of a variety of controllable parameters at low production cost and the glass layer can be loaded with other dopants such as Ge, B, and Ti. Many researches have been carried out in the fabrications of Ge-doped silica films, such as sol–gel, PECVD, magnetron rf-sputtering and VAD, however, few investigations on the characterization of the films. ∗
Corresponding author. Tel.: +86-431-8499017; fax: +86-431-8981524. E-mail address: [email protected] (L. Zhang). 0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.12.003
In this letter, we represent the composition of the FHD deposited Ge-doped silica films and report on the analysis of some experimental data, which is then used to elucidate the influence of processing conditions on the properties of the products. 2. Experimental The Ge-doped silica films were deposited on (1 0 0) Si substrates using flame hydrolysis deposition. In this study, SiCl4 , and GeCl4 mixtures carried by He gas were used as source gas, and introduced into the oxy/hydrogen torch through mass flow controllers, respectively, where they are subsequently hydrolyzed. The temperature of the substrates was kept constant during deposition by water-cooling the substrate holder. The hydrolysis process is described by the following reactions: SiCl4 + 2H2 + O2 = SiO2 + 4HCl GeCl4 + 2H2 + O2 = GeO2 + 4HCl As a consequence, SiO2 –GeO2 glass particles were formed on Si substrate placed on the turntable. The most prominent feature of the Ge-doped silica film is the remarkable dependence of the GeO2 concentration. During
318
L. Zhang et al. / Materials Science and Engineering B107 (2004) 317–320
Fig. 1. XPS survey spectrum of the fabricated film annealed to 1150 ◦ C for 2 h (Ge:Si = 10:90).
deposition process, we adjusted the GeCl4 flow rate varying between 6 and 14 sccm, SiCl4 flow rate was kept at 20 sccm, while the value of H2 and O2 keeping constant. After that, the Si wafers with the porous particles were put into electric furnace heating up to different temperatures of 1000, 1050, 1100, and 1150 ◦ C for 2 h for consolidation. The refractive indices of the films and different annealing temperatures were contrasted from Variable Angle Spectroscopic Ellipsometer (VASE), and we obtained the relationship between different GeO2 content and the refractive index at 1550 nm in our experiment. VASE data were acquired at four angles of incidence (55, 60, 65, 70◦ ) over the spectral
range 250–1700 nm. The chemical compositions of the films were analyzed by X-ray photoelectron spectrometer (XPS) on VG ESCALAB MK II in vacuum at a base pressure of 1 × 10−7 Pa with the resolution of 0.2 eV using a Mg K␣ source. We also measured the optical losses of the samples at 1550 nm.
3. Results and discussion Fig. 1 shows the XPS spectrum of the fabricated film that have been annealed to 1150 ◦ C for 2 h from GeCl4 ,
Fig. 2. Comparison of the refractive indices of the films annealed to 1000, 1050, 1100, and 1150 ◦ C for 2 h, respectively.
L. Zhang et al. / Materials Science and Engineering B107 (2004) 317–320
319
Fig. 3. The dependence of the refractive indices at 1550 nm of the films on GeO2 /(GeO2 + SiO2 ) weight percent (SiCi4 : 20 sccm, GeCl4 varies from 0, 6 to 14 sccm).
SiCl4 , and H2 /O2 gas mixtures. We can observe the peaks of Ge, Si, O, and C elements in the spectrum. It indicates that germanium has been incorporated into the matrix of SiO2 . The quantification of peaks of Ge 3d and Si 2p for the 1150 ◦ C-annealed film gives the ratio of Ge:Si as 10:90. Fig. 2 shows the comparison of refractive indices of the films (deposited above) annealed to different temperatures (1000, 1050, 1100, and 1150 ◦ C) on wavelength by VASE. VASE data were acquired at four angles of incidence (55, 60, 65, 70◦ ) over the spectral range 250–1700 nm. As the
wavelength increases, the refractive index of the film decreases. And when the annealing temperature increased, the refractive index also rises. It can be observed that the curve is almost identical at lower temperatures, while is marked secerned at higher temperatures. The melting point of GeO2 is 1115 ◦ C, therefore, the nearer to 1115 ◦ C, the more interfusion with SiO2 , and the higher the refractive index. Whats more, the increase in the film density is the reason for the higher refractive index. The refractive indices of 1000, 1050, 1100, and 1150 ◦ C around 1550 nm are 1.4586, 1.4603, 1.4621, and 1.4639, respectively.
Fig. 4. The dependence of the extinction coefficient of the 1150 ◦ C-annealed films on wavelength.
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L. Zhang et al. / Materials Science and Engineering B107 (2004) 317–320
With the increase of GeO2 /(GeO2 + SiO2 ) weight percent, the changes in refractive indices at 1550 nm of the 1150 ◦ C-annealed films are shown in Fig. 3. The refractive indices of the samples increase from 1.4513 (pure SiO2 annealed to 1150 ◦ C) to 1.4905 with the increasing GeCl4 flow ratio varied from 0, 6 to 14 sccm, which indicates that the amount of GeO2 incorporated into the films increase. It can be concluded that they are approximately linear correlation. For the 10GeO2 –90SiO2 film, which contains 10 mol% of GeO2 , the refractive index is 0.868% increased. When the flow rate of GeCl4 is equal to 14 sccm, the GeO2 weight percent is 54.21% (Ge:Si = 40:60) measured by XPS, and the refractive index of the film increases to 1.4905. The increase in the refractive indices of the films with the increasing GeO2 weight percent is due to the replacement of Si–O by Ge–O and the higher refractive index of GeO2 . Fig. 4 shows the dependence of the extinction coefficient of the 1150 ◦ C-annealed film (Ge:Si = 10:90) on wavelength (250–1700 nm). It can be observed that the extinction coefficient of the film annealed to 1150 ◦ C for 2 h is small as a whole, and the maximum is 0.00448 at 274 nm. When the wavelength is larger than 1024 nm, the extinction coefficient becomes smaller and smaller and down to zero. It is clear that the value is beyond the range of precision. The extinction coefficient appears to maintain the magnitude of 10−6 in a great measure, and the minimum 1.4961 × 10−6 that can be measured. The propagation loss evaluated from the extinction coefficient is 0.527 dB/cm. This value is small enough to fabricate integrated optical circuits. We can choose pure SiO2 (annealed to 1400 ◦ C for 2 h, n = 1.4564) as buffer layer [9] and Ge-doped SiO2 (described above, n = 1.4639) as core layer for waveguide. The difference between them is n = 0.515%. These films are used for channel waveguide exposed to UV laser light through a phase mask, as introduced elsewhere.
4. Conclusions Glass Ge-doped SiO2 films have been deposited on Si substrates from GeCl4 , SiCl4 , and H2 /O2 using FHD at various GeCl4 flow rate. The XPS survey spectrum of the fabricated film is given to indicate that germanium has been incorporated into the matrix of SiO2 , and the ratio of Ge and Si was estimated from the Ge 3d and Si 2p peaks of the XPS spectrum. We contrasted the change of the refractive index on various annealing temperatures. The concentration of GeO2 in the deposited film was proportional to the refractive index at 1550 nm approximately in our experimental conditions. We also measured the extinction coefficient of the samples and obtained the optical loss. When the GeO2 content to SiO2 is equal to 16.28% (10GeO2 –90SiO2 ), the optical loss of the film is less than 0.527 dB/cm at 1550 nm. These values are fit for the integrated optical circuit. Acknowledgements This work was supported by the Major State Basic Research Program under Grant No. G2000036602.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
M. Kawachi, Optical Quantum Electron. 22 (1990) 391–416. S. Valette, J. Modern Optics 35 (6) (1988) 993–1005. Y.P. Li, C.H. Henry, IEE Proc. Optoelectron. 143 (5) (1996) 263–280. S. Valette, J.P. Jadot, et al., Solid State Technol. 32 (1989) 69–75. M. Kawachi, M. Yasu, T. Edahiro, Electron. Lett. 19 (15) (1983) 583–584. T. Kominato, Y. Ohmori, et al., Electron. Lett. 26 (5) (1990) 327–328. M. Kawachi, M. Yasu, M. Kobayashi, Jpn. J. Appl. Phys. 22 (12) (1983) 1932. N. Takato, M. Yasu, M. Kawachi, Electron. Lett. 22 (6) (1986) 321– 322. L. Zhang, W. Xie, et al., Opt. Mat. 22 (2003) 283–287.