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High-quality multiple quantum wells selectively grown with tapered masks by ultra-low-pressure MOCVD
Optical Materials 28 (2006) 1037–1040 www.elsevier.com/locate/optmat
High-quality multiple quantum wells selectively grown with tapered masks by ultra-low-pressure MOCVD Q. Zhao *, J.Q. Pan, J. Zhang, W. Wang National Center of Optoelectronics Technology, Institute of Semiconductor, Chinese Academy of Sciences, P.O. Box 912, 100083 Beijing, PR China Received 12 November 2004; accepted 6 June 2005 Available online 31 August 2005
Abstract An InGaAsP/InGaAsP multiple quantum wells (MQWs) selectively grown by ultra-low-pressure (22 mbar) metal-organic chemical vapor deposition was investigated in this article. A 46 nm photoluminescence peak wavelength shift was obtained with a small mask width variation (15–30 lm). High-quality crystal layers with a photoluminescence (PL) full-width-at-half-maximum (FWHM) of less than 30 meV were achieved. Using novel tapered masks, the transition-effect of the tapered region was also studied. The energy detuning of the tapered region was observed to be saturated with the larger ratio of the mask width divided to the tapered region length. Ó 2005 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 81.15.Gh; 78.55.Cr Keywords: Selective-area growth; Ultra-low-pressure; Metal-organic chemical vapor deposition; Tapered mask; Photoluminescence
1. Introduction Metal-organic chemical vapor deposition (MOCVD) selective-area growth (SAG) method is one of the most promising techniques for fabricating integrated optical waveguide devices because different in-plane band gap energy of MQWs structures can be easily adjusted by changing the dielectric mask width during simultaneous epitaxy [1]. And its application for the fabrication of photonic devices was first discussed by Azoulay et al. [2]. In the last 20 years, the SAG technique has been widely applied in compound semiconductor growth. InGaAs(P)/InGaAsP MQWs system has been quite extensively studied both theoretically and experimentally as long-wavelength lasers or EA-type modulators [3–6]. However, the understanding of the SAG process
*
Corresponding author. Tel.: +861082305191; fax: +861082304004. E-mail address: [email protected] (Q. Zhao).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.06.016
in MOCVD, such as lateral control of thickness, and layer composition modulation, etc., is far from complete up to now. Therefore, in light of the above-mentioned background, it is necessary to investigate the growth conditions and mechanisms of band gap engineered structures for InGaAs(P)/InGaAsP MQWs system with optimized epitaxial parameters. It is well known that low-pressure is beneficial to obtain abrupt hetero-interface and good homogeneity. However, that technique generally leads low growth rates. In this study, high crystalline quality InGaAsP/ InGaAsP MQWs were selective-area grown on maskpatterned planar InP substrates, which with a photoluminescence (PL) full-width-at-half-maximum (FWHM) of less than 30 meV were achieved under the optimizing ultra-low-pressure conditions. Also, the growth rates were kept as high as 1.8 lm/h. In order to study the uniformity of the MQWs grown in the selective area, novel tapered masks were employed. The saturation of the energy detuning in the tapered region was observed.
1038
Q. Zhao et al. / Optical Materials 28 (2006) 1037–1040
Tapered Region
Wm
Wg
Mask Region
Fig. 1. Schematic diagram of the dual SiO2 stripe mask used during the selective growth of the InGaAsP/InGaAsP MQWs layers.
2. Experimental A horizontal ultra-low-pressure (22 mbar) MOCVD reactor was used. The reactive precursors were trimethylindium (TMIn), triethylgarium (TEGa), arsine (AsH3), and phosphine (PH3). The samples were grown at substrate temperature of 655 °C, while using V/III ratios of 250 and keeping the growth rates for the maskless area at 1.8 lm/h. SAG process was carried out on patterned substrates. A 200-nm thick SiO2 dielectric films were deposited on the S-doped (100) InP substrates by plasma-enhanced chemical vapor deposition (PECVD). Tapered masks were patterned parallel to the [0 1 1] direction by the conventional photolithographic technology and etching. The schematic view of the masks is shown in Fig. 1. The mask width (Wm) was varied from 15 to 30 lm with 600 lm length, while the width of the gap region (Wg) between the masks was fixed at 15 lm. The tapered region length (Lt) was varied from 20 to 50 lm. Micro-photoluminescence (l-PL) excited by an Ar+ laser (k = 514.5 nm) with InGaAs photodiode detector was used to evaluate the crystalline quality of the samples at room temperature. The excited spot was focused to a diameter of 4 lm.
3. Results and discussion Fig. 2 shows the PL spectra of the selectively grown InGaAsP/InGaAsP MQWs with the mask width varied from 15 to 30 lm measured at room temperature. With the increasing of the Wm, the PL peaks were red shifted from 1532 to 1578 nm, which corresponds to an Eg change of 23 meV covered the whole C-bands of the fiber-optic communication spectrum. Nevertheless, two different regimes were obviously suggested according to the spectra. From the mask width of 15–22 lm, the PL peak shift was about 10 nm, while 36 nm of PL peak
Fig. 2. The PL spectra of the selectively grown InGaAsP/InGaAsP MQWs with the mask width varied from 15 to 30 lm measured at room temperature (* in the inset means the excited spots).
shift was obtained with the mask varied from 22 to 30 lm. It was well known that the SAG process in MOCVD was basically due to the competitive mechanism of dominant lateral gas phase diffusion and surface atom migration. However, the effect of surface kinetics under our experimental condition was quite weak due to the wider gap region width Wg. Therefore, the diffusion effects of the lateral gas phase played an important role in the SAG process in MOCVD. In the ultra-lowpressure, the reagents particles can easily diffuse out of stagnation gas phase layer thanks to the large mean free path of the particles. In the case of narrower stripe masks (15–22 lm), reactive sources concentration grads was not enough to make up the migration of reagents by gas phase diffusion, which leaded the effect of selective growth process was inferior to that in the wider stripe masks (22–30 lm). It can be recognized that there is an optimization value for the width of masks to obtain the appropriate energy detuning. The PL intensity, however, slightly degraded with the increasing of mask width. As mentioned above, the thickness of MQWs layers in the gap region enhanced with the increasing of the mask width by lateral gas phase diffusion. The principal mechanism of selectively grown MOCVD lies in the lateral gas phase diffusion group-III precursors [7]. The associated reduced V/III ratio or the compositional variety in the gap region probably caused the intensity degradation. On the other hand, the FWHM of the PL spectra of the selective grown MQWs as a function of mask width is plotted in Fig. 3. The FWHM was rather small and nearly kept unchanged with the increasing of the mask width, which demonstrated that high-quality, and high-uniformity crystal can be obtained in the ultra-low-pressure growth conditions. In order to study the transition-effect in the selective growth process, a novel tapered mask was employed.
Q. Zhao et al. / Optical Materials 28 (2006) 1037–1040 40
FWHM (meV)
35
30
25
20
15 0
5
10
15
20
25
30
Mask Width: Wm (μm) Fig. 3. The FWHM profile of the PL spectra shown in Fig. 2, as a function of the mask width.
Fig. 4 gives the change of the PL wavelength with various mask width versus the distance of the excited spot from the center of the selective area. With the increasing of the distance, the PL wavelength was blue shifted. 50 45
FWHM (meV)
40 35 30 25 20 15
1039
These PL peak shifts were due to the decrease of indium content by vapor-phase diffusion [3,8,9]. However, different kPL shifting tendency in the tapered regions were indicated from the plot. The DkPL was rather small (about 22 nm) with the narrower Wm (15–22 lm), whereas 50 nm wavelength shifts were achieved with the wider Wm (30 lm). Furthermore, the PL peak wavelength was slightly changed when the excited spots from the center of the selective area were over 100 lm. Accordingly, the gas phase diffusion length was estimated to about 100 lm. Note that the gap region width Wg was chosen be much smaller than the gas phase diffusion length in our SAG process conditions; it is crucial to obtain highly uniform selective grown MQWs. Fig. 4 also shows the corresponding FWHM of this PL. The results indicated that the uniform thickness interface and good homogeneity in composition of tapered region was almost as good as that of normally grown MQWs layers on the maskless area, which approved that this structure was fit for device application. The relationship between the PL wavelength shift and mask width had been intensively studied [10–12]. However, there are few reports on the effect of transition area between SAG and maskless region. Actually, the transition area length is also a crucial parameter for the dense photonic integrated circuits (PICs). Therefore, a better understanding of the transition-effect in the tapered region for the optimization of mask design is required. In order to study the co-influence of Wm and Lt, different Wm values of 15, 22, 30 lm, and Lt values of 20, 30, 50 lm are employed, respectively. The energy detuning (DEd) in the tapered region as a function of Wm/Lt ratio was further investigated, as shown in Fig. 5, saturation was observed. In the range of the Wm/Lt ratio less than 0.6, the DEd was linearly increased, however, it increased slightly in the range of Wm/Lt ratio larger than 0.75. The saturation of the energy detuning was attributed to the
10 1580
30 Wm
1560
25
Energy Detuning (meV)
PL wavelength (nm)
Excited Spots
1540
1520 Wm=15μm Wm=22μm Wm =30μm
1500 0
50
100
150
20
15
10
200
Distance from the center of the selective area (μm)
5 0.3
Fig. 4. The PL wavelength shift and the PL FWHM with various mask width versus the distance of the excited spot from the center of the selective area. The position of the excited spots is shown in the inset.
0.6
0.9
1.2
1.5
Wm/Lt Fig. 5. The energy detuning in the tapered region, as a function of Wm/Lt ratio.
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Q. Zhao et al. / Optical Materials 28 (2006) 1037–1040
weakening of the lateral gas phase diffusion effects with the larger Wm/Lt ratio, which demonstrated the selectivity was not only influenced by mask width, but also by transition area length. Even in our ultra-low-pressure conditions, it was illuminated that the transition-effect of the tapered region also can be controlled in a relatively small area by reasonably optimizing the width of the mask, and the length of the tapered region, respectively. In practice, weaker transition-effect is very advantage to obtain good uniformity epitaxial layers for fabricating high-performance integrated optical devices.
4. Conclusion In this work, ultra-low-pressure (22 mbar) selective MOCVD was performed. The 46 nm wavelength shift with high-crystalline quality InGaAsP/InGaAsP MQWs of FWHM below 30 meV was obtained with small mask width variation (15–30 lm). Moreover, in order to study the transition-effect between the selective region and the maskless region, a novel tapered mask was employed. The PL wavelength shift tendency in the tapered region was enhanced with the wider mask width about 30 lm. The gas phase diffusion length was estimated to about 100 lm. The energy detuning in the tapered region as a function of Wm/Lt ratio was also investigated. The saturation about 25 meV was observed with the ration larger than 0.75.
Acknowledgements This work was supported by National 973 project (Grant No. G20000683-1) and National Natural Science Foundation of China (Grant No. 69896260).
References [1] T. Sasaki, M. Kitamura, I. Mito, J. Cryst. Growth 132 (1993) 435. [2] R. Azoulay, N. Bouadma, J.C. Bouley, L. Dugrand, J. Cryst. Growth 55 (1981) 229. [3] M. Aoki, M. Suzuki, M. Takahashi, H. Sano, T. Ido, T. Kawano, A. Takai, Electron. Lett. 28 (1992) 1157. [4] M. Aoki, M. Takahashi, M. Suzuki, H. Sano, K. Uomi, T. Kawano, A. Takai, IEEE Photon. Technol. Lett. 4 (1992) 580. [5] M. Aoki, M. Suzuki, H. Sano, T. Kawano, T. Ido, T. Taniwatari, K. Uomi, A. Takai, IEEE J. Quant. Electron. 29 (1993) 2088. [6] M.K. Chin, W.S.C. Chang, IEEE J. Quant. Electron. 29 (1993) 2476. [7] M. Gibbon, J.P. Stagg, C.G. Cureton, E.J. Thrush, C.J. Jones, R.E. Mallard, R.E. Pritchard, N. Collis, A. Chew, Semicond. Sci. Technol. 8 (1991) 998. [8] M. Tsuji, K. Makita, T. Takeuchi, K. Taguchi, J. Cryst. Growth 170 (1996) 669. [9] M. Tsuji, K. Makita, T. Takeuchi, K. Taguchi, J. Cryst. Growth 162 (1996) 25. [10] T. Kihara, Y. Nitta, H. Suda, K. Miki, K. Shimomura, J. Cryst. Growth 221 (2000) 198. [11] T. Tsuchiya, J. Shimizu, M. Shirai, M. Aoki, J. Cryst. Growth 248 (2003) 387. [12] F.N. Racedo, M.P. Pires, B. Yavich, L.C.D. Goncalves, P.L. Souza, Mater. Sci. Eng. B 74 (2000) 13.
High-quality multiple quantum wells selectively grown with tapered masks by ultra-low-pressure MOCVD Q. Zhao *, J.Q. Pan, J. Zhang, W. Wang National Center of Optoelectronics Technology, Institute of Semiconductor, Chinese Academy of Sciences, P.O. Box 912, 100083 Beijing, PR China Received 12 November 2004; accepted 6 June 2005 Available online 31 August 2005
Abstract An InGaAsP/InGaAsP multiple quantum wells (MQWs) selectively grown by ultra-low-pressure (22 mbar) metal-organic chemical vapor deposition was investigated in this article. A 46 nm photoluminescence peak wavelength shift was obtained with a small mask width variation (15–30 lm). High-quality crystal layers with a photoluminescence (PL) full-width-at-half-maximum (FWHM) of less than 30 meV were achieved. Using novel tapered masks, the transition-effect of the tapered region was also studied. The energy detuning of the tapered region was observed to be saturated with the larger ratio of the mask width divided to the tapered region length. Ó 2005 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 81.15.Gh; 78.55.Cr Keywords: Selective-area growth; Ultra-low-pressure; Metal-organic chemical vapor deposition; Tapered mask; Photoluminescence
1. Introduction Metal-organic chemical vapor deposition (MOCVD) selective-area growth (SAG) method is one of the most promising techniques for fabricating integrated optical waveguide devices because different in-plane band gap energy of MQWs structures can be easily adjusted by changing the dielectric mask width during simultaneous epitaxy [1]. And its application for the fabrication of photonic devices was first discussed by Azoulay et al. [2]. In the last 20 years, the SAG technique has been widely applied in compound semiconductor growth. InGaAs(P)/InGaAsP MQWs system has been quite extensively studied both theoretically and experimentally as long-wavelength lasers or EA-type modulators [3–6]. However, the understanding of the SAG process
*
Corresponding author. Tel.: +861082305191; fax: +861082304004. E-mail address: [email protected] (Q. Zhao).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.06.016
in MOCVD, such as lateral control of thickness, and layer composition modulation, etc., is far from complete up to now. Therefore, in light of the above-mentioned background, it is necessary to investigate the growth conditions and mechanisms of band gap engineered structures for InGaAs(P)/InGaAsP MQWs system with optimized epitaxial parameters. It is well known that low-pressure is beneficial to obtain abrupt hetero-interface and good homogeneity. However, that technique generally leads low growth rates. In this study, high crystalline quality InGaAsP/ InGaAsP MQWs were selective-area grown on maskpatterned planar InP substrates, which with a photoluminescence (PL) full-width-at-half-maximum (FWHM) of less than 30 meV were achieved under the optimizing ultra-low-pressure conditions. Also, the growth rates were kept as high as 1.8 lm/h. In order to study the uniformity of the MQWs grown in the selective area, novel tapered masks were employed. The saturation of the energy detuning in the tapered region was observed.
1038
Q. Zhao et al. / Optical Materials 28 (2006) 1037–1040
Tapered Region
Wm
Wg
Mask Region
Fig. 1. Schematic diagram of the dual SiO2 stripe mask used during the selective growth of the InGaAsP/InGaAsP MQWs layers.
2. Experimental A horizontal ultra-low-pressure (22 mbar) MOCVD reactor was used. The reactive precursors were trimethylindium (TMIn), triethylgarium (TEGa), arsine (AsH3), and phosphine (PH3). The samples were grown at substrate temperature of 655 °C, while using V/III ratios of 250 and keeping the growth rates for the maskless area at 1.8 lm/h. SAG process was carried out on patterned substrates. A 200-nm thick SiO2 dielectric films were deposited on the S-doped (100) InP substrates by plasma-enhanced chemical vapor deposition (PECVD). Tapered masks were patterned parallel to the [0 1 1] direction by the conventional photolithographic technology and etching. The schematic view of the masks is shown in Fig. 1. The mask width (Wm) was varied from 15 to 30 lm with 600 lm length, while the width of the gap region (Wg) between the masks was fixed at 15 lm. The tapered region length (Lt) was varied from 20 to 50 lm. Micro-photoluminescence (l-PL) excited by an Ar+ laser (k = 514.5 nm) with InGaAs photodiode detector was used to evaluate the crystalline quality of the samples at room temperature. The excited spot was focused to a diameter of 4 lm.
3. Results and discussion Fig. 2 shows the PL spectra of the selectively grown InGaAsP/InGaAsP MQWs with the mask width varied from 15 to 30 lm measured at room temperature. With the increasing of the Wm, the PL peaks were red shifted from 1532 to 1578 nm, which corresponds to an Eg change of 23 meV covered the whole C-bands of the fiber-optic communication spectrum. Nevertheless, two different regimes were obviously suggested according to the spectra. From the mask width of 15–22 lm, the PL peak shift was about 10 nm, while 36 nm of PL peak
Fig. 2. The PL spectra of the selectively grown InGaAsP/InGaAsP MQWs with the mask width varied from 15 to 30 lm measured at room temperature (* in the inset means the excited spots).
shift was obtained with the mask varied from 22 to 30 lm. It was well known that the SAG process in MOCVD was basically due to the competitive mechanism of dominant lateral gas phase diffusion and surface atom migration. However, the effect of surface kinetics under our experimental condition was quite weak due to the wider gap region width Wg. Therefore, the diffusion effects of the lateral gas phase played an important role in the SAG process in MOCVD. In the ultra-lowpressure, the reagents particles can easily diffuse out of stagnation gas phase layer thanks to the large mean free path of the particles. In the case of narrower stripe masks (15–22 lm), reactive sources concentration grads was not enough to make up the migration of reagents by gas phase diffusion, which leaded the effect of selective growth process was inferior to that in the wider stripe masks (22–30 lm). It can be recognized that there is an optimization value for the width of masks to obtain the appropriate energy detuning. The PL intensity, however, slightly degraded with the increasing of mask width. As mentioned above, the thickness of MQWs layers in the gap region enhanced with the increasing of the mask width by lateral gas phase diffusion. The principal mechanism of selectively grown MOCVD lies in the lateral gas phase diffusion group-III precursors [7]. The associated reduced V/III ratio or the compositional variety in the gap region probably caused the intensity degradation. On the other hand, the FWHM of the PL spectra of the selective grown MQWs as a function of mask width is plotted in Fig. 3. The FWHM was rather small and nearly kept unchanged with the increasing of the mask width, which demonstrated that high-quality, and high-uniformity crystal can be obtained in the ultra-low-pressure growth conditions. In order to study the transition-effect in the selective growth process, a novel tapered mask was employed.
Q. Zhao et al. / Optical Materials 28 (2006) 1037–1040 40
FWHM (meV)
35
30
25
20
15 0
5
10
15
20
25
30
Mask Width: Wm (μm) Fig. 3. The FWHM profile of the PL spectra shown in Fig. 2, as a function of the mask width.
Fig. 4 gives the change of the PL wavelength with various mask width versus the distance of the excited spot from the center of the selective area. With the increasing of the distance, the PL wavelength was blue shifted. 50 45
FWHM (meV)
40 35 30 25 20 15
1039
These PL peak shifts were due to the decrease of indium content by vapor-phase diffusion [3,8,9]. However, different kPL shifting tendency in the tapered regions were indicated from the plot. The DkPL was rather small (about 22 nm) with the narrower Wm (15–22 lm), whereas 50 nm wavelength shifts were achieved with the wider Wm (30 lm). Furthermore, the PL peak wavelength was slightly changed when the excited spots from the center of the selective area were over 100 lm. Accordingly, the gas phase diffusion length was estimated to about 100 lm. Note that the gap region width Wg was chosen be much smaller than the gas phase diffusion length in our SAG process conditions; it is crucial to obtain highly uniform selective grown MQWs. Fig. 4 also shows the corresponding FWHM of this PL. The results indicated that the uniform thickness interface and good homogeneity in composition of tapered region was almost as good as that of normally grown MQWs layers on the maskless area, which approved that this structure was fit for device application. The relationship between the PL wavelength shift and mask width had been intensively studied [10–12]. However, there are few reports on the effect of transition area between SAG and maskless region. Actually, the transition area length is also a crucial parameter for the dense photonic integrated circuits (PICs). Therefore, a better understanding of the transition-effect in the tapered region for the optimization of mask design is required. In order to study the co-influence of Wm and Lt, different Wm values of 15, 22, 30 lm, and Lt values of 20, 30, 50 lm are employed, respectively. The energy detuning (DEd) in the tapered region as a function of Wm/Lt ratio was further investigated, as shown in Fig. 5, saturation was observed. In the range of the Wm/Lt ratio less than 0.6, the DEd was linearly increased, however, it increased slightly in the range of Wm/Lt ratio larger than 0.75. The saturation of the energy detuning was attributed to the
10 1580
30 Wm
1560
25
Energy Detuning (meV)
PL wavelength (nm)
Excited Spots
1540
1520 Wm=15μm Wm=22μm Wm =30μm
1500 0
50
100
150
20
15
10
200
Distance from the center of the selective area (μm)
5 0.3
Fig. 4. The PL wavelength shift and the PL FWHM with various mask width versus the distance of the excited spot from the center of the selective area. The position of the excited spots is shown in the inset.
0.6
0.9
1.2
1.5
Wm/Lt Fig. 5. The energy detuning in the tapered region, as a function of Wm/Lt ratio.
1040
Q. Zhao et al. / Optical Materials 28 (2006) 1037–1040
weakening of the lateral gas phase diffusion effects with the larger Wm/Lt ratio, which demonstrated the selectivity was not only influenced by mask width, but also by transition area length. Even in our ultra-low-pressure conditions, it was illuminated that the transition-effect of the tapered region also can be controlled in a relatively small area by reasonably optimizing the width of the mask, and the length of the tapered region, respectively. In practice, weaker transition-effect is very advantage to obtain good uniformity epitaxial layers for fabricating high-performance integrated optical devices.
4. Conclusion In this work, ultra-low-pressure (22 mbar) selective MOCVD was performed. The 46 nm wavelength shift with high-crystalline quality InGaAsP/InGaAsP MQWs of FWHM below 30 meV was obtained with small mask width variation (15–30 lm). Moreover, in order to study the transition-effect between the selective region and the maskless region, a novel tapered mask was employed. The PL wavelength shift tendency in the tapered region was enhanced with the wider mask width about 30 lm. The gas phase diffusion length was estimated to about 100 lm. The energy detuning in the tapered region as a function of Wm/Lt ratio was also investigated. The saturation about 25 meV was observed with the ration larger than 0.75.
Acknowledgements This work was supported by National 973 project (Grant No. G20000683-1) and National Natural Science Foundation of China (Grant No. 69896260).
References [1] T. Sasaki, M. Kitamura, I. Mito, J. Cryst. Growth 132 (1993) 435. [2] R. Azoulay, N. Bouadma, J.C. Bouley, L. Dugrand, J. Cryst. Growth 55 (1981) 229. [3] M. Aoki, M. Suzuki, M. Takahashi, H. Sano, T. Ido, T. Kawano, A. Takai, Electron. Lett. 28 (1992) 1157. [4] M. Aoki, M. Takahashi, M. Suzuki, H. Sano, K. Uomi, T. Kawano, A. Takai, IEEE Photon. Technol. Lett. 4 (1992) 580. [5] M. Aoki, M. Suzuki, H. Sano, T. Kawano, T. Ido, T. Taniwatari, K. Uomi, A. Takai, IEEE J. Quant. Electron. 29 (1993) 2088. [6] M.K. Chin, W.S.C. Chang, IEEE J. Quant. Electron. 29 (1993) 2476. [7] M. Gibbon, J.P. Stagg, C.G. Cureton, E.J. Thrush, C.J. Jones, R.E. Mallard, R.E. Pritchard, N. Collis, A. Chew, Semicond. Sci. Technol. 8 (1991) 998. [8] M. Tsuji, K. Makita, T. Takeuchi, K. Taguchi, J. Cryst. Growth 170 (1996) 669. [9] M. Tsuji, K. Makita, T. Takeuchi, K. Taguchi, J. Cryst. Growth 162 (1996) 25. [10] T. Kihara, Y. Nitta, H. Suda, K. Miki, K. Shimomura, J. Cryst. Growth 221 (2000) 198. [11] T. Tsuchiya, J. Shimizu, M. Shirai, M. Aoki, J. Cryst. Growth 248 (2003) 387. [12] F.N. Racedo, M.P. Pires, B. Yavich, L.C.D. Goncalves, P.L. Souza, Mater. Sci. Eng. B 74 (2000) 13.