- Email: [email protected]
s electroabsorption modulator integrated with a tunable DBR laser
ARTICLE IN PRESS
Journal of Crystal Growth 298 (2007) 672–675 www.elsevier.com/locate/jcrysgro
Selective-area MOVPE growth for 10 Gbit/s electroabsorption modulator integrated with a tunable DBR laser Sung-Bock Kima,, Jae-Sik Sima, Ki Soo Kima, Eun-Deok Sima, Sang-Wan Ryub, Hong Lee Parkc a
Optical Devices Group, IT Convergence and Components Laboratory, Electronics and Telecommunications Research Institute, 161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-700, South Korea b Department of Physics, Chonnam National University, 300 Yongbong-Dong, Buk-Gu, Gwangju 500-757, South Korea c Department of Physics, Yonsei University, 134 Sinchon-Dong, Seodaemoon-Gu, Seoul 120-749, South Korea Available online 8 December 2006
Abstract We have designed and fabricated electroabsorption modulator integrated monolithically with a distributed Bragg reflector laser using selective area growth (SAG) by metalorganic vapor phase epitaxy (MOVPE). Through the bandgap engineering due to the growth rate enhancement and compositional variation from the difference of migration and/or diffusion length of species by various SAG mask pattern, we have achieved the photoluminescence peak wavelengths of active layer in the laser, modulator, and Bragg grating regions to 1.557, 1.503, and 1.442 mm, respectively. The electroabsorption modulator-integrated laser shows a good performance including threshold current of below 6 mA, output power of 5 mW for 45 mA current injected in the active region, and high side-mode suppression ratio over 45 dB with the 3-dB modulation bandwidth over 11 GHz. r 2006 Elsevier B.V. All rights reserved. Keywords: A3. Thin film/epitaxy growth; A3. Selective epitaxy
1. Introduction Photonic integrated circuits (PICs) offer significant performance advantages over discrete devices, adding flexibility and functionality while reducing final system cost and size [1,2]. To fabricate monolithic integrated devices, it is necessary to obtain large bandgap energy shift between the functional elements. Selective area growth (SAG) is an important technique for monolithic integration of photonic devices in III–V compound semiconductor [3,4]. The SAG using metalorganic vapor phase epitaxy (MOVPE) can realize bandgap engineering on a single wafer due to the growth rate enhancement and composition variation from the difference of vapor phase diffusion length and surface migration length of group III species by changing the dielectric mask geometry [5]. Corresponding author. Fax: +82 42 860 6248.
E-mail address: [email protected] (S.-B. Kim). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.210
There are several reports on integration of a singlefrequency distributed feedback (DFB) laser with an electroabsorption (EA) modulator [6,7]. Recently, multiple wavelength system with tunable light sources has received great attention. In particular, the EA modulator integrated with a tunable LD is a key device for wavelength division multiplexing (WDM) system since the wavelength of the DBR laser can be tuned over a range of 5 nm by controlling the Bragg section current [8,9]. In this work, we report the more 100 nm photoluminescence (PL) wavelength shift using the specially designed mask and active layer with SAG technique. High crystalline quality strained InGaAsP/InGaAsP multiple quantum wells (MQWs) with PL full-width at halfmaximum (FWHM) of less than 43 meV at room temperature are selectively grown on patterned region. The experimental results of SAG led to the development of a DBR laser integrated with 10 Gbit/s EA modulator.
ARTICLE IN PRESS S.-B. Kim et al. / Journal of Crystal Growth 298 (2007) 672–675
2. Selective-area growth for monolithic integration In order to integrate a DBR laser with EA modulator, three distinct regions with different transition energies were used for the active, tuning, and modulator regions of device. Fig. 1(a) shows geometry of a SiNx mask pattern that is specially designed in order to obtain a good crystal quality and a proper wavelength of each layer. The schematic diagram of the device composed of four sections (active, phase control, DBR, and modulator section) is shown in Fig. 1(b). However, SAG sections including unmasked region are divided into three sections because the phase control section and DBR section are localized in the same region called the tuning region. The SAG was performed by low-pressure MOVPE on a patterned n-InP substrate. The growth temperature and the reactor pressure were kept at 630 1C and 100 mbar, respectively. In order to adjust the PL wavelengths of an active, modulator, and tuning regions to 1.55, 1.50, and 1.45 mm, respectively, the MQW consists of seven periods of 6-nm-thick lattice-matched InGaAsP wells (lPL ¼ 1.58 mm) and 10-nm-thick tensile-strained (0.6%) InGaAsP barriers (lPL ¼ 1.20 mm) in the unmasked region.
a
673
The MQW was embedded in between two 55-nm-thick lattice-matched InGaAsP separate confinement heterostructure (SCH) layers (lPL ¼ 1.10 mm). Fig. 2 shows the field emission scanning electron microscope (FE-SEM) cross-sectional images of MQWs in an active, tuning, and modulator region indicated in Fig. 1(a). The growth rate enhancement factors of the MQW with SCH at an active region and a modulator region are estimated at around 1.43 and 1.16, respectively. Also, the bandgap wavelengths of each region were determined by spatial resolution PL (m-PL) with a beam diameter of about 1 mm. Fig. 3 shows the PL characteristics at room temperature for these structures. PL measurement indicates transition energies corresponding to a wavelength of 1.557, 1.503, and 1.442 mm with PL FWHM of less than 43 meV. These energy shifts very well agree with the design scheme. Delprat et al. [10,11] reported that PL wavelength shift between the active region and the tuning region was 85 nm with the growth rate enhancement factor of around 1.8. In our study, however, PL wavelength shift over 110 nm was obtained with lower growth rate enhancement factor (1.43). This is similar to Zhu et al.’s [12] result where they report wavelength shift of 80 nm with growth rate
400µm
500µm
250µm
active region
tuning region
modulator region 30µm
70µm 50µm
50µm
70µm
30µm
b
active
p-InP
PC
DBR
modulator
Bragg mirror
active MQW n-InP HR
AR
Fig. 1. (a) Geometry of the SiNx SAG mask specially patterned on the substrate before growth and (b) schematic diagram of an EA modulator integrated with a tunable DBR laser.
Fig. 2. FE-SEM photographs of MQW in (a) active, (b) tuning, and (c) modulator regions.
ARTICLE IN PRESS S.-B. Kim et al. / Journal of Crystal Growth 298 (2007) 672–675
674
condition. The rear facet is HR (reflectivity is around 90%) coated with three pair of SiO2/TiO2.
tuning region
PL intensity(a.u.)
1.442µm
4. Device characteristics
FWHM=42.6meV
FWHM=40.9meV
modulator region 1.503µm active region 1.557µm FWHM=42.1meV
1250 1300 1350 1400 1450 1500 1550 1600 1650 wavelength(nm)
Fig. 4 shows the typical current–light characteristics of the integrated device measured under continuous wave operation at room temperature. In spite of the low PL intensity of the active region (Fig. 3), the threshold current is 5.7 mA and the output power is 5 mW for 65 mA current injected in the active section. The inset shows the optical spectrum of the laser with a large side mode suppression ratio (SMSR) of 45 dB at a 60 mA laser current level. The tuning characteristics as a function of Bragg current are illustrated in Fig. 5. As shown in Fig. 5, the wavelength 6
Fig. 3. PL spectra of active, modulator, and tuning regions at room temperature.
@ room temp. CW-operation
5
The DBR laser consisted of a 400-mm-long active section, a 150-mm-long phase control section, and a 200mm-long DBR section, while the EA modulator had a length of 120 mm. Each section was electrically isolated with a 20-mm-long isolation trench. The isolation resistance of 10 kO was required. The DBR grating was inscribed by holographic exposure and reactive ion etching. The integrated device was formed in the planar buried heterostructure (PBH) with p–n–p current blocking layer for the lateral and electrical confinements on the device, so current leakage was effectively blocked. A 1.5-mm-thick pInP cladding layer and 0.1-mm-thick p+-InGaAs contact layer were overgrown successively. N- and p-metal contact electrodes were deposited afterwards. For the final step, the front facet was AR coated with two pair of TiO2/SiO2. Reflectivity of less than 1% is obtained with an optimized
4
3 SMSR=45 dB
2
1545
1
1550 1555 Wavelength (nm)
0 0
10 20 30 40 50 60 70 80 Current (mA)
Fig. 4. Output power versus current injected in the active section with, in inset, the optical spectrum for an injected current of 60 mA.
1557.5 PC current = 0 mA 1557.0 Wavelength (nm)
3. Device fabrication
Output power (mW)
enhancement factor of about 1.34. Energy shift can be explained by two reasons: (1) increment of well width and (2) increment of indium composition in MQWs, mentioned by other papers [13,14]. When the composition was not varying but the increment of thickness exists, we calculated that the wavelength shift did not exceed 40 nm. It implies that PL shift over 70 nm was originated from increment of In-composition by difference of migration/diffusion length of group III species. It could be assumed that y remains constant in SAG of In1xGaxAsyP1y material [15,16]. Also, the In content increased with increasing mask width because the migration length of indium is much longer than that of gallium. For bandgap engineering, it means that the control of migration/diffusion length of group III species is more effective than that of thickness variation in our SAG condition.
1556.5 1556.0
Maximum tuning range: 3.45 nm
1555.5 1555.0 1554.5 1554.0 1553.5 0
10
20 DBR current (mA)
30
40
Fig. 5. Dependence of the wavelength on the Bragg section current.
ARTICLE IN PRESS S.-B. Kim et al. / Journal of Crystal Growth 298 (2007) 672–675
respectively. The integrated device was fabricated in conventional PBH with p–n–p current blocking layer. The integrated device with SAG shows a good performance including threshold current of 5.7 mA, total tuning range of 3.45 nm, and 3 dB cut-off frequency exceeding 11.6 GHz. These device characteristics are highly adjustable for highspeed data transmission applications in WDM system.
3
0 E/O Response (dB)
675
-3
-6
References
Small signal RF response -9
-12 0
2
4
6
8
10
12
14
Frequency (GHz) Fig. 6. A small signal E/O response as a function of frequency.
of DBR laser was tuned from 1557.25 to 1553.8 nm by DBR current, where the currents injected in active and in phase control sections were kept at 50 and 0 mA, respectively. This result indicates that the fabricated DBR laser is applicable as a tunable WDM transmitter. A static extinction ratio over 20 dB at an applied modulator bias voltage of 3 V was also observed. For high-speed measurement over 10 GHz, we measured E/O bandwidth using an Anritsu 87300C 65 GHz vector analyzer and a calibrated receiver. The frequency response reached the laser current of 50 mA. The E/O response is shown in Fig. 6, which reveals a 3 dB small signal bandwidth of about 11.6 GHz. 5. Conclusion The monolithic integration of a tunable DBR laser EA modulator has been achieved by selective area MOVPE. By using the specially designed mask pattern and epilayer, we could adjust the PL wavelengths of an active, modulator, and tuning regions to 1.557, 1.503, and 1.442 mm,
[1] T.L. Koch, U. Koren, IEEE J. Quantum Electron. 27 (1991) 641. [2] S. Charbonneau, E.S. Koteles, P.J. Poole, J.J. He, G.C. Aers, J. Haysom, M. Buchanan, Y. Feng, A. Delage, F. Yang, M. Davies, R.D. Goldberg, P.G. Piva, I.V. Mitchell, IEEE J. Select. Topics Quantum Electron. 4 (1998) 772. [3] T. Van Caenegem, D. Van Thourhout, M. Galarza, S. Verstuyft, I. Moerman, P. Van Daele, R. Baets, P. Demeester, C.G.P. Herben, X.J.M. Leijtens, M.K. Smit, Electron. Lett. 37 (2001) 296. [4] Q. Zhao, J.Q. Pan, J. Zhang, H.L. Zhu, W. Wang, J. Crystal Growth 288 (2006) 27. [5] 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 (1993) 998. [6] M. Aoki, M. Suzuki, H. Sano, T. Kawano, T. Ido, T. Taniwatari, K. Uomi, A. Takai, IEEE J. Quantum Electron. 29 (1993) 2088. [7] Y.-H. Kwon, J.-S. Choe, J.-S. Sim, S.-B. Kim, K.-S. Choi, B.-S. Choi, in: Technical Digest OECC 2005, Seoul, Korea, Paper 6F1-5, 2005, p. 316. [8] J.C. Cartledge, J. Lightwave Technol. 14 (1996) 480. [9] S.H. Oh, H. Ko, K.S. Kim, J.-M. Lee, C.W. Lee, O.K. Kwon, S. Park, M.H. Park, ETRI J. 27 (2005) 551. [10] D. Delprat, L. Silvestre, A. Ougazzaden, F. Delorme, S. Slempks, A. Ramdane, in: IPRM’96, Schwabisch Gmund, Germany, Paper TuP-C4, 1996, p. 162. [11] D. Delprat, A. Ramdane, L. Silvestre, A. Ougazzaden, F. Delorme, S. Slempks, IEEE Photon. Technol. Lett. 9 (1997) 898. [12] J.T. Zhu, L. Billia, D. Bour, S. Corzine, G. Ho¨fler, J. Crystal Growth 272 (2004) 576. [13] W. Qiu, W. Wang, J. Dong, F. Zhou, J. Jhang, J. Crystal Growth 250 (2003) 583. [14] G. Ru, X. Yu, Z. Chen, J. Zhang, J. Lin, F.-S. Choa, J. Crystal Growth 285 (2005) 499. [15] M.-S. Kim, C. Caneau, E. Colas, R. Bhat, J. Crystal Growth 123 (1992) 69. [16] T. Sasaki, M. Kitamura, I. Mito, J. Crystal Growth 132 (1993) 435.
Journal of Crystal Growth 298 (2007) 672–675 www.elsevier.com/locate/jcrysgro
Selective-area MOVPE growth for 10 Gbit/s electroabsorption modulator integrated with a tunable DBR laser Sung-Bock Kima,, Jae-Sik Sima, Ki Soo Kima, Eun-Deok Sima, Sang-Wan Ryub, Hong Lee Parkc a
Optical Devices Group, IT Convergence and Components Laboratory, Electronics and Telecommunications Research Institute, 161 Gajeong-Dong, Yuseong-Gu, Daejeon 305-700, South Korea b Department of Physics, Chonnam National University, 300 Yongbong-Dong, Buk-Gu, Gwangju 500-757, South Korea c Department of Physics, Yonsei University, 134 Sinchon-Dong, Seodaemoon-Gu, Seoul 120-749, South Korea Available online 8 December 2006
Abstract We have designed and fabricated electroabsorption modulator integrated monolithically with a distributed Bragg reflector laser using selective area growth (SAG) by metalorganic vapor phase epitaxy (MOVPE). Through the bandgap engineering due to the growth rate enhancement and compositional variation from the difference of migration and/or diffusion length of species by various SAG mask pattern, we have achieved the photoluminescence peak wavelengths of active layer in the laser, modulator, and Bragg grating regions to 1.557, 1.503, and 1.442 mm, respectively. The electroabsorption modulator-integrated laser shows a good performance including threshold current of below 6 mA, output power of 5 mW for 45 mA current injected in the active region, and high side-mode suppression ratio over 45 dB with the 3-dB modulation bandwidth over 11 GHz. r 2006 Elsevier B.V. All rights reserved. Keywords: A3. Thin film/epitaxy growth; A3. Selective epitaxy
1. Introduction Photonic integrated circuits (PICs) offer significant performance advantages over discrete devices, adding flexibility and functionality while reducing final system cost and size [1,2]. To fabricate monolithic integrated devices, it is necessary to obtain large bandgap energy shift between the functional elements. Selective area growth (SAG) is an important technique for monolithic integration of photonic devices in III–V compound semiconductor [3,4]. The SAG using metalorganic vapor phase epitaxy (MOVPE) can realize bandgap engineering on a single wafer due to the growth rate enhancement and composition variation from the difference of vapor phase diffusion length and surface migration length of group III species by changing the dielectric mask geometry [5]. Corresponding author. Fax: +82 42 860 6248.
E-mail address: [email protected] (S.-B. Kim). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.10.210
There are several reports on integration of a singlefrequency distributed feedback (DFB) laser with an electroabsorption (EA) modulator [6,7]. Recently, multiple wavelength system with tunable light sources has received great attention. In particular, the EA modulator integrated with a tunable LD is a key device for wavelength division multiplexing (WDM) system since the wavelength of the DBR laser can be tuned over a range of 5 nm by controlling the Bragg section current [8,9]. In this work, we report the more 100 nm photoluminescence (PL) wavelength shift using the specially designed mask and active layer with SAG technique. High crystalline quality strained InGaAsP/InGaAsP multiple quantum wells (MQWs) with PL full-width at halfmaximum (FWHM) of less than 43 meV at room temperature are selectively grown on patterned region. The experimental results of SAG led to the development of a DBR laser integrated with 10 Gbit/s EA modulator.
ARTICLE IN PRESS S.-B. Kim et al. / Journal of Crystal Growth 298 (2007) 672–675
2. Selective-area growth for monolithic integration In order to integrate a DBR laser with EA modulator, three distinct regions with different transition energies were used for the active, tuning, and modulator regions of device. Fig. 1(a) shows geometry of a SiNx mask pattern that is specially designed in order to obtain a good crystal quality and a proper wavelength of each layer. The schematic diagram of the device composed of four sections (active, phase control, DBR, and modulator section) is shown in Fig. 1(b). However, SAG sections including unmasked region are divided into three sections because the phase control section and DBR section are localized in the same region called the tuning region. The SAG was performed by low-pressure MOVPE on a patterned n-InP substrate. The growth temperature and the reactor pressure were kept at 630 1C and 100 mbar, respectively. In order to adjust the PL wavelengths of an active, modulator, and tuning regions to 1.55, 1.50, and 1.45 mm, respectively, the MQW consists of seven periods of 6-nm-thick lattice-matched InGaAsP wells (lPL ¼ 1.58 mm) and 10-nm-thick tensile-strained (0.6%) InGaAsP barriers (lPL ¼ 1.20 mm) in the unmasked region.
a
673
The MQW was embedded in between two 55-nm-thick lattice-matched InGaAsP separate confinement heterostructure (SCH) layers (lPL ¼ 1.10 mm). Fig. 2 shows the field emission scanning electron microscope (FE-SEM) cross-sectional images of MQWs in an active, tuning, and modulator region indicated in Fig. 1(a). The growth rate enhancement factors of the MQW with SCH at an active region and a modulator region are estimated at around 1.43 and 1.16, respectively. Also, the bandgap wavelengths of each region were determined by spatial resolution PL (m-PL) with a beam diameter of about 1 mm. Fig. 3 shows the PL characteristics at room temperature for these structures. PL measurement indicates transition energies corresponding to a wavelength of 1.557, 1.503, and 1.442 mm with PL FWHM of less than 43 meV. These energy shifts very well agree with the design scheme. Delprat et al. [10,11] reported that PL wavelength shift between the active region and the tuning region was 85 nm with the growth rate enhancement factor of around 1.8. In our study, however, PL wavelength shift over 110 nm was obtained with lower growth rate enhancement factor (1.43). This is similar to Zhu et al.’s [12] result where they report wavelength shift of 80 nm with growth rate
400µm
500µm
250µm
active region
tuning region
modulator region 30µm
70µm 50µm
50µm
70µm
30µm
b
active
p-InP
PC
DBR
modulator
Bragg mirror
active MQW n-InP HR
AR
Fig. 1. (a) Geometry of the SiNx SAG mask specially patterned on the substrate before growth and (b) schematic diagram of an EA modulator integrated with a tunable DBR laser.
Fig. 2. FE-SEM photographs of MQW in (a) active, (b) tuning, and (c) modulator regions.
ARTICLE IN PRESS S.-B. Kim et al. / Journal of Crystal Growth 298 (2007) 672–675
674
condition. The rear facet is HR (reflectivity is around 90%) coated with three pair of SiO2/TiO2.
tuning region
PL intensity(a.u.)
1.442µm
4. Device characteristics
FWHM=42.6meV
FWHM=40.9meV
modulator region 1.503µm active region 1.557µm FWHM=42.1meV
1250 1300 1350 1400 1450 1500 1550 1600 1650 wavelength(nm)
Fig. 4 shows the typical current–light characteristics of the integrated device measured under continuous wave operation at room temperature. In spite of the low PL intensity of the active region (Fig. 3), the threshold current is 5.7 mA and the output power is 5 mW for 65 mA current injected in the active section. The inset shows the optical spectrum of the laser with a large side mode suppression ratio (SMSR) of 45 dB at a 60 mA laser current level. The tuning characteristics as a function of Bragg current are illustrated in Fig. 5. As shown in Fig. 5, the wavelength 6
Fig. 3. PL spectra of active, modulator, and tuning regions at room temperature.
@ room temp. CW-operation
5
The DBR laser consisted of a 400-mm-long active section, a 150-mm-long phase control section, and a 200mm-long DBR section, while the EA modulator had a length of 120 mm. Each section was electrically isolated with a 20-mm-long isolation trench. The isolation resistance of 10 kO was required. The DBR grating was inscribed by holographic exposure and reactive ion etching. The integrated device was formed in the planar buried heterostructure (PBH) with p–n–p current blocking layer for the lateral and electrical confinements on the device, so current leakage was effectively blocked. A 1.5-mm-thick pInP cladding layer and 0.1-mm-thick p+-InGaAs contact layer were overgrown successively. N- and p-metal contact electrodes were deposited afterwards. For the final step, the front facet was AR coated with two pair of TiO2/SiO2. Reflectivity of less than 1% is obtained with an optimized
4
3 SMSR=45 dB
2
1545
1
1550 1555 Wavelength (nm)
0 0
10 20 30 40 50 60 70 80 Current (mA)
Fig. 4. Output power versus current injected in the active section with, in inset, the optical spectrum for an injected current of 60 mA.
1557.5 PC current = 0 mA 1557.0 Wavelength (nm)
3. Device fabrication
Output power (mW)
enhancement factor of about 1.34. Energy shift can be explained by two reasons: (1) increment of well width and (2) increment of indium composition in MQWs, mentioned by other papers [13,14]. When the composition was not varying but the increment of thickness exists, we calculated that the wavelength shift did not exceed 40 nm. It implies that PL shift over 70 nm was originated from increment of In-composition by difference of migration/diffusion length of group III species. It could be assumed that y remains constant in SAG of In1xGaxAsyP1y material [15,16]. Also, the In content increased with increasing mask width because the migration length of indium is much longer than that of gallium. For bandgap engineering, it means that the control of migration/diffusion length of group III species is more effective than that of thickness variation in our SAG condition.
1556.5 1556.0
Maximum tuning range: 3.45 nm
1555.5 1555.0 1554.5 1554.0 1553.5 0
10
20 DBR current (mA)
30
40
Fig. 5. Dependence of the wavelength on the Bragg section current.
ARTICLE IN PRESS S.-B. Kim et al. / Journal of Crystal Growth 298 (2007) 672–675
respectively. The integrated device was fabricated in conventional PBH with p–n–p current blocking layer. The integrated device with SAG shows a good performance including threshold current of 5.7 mA, total tuning range of 3.45 nm, and 3 dB cut-off frequency exceeding 11.6 GHz. These device characteristics are highly adjustable for highspeed data transmission applications in WDM system.
3
0 E/O Response (dB)
675
-3
-6
References
Small signal RF response -9
-12 0
2
4
6
8
10
12
14
Frequency (GHz) Fig. 6. A small signal E/O response as a function of frequency.
of DBR laser was tuned from 1557.25 to 1553.8 nm by DBR current, where the currents injected in active and in phase control sections were kept at 50 and 0 mA, respectively. This result indicates that the fabricated DBR laser is applicable as a tunable WDM transmitter. A static extinction ratio over 20 dB at an applied modulator bias voltage of 3 V was also observed. For high-speed measurement over 10 GHz, we measured E/O bandwidth using an Anritsu 87300C 65 GHz vector analyzer and a calibrated receiver. The frequency response reached the laser current of 50 mA. The E/O response is shown in Fig. 6, which reveals a 3 dB small signal bandwidth of about 11.6 GHz. 5. Conclusion The monolithic integration of a tunable DBR laser EA modulator has been achieved by selective area MOVPE. By using the specially designed mask pattern and epilayer, we could adjust the PL wavelengths of an active, modulator, and tuning regions to 1.557, 1.503, and 1.442 mm,
[1] T.L. Koch, U. Koren, IEEE J. Quantum Electron. 27 (1991) 641. [2] S. Charbonneau, E.S. Koteles, P.J. Poole, J.J. He, G.C. Aers, J. Haysom, M. Buchanan, Y. Feng, A. Delage, F. Yang, M. Davies, R.D. Goldberg, P.G. Piva, I.V. Mitchell, IEEE J. Select. Topics Quantum Electron. 4 (1998) 772. [3] T. Van Caenegem, D. Van Thourhout, M. Galarza, S. Verstuyft, I. Moerman, P. Van Daele, R. Baets, P. Demeester, C.G.P. Herben, X.J.M. Leijtens, M.K. Smit, Electron. Lett. 37 (2001) 296. [4] Q. Zhao, J.Q. Pan, J. Zhang, H.L. Zhu, W. Wang, J. Crystal Growth 288 (2006) 27. [5] 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 (1993) 998. [6] M. Aoki, M. Suzuki, H. Sano, T. Kawano, T. Ido, T. Taniwatari, K. Uomi, A. Takai, IEEE J. Quantum Electron. 29 (1993) 2088. [7] Y.-H. Kwon, J.-S. Choe, J.-S. Sim, S.-B. Kim, K.-S. Choi, B.-S. Choi, in: Technical Digest OECC 2005, Seoul, Korea, Paper 6F1-5, 2005, p. 316. [8] J.C. Cartledge, J. Lightwave Technol. 14 (1996) 480. [9] S.H. Oh, H. Ko, K.S. Kim, J.-M. Lee, C.W. Lee, O.K. Kwon, S. Park, M.H. Park, ETRI J. 27 (2005) 551. [10] D. Delprat, L. Silvestre, A. Ougazzaden, F. Delorme, S. Slempks, A. Ramdane, in: IPRM’96, Schwabisch Gmund, Germany, Paper TuP-C4, 1996, p. 162. [11] D. Delprat, A. Ramdane, L. Silvestre, A. Ougazzaden, F. Delorme, S. Slempks, IEEE Photon. Technol. Lett. 9 (1997) 898. [12] J.T. Zhu, L. Billia, D. Bour, S. Corzine, G. Ho¨fler, J. Crystal Growth 272 (2004) 576. [13] W. Qiu, W. Wang, J. Dong, F. Zhou, J. Jhang, J. Crystal Growth 250 (2003) 583. [14] G. Ru, X. Yu, Z. Chen, J. Zhang, J. Lin, F.-S. Choa, J. Crystal Growth 285 (2005) 499. [15] M.-S. Kim, C. Caneau, E. Colas, R. Bhat, J. Crystal Growth 123 (1992) 69. [16] T. Sasaki, M. Kitamura, I. Mito, J. Crystal Growth 132 (1993) 435.