- Email: [email protected]
High-performance EML grown on taper-masked pattern substrates by ultra-low-pressure MOCVD
ARTICLE IN PRESS
Journal of Crystal Growth 288 (2006) 27–31 www.elsevier.com/locate/jcrysgro
High-performance EML grown on taper-masked pattern substrates by ultra-low-pressure MOCVD Q. Zhao, J.Q. Pan, J. Zhang, H.L. Zhu, W. Wang Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, 100083 Beijing, PR China Available online 19 January 2006
Abstract A novel in-plane bandgap energy controlling technique by ultra-low pressure (22 mbar) selective area growth (SAG) has been developed. To our knowledge, this is the lowest pressure condition during SAG process ever reported. In this work, high crystalline quality InGaAsP–InP MQWs with a photoluminescence (PL) full-width at half-maximum (FWHM) of less than 35 meV are selectively grown on mask-patterned planar InP substrates by ultra-low pressure (22 mbar) metal-organic chemical vapor deposition (MOCVD). In order to study the uniformity of the MQWs grown in the selective area, novel tapered masks are designed and used. Through optimizing growth conditions, a wide wavelength shift of over 80 nm with a rather small mask width variation (0–30 mm) is obtained. The mechanism of ultra-low pressure SAG is detailed by analyzing the effect of various mask designs and quantum well widths. This powerful technique is then applied to fabricate an electroabsorption-modulated laser (EML). Superior device characteristics are achieved, such as a low threshold current of 19 mA and an output power of 7 mW. r 2005 Elsevier B.V. All rights reserved. PACS: 68.65.Fg; 81.15.Gh; 78.55.Cr Keywords: A1. Selective area growth; A2. Ultra-low pressure; A3. InGaAsP; A4. Tapered mask; A5. Integrated device
1. Introduction Recently, interest in external modulation of 1.55 mm wavelength lasers for high-speed telecommunication application has increased because of the achievable high RF frequency and low drive voltage. An electroabsorptionmodulated laser (EML) can further lead to compact devices with high coupling efficiency and low packaging costs [1,2]. However, one major difficulty in fabricating monolithic integrated devices is to obtain large bandgap energy shift between the functional elements. More recently, a novel integration method for bandgap energy controlling during simultaneous selective area growth (SAG) process in metal-organic chemical vapor deposition (MOCVD) has received great attention [3–5]. Different bandgaps of MQW structures can be easily achieved in a substrate simultaneously by changing the dielectric mask geometry. The bandgap energy detuning mainly depends Corresponding author. Tel.: +86 10 82304610; fax: +86 10 82305033.
E-mail address: [email protected] (Q. Zhao). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.12.027
on thickness growth enhancement, additionally, with small composition modulation. High optical coupling efficiency is maintained due to the perfect and uniform wave-guide structure using this technique. On the other hand, it is well known that low pressure is beneficial to obtaining abrupt hetero-interfaces with good homogeneity. However, this technique generally suffers from low growth rates. In this work, we develop an ultralow pressure (22 mbar) SAG technique that maintains a high growth rate. To our knowledge, this is the lowest pressure condition during SAG process ever reported. High crystalline quality InGaAsP/InGaAsP MQWs with a photoluminescence (PL) full-width at half-maximum (FWHM) of less than 35 meV are selectively grown on mask-patterned planar InP substrates under ultra-low pressure (22 mbar) MOCVD. A wide wavelength shift of about 80 nm has been obtained through optimizing growth conditions. The mechanism of ultra-low-pressure SAG is detailed by analyzing the effect of various mask designs and quantum well widths. A 1.55 mm MQW ridge-waveguide EML with a room temperature threshold
ARTICLE IN PRESS Q. Zhao et al. / Journal of Crystal Growth 288 (2006) 27–31
28
current of 19 mA, and output power of about 7 mW is fabricated by this technique. 2. Experimental procedure First, a 200-nm-thick SiO2 dielectric film is deposited on the S-doped (1 0 0) InP substrates by plasma-enhanced chemical vapor deposition (PECVD). In order to keep the uniformity of the MQWs materials grown in the selective area, novel tapered masks are employed. The tapered masks are 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) is varied from 15 to 30 mm with 600 mm length, while the width of the gap region (Wg) between the masks is fixed at 15 mm. The length of the tapered region (Lt) is 50 mm. Then the SAG process is carried out on the
Tapered Region
Wm
Wg
Mask Region
Fig. 1. Schematic view of the tapered mask pattern.
patterned substrate. A horizontal flow, MOCVD AIX 200 reactor with a rotating susceptor is employed for preparing samples. The reactive precursors are trimethylindium (TMIn), triethylgarium (TEGa), arsine (AsH3), and phosphine (PH3). The samples are grown at 655 1C, and with V/III ratios of 250. The typical growth rates on unmasked region (planar growth) are kept as high as 1.8 mm/h for InP and 2.55 mm/h for InGaAsP, respectively. Micro-photoluminescence (m-PL) excited by an Ar+ laser (l ¼ 514:5 nm) with InGaAs photodiode detector is used to evaluate the crystalline quality of the samples at room temperature. The excited spot is focused to a diameter of 4 mm. 3. Results and discussions Fig. 2 shows the diagram of mechanism of the SAG MOCVD process. There are three gas supply paths during the selective MOCVD, which are vertical gas phase diffusion (VGPD), lateral gas phase diffusion (LGPD), and surface atom migration (SAM), respectively. Of course, the adatoms can be desorbed. Because the MOCVD process cannot occur on the surface of the dielectric masks, the LGPD effect is driven by the lateral elements concentration gradient. The SAG process in MOCVD is basically due to the competitive mechanism of LGPD and SAM. Sasaki et al. [6] found that SAM effect governed the growth process in the case of narrow gap region when the width of the gap region (Wg) is less than 2 mm. For our case, however, Wg is set to be much larger than that of narrow-stripe SAG, so, the SAM effect can be negligible because the particles absorbed on the masks can move only less than several microns in the selective region, on the other hand, these particles form the growth spike at the edge of unmasked region as shown in Fig. 2. With the ultra-low pressure in this work, the effect of LGPD is further enhanced thanks to the large mean free path of the reagents particles. Therefore, the diffusion effects of the lateral gas phase play a dominant role in the SAG process during MOCVD growth.
(1) VGPD (2) LGPD TEGa
TMIn
(4) Desorption (100)
Dielectric Mask
(3) SAM
d Wm
d0 Wg
Growth Spike
(1) VGPD : Vertical Gas Phase Diffusion (2) LGPD : Lateral Gas Phase Diffusion (3) SAM : Surface Atom Migration (4) Desorption Fig. 2. Diagram of the SAG MOCVD mechanism.
ARTICLE IN PRESS Q. Zhao et al. / Journal of Crystal Growth 288 (2006) 27–31
0.840 0.835
1550 Energy (eV)
PL Peak Wavelength (nm)
1600
29
1500
1450
0.830 0.825 0.820 0.815 0.810
60 56
1.0 50
0.8 0.6
40 0.4
FWHM (meV)
Normalized PL Intensity
(a) 1400 1.2
60
62 64 Wwell (angstrom)
66
68
70
Fig. 4. The bandgap energy in the unmasked region as a function of the well’s width.
50
0.2
30 45
0.0 30
Fig. 3. (a) Dependence of the photoluminescence wavelength on mask width Wm and (b) dependence of the normalized PL intensity and FWHM on mask width Wm.
The Wm dependence of the PL peak wavelength, the corresponding normalized PL intensity as well as the PL FWHM from selectively grown MQW are shown in Fig. 3 A wide peak red shift from 1485 to 1568 nm is obviously observed, and this wavelength range covers the entire Cbands of the fiber-optic communication spectrum. Over 80 nm wavelength windows has been obtained with a rather small mask width variation (0–30 mm), which makes it possible to fabricate monolithic integration of laser/ modulator. The PL peak shift is mainly due to the increase of indium content by the effect of the LGPD [7–9]. The corresponding normalized stronger PL intensity and the narrower PL FWHM indicate that MQW with high quality, and uniformity crystal could be obtained by the ultra-low pressure growth conditions. In order to evaluate the influence of thickness enhancement, the bandgap energy in the maskless region as a function of the wells width is further investigated, as shown in Fig. 4, a clear saturation is also observed. For the quantum wells width wider than 6.4 nm, the bandgap energy almost remains constant. For fabricating integrated optical devices, appropriate selectivity and PL wavelength of each functional element should be considered simultaneously. Compared with selectivity, the influence of well width to the PL wavelength of each functional element is more significant. The change of the PL wavelength and FWHM as a function of the distance of the excited spot from the center
40 FWHM (meV)
10 20 Mask Width Wm (um)
35 30 25 20 15 10 1580 Excited Spots
PL wavelength (nm)
0 (b)
58
Wm
1560
1540
1520 Wm=15 µm Wm=22 µm
1500
Wm=30 µm
0 50 100 150 200 Distance from the center of the selective area (µm) Fig. 5. 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.
of the selective area to maskless area is shown in Fig. 5. It is clearly observed that the wavelength shift (DlPL) is rather small (about 22 nm) for the narrower Wm (15–22 mm),
ARTICLE IN PRESS Q. Zhao et al. / Journal of Crystal Growth 288 (2006) 27–31
30
p Electrode InGaAsP/InGaAsP MQW structure n-InP Sub.
DFB
n Electrode
EAM
first-order grating InGaAsP optical confinement layers InP Buffer InGaAsP/InGaAsP MQW structure Fig. 6. The configuration of EML fabricated by ultra-low-pressure SAG.
7 6 5 4 3 10dB/div
Power (mW)
whereas 50 nm wavelength shifts are achieved for the wider Wm (30 mm). Furthermore, the PL peak wavelength is slightly changed when the excited spots distance from the center of the selective area is over 100 mm. For our SAG growth process, the gap region width Wg is chosen to be much smaller than the gas phase diffusion length, it is crucial to obtain highly uniform selectively grown MQWs materials. Fig. 5 also shows the corresponding FWHM of this PL. The results indicated that the hetero-interface and homogeneity in composition of tapered region is almost as good as that of normally grown MQWs layers on the maskless area, which approves that this structure can be used as device fabrication. Considering the appropriate energy detuning between the modulator and laser, we chose W m ¼ 22 mm as an optimized SAG condition. The lPL are 1485 and 1547 nm for the modulator with FWHM of 31 meV and laser with FWHM of 30 meV, respectively. A 62 nm wavelength red shift around the 1.5 mm wavelength window between laser section and modulator section is obtained. Then, a MQW EML is fabricated based on the ultra-low pressure bandgap energy control technique. The schematic of EML is shown in Fig. 6. The absorption layers of EAMs and laser active layers consist one continuous InGaAsP/InGaAsP strained MQW structure and InGaAsP optical confinement layers with different thickness caused by SAG process (see the inset in Fig. 6). The MQW absorption layer consists of six pair of undoped 5.6-nm-thick 0.7% compressive strain InGaAsP wells separated by 9.6-nm-thick lattice-matched InGaAsP barriers, embedded in between 100-nm-thick lattice-matched InGaAsP (lPL ¼ 1200 nm) optical confinement layers. The typical I–L curve of the integrated device is shown in Fig. 7. Continuous wave threshold current of 19 mA, for the DFB laser of length 300 mm and the modulators of length 150 mm, is measured at room temperature. About 7 mW output power is available at the modulator facet at the reverse bias voltage of 0 V. The inset of Fig. 7 shows the side mode suppression ratio (SMSR) over 40 dB is achieved. These properties reflect the high crystalline quality of SAG and the high optical coupling efficiency between the functional elements. It is
2 1
1.540 1.545 1.550 1.555 Wavelength (µm)
0 0
20
40
60 Current(mA)
80
100
Fig. 7. CW output power from the modulator facet versus laser drive current. The inset illustrates lasing spectrum of the integrated device (I LD ¼ 80 mA).
recognized that the SAG method successfully preserves the performance of the discrete functional elements. 4. Conclusion A growth technique of ultra-low pressure SAG process for the precisely control of bandgap energy shift is developed. High crystalline quality InGaAsP/InGaAsP MQWs with PL FWHM of 30 meV is selectively area grown under optimizing ultra-low pressure conditions (22 mbar). By applying the technique, the InGaAsP/ InGaAsP MQW EML is successfully fabricated. This method not only greatly simplifies the integration process, but also achieves high device performance, such as low threshold current of 19 mA, about 7 mW output power. These experimental results demonstrate that ultra-lowpressure SAG integration scheme is a promising approach for fabricating high-speed transmission photonic integrated circuits (PICs).
ARTICLE IN PRESS Q. Zhao et al. / Journal of Crystal Growth 288 (2006) 27–31
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] A. Ramdane, F. Devaux, N. Souli, D. Delprat, A. Ougazzaden, IEEE J. Select. Top. Quantum Electron. 2 (1996) 326. [2] A. Ramdane, P. Krauz, E.V.K. Rao, A. Hamouli, A. Ougazzaden, D. Robein, A. Gloukhian, M. Carre, IEEE Photon. Technol. Lett. 7 (1995) 1016.
31
[3] Y. Sakata, T. Hosoda, Y. Sasaki, S. Kitamura, M. Yamamoto, Y. Inomoto, K. Komatsu, IEEE J. Quantum Electron. 35 (1999) 368. [4] S. Sudo, Y. Yokoyama, T. Nakazaki, K. Mori, K. Kudo, M. Yamaguchi, T. Sasaki, J. Crystal Growth 221 (2000) 189. [5] E.H. Sargent, Solid State Electron. 44 (2000) 147. [6] T. Sasaki, M. Kitamura, I. Mito, J. Crystal Growth 132 (1993) 435. [7] M. Aoki, M. Suzuki, M. Takahashi, H. Sano, T. Ido, T. Kawano, A. Takai, Electron. Lett. 28 (1992) 1157. [8] M. Tsuji, K. Makita, T. Takeuchi, K. Taguchi, J. Crystal Growth 170 (1996) 669. [9] M. Tsuji, K. Makita, T. Takeuchi, K. Taguchi, J. Crystal Growth 162 (1996) 25.
Journal of Crystal Growth 288 (2006) 27–31 www.elsevier.com/locate/jcrysgro
High-performance EML grown on taper-masked pattern substrates by ultra-low-pressure MOCVD Q. Zhao, J.Q. Pan, J. Zhang, H.L. Zhu, W. Wang Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, 100083 Beijing, PR China Available online 19 January 2006
Abstract A novel in-plane bandgap energy controlling technique by ultra-low pressure (22 mbar) selective area growth (SAG) has been developed. To our knowledge, this is the lowest pressure condition during SAG process ever reported. In this work, high crystalline quality InGaAsP–InP MQWs with a photoluminescence (PL) full-width at half-maximum (FWHM) of less than 35 meV are selectively grown on mask-patterned planar InP substrates by ultra-low pressure (22 mbar) metal-organic chemical vapor deposition (MOCVD). In order to study the uniformity of the MQWs grown in the selective area, novel tapered masks are designed and used. Through optimizing growth conditions, a wide wavelength shift of over 80 nm with a rather small mask width variation (0–30 mm) is obtained. The mechanism of ultra-low pressure SAG is detailed by analyzing the effect of various mask designs and quantum well widths. This powerful technique is then applied to fabricate an electroabsorption-modulated laser (EML). Superior device characteristics are achieved, such as a low threshold current of 19 mA and an output power of 7 mW. r 2005 Elsevier B.V. All rights reserved. PACS: 68.65.Fg; 81.15.Gh; 78.55.Cr Keywords: A1. Selective area growth; A2. Ultra-low pressure; A3. InGaAsP; A4. Tapered mask; A5. Integrated device
1. Introduction Recently, interest in external modulation of 1.55 mm wavelength lasers for high-speed telecommunication application has increased because of the achievable high RF frequency and low drive voltage. An electroabsorptionmodulated laser (EML) can further lead to compact devices with high coupling efficiency and low packaging costs [1,2]. However, one major difficulty in fabricating monolithic integrated devices is to obtain large bandgap energy shift between the functional elements. More recently, a novel integration method for bandgap energy controlling during simultaneous selective area growth (SAG) process in metal-organic chemical vapor deposition (MOCVD) has received great attention [3–5]. Different bandgaps of MQW structures can be easily achieved in a substrate simultaneously by changing the dielectric mask geometry. The bandgap energy detuning mainly depends Corresponding author. Tel.: +86 10 82304610; fax: +86 10 82305033.
E-mail address: [email protected] (Q. Zhao). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.12.027
on thickness growth enhancement, additionally, with small composition modulation. High optical coupling efficiency is maintained due to the perfect and uniform wave-guide structure using this technique. On the other hand, it is well known that low pressure is beneficial to obtaining abrupt hetero-interfaces with good homogeneity. However, this technique generally suffers from low growth rates. In this work, we develop an ultralow pressure (22 mbar) SAG technique that maintains a high growth rate. To our knowledge, this is the lowest pressure condition during SAG process ever reported. High crystalline quality InGaAsP/InGaAsP MQWs with a photoluminescence (PL) full-width at half-maximum (FWHM) of less than 35 meV are selectively grown on mask-patterned planar InP substrates under ultra-low pressure (22 mbar) MOCVD. A wide wavelength shift of about 80 nm has been obtained through optimizing growth conditions. The mechanism of ultra-low-pressure SAG is detailed by analyzing the effect of various mask designs and quantum well widths. A 1.55 mm MQW ridge-waveguide EML with a room temperature threshold
ARTICLE IN PRESS Q. Zhao et al. / Journal of Crystal Growth 288 (2006) 27–31
28
current of 19 mA, and output power of about 7 mW is fabricated by this technique. 2. Experimental procedure First, a 200-nm-thick SiO2 dielectric film is deposited on the S-doped (1 0 0) InP substrates by plasma-enhanced chemical vapor deposition (PECVD). In order to keep the uniformity of the MQWs materials grown in the selective area, novel tapered masks are employed. The tapered masks are 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) is varied from 15 to 30 mm with 600 mm length, while the width of the gap region (Wg) between the masks is fixed at 15 mm. The length of the tapered region (Lt) is 50 mm. Then the SAG process is carried out on the
Tapered Region
Wm
Wg
Mask Region
Fig. 1. Schematic view of the tapered mask pattern.
patterned substrate. A horizontal flow, MOCVD AIX 200 reactor with a rotating susceptor is employed for preparing samples. The reactive precursors are trimethylindium (TMIn), triethylgarium (TEGa), arsine (AsH3), and phosphine (PH3). The samples are grown at 655 1C, and with V/III ratios of 250. The typical growth rates on unmasked region (planar growth) are kept as high as 1.8 mm/h for InP and 2.55 mm/h for InGaAsP, respectively. Micro-photoluminescence (m-PL) excited by an Ar+ laser (l ¼ 514:5 nm) with InGaAs photodiode detector is used to evaluate the crystalline quality of the samples at room temperature. The excited spot is focused to a diameter of 4 mm. 3. Results and discussions Fig. 2 shows the diagram of mechanism of the SAG MOCVD process. There are three gas supply paths during the selective MOCVD, which are vertical gas phase diffusion (VGPD), lateral gas phase diffusion (LGPD), and surface atom migration (SAM), respectively. Of course, the adatoms can be desorbed. Because the MOCVD process cannot occur on the surface of the dielectric masks, the LGPD effect is driven by the lateral elements concentration gradient. The SAG process in MOCVD is basically due to the competitive mechanism of LGPD and SAM. Sasaki et al. [6] found that SAM effect governed the growth process in the case of narrow gap region when the width of the gap region (Wg) is less than 2 mm. For our case, however, Wg is set to be much larger than that of narrow-stripe SAG, so, the SAM effect can be negligible because the particles absorbed on the masks can move only less than several microns in the selective region, on the other hand, these particles form the growth spike at the edge of unmasked region as shown in Fig. 2. With the ultra-low pressure in this work, the effect of LGPD is further enhanced thanks to the large mean free path of the reagents particles. Therefore, the diffusion effects of the lateral gas phase play a dominant role in the SAG process during MOCVD growth.
(1) VGPD (2) LGPD TEGa
TMIn
(4) Desorption (100)
Dielectric Mask
(3) SAM
d Wm
d0 Wg
Growth Spike
(1) VGPD : Vertical Gas Phase Diffusion (2) LGPD : Lateral Gas Phase Diffusion (3) SAM : Surface Atom Migration (4) Desorption Fig. 2. Diagram of the SAG MOCVD mechanism.
ARTICLE IN PRESS Q. Zhao et al. / Journal of Crystal Growth 288 (2006) 27–31
0.840 0.835
1550 Energy (eV)
PL Peak Wavelength (nm)
1600
29
1500
1450
0.830 0.825 0.820 0.815 0.810
60 56
1.0 50
0.8 0.6
40 0.4
FWHM (meV)
Normalized PL Intensity
(a) 1400 1.2
60
62 64 Wwell (angstrom)
66
68
70
Fig. 4. The bandgap energy in the unmasked region as a function of the well’s width.
50
0.2
30 45
0.0 30
Fig. 3. (a) Dependence of the photoluminescence wavelength on mask width Wm and (b) dependence of the normalized PL intensity and FWHM on mask width Wm.
The Wm dependence of the PL peak wavelength, the corresponding normalized PL intensity as well as the PL FWHM from selectively grown MQW are shown in Fig. 3 A wide peak red shift from 1485 to 1568 nm is obviously observed, and this wavelength range covers the entire Cbands of the fiber-optic communication spectrum. Over 80 nm wavelength windows has been obtained with a rather small mask width variation (0–30 mm), which makes it possible to fabricate monolithic integration of laser/ modulator. The PL peak shift is mainly due to the increase of indium content by the effect of the LGPD [7–9]. The corresponding normalized stronger PL intensity and the narrower PL FWHM indicate that MQW with high quality, and uniformity crystal could be obtained by the ultra-low pressure growth conditions. In order to evaluate the influence of thickness enhancement, the bandgap energy in the maskless region as a function of the wells width is further investigated, as shown in Fig. 4, a clear saturation is also observed. For the quantum wells width wider than 6.4 nm, the bandgap energy almost remains constant. For fabricating integrated optical devices, appropriate selectivity and PL wavelength of each functional element should be considered simultaneously. Compared with selectivity, the influence of well width to the PL wavelength of each functional element is more significant. The change of the PL wavelength and FWHM as a function of the distance of the excited spot from the center
40 FWHM (meV)
10 20 Mask Width Wm (um)
35 30 25 20 15 10 1580 Excited Spots
PL wavelength (nm)
0 (b)
58
Wm
1560
1540
1520 Wm=15 µm Wm=22 µm
1500
Wm=30 µm
0 50 100 150 200 Distance from the center of the selective area (µm) Fig. 5. 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.
of the selective area to maskless area is shown in Fig. 5. It is clearly observed that the wavelength shift (DlPL) is rather small (about 22 nm) for the narrower Wm (15–22 mm),
ARTICLE IN PRESS Q. Zhao et al. / Journal of Crystal Growth 288 (2006) 27–31
30
p Electrode InGaAsP/InGaAsP MQW structure n-InP Sub.
DFB
n Electrode
EAM
first-order grating InGaAsP optical confinement layers InP Buffer InGaAsP/InGaAsP MQW structure Fig. 6. The configuration of EML fabricated by ultra-low-pressure SAG.
7 6 5 4 3 10dB/div
Power (mW)
whereas 50 nm wavelength shifts are achieved for the wider Wm (30 mm). Furthermore, the PL peak wavelength is slightly changed when the excited spots distance from the center of the selective area is over 100 mm. For our SAG growth process, the gap region width Wg is chosen to be much smaller than the gas phase diffusion length, it is crucial to obtain highly uniform selectively grown MQWs materials. Fig. 5 also shows the corresponding FWHM of this PL. The results indicated that the hetero-interface and homogeneity in composition of tapered region is almost as good as that of normally grown MQWs layers on the maskless area, which approves that this structure can be used as device fabrication. Considering the appropriate energy detuning between the modulator and laser, we chose W m ¼ 22 mm as an optimized SAG condition. The lPL are 1485 and 1547 nm for the modulator with FWHM of 31 meV and laser with FWHM of 30 meV, respectively. A 62 nm wavelength red shift around the 1.5 mm wavelength window between laser section and modulator section is obtained. Then, a MQW EML is fabricated based on the ultra-low pressure bandgap energy control technique. The schematic of EML is shown in Fig. 6. The absorption layers of EAMs and laser active layers consist one continuous InGaAsP/InGaAsP strained MQW structure and InGaAsP optical confinement layers with different thickness caused by SAG process (see the inset in Fig. 6). The MQW absorption layer consists of six pair of undoped 5.6-nm-thick 0.7% compressive strain InGaAsP wells separated by 9.6-nm-thick lattice-matched InGaAsP barriers, embedded in between 100-nm-thick lattice-matched InGaAsP (lPL ¼ 1200 nm) optical confinement layers. The typical I–L curve of the integrated device is shown in Fig. 7. Continuous wave threshold current of 19 mA, for the DFB laser of length 300 mm and the modulators of length 150 mm, is measured at room temperature. About 7 mW output power is available at the modulator facet at the reverse bias voltage of 0 V. The inset of Fig. 7 shows the side mode suppression ratio (SMSR) over 40 dB is achieved. These properties reflect the high crystalline quality of SAG and the high optical coupling efficiency between the functional elements. It is
2 1
1.540 1.545 1.550 1.555 Wavelength (µm)
0 0
20
40
60 Current(mA)
80
100
Fig. 7. CW output power from the modulator facet versus laser drive current. The inset illustrates lasing spectrum of the integrated device (I LD ¼ 80 mA).
recognized that the SAG method successfully preserves the performance of the discrete functional elements. 4. Conclusion A growth technique of ultra-low pressure SAG process for the precisely control of bandgap energy shift is developed. High crystalline quality InGaAsP/InGaAsP MQWs with PL FWHM of 30 meV is selectively area grown under optimizing ultra-low pressure conditions (22 mbar). By applying the technique, the InGaAsP/ InGaAsP MQW EML is successfully fabricated. This method not only greatly simplifies the integration process, but also achieves high device performance, such as low threshold current of 19 mA, about 7 mW output power. These experimental results demonstrate that ultra-lowpressure SAG integration scheme is a promising approach for fabricating high-speed transmission photonic integrated circuits (PICs).
ARTICLE IN PRESS Q. Zhao et al. / Journal of Crystal Growth 288 (2006) 27–31
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] A. Ramdane, F. Devaux, N. Souli, D. Delprat, A. Ougazzaden, IEEE J. Select. Top. Quantum Electron. 2 (1996) 326. [2] A. Ramdane, P. Krauz, E.V.K. Rao, A. Hamouli, A. Ougazzaden, D. Robein, A. Gloukhian, M. Carre, IEEE Photon. Technol. Lett. 7 (1995) 1016.
31
[3] Y. Sakata, T. Hosoda, Y. Sasaki, S. Kitamura, M. Yamamoto, Y. Inomoto, K. Komatsu, IEEE J. Quantum Electron. 35 (1999) 368. [4] S. Sudo, Y. Yokoyama, T. Nakazaki, K. Mori, K. Kudo, M. Yamaguchi, T. Sasaki, J. Crystal Growth 221 (2000) 189. [5] E.H. Sargent, Solid State Electron. 44 (2000) 147. [6] T. Sasaki, M. Kitamura, I. Mito, J. Crystal Growth 132 (1993) 435. [7] M. Aoki, M. Suzuki, M. Takahashi, H. Sano, T. Ido, T. Kawano, A. Takai, Electron. Lett. 28 (1992) 1157. [8] M. Tsuji, K. Makita, T. Takeuchi, K. Taguchi, J. Crystal Growth 170 (1996) 669. [9] M. Tsuji, K. Makita, T. Takeuchi, K. Taguchi, J. Crystal Growth 162 (1996) 25.