Heterogeneously integrated photonic-crystal lasers on silicon for on/off chip optical interconnects Koji Takeda,1,2,* Tomonari Sato,1 Takuro Fujii,1,2 Eiichi Kuramochi,1,3 Masaya Notomi,1,3 Koichi Hasebe,1,2 Takaaki Kakitsuka,1,2 and Shinji Matsuo1,2 2
1 Nanophotonics Center, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan NTT Device Technology Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan 3 NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan * [email protected]
Abstract: We demonstrate the continuous-wave operation of lambda-scale embedded active-region photonic-crystal (LEAP) lasers at room temperature, which we fabricated on a Si wafer. The on-Si LEAP lasers exhibit a threshold current of 31 μA, which is the lowest reported value for any type of semiconductor laser on Si. This reveals the great potential of LEAP lasers as light sources for on- or off-chip optical interconnects with ultra-low power consumption in future information communication technology devices including CMOS processors. ©2015 Optical Society of America OCIS codes: (250.5960) Semiconductor lasers; (140.3948) Microcavity devices; (230.5298) Photonic crystals; (200.4650) Optical interconnects
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
12. 13.
Ministry of Internal Affairs and Communications, Japan, http://www.soumu.go.jp/ C. F. Lam, H. Liu, B. Koley, X. Zhao, V. Kamalov, and V. Gill, “Fiber optic communication technologies: what’s needed for datacenter network operations,” IEEE Commun. Mag. 48(7), 32–39 (2010). A. Benner, “Optical interconnect opportunities in supercomputers and high end computing,” in Optical Fiber Communication Conference (OFC), 2012 OSA Technical Digest Series (Optical Society of America, 2012), paper OTu2B. D. Miller, “Device Requirements for Optical Interconnects to Silicon Chips,” Proc. IEEE 97(7), 1166–1185 (2009). International Technology Roadmap for Semiconductors, http://www.itrs.net/ Y. A. Vlsaov, “Silicon photonics for next generation computing systems,” in Proceedings of European Conference on Optical Communications, paper SC2, 2008. W. Kobayashi, T. Ito, T. Yamanaka, T. Fujisawa, Y. Shibata, T. Kurosaki, M. Kohtoku, T. Tadokoro, and H. Sanjoh, “50-Gb/s direct modulation of a 1.3-μm InGaAlAs-based DFB laser with a ridge waveguide structure,” IEEE J. Sel. Top. Quantum Electron. 19(4), 1500908 (2013). K. Adachi, K. Shinoda, T. Kitatani, T. Fukamachi, Y. Matsuoka, T. Sugawara, and S. Tsuji, “25-Gb/s multichannel 1.3-μm surface-emitting lens-integrated DFB laser arrays,” J. Lightwave Technol. 29(19), 2899– 2905 (2011). S. Matsuo, T. Fujii, K. Hasebe, K. Takeda, T. Sato, and T. Kakitsuka, “Directly modulated buried heterostructure DFB laser on SiO₂/Si substrate fabricated by regrowth of InP using bonded active layer,” Opt. Express 22(10), 12139–12147 (2014). W. Hofmann, P. Moser, and D. Bimberg, “Energy-efficient VCSELs for interconnects,” Photon. J. 4(2), 652–656 (2012). S. Imai, K. Takaki, S. Kamiya, H. Shimizu, J. Yoshida, Y. Kawakita, T. Takagi, K. Hiraiwa, H. Shimizu, T. Suzuki, N. Iwai, T. Ishikawa, N. Tsukiji, and A. Kasukawa, “Recorded low power dissipation in highly reliable 1060-nm VCSELs for “Green” optical interconnection,” IEEE J. Sel. Top. Quantum Electron. 17(6), 1614–1620 (2011). S. Matsuo, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato, Y. Kawaguchi, and M. Notomi, “High-speed ultracompact buried heterostructure photonic-crystal laser with 13 fJ of energy consumed per bit transmitted,” Nat. Photonics 4(9), 648–654 (2010). S. Matsuo, A. Shinya, C.-H. Chen, K. Nozaki, T. Sato, Y. Kawaguchi, H. Taniyama, and M. Notomi, “20-Gbit/s directly modulated photonic crystal nanocavity laser with ultra-low power consumption,” Opt. Express 19(3), 2242–2250 (2011).
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 702
14. K. Takeda, T. Sato, A. Shinya, K. Nozaki, W. Kobayashi, H. Taniyama, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo, “Few-fJ/bit data transmissions using directly modulated lambda-scale embedded active region photonic-crystal lasers,” Nat. Photonics 7(7), 569–575 (2013). 15. S. Matsuo, T. Sato, K. Takeda, A. Shinya, K. Nozaki, H. Taniyama, M. Notomi, K. Hasebe, and T. Kakitsuka, “Ultralow operating energy electrically driven photonic crystal lasers,” IEEE J. Sel. Top. Quantum Electron. 19(4), 4900311 (2013). 16. K. Nozaki, S. Matsuo, K. Takeda, T. Sato, T. Fujii, E. Kuramochi, and M. Notomi, “High-responsivity 1.7-μmlong InGaAs photodetectors based on photonic crystal with ultrasmall buried heterostructure,” in Conference on Lasers and Electro Optics (CLEO), 2014 OSA Technical Digest Series (Optical Society of America, 2014), paper STh4I.3. 17. K. Takeda, T. Sato, A. Shinya, K. Nozaki, H. Taniyama, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo, “Integrated On-Chip Optical Links Using Photonic-Crystal Lasers and Photodetectors with Current Blocking Trenches,” in Optical Fiber Communication Conference (OFC), 2013 OSA Technical Digest Series (Optical Society of America, 2013), paper OM2J.5. 18. K. Takeda, T. Sato, A. Shinya, K. Nozaki, H. Taniyama, K. Hasebe, T. Kakitsuka, M. Notomi, and S. Matsuo, “5.5-fJ/bit direct modulation of lambda-scale embedded active region photonic-crystal lasers,” in Proceedings of Optical Interconnects Conference, (IEEE, 2013) pp. 104 – 105. 19. B. G. Lee, A. Biberman, J. Chan, and K. Bergman, “High-performance modulators and switches for silicon photonic networks-on-chip,” IEEE J. Sel. Top. Quantum Electron. 16(1), 6–22 (2010). 20. C. Monat, C. Seassal, X. Letartre, P. Viktorovitch, P. Regreny, M. Gendry, P. Rojo-Romeo, G. Hollinger, E. Jalaguier, S. Pocas, and B. Aspar, “InP 2D photonic crystal microlasers on silicon wafer: room temperature operation at 1.55 μm,” Electron. Lett. 37(12), 764–766 (2001). 21. K. Tanabe, M. Nomura, D. Guimard, S. Iwamoto, and Y. Arakawa, “Room temperature continuous wave operation of InAs/GaAs quantum dot photonic crystal nanocavity laser on silicon substrate,” Opt. Express 17(9), 7036–7042 (2009). 22. Y. Halioua, A. Bazin, P. Monnier, T. J. Karle, G. Roelkens, I. Sagnes, R. Raj, and F. Raineri, “Hybrid III-V semiconductor/silicon nanolaser,” Opt. Express 19(10), 9221–9231 (2011). 23. T. Tsuchizawa, K. Yamada, T. Watanabe, S. Park, H. Nishi, R. Kou, H. Shinojima, and S. Itabashi, “Monolithic integration of silicon-, germanium-, and silica-based optical devices for telecommunications applications,” IEEE J. Sel. Top. Quantum Electron. 17(3), 516–525 (2011). 24. D. Di Liang, D. Chapman, Y. Li, D. Oakley, T. Napoleone, P. Juodawlkis, C. Brubaker, C. Mann, H. Bar, O. Raday, and J. Bowers, “Uniformity study of wafer-scale InP-to-silicon hybrid integration,” Appl. Phys., A Mater. Sci. Process. 103, 213–218 (2011). 25. G. Björk, A. Karlsson, and Y. Yamamoto, “On the linewidth of microcavity lasers,” Appl. Phys. Lett. 60(3), 304–306 (1992).
1. Introduction The power consumption of information communication technology (ICT) equipment is a serious problem because of the huge demand for services requiring large capacity communication such as cloud computing and video streaming [1]. These services require large datacenters that are increasing the volume of traffic. Optical communication technologies have been widely used in telecom networks, and active optical cables (AOCs) have brought the optical interconnects in datacenters and supercomputers [2, 3]. These optical communication technologies have contributed greatly to reducing the power consumption of communications and datacenters. To reduce the power dissipation of ICT equipment, there have been several attempts to introduce optical interconnects at much shorter distances: inside each instrument, on circuit boards, between complementary metal oxide semiconductor (CMOS) chips, and also on CMOS chips [4]. Optical interconnects on CMOS chips are especially attractive because copper interconnects constitute a bottleneck as regards power consumption and speed in current advanced CMOS chips after their continuous improvement under the scaling law [5]. If energy-efficient optical interconnects were realized on CMOS chips, the power consumption of CMOS chips would be significantly reduced [6]. To realize such interconnects, we need a directly modulated laser with ultralow power consumption because the direct modulation of a light source is the most power efficient scheme for transmitting data in optical communication.
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 703
Fig. 1. Integration schemes of LEAP lasers and connecting waveguides: vertical coupling. The LEAP laser emits light in the direction normal to the wafer, which can be coupled by a grating coupler, and converted to the lateral direction. Part of LEAP laser in the schematic is hidden to show the layer stack of the LEAP laser and the grating coupler.
The size of the active region is an important parameter for achieving light sources with such a low power consumption. Two major types of directly modulated lasers have been used for optical interconnects: distributed feedback (DFB) lasers and vertical-cavity surface emitting lasers (VCSELs). Typical DFB lasers occupy a few 100 square microns and their energy cost for generating a single bit of data is of the order of 1 pJ [7–9]. On the other hand, VCSELs have active sizes that are a few μm in diameter and the energy cost can be reduced to 81 fJ/bit [10, 11]. However, to reduce the energy cost to 10 fJ/bit [4], we need to further reduce the size of the active region. In this context, we have studied lambda-scale embedded active-region photonic-crystal (LEAP) lasers, which have a wavelength order active region. The carrier and optical confinement in the active region are achieved respectively by buried heterostructures and a photonic crystal (PhC), which is a hexagonal array of air holes [12]. Optically pumped LEAP lasers exhibit direct modulation at a bit rate of 20 Gbit/s [13]. The first continuous-wave (CW) operation of electrically driven LEAP lasers was achieved in 2012, and the threshold current was reduced to 4.8 μA, which was the lowest reported value for any semiconductor laser [14]. Energy efficient direct modulation was demonstrated with a bit rate of 12.5 Gbit/s and an energy cost of 14 fJ/bit [15]. By using the same fabrication processes, photonic-crystal (PhC) photodiodes (PDs) could be fabricated on the same wafers [16], and thus on-chip optical links consisting of LEAP lasers and PhC PDs were demonstrated [17, 18]. However, these results were achieved using a device formed on InP wafers. Lasers with similar characteristics on Si were needed if we were to realize optical interconnects on CMOS chips or between CMOS chips. This is because the use of optical filters and switches fabricated by employing Si photonics technologies are expected to greatly reduce the total power consumption of optical networks during on/off chip data transmission [6, 19]. There were several reports about PhC lasers on Si, but the CW operation was achieved only by the optical pumping [20–22]. To use them as the light sources, electrically-driven PhC lasers on Si are desired. However, it is difficult to achieve because the membrane structure increases both electrical and thermal resistances, which increase the active region temperature during the current injection [12]. In this paper, we report the CW operation of electrically-driven LEAP lasers on Si at room temperature. Oxygen-plasma assisted wafer bonding is used to fabricate the on-Si lasers. The bonding is carried out at an interface between Si and SiO2. An air-bridge structure is employed to maintain the quality factor of the cavity, and we achieved continuous-wave operation of the LEAP lasers with a threshold current of 31 μA, which is the smallest value for any type of semiconductor laser on Si.
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 704
Fig. 2. Process for fabricating LEAP lasers on Si. (a) An InGaAs etchstop layer and an InGaAsP MQW are grown on an InP substrate. (b) Etching and regrowth form a wavelengthscale BH, followed by the ion implantation of Si and diffusion of Zn to form lateral p-i-n junctions. (c) Deposition of 2-μm-thick SiO2 followed by CMP. (d) The processed InP wafer is bonded on a bulk Si substrate using oxygen-plasma assisted bonding. (e) The InP substrate and InGaAs etchstop are removed, and PhC holes are dry etched. (f) Finally metals are deposited and the SiO2 layer is partially etched with a wet etching solution.
2. Device design and fabrication There are several possible ways to integrate connecting optical waveguides and LEAP lasers on Si, and Fig. 1 is a schematic showing one potential integration scheme. Without coupling waveguides, the LEAP lasers have light emission in the direction normal to the wafer [12], so we can couple the light to the silicon photonics by using grating couplers. After coupling the light to the Si wire waveguides, they can be used as low-loss passive waveguides. When a relatively long distance is required such as chip-to-chip interconnection on a printed circuit board (PCB), spot-size converter integration is attractive because we can obtain a low coupling loss by using an inversely tapered Si waveguide and SiOx passive waveguides [23]. By integrating optical filters, switches and waveguide-type Ge photodetectors using Si photonics technologies, a photonic network incorporating the LEAP lasers can be realized [13]. We carried out wafer bonding and achieved the continuous-wave operation of LEAP lasers on Si as the first step towards realizing future waveguide-coupled optical light sources as well as photonic networks on Si. The device fabrication procedures are summarized in Fig. 2. All processes were performed using a combination of InP and Si wafers with a wafer size of 2 inches. First we used metalorganic vapor phase epitaxy (MOVPE) to form InGaAsP 6-quantum well (QW) wavelength-scale buried heterostructures on an InP substrate with an InGaAs etch stop layer (Fig. 2(a)). The region where the active layer remained was defined using electron-beam lithography, and mesa structures were formed with dry etching. Wavelength-scale embedded active regions were formed on InP wafers by MOVPE regrowth. n-InP regions were defined by Si ion implantation and annealing to activate dopants. We defined the p-InP regions by the thermal diffusion of Zn (Fig. 2(b)). The next step was a wafer bonding process. 2-μm-thick SiO2 was deposited on the InP that had undergone the abovementioned processes. The InP wafers were bonded on Si wafers with an O2 plasma assisted bonding technique [24] after surface planarization by a chemicalmechanical polishing (CMP) technique. The InP substrates were removed by polishing and selective wet etching. By using these procedures, we successfully transferred III-V thin films, which had embedded active regions and lateral p-i-n junctions, to Si wafers. Figure 3 shows transmission electron microscopic (TEM) images of the bonded wafer after the InP substrate
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 705
Fig. 3. Cross sectional transmission electron microscopy (TEM) image of the bonded wafers. The InP substrate had already been removed in these images. (a) Large area view, and (b) magnified view at the bonding interface.
Fig. 4. (a) Schematic and (b) scanning electron microscopic view of fabricated on-Si LEAP lasers. The active region is as small as 2.87 × 0.3 × 0.15 μm3. Current blocking trenches were used with a trench width of 200 nm. A 2-μm-thick SiO2 layer was used as the bonding interface as well as the sacrificial layer to form the air-bridge structure.
had been removed. To avoid optical coupling between the III-V layers and Si substrates, we used a SiO2 thickness of 2 μm, which has been widely used in silicon-on-insulator (SOI) based Si photonics. After the bonding, PhC holes were etched by further EB lithography and dry etching. Finally, we fabricated electrodes using lift-off processes. Air-bridge structures were formed by the selective wet etching of SiO2. Figure 4(a) shows a schematic of the fabricated device. Figure 4(b) shows a scanning electron microscope (SEM) image of the device. The left and right trapezoidal regions were nand p-type doped, respectively. The active region was 2.87 × 0.3 × 0.15 μm3. The InP slab was 250 nm thick, and the PhC lattice constant was 410 nm. We also employed current blocking trenches with a designed width of 200 nm. 3. Device characteristics The fabricated devices were mounted on a temperature-controlled stage with the temperature kept at 25°C. Output light was collected from the direction perpendicular to the wafer with a microscope objective lens and a multi-mode fiber. The output power and applied voltage were measured as a function of injected current (LI-V characteristic), and the result is shown in Fig. 5. We achieved the continuous wave operation of the current driven LEAP lasers on Si at room temperature with a threshold current of 31 μA (see inset), which is the smallest value yet reported for any semiconductor laser on Si. The
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 706
Fig. 5. (a) Light output and applied voltage versus injected current (L-I-V). A threshold current of 31 μA was obtained. (b) Lasing spectrum of the LEAP laser at an injected current of 100 μA. The lasing wavelength was 1501 nm.
Fig. 6. (a) Superimposed spectra of the LEAP lasers. The injected current was varied from 20 to 100 μA. (b) Peak wavelength and 3-dB bandwidth versus injected current. The bandwidth was measured with an optical spectrum analyzer at a resolution of 0.02 nm.
maximum output power was 0.27 μW at an injected current of 200 μA, which included the optical coupling loss of the measurement setup (approximately 10 dB). The maximum output power from the device was estimated to be a few μW considering the coupling loss. The differential resistance was approximately 5.4 kΩ at the threshold current. The lasing spectrum of the LEAP laser is shown in Fig. 5(b) with an injected current of 100 μA. We obtained single-mode lasing at a lasing wavelength of approximately 1501 nm. Superimposed lasing spectra are shown in Fig. 6(a) with an injected current ranging from 20 to 100 μA. Peak wavelengths and full-width at half-maximum (FWHM) linewidths were extracted from the spectra, and are summarized in Fig. 6(b). When the injected current was increased, a blue shift was observed in the wavelength until the current reached the threshold caused by the carrier plasma effect. We also observed a red shift in the wavelength, which was due to heat generation, above the lasing threshold. A clear kink in the decreasing linewidth was observed at around the lasing threshold showing the lasing behavior of #227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 707
nanocavity lasers [25]. The linewidth at the threshold was 0.07 nm. These results for the L-IV characteristic and spectral behavior are clear evidence that the LEAP lasers on Si are lasing at room temperature. The threshold current was a little larger than our previous report [14], which was due to the wavelength detuning between the lasing wavelength of 1501 nm and the photoluminescence peak of 1550 nm in this particular device. 4. Conclusion We reported the continuous-wave operation of LEAP lasers fabricated on Si wafers. Oxygenplasma assisted wafer bonding was carried out at an interface between SiO2 and Si. Current blocking trenches and air-bridge structures were used as previously reported. We achieved the lowest reported threshold current of 31 μA, and the device exhibited single mode lasing at a wavelength around 1501 nm. We think the CW operation of on-Si LEAP lasers is an important step towards realizing on-CMOS-chip optical interconnects, and future integration with waveguides will enable the LEAP lasers to be applied to off-chip optical interconnects. Acknowledgments We thank Mr. K. Ishibashi, Mr. Y. Shouji, and Mr. Y. Yokoyama for assistance with device fabrication.
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 708
1 Nanophotonics Center, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan NTT Device Technology Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan 3 NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa, 243-0198, Japan * [email protected]
Abstract: We demonstrate the continuous-wave operation of lambda-scale embedded active-region photonic-crystal (LEAP) lasers at room temperature, which we fabricated on a Si wafer. The on-Si LEAP lasers exhibit a threshold current of 31 μA, which is the lowest reported value for any type of semiconductor laser on Si. This reveals the great potential of LEAP lasers as light sources for on- or off-chip optical interconnects with ultra-low power consumption in future information communication technology devices including CMOS processors. ©2015 Optical Society of America OCIS codes: (250.5960) Semiconductor lasers; (140.3948) Microcavity devices; (230.5298) Photonic crystals; (200.4650) Optical interconnects
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
12. 13.
Ministry of Internal Affairs and Communications, Japan, http://www.soumu.go.jp/ C. F. Lam, H. Liu, B. Koley, X. Zhao, V. Kamalov, and V. Gill, “Fiber optic communication technologies: what’s needed for datacenter network operations,” IEEE Commun. Mag. 48(7), 32–39 (2010). A. Benner, “Optical interconnect opportunities in supercomputers and high end computing,” in Optical Fiber Communication Conference (OFC), 2012 OSA Technical Digest Series (Optical Society of America, 2012), paper OTu2B. D. Miller, “Device Requirements for Optical Interconnects to Silicon Chips,” Proc. IEEE 97(7), 1166–1185 (2009). International Technology Roadmap for Semiconductors, http://www.itrs.net/ Y. A. Vlsaov, “Silicon photonics for next generation computing systems,” in Proceedings of European Conference on Optical Communications, paper SC2, 2008. W. Kobayashi, T. Ito, T. Yamanaka, T. Fujisawa, Y. Shibata, T. Kurosaki, M. Kohtoku, T. Tadokoro, and H. Sanjoh, “50-Gb/s direct modulation of a 1.3-μm InGaAlAs-based DFB laser with a ridge waveguide structure,” IEEE J. Sel. Top. Quantum Electron. 19(4), 1500908 (2013). K. Adachi, K. Shinoda, T. Kitatani, T. Fukamachi, Y. Matsuoka, T. Sugawara, and S. Tsuji, “25-Gb/s multichannel 1.3-μm surface-emitting lens-integrated DFB laser arrays,” J. Lightwave Technol. 29(19), 2899– 2905 (2011). S. Matsuo, T. Fujii, K. Hasebe, K. Takeda, T. Sato, and T. Kakitsuka, “Directly modulated buried heterostructure DFB laser on SiO₂/Si substrate fabricated by regrowth of InP using bonded active layer,” Opt. Express 22(10), 12139–12147 (2014). W. Hofmann, P. Moser, and D. Bimberg, “Energy-efficient VCSELs for interconnects,” Photon. J. 4(2), 652–656 (2012). S. Imai, K. Takaki, S. Kamiya, H. Shimizu, J. Yoshida, Y. Kawakita, T. Takagi, K. Hiraiwa, H. Shimizu, T. Suzuki, N. Iwai, T. Ishikawa, N. Tsukiji, and A. Kasukawa, “Recorded low power dissipation in highly reliable 1060-nm VCSELs for “Green” optical interconnection,” IEEE J. Sel. Top. Quantum Electron. 17(6), 1614–1620 (2011). S. Matsuo, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato, Y. Kawaguchi, and M. Notomi, “High-speed ultracompact buried heterostructure photonic-crystal laser with 13 fJ of energy consumed per bit transmitted,” Nat. Photonics 4(9), 648–654 (2010). S. Matsuo, A. Shinya, C.-H. Chen, K. Nozaki, T. Sato, Y. Kawaguchi, H. Taniyama, and M. Notomi, “20-Gbit/s directly modulated photonic crystal nanocavity laser with ultra-low power consumption,” Opt. Express 19(3), 2242–2250 (2011).
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 702
14. K. Takeda, T. Sato, A. Shinya, K. Nozaki, W. Kobayashi, H. Taniyama, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo, “Few-fJ/bit data transmissions using directly modulated lambda-scale embedded active region photonic-crystal lasers,” Nat. Photonics 7(7), 569–575 (2013). 15. S. Matsuo, T. Sato, K. Takeda, A. Shinya, K. Nozaki, H. Taniyama, M. Notomi, K. Hasebe, and T. Kakitsuka, “Ultralow operating energy electrically driven photonic crystal lasers,” IEEE J. Sel. Top. Quantum Electron. 19(4), 4900311 (2013). 16. K. Nozaki, S. Matsuo, K. Takeda, T. Sato, T. Fujii, E. Kuramochi, and M. Notomi, “High-responsivity 1.7-μmlong InGaAs photodetectors based on photonic crystal with ultrasmall buried heterostructure,” in Conference on Lasers and Electro Optics (CLEO), 2014 OSA Technical Digest Series (Optical Society of America, 2014), paper STh4I.3. 17. K. Takeda, T. Sato, A. Shinya, K. Nozaki, H. Taniyama, M. Notomi, K. Hasebe, T. Kakitsuka, and S. Matsuo, “Integrated On-Chip Optical Links Using Photonic-Crystal Lasers and Photodetectors with Current Blocking Trenches,” in Optical Fiber Communication Conference (OFC), 2013 OSA Technical Digest Series (Optical Society of America, 2013), paper OM2J.5. 18. K. Takeda, T. Sato, A. Shinya, K. Nozaki, H. Taniyama, K. Hasebe, T. Kakitsuka, M. Notomi, and S. Matsuo, “5.5-fJ/bit direct modulation of lambda-scale embedded active region photonic-crystal lasers,” in Proceedings of Optical Interconnects Conference, (IEEE, 2013) pp. 104 – 105. 19. B. G. Lee, A. Biberman, J. Chan, and K. Bergman, “High-performance modulators and switches for silicon photonic networks-on-chip,” IEEE J. Sel. Top. Quantum Electron. 16(1), 6–22 (2010). 20. C. Monat, C. Seassal, X. Letartre, P. Viktorovitch, P. Regreny, M. Gendry, P. Rojo-Romeo, G. Hollinger, E. Jalaguier, S. Pocas, and B. Aspar, “InP 2D photonic crystal microlasers on silicon wafer: room temperature operation at 1.55 μm,” Electron. Lett. 37(12), 764–766 (2001). 21. K. Tanabe, M. Nomura, D. Guimard, S. Iwamoto, and Y. Arakawa, “Room temperature continuous wave operation of InAs/GaAs quantum dot photonic crystal nanocavity laser on silicon substrate,” Opt. Express 17(9), 7036–7042 (2009). 22. Y. Halioua, A. Bazin, P. Monnier, T. J. Karle, G. Roelkens, I. Sagnes, R. Raj, and F. Raineri, “Hybrid III-V semiconductor/silicon nanolaser,” Opt. Express 19(10), 9221–9231 (2011). 23. T. Tsuchizawa, K. Yamada, T. Watanabe, S. Park, H. Nishi, R. Kou, H. Shinojima, and S. Itabashi, “Monolithic integration of silicon-, germanium-, and silica-based optical devices for telecommunications applications,” IEEE J. Sel. Top. Quantum Electron. 17(3), 516–525 (2011). 24. D. Di Liang, D. Chapman, Y. Li, D. Oakley, T. Napoleone, P. Juodawlkis, C. Brubaker, C. Mann, H. Bar, O. Raday, and J. Bowers, “Uniformity study of wafer-scale InP-to-silicon hybrid integration,” Appl. Phys., A Mater. Sci. Process. 103, 213–218 (2011). 25. G. Björk, A. Karlsson, and Y. Yamamoto, “On the linewidth of microcavity lasers,” Appl. Phys. Lett. 60(3), 304–306 (1992).
1. Introduction The power consumption of information communication technology (ICT) equipment is a serious problem because of the huge demand for services requiring large capacity communication such as cloud computing and video streaming [1]. These services require large datacenters that are increasing the volume of traffic. Optical communication technologies have been widely used in telecom networks, and active optical cables (AOCs) have brought the optical interconnects in datacenters and supercomputers [2, 3]. These optical communication technologies have contributed greatly to reducing the power consumption of communications and datacenters. To reduce the power dissipation of ICT equipment, there have been several attempts to introduce optical interconnects at much shorter distances: inside each instrument, on circuit boards, between complementary metal oxide semiconductor (CMOS) chips, and also on CMOS chips [4]. Optical interconnects on CMOS chips are especially attractive because copper interconnects constitute a bottleneck as regards power consumption and speed in current advanced CMOS chips after their continuous improvement under the scaling law [5]. If energy-efficient optical interconnects were realized on CMOS chips, the power consumption of CMOS chips would be significantly reduced [6]. To realize such interconnects, we need a directly modulated laser with ultralow power consumption because the direct modulation of a light source is the most power efficient scheme for transmitting data in optical communication.
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 703
Fig. 1. Integration schemes of LEAP lasers and connecting waveguides: vertical coupling. The LEAP laser emits light in the direction normal to the wafer, which can be coupled by a grating coupler, and converted to the lateral direction. Part of LEAP laser in the schematic is hidden to show the layer stack of the LEAP laser and the grating coupler.
The size of the active region is an important parameter for achieving light sources with such a low power consumption. Two major types of directly modulated lasers have been used for optical interconnects: distributed feedback (DFB) lasers and vertical-cavity surface emitting lasers (VCSELs). Typical DFB lasers occupy a few 100 square microns and their energy cost for generating a single bit of data is of the order of 1 pJ [7–9]. On the other hand, VCSELs have active sizes that are a few μm in diameter and the energy cost can be reduced to 81 fJ/bit [10, 11]. However, to reduce the energy cost to 10 fJ/bit [4], we need to further reduce the size of the active region. In this context, we have studied lambda-scale embedded active-region photonic-crystal (LEAP) lasers, which have a wavelength order active region. The carrier and optical confinement in the active region are achieved respectively by buried heterostructures and a photonic crystal (PhC), which is a hexagonal array of air holes [12]. Optically pumped LEAP lasers exhibit direct modulation at a bit rate of 20 Gbit/s [13]. The first continuous-wave (CW) operation of electrically driven LEAP lasers was achieved in 2012, and the threshold current was reduced to 4.8 μA, which was the lowest reported value for any semiconductor laser [14]. Energy efficient direct modulation was demonstrated with a bit rate of 12.5 Gbit/s and an energy cost of 14 fJ/bit [15]. By using the same fabrication processes, photonic-crystal (PhC) photodiodes (PDs) could be fabricated on the same wafers [16], and thus on-chip optical links consisting of LEAP lasers and PhC PDs were demonstrated [17, 18]. However, these results were achieved using a device formed on InP wafers. Lasers with similar characteristics on Si were needed if we were to realize optical interconnects on CMOS chips or between CMOS chips. This is because the use of optical filters and switches fabricated by employing Si photonics technologies are expected to greatly reduce the total power consumption of optical networks during on/off chip data transmission [6, 19]. There were several reports about PhC lasers on Si, but the CW operation was achieved only by the optical pumping [20–22]. To use them as the light sources, electrically-driven PhC lasers on Si are desired. However, it is difficult to achieve because the membrane structure increases both electrical and thermal resistances, which increase the active region temperature during the current injection [12]. In this paper, we report the CW operation of electrically-driven LEAP lasers on Si at room temperature. Oxygen-plasma assisted wafer bonding is used to fabricate the on-Si lasers. The bonding is carried out at an interface between Si and SiO2. An air-bridge structure is employed to maintain the quality factor of the cavity, and we achieved continuous-wave operation of the LEAP lasers with a threshold current of 31 μA, which is the smallest value for any type of semiconductor laser on Si.
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 704
Fig. 2. Process for fabricating LEAP lasers on Si. (a) An InGaAs etchstop layer and an InGaAsP MQW are grown on an InP substrate. (b) Etching and regrowth form a wavelengthscale BH, followed by the ion implantation of Si and diffusion of Zn to form lateral p-i-n junctions. (c) Deposition of 2-μm-thick SiO2 followed by CMP. (d) The processed InP wafer is bonded on a bulk Si substrate using oxygen-plasma assisted bonding. (e) The InP substrate and InGaAs etchstop are removed, and PhC holes are dry etched. (f) Finally metals are deposited and the SiO2 layer is partially etched with a wet etching solution.
2. Device design and fabrication There are several possible ways to integrate connecting optical waveguides and LEAP lasers on Si, and Fig. 1 is a schematic showing one potential integration scheme. Without coupling waveguides, the LEAP lasers have light emission in the direction normal to the wafer [12], so we can couple the light to the silicon photonics by using grating couplers. After coupling the light to the Si wire waveguides, they can be used as low-loss passive waveguides. When a relatively long distance is required such as chip-to-chip interconnection on a printed circuit board (PCB), spot-size converter integration is attractive because we can obtain a low coupling loss by using an inversely tapered Si waveguide and SiOx passive waveguides [23]. By integrating optical filters, switches and waveguide-type Ge photodetectors using Si photonics technologies, a photonic network incorporating the LEAP lasers can be realized [13]. We carried out wafer bonding and achieved the continuous-wave operation of LEAP lasers on Si as the first step towards realizing future waveguide-coupled optical light sources as well as photonic networks on Si. The device fabrication procedures are summarized in Fig. 2. All processes were performed using a combination of InP and Si wafers with a wafer size of 2 inches. First we used metalorganic vapor phase epitaxy (MOVPE) to form InGaAsP 6-quantum well (QW) wavelength-scale buried heterostructures on an InP substrate with an InGaAs etch stop layer (Fig. 2(a)). The region where the active layer remained was defined using electron-beam lithography, and mesa structures were formed with dry etching. Wavelength-scale embedded active regions were formed on InP wafers by MOVPE regrowth. n-InP regions were defined by Si ion implantation and annealing to activate dopants. We defined the p-InP regions by the thermal diffusion of Zn (Fig. 2(b)). The next step was a wafer bonding process. 2-μm-thick SiO2 was deposited on the InP that had undergone the abovementioned processes. The InP wafers were bonded on Si wafers with an O2 plasma assisted bonding technique [24] after surface planarization by a chemicalmechanical polishing (CMP) technique. The InP substrates were removed by polishing and selective wet etching. By using these procedures, we successfully transferred III-V thin films, which had embedded active regions and lateral p-i-n junctions, to Si wafers. Figure 3 shows transmission electron microscopic (TEM) images of the bonded wafer after the InP substrate
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 705
Fig. 3. Cross sectional transmission electron microscopy (TEM) image of the bonded wafers. The InP substrate had already been removed in these images. (a) Large area view, and (b) magnified view at the bonding interface.
Fig. 4. (a) Schematic and (b) scanning electron microscopic view of fabricated on-Si LEAP lasers. The active region is as small as 2.87 × 0.3 × 0.15 μm3. Current blocking trenches were used with a trench width of 200 nm. A 2-μm-thick SiO2 layer was used as the bonding interface as well as the sacrificial layer to form the air-bridge structure.
had been removed. To avoid optical coupling between the III-V layers and Si substrates, we used a SiO2 thickness of 2 μm, which has been widely used in silicon-on-insulator (SOI) based Si photonics. After the bonding, PhC holes were etched by further EB lithography and dry etching. Finally, we fabricated electrodes using lift-off processes. Air-bridge structures were formed by the selective wet etching of SiO2. Figure 4(a) shows a schematic of the fabricated device. Figure 4(b) shows a scanning electron microscope (SEM) image of the device. The left and right trapezoidal regions were nand p-type doped, respectively. The active region was 2.87 × 0.3 × 0.15 μm3. The InP slab was 250 nm thick, and the PhC lattice constant was 410 nm. We also employed current blocking trenches with a designed width of 200 nm. 3. Device characteristics The fabricated devices were mounted on a temperature-controlled stage with the temperature kept at 25°C. Output light was collected from the direction perpendicular to the wafer with a microscope objective lens and a multi-mode fiber. The output power and applied voltage were measured as a function of injected current (LI-V characteristic), and the result is shown in Fig. 5. We achieved the continuous wave operation of the current driven LEAP lasers on Si at room temperature with a threshold current of 31 μA (see inset), which is the smallest value yet reported for any semiconductor laser on Si. The
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 706
Fig. 5. (a) Light output and applied voltage versus injected current (L-I-V). A threshold current of 31 μA was obtained. (b) Lasing spectrum of the LEAP laser at an injected current of 100 μA. The lasing wavelength was 1501 nm.
Fig. 6. (a) Superimposed spectra of the LEAP lasers. The injected current was varied from 20 to 100 μA. (b) Peak wavelength and 3-dB bandwidth versus injected current. The bandwidth was measured with an optical spectrum analyzer at a resolution of 0.02 nm.
maximum output power was 0.27 μW at an injected current of 200 μA, which included the optical coupling loss of the measurement setup (approximately 10 dB). The maximum output power from the device was estimated to be a few μW considering the coupling loss. The differential resistance was approximately 5.4 kΩ at the threshold current. The lasing spectrum of the LEAP laser is shown in Fig. 5(b) with an injected current of 100 μA. We obtained single-mode lasing at a lasing wavelength of approximately 1501 nm. Superimposed lasing spectra are shown in Fig. 6(a) with an injected current ranging from 20 to 100 μA. Peak wavelengths and full-width at half-maximum (FWHM) linewidths were extracted from the spectra, and are summarized in Fig. 6(b). When the injected current was increased, a blue shift was observed in the wavelength until the current reached the threshold caused by the carrier plasma effect. We also observed a red shift in the wavelength, which was due to heat generation, above the lasing threshold. A clear kink in the decreasing linewidth was observed at around the lasing threshold showing the lasing behavior of #227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 707
nanocavity lasers [25]. The linewidth at the threshold was 0.07 nm. These results for the L-IV characteristic and spectral behavior are clear evidence that the LEAP lasers on Si are lasing at room temperature. The threshold current was a little larger than our previous report [14], which was due to the wavelength detuning between the lasing wavelength of 1501 nm and the photoluminescence peak of 1550 nm in this particular device. 4. Conclusion We reported the continuous-wave operation of LEAP lasers fabricated on Si wafers. Oxygenplasma assisted wafer bonding was carried out at an interface between SiO2 and Si. Current blocking trenches and air-bridge structures were used as previously reported. We achieved the lowest reported threshold current of 31 μA, and the device exhibited single mode lasing at a wavelength around 1501 nm. We think the CW operation of on-Si LEAP lasers is an important step towards realizing on-CMOS-chip optical interconnects, and future integration with waveguides will enable the LEAP lasers to be applied to off-chip optical interconnects. Acknowledgments We thank Mr. K. Ishibashi, Mr. Y. Shouji, and Mr. Y. Yokoyama for assistance with device fabrication.
#227167 - $15.00 USD (C) 2014 OSA
Received 21 Nov 2014; revised 26 Dec 2014; accepted 30 Dec 2014; published 13 Jan 2015 29 Dec 2014 | Vol. 22, No. 26 | DOI:10.1364/OE.22.0702 | OPTICS EXPRESS 708
