Lateral Current-injection Membrane Lasers Fabricated on a Silicon Substrate

1. Introduction

The power consumption in information and communication technology devices has reached a critical level with the explosive increase in network traffic. The power dissipation in large-scale datacenters is estimated to exceed 1% of all power consumption. It is therefore essential to cut the power consumption of both networks and devices. Data transmission within datacenters consumes 70% of all network traffic handled by datacenters [1]. Optical interconnection, in which the electrical data transmission is replaced with optical data transmission, has been actively developed to reduce datacenter power consumption. Vertical cavity surface emitting lasers (VCSELs) have already been commercialized as the transmitters for intra-rack and inter-board data transmission. Furthermore, optical links for intra-chip data transmission are also attracting attention because the performance of chip-to-chip and intra-chip data transmission for large-scale integrated circuits is facing the limit.

To introduce optical links into short-reach data transmission, it is critical to reduce the power consumption and large-scale integration of optical devices, including transmitters and receivers. Wavelength division multiplexing (WDM) is a promising technology to realize large-capacity and low-cost optical links. However, the VCSELs currently used in optical interconnections are inappropriate for single-mode transmission and the WDM network. It is thus essential to achieve high-density integration of single-mode lasers such as distributed feedback (DFB) lasers in order to introduce the WDM network into short-reach optical interconnections.

Efforts to achieve high-density integration include the development of silicon photonics to realize low-cost and high-density optical device integration on large-diameter silicon wafers [2]. The use of silicon photonics is expanding from passive components such as optical waveguides and filters to active components such as modulators and germanium-based photodiodes. In addition, the fusion of optical and electronic circuits is promising. Transceivers consisting of silicon optical devices and CMOS (complementary metal-oxide-semiconductor) transistors integrated on silicon wafers have already been commercialized.

Meanwhile, the integration of lasers on silicon remains a challenge. Silicon is an indirect transition semiconductor; it is not appropriate for light sources because of its extremely low emission efficiency. Some approaches have been proposed to achieve emission of silicon-based materials. These approaches include adding impurities, enhancing the emission rate by using fine structures, and using germanium-based lasers.

However, commercialized lasers have not been achieved yet. The standard approach is hybrid integration of compound semiconductor-based lasers and silicon-based optical devices. The commercialized transceivers still employ semiconductor lasers as extra light sources. There are two ways of achieving hybrid integration: laser bonding on silicon optical circuits, and laser fabrication on compound semiconductor active regions bonded on silicon waveguides. The former approach requires fine alignment between the lasers and silicon waveguides with submicron order accuracy. This requirement will raise the fabrication cost as the integration density increases.

In the latter approach, laser waveguides are determined by the silicon waveguide; therefore, the alignment accuracy of bonding processes is eased. However, it is difficult to achieve compact lasers because the optical confinement in the active layer is low; most optical power is confined in the silicon waveguides. Therefore, it is essential to make progress in both the fabrication process and the development of the laser structure in order to realize high-density integration of lasers with low-power consumption on silicon substrates.

2. Integration of compound semiconductors on a silicon substrate

NTT has developed novel techniques to integrate semiconductor lasers on silicon. The concept of the integration is shown in Fig. 1. First, silicon photonic circuits are fabricated on a large-diameter silicon dioxide and silicon (SiO₂/Si) substrate. Next, membrane compound semiconductor layers are bonded on the silicon photonic circuits. We call this bonded compound semiconductor layer a template. The lasers are fabricated on this template. The lasers can be fabricated by using alignment marks made on a silicon substrate. It is easy to achieve fine alignment between the silicon waveguides and the lasers. This technique will enable high-density integration of all optical devices on a large-diameter silicon wafer. With further development of the integration of photonic and electronic circuits, it will be possible to achieve compact transceivers on a large-scale silicon wafer.

Fig. 1. Concept of laser integration on a silicon wafer and photonics-electronics integration.

The process used to fabricate the membrane lasers is shown in Fig. 2; it is the same as that described in our previous reports [3–4]. An active layer is grown on an indium phosphide (InP) substrate to be bonded with an SiO₂/Si substrate. The InP-substrate is removed to form a membrane layer. The buried heterostructure (BH) is formed by means of waveguide etching and crystal regrowth on the template. The lateral current injection structure is formed by selective impurity doping. Electrodes and Bragg gratings are formed on the surface of the laser. This laser features high optical confinement in the active region thanks to the large difference in the refractive index between SiO₂ and the thin compound semiconductor layers. In addition, the BH provides strong carrier confinement in the active region to enhance the carrier-photon interaction. This feature contributes to reducing the footprint and power consumption of lasers [5]. We previously reported the energy cost of 171 fJ/bit, which was the smallest value of all DFB lasers [6]. These approaches enable the high-density integration of silicon optical circuits and high-performance lasers on a silicon wafer.

Fig. 2. Membrane laser fabrication process.

3. Membrane lasers on a silicon substrate

We fabricated the lasers using the fabrication process described in the previous section [7]. The most challenging step is growing the crystal on the bonded wafer consisting of silicon, SiO₂, and compound semiconductors. The environmental temperature varies between room temperature and 600°C during the fabrication process. Meanwhile, the thermal expansion coefficients of the silicon, SiO₂, and InP are different. This difference causes thermal stress in the epitaxial layer during the fabrication. The stress could cause serious defects in the active region, which would degrade the lasing characteristics. We were able to suppress the effect of thermal stress by using a thin InP-based template. The critical thickness of the InP layer was theoretically estimated to be 430 nm to achieve crystal growth under the temperature variation during the fabrication process. This thickness is sufficient to realize optimized membrane lasers. We used a 250-nm-thick InP-based template to fabricate the lasers.

The template is fabricated using a 2-inch-diameter thermally oxidized silicon wafer and an InP wafer. The active layer, consisting of six InGaAsP (indium gallium arsenide phosphide)-based quantum wells, is grown on an InP substrate by metal-organic vapor phase epitaxy (MOVPE). Covalent bonding assisted by O₂ plasma is used to bond the epitaxial layer and the SiO₂/Si wafer. After the bonding process, the BH is fabricated by chemical etching and MOVPE regrowth. The p-i-n diode structure is formed by Si-ion implantation for n-doped regions and Zn thermal diffusion for p-doped regions. The surface grating is patterned on the 130-nm-thick SiO₂ layer above the active region waveguide. A cross section of the fabricated BH is shown in Fig. 3(a). The active region was successfully buried with the InP layer. No cracks or dislocations are observed. The photoluminescence spectrum of the fabricated BH on the SiO₂/Si layer is shown in Fig. 3(b). The emission spectrum of BH active layers grown on InP is superimposed. The spectra are almost the same; no serious degradation is observed after the laser fabrication. These results verify that our approach is applicable to laser fabrication.

Fig. 3. (a) Cross section and (b) photoluminescence spectra of BH fabricated on SiO₂/Si.

We fabricated the DFB laser by using this active layer. The laser cavity is formed by the surface grating and etching mirrors at the facets. The coupling coefficient of the grating was designed to be 150 cm⁻¹. Single-mode lasing with a side mode suppression ratio over 40 dB is observed. The current-output power characteristics of the fabricated laser with a cavity length of 120 μm are plotted in Fig. 4(a). The threshold current was 1.8 mA. Lasing without kinks is observed at temperatures up to 100°C. An eye diagram of 40-Gbit/s direct modulation is shown in Fig. 4(b). The bias current and modulation voltage were 15 mA and 1.31 V, which corresponds to an energy cost of 848 fJ/bit. The modulation efficiency was 6.0 GHz/mA^0.5. This is the first 40-Gbit/s direct modulation of membrane lasers.

Fig. 4. (a) Current-output characteristics and (b) modulation waveform of membrane laser integrated on silicon.

The integration of the lasers and silicon/silica-based waveguide is a significant step forward toward realizing photonic integrated circuits on silicon. A distributed reflector (DR) laser integrated with a spot-size converter (SSC) and a SiO_x waveguide [8] is shown in Fig. 5(a). The DR laser consists of a front DFB section and a rear distributed Bragg reflector (DBR) section. The rear DBR mirror suppresses the output from the rear section, which can enhance the effectiveness. A compact cavity can be achieved by employing a DBR with high reflectivity. The BH active region has almost the same structure as that of the previous DFB laser. The surface grating is patterned on the InP surface above the DFB and DBR section. The coupling coefficient is designed to be 1500 cm⁻¹. The SSC section consists of an InP tapered waveguide section covered with the output SiO_x waveguide and is formed in front of the DR laser. The laser beam, strongly confined in the BH active region, spreads in the tapered region to convert into the propagation mode of the SiO_x waveguide. The DFB length and the SSC taper length are 50 μm and 300 μm, respectively. The measured coupling between the laser and optical fiber is 2.7 dB, which is almost 6 dB smaller than the coupling between the laser without the SSC and a lensed fiber. The modulation efficiency of the fabricated lasers integrated on silicon is plotted in Fig. 5(b). The DFB lasers with cavity lengths of 73 μm and 120 μm, and the DR laser with a cavity length of 50 μm are compared. The modulation efficiency of the DR laser was enhanced to 9.43 GHz/mA^0.5 thanks to the short cavity. The energy cost was also improved to 132 fJ/bit under the modulation speed of 25.8 Gbit/s.

Fig. 5. (a) Structure of SSC-integrated membrane laser and (b) modulation efficiency of membrane lasers.

4. Summary

We reported the development of semiconductor lasers fabricated on a silicon substrate. The crystal growth on the bonded wafer makes it possible to achieve membrane lasers operating with low power consumption comparable to VCSELs. We also successfully integrated the laser and the silica-based waveguide with high coupling efficiency. Our next challenges are to achieve high-density integration of the lasers and a larger wafer size. The integration of light sources and silicon optical circuits would greatly expand the application area of silicon photonics. These technologies would pave the way to achieving large-scale optical circuits for optical interconnections. The ultimate target in the future is the integration of electronics and optical circuits.

References

	Takaaki Kakitsuka Senior Research Engineer, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center. He received a B.S. and M.S. in physics in 1994 and 1996 and a Dr. Eng. in 2012 from Kyushu University, Fukuoka. In 1996, he joined NTT Opto-electronics Laboratories, where he engaged in research on semiconductor lasers and optical functional devices. From 2009 to 2011, he was with the Research and Development Planning Department. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE), the Institute of Electrical and Electronics Engineers (IEEE) Photonics Society, the Japan Society of Applied Physics (JSAP), and the Physical Society of Japan.
	Takuro Fujii Researcher, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center. He received a B.E. and M.E. in engineering from Keio University, Kanagawa, in 2010 and 2012. He joined NTT Photonics Laboratories in 2012. He has been researching MOVPE growth of III-V semiconductors and the development of III-V semiconductor lasers on Si for photonic integrated circuits. He is a member of JSAP.
	Koji Takeda Research Engineer, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center. He received his B.S., M.S., and Ph.D. in electronics engineering from the University of Tokyo in 2005, 2007, and 2010. He received a research fellowship for young scientists from the Japan Society for the Promotion of Science for the years 2008 to 2010. He joined NTT Photonics Laboratories in 2010. His current research interests include ultralow-power optical interconnects, InP photonic integrated circuits, and photonic crystal lasers. He received the Best Student Paper Award from the IEEE Photonics Society in 2009, and the Outstanding Student Presentation Award from JSAP in 2010. He is a member of the IEEE Photonics Society, JSAP, and IEICE.
	Hidetaka Nishi Researcher, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center. He received a B.S. and M.S. in mechanical science and engineering from Tokyo Institute of Technology in 2005 and 2007. In 2007, he joined NTT Microsystem Integration Laboratories. Since then, he has been conducting research on integrated silicon photonic devices. He is a member of the Optical Society of America (OSA) and JSAP.
	Tomonari Sato Manager, IT Innovation Department, NTT EAST Corporation. He received his B.E., M.E., and Ph.D. in engineering from the University of Tsukuba, Ibaraki, in 2001, 2003, and 2009. In 2003, he joined NTT Photonics Laboratories. He has been engaged in research on MOVPE growth of III-V semiconductors and the development of semiconductor lasers for sensor applications and photonic crystal lasers. He moved to NTT EAST in July 2014. He is a member of JSAP.
	Koichi Hasebe Research Engineer, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center. He received an M.E. and Ph.D. in electronics and applied physics from Tokyo Institute of Technology in 2005 and 2008. He received a research fellowship for young scientists from the Japan Society for the Promotion of Science for the years 2006 to 2009. In 2008, he was a postdoctoral fellow and visiting researcher at Tokyo Institute of Technology and the University of California, Berkeley, USA. He has been with NTT Photonics Laboratories since 2009. His current research interests include next-generation access systems, InP photonic functional devices, and nano-microcavity semiconductor lasers. He is a member of IEICE and OSA.
	Tai Tsuchizawa Senior Research Engineer, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center. He received a B.S. and M.S. in physics from Sophia University, Tokyo, in 1984 and 1986 and a Ph.D. from the University of Tokyo in 1990. Since joining NTT in 1999, he has been studying electron cyclotron resonance plasma technology and its application to an etching process for microfabrication. His current research interests include fabrication technologies for silicon-based optoelectronics devices. He is a member of JSAP.
	Tsuyoshi Yamamoto Senior Research Engineer, Supervisor, Group Leader, Materials and Devices Laboratory, NTT Device Technology Laboratories. He received a B.E. in electrical engineering from Kansai University, Osaka, in 1991. He joined NTT Communication Switching Laboratories in 1991, where he engaged in R&D of several optical interconnection systems using free-space optics. He has recently been involved in researching and developing several photonic-integrated devices based on silicon photonics and III-V semiconductor technologies. During 1998–1999, he was a visiting research engineer at the Department of Electrical and Computer Engineering, McGill University, Quebec, Canada. During 2007–2011, he was the director of the Optical Node System Department of the Optical Communication Systems Group at NTT Electronics Corp. He received Microoptics Conference Best Paper Awards in 2003 and 2009. He is a member of IEEE.
	Koji Yamada Senior Research Engineer, Supervisor, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center*. He received his B.E., M.E. and Ph.D. in nuclear engineering from Kyushu University in 1986, 1988, and 2003. He joined NTT in 1988, where he engaged in studies on accelerator physics and engineering for synchrotron light sources, as well as silicon-based photonic platforms. He was a member of the Photonics Electronics Technology Research Association (PETRA) from 2010 to 2015 and served as a leader of the photonic wiring and waveguide research team for the Photonics-Electronics Convergent System Technology (PECST) project. He is a member of IEEE, IEICE, JSAP, the Atomic Energy Society of Japan, and the Particle Accelerator Society of Japan. *He joined the National Institute of Advanced Industrial Science and Technology (AIST) in 2015, where he continues his research on silicon-based photonic platforms.
	Shinji Matsuo Senior Distinguished Researcher, Materials and Devices Laboratory, NTT Device Technology Laboratories; and NTT Nanophotonics Center. He received a B.E. and M.E. in electrical engineering from Hiroshima University in 1986 and 1988 and a Ph.D. in electronics and applied physics from Tokyo Institute of Technology in 2008. In 1988, he joined NTT Opto-electronics Laboratories, where he researched photonic functional devices using multiple quantum well pin modulators and VCSELs. In 1997, he researched optical networks using WDM technologies at NTT Network Innovation Laboratories. Since 2000, he has been researching high-speed tunable optical filters and lasers for photonic packet switching at NTT Photonics Laboratories. He is a member of the IEEE Photonics Society, JSAP, and IEICE.