We demonstrate direct modulation of an InAs/GaAs quantum dot (QD) laser on Si. A Fabry–Pérot QD laser was integrated on Si by an ultraviolet-activated direct bonding method, and a cavity was formed using cleaved facets without HR/AR coatings. The bonded laser was operated under continuous-wave pumping at room temperature with a threshold current of 41 mA and a maximum output power of 30 mW (single facet). Even with such a simple device structure and fabrication process, our bonded laser is directly modulated using a 10 Gbps non-return-to-zero signal with an extinction ratio of 1.9 dB at room temperature. Furthermore, 6 Gbps modulation with an extinction ratio of 4.5 dB is achieved at temperatures up to 60 °C without any current or voltage adjustment. These results of device performances indicate an encouraging demonstration on III-V QD lasers on Si for the applications of the photonic integrated circuits.
© 2016 Optical Society of America
The rapid development of Si-based optoelectronic devices indicates the importance of photonic integrated circuits (PICs), which are advantageous for high-speed data computation and communication with low power consumption, and are thus an excellent solution for meeting the ever-increasing volumes of internet traffic [1,2]. Light sources used in silicon photonics are currently based on sophisticated III-V semiconductor compound lasers, such as quantum dot (QD) lasers . QD lasers exhibit properties such as low lasing thresholds, temperature stability, and large modulation bandwidths [4,5], are extremely suitable for PIC applications, and offer the potential of high-density integration. Although there have been several successful demonstrations of heteroepitaxial growth for the heterointegration of QD lasers and Si [6–9], the dissimilarities between III-V materials and Si still make this method challenging. Wafer bonding technology [10–13], on the other hand, is a promising way to integrate III-V materials and Si with a simple fabrication process which is not subject to the materials’ nature. We have presented QD lasers on Si using metal bonding , direct bonding , and metal-stripe bonding that directly bonded a semiconductor and metal . The bonded QD lasers exhibit comparable performances on lasing thresholds and temperature stability  as those of the as-grown lasers.
High-speed transceivers with low power consumption are desirable in optical communication systems. The direct modulation of laser diodes is thus fascinating for high-speed communication due to its low cost, simpler fabrication, and system compactness compared to the use of external electro-absorption or electro-optic modulators integrated with lasers . There have been a few publications on wafer-bonded quantum well (QW) lasers on Si [19–21], and, through using careful device design, high-speed modulation of 28 Gbps  has been achieved (using short-cavity QW lasers). Besides, among these studies, only one group further showed the direct modulation at various temperatures, which is also an important issue in the operation of communication systems. They demonstrated 25.8 Gbps modulation of a 73-um-long QW laser on Si at 50 °C, and 20 Gbps modulation was also achieved at 70 °C with different bias currents and voltages . However, there have been no reports of the direct modulation of wafer-bonded QD lasers on Si so far. In this paper, we present the first demonstration of the direct modulation of InAs/GaAs QD lasers on Si, and also their modulated operation at various temperatures, which is rarely discussed in other wafer-bonded QW lasers on Si. Ridge-type laser was integrated onto Si using ultraviolet-activated direct bonding, which was adapted here for strong bonding strength. A Fabry–Pérot cavity was formed by manual cleaving, resulting in a cavity length of 500 μm. The bonded laser operates under continuous-wave (CW) pumping at room temperature with a threshold current of 41 mA, and with a maximum output power detected from a single facet of 30 mW. The bonded laser is further directly modulated with a 10 Gbps non-return-to-zero (NRZ) signal at room temperature, and the operating temperature can be increased to 60 °C for 6 Gbps modulation without any current or voltage adjustment. With simple device structure and fabrication, our results of the device performances are an encouraging demonstration for III-V QD lasers on Si for silicon photonics and PIC applications.
Figure 1 shows a schematic flow diagram of the fabrication process, the device structure, as well as a corresponding cross-sectional scanning electron microscope (SEM) image for the QD lasers on Si substrate. A double heterostructure InAs/GaAs QD laser structure was grown on a GaAs (100) substrate by molecular beam epitaxy, and consisted of a GaAs layer embedded with eight layers of self-assembled InAs QDs with a density of 6 × 1010 cm−2 per layer. Here p-type modulation doping in the InAs/GaAs QD core region was introduced to achieve high temperature stability . The InAs/GaAs core layer was clad in p-type and n-type Al0.4Ga0.6As layers with doping densities of orders of 1018 and 1017 cm−3, respectively; a 1-μm-thick Al0.7Ga0.3As etch-stop layer was grown between the GaAs substrate and the lower cladding layer. The laser wafer and the epi-ready phosphor-doped Si wafer with a doping concentration of 2 × 1019 cm−3 were coated with photoresist to protect the surfaces from particles and contaminants generated from the dicing process. The laser and Si wafers were then diced into 0.64 and 1 cm2 dies, respectively. Next, the laser and Si dies were dipped into acetone to remove the photoresist and organic contaminants on the surfaces of the laser and Si dies. This process was then followed by cleaning with methanol and deionized water to remove any residual acetone. The native oxide on the laser and the Si dies were etched using a HF solution (20 vol %) at room temperature. Prior to the bonding process, both the laser and Si dies were treated using ultraviolet ozone at room temperature for 5 minutes [22,23] to hydrophilize the bonding surfaces and enhance the bonding strength . The two die pieces were then immediately brought into contact with each other and annealed at 300 °C in ambient air for 3 h under a uniaxial pressure of 0.1 MPa. After bonding, the GaAs substrate was then removed by chemical wet etching with H3PO4–H2O2 (3:7 vol), followed by 50% citric acid–H2O2 mixture (4:1 vol) [25–27]; a photoresist was applied to the edges of the laser die prior to the etch process in order to avoid undercutting of the QD laser structure during the layer transfer process. The compositions of the H3PO4–H2O2 and citric acid–H2O2 mixtures were chosen here to maximize the etch rate of GaAs and the etch selectivity between GaAs and AlGaAs, respectively. Finally, the etch-stop AlGaAs layer was removed using a HF solution (20 vol %) at room temperature.
The ridge laser structures were then fabricated with a top width of 5 μm using photolithography and wet etching with a 50% citric acid–H2O2 mixture (10:1 vol.), where the composition ratio was chosen to maximize the etch rate for both GaAs and AlGaAs. It should be noted that the undercut in the III-V mesa shown in the SEM image resulted from etching angle dependence on the crystallographic orientation of GaAs . Besides, the metal layer is only partially discontinuous due to the curved sidewall, which is evidenced by the inset of the SEM image in Fig. 1(b) indicating a continuous metal layer. A 100-nm-thick SiO2 passivation layer was deposited on the top of the ridge laser by magnetron sputtering, and a 2-μm-wide open window was left for current injection. The bottom electrode was deposited using electron beam evaporation, and consists of Ti (20 nm)/Pt (30 nm)/Au (500 nm). Top electrode, with a width of 100 μm, was next deposited in a similar way, but with a thicker 1800-nm-thick Au layer prepared for chip assembly to submounts. The cross-sectional SEM image shown in Fig. 1(b) indicates that the III-V laser structure is firmly bonded to the Si substrate without any gaps, and there are no clear defects at the interface. The bonded structure was finished by annealing at 350 °C in ambient air for 30 minutes to form Ohmic contacts. Fabry–Pérot cavities with a length Lc of 500 μm were then formed using manual cleaving. Incidentally, the yield of the cleaving process is around 8% to 10%, since Si owns stronger elemental bonds than GaAs, which makes a difficult cleaving process on Si-based lasers, and thus influences the quality of as-cleaved facets. Then the samples were further diced into 500 × 500 μm2 laser chips, which were assembled to submounts using die bonding and wire bonding for the following modulation experiments. It should be noted that neither an anti-reflection coating nor a high-reflection coating was applied to the cleaved edges of the device in this study.
3. Static characteristics
The laser device on a submount was directly mounted on a metal plate heat sink connected to a temperature controller, and a CW current was applied using a probing station. The light–current–voltage characteristics of the laser-diode device are shown in Fig. 2. The Ohmic behavior of the bonded GaAs/Si interface is evidenced by the fact that the current–voltage curve shows a p-n diode behavior, and the measured differential device resistance is 5.2 Ω. Compared to current-voltage characteristics of general laser diodes and our previous work of direct-fusion-bonded lasers on Si , a large operating voltage is applied here for compensating the voltage drop across the GaAs/Si interface, as well as for breaking down a thin oxide layer that forms on the bonding interface by the ultraviolet-ozone-activated surfaces boned in ambient atmosphere. The clear kink in the light–current curve indicates a threshold current of 41 mA at room temperature for the bonded laser, and the maximum output power collected from a single facet is 30 mW at an injection current of 200 mA. The slope efficiency of the bonded laser is 16% (32%) W/A, and the wall-plug efficiency is around 3% (6%) by accounting the output optical power from a single facet (double facets). Figures 3(a) and 3(b) show light-current curves measured at several temperatures, and the temperature dependence of the threshold current, respectively. No significant change in the threshold current was observed up to 60 °C, and the characteristic temperature T0 of the device is 340 K under such conditions. For operation at higher temperatures, the bonded laser shows a threshold current of 57 mA at 70 °C, and the corresponding T0 is thus reduced to 140 K. In addition, the bonded laser could not be operated at temperatures over 80 °C, most likely due to the presence of the thin oxide layer on the bonding interface in this case, which possibly results in an insufficient thermal dissipation especially for CW current injection that usually causes a self-heating effect on the laser and thus hinders higher-temperature operation. In addition to this effect, several heating steps during the device fabrication process as well as the chip assembly may induce thermal stresses on the bonding interface, which could also lead to significant impact to the thermal properties of the bonded laser. The lasing emission was coupled through a collimator connecting to a single-mode fiber with a coupling efficiency of around 10%, and then transmitted to an optical spectrum analyzer for evaluating electroluminescence spectra. Lasing emission is observed at around 1.3 μm as shown in Fig. 4(a), which is associated with the ground-state transition of the InAs QDs. Figure 4(b) shows the electroluminescence spectra at a CW-pumped current of 100 mA at several temperatures for the bonded laser, indicating a wavelength shift with the temperature increase of 0.49 nm/°C.
4. Dynamic characteristics
High-speed direct modulation of the bonded laser on Si is demonstrated by evaluating the bit rate performance of the laser. The ground-signal-ground probe contacting to the submount with a 50-Ω termination was connected to a DC source and a pulse pattern generator using a bias tee. The output signal was collected using a detector connected to the oscilloscope for data analysis. A large-signal modulation measurement was performed using an Anritsu MT1810A Signal Analyzer and a 211-1 NRZ pseudorandom bit sequence (PRBS) pattern. The bonded laser was then driven above threshold with a bias current of 100 mA, which was applied for obtaining optimized eye diagrams, under the driving electrical signal with a 2.5 V swing on the voltage. Figure 5 shows the measured eye diagrams for 2.5 Gbps to 10 Gbps modulation at room temperature. The eye patterns indicate that the bonded laser could be modulated at up to 10 Gbps with an extinction ratio of 1.9 dB. We further examined the modulation performances at varied temperatures. Figure 6 shows the 6 Gbps modulation of the bonded laser on Si at 40 °C and 60 °C under the same operation conditions without any current or voltage adjustment, where the 60 °C eye diagram shows a clear eye opening with an extinction ratio of 4.5 dB. The modulation speed in our device is limited by several factors. First, the thin passivation layer and the broad-area contact pad usually results in large parasitic capacitances that act to lower the modulation speed of the laser device. Besides, the large resistance stemming from the thin oxide layer on the bonding interface would also decrease the modulation speed. Furthermore, losses resulting from errors during fabrication, in the cavity lengths, as well as imperfections in the as-cleaved facets will also limit the modulation performance in our bonded lasers. Higher modulation speed can be expected via the optimization of the device structure, and the introduction of more sophisticated fabrication techniques for obtaining smaller active region volumes and low-loss cavities.
We have demonstrated the first CW-pumped direct-bonded InAs/GaAs QD laser on Si, and furthermore performed the first modulation experiment on such as device. Our device is directly modulated with a bit rate of 10 Gbps at room temperature, and 6 Gbps modulation is achieved up to 60 °C without any current or voltage adjustment. The experimental results of the device performance give an encouraging demonstration for III-V QD lasers on Si for silicon photonics applications.
New Energy and Industrial Technology Development Organization (NEDO); the Project for Developing Innovation Systems of Ministry of Education, Culture, Sports, Science and Technology (MEXT); and Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Specially Promoted Research (15H05700).
The authors thank Bongyong Jang, Satomi Ishida, Takeo Kageyama, Mark Holmes, Munetaka Arita, and Satoshi Kanbe for their technical support and scientific discussion.
References and links
1. A. Liu and M. Paniccia, “Advances in silicon photonic devices for silicon-based optoelectronic applications,” Physica E 35(2), 223–228 (2006). [CrossRef]
2. Y. Arakawa, T. Nakamura, Y. Urino, and T. Fujita, “Silicon photonics for next generation system integration platform,” IEEE Commun. Mag. 51(3), 72–77 (2013). [CrossRef]
3. Y. Arakawa and H. Sakaki, “Multidimensional quantum well laser and temperature dependence of its threshold current,” Appl. Phys. Lett. 40(11), 939–941 (1982). [CrossRef]
4. K. Takada, Y. Tanaka, T. Matsumoto, M. Ekawa, H. Z. Song, Y. Nakata, M. Yamaguchi, K. Nishi, T. Yamamoto, M. Sugawara, and Y. Arakawa, “Wide-temperature-range 10.3 Gbit/s operations of 1.3 μm high-density quantum-dot DFB lasers,” Electron. Lett. 47(3), 1–2 (2011). [CrossRef]
5. Y. Tanaka, M. Ishida, K. Takada, T. Yamamoto, H. Z. Song, Y. Nakata, M. Yamaguchi, K. Nishi, M. Sugawara, and Y. Arakawa, “25 Gbps direct modulation in 1.3-μm InAs/GaAs high-density quantum-dot lasers,” Proc. CLEO, 2010, paper CTuZ1. [CrossRef]
6. A. D. Lee, Q. Jiang, M. Tang, Y. Zhang, A. J. Seeds, and H. Liu, “InAs/GaAs quantum-dot lasers monolithically grown on Si, Ge, and Ge-on-Si substrates,” IEEE J. Sel. Top. Quantum Electron. 19(4), 1901107 (2013). [CrossRef]
7. M. Tang, S. Chen, J. Wu, Q. Jiang, V. G. Dorogan, M. Benamara, Y. I. Mazur, G. J. Salamo, A. Seeds, and H. Liu, “1.3-μm InAs/GaAs quantum-dot lasers monolithically grown on Si substrates using InAlAs/GaAs dislocation filter layers,” Opt. Express 22(10), 11528–11535 (2014). [CrossRef] [PubMed]
8. A. Y. Liu, C. Zhang, J. Norman, A. Snyder, D. Lubyshev, J. M. Fastenau, A. W. K. Liu, A. C. Gossard, and J. E. Bowers, “High performance continuous wave 1.3μm quantum dot lasers on silicon,” Appl. Phys. Lett. 104(4), 041104 (2014). [CrossRef] [PubMed]
9. S. Chen, W. Li, J. Wu, Q. Jiang, M. Tang, S. Shutts, S. N. Elliott, A. Sobiesierski, A. J. Seeds, I. Ross, P. M. Smowton, and H. Liu, “Electrically pumped continuous-wave III-V quantum dot lasers on silicon,” Nat. Photonics 10(5), 307–311 (2016). [CrossRef]
10. A. W. Fang, H. Park, O. Cohen, R. Jones, M. J. Paniccia, and J. E. Bowers, “Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Opt. Express 14(20), 9203–9210 (2006). [CrossRef] [PubMed]
12. D. Liang and J. E. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics 4(8), 511–517 (2010). [CrossRef]
13. S. Stanković, R. Jones, M. N. Sysak, J. M. Heck, G. Roelkens, and D. Van Thourhout, “1310-nm hybrid III–V/Si Fabry–Pérot laser based on adhesive bonding,” IEEE Photonics Technol. Lett. 23(23), 1781–1783 (2011). [CrossRef]
14. K. Tanabe, D. Guimard, D. Bordel, S. Iwamoto, and Y. Arakawa, “Electrically pumped 1.3 microm room-temperature InAs/GaAs quantum dot lasers on Si substrates by metal-mediated wafer bonding and layer transfer,” Opt. Express 18(10), 10604–10608 (2010). [CrossRef] [PubMed]
16. Y. H. Jhang, K. Tanabe, S. Iwamoto, and Y. Arakawa, “InAs/GaAs quantum dot lasers on silicon-on-insulator substrates by metal-stripe wafer bonding,” IEEE Photonics Technol. Lett. 27(8), 875–878 (2015). [CrossRef]
17. K. Tanabe, T. Rae, K. Watanabe, and Y. Arakawa, “High-temperature 1.3 μm InAs/GaAs quantum dot lasers on Si substrates fabricated by wafer bonding,” Appl. Phys. Express 6(8), 082703 (2013). [CrossRef]
18. R. S. Tucker, “Green optical communications – Part I: energy limitations intransport,” IEEE J. Sel. Top. Quantum Electron. 17(2), 245–260 (2011). [CrossRef]
19. C. Zhang, S. Srinivasan, Y. Tang, M. J. R. Heck, M. L. Davenport, and J. E. Bowers, “Low threshold and high speed short cavity distributed feedback hybrid silicon lasers,” Opt. Express 22(9), 10202–10209 (2014). [CrossRef] [PubMed]
20. S. Matsuo, T. Fujii, K. Hasebe, K. Takeda, T. Sato, and T. Kakitsuka, “Directly modulated DFB laser on SiO2/Si substrate for datacenter networks,” J. Lightwave Technol. 33(6), 1217–1222 (2015). [CrossRef]
21. A. Abbasi, J. Verbist, J. Van Kerrebrouck, F. Lelarge, G.-H. Duan, X. Yin, J. Bauwelinck, G. Roelkens, and G. Morthier, “28 Gb/s direct modulation heterogeneously integrated C-band InP/SOI DFB laser,” Opt. Express 23(20), 26479–26485 (2015). [CrossRef] [PubMed]
22. Z. Tang, P. Peng, T. Shi, G. Liao, L. Nie, and S. Liu, “Effect of nanoscale surface topography on low temperature direct wafer bonding process with UV activation,” Sens. Actuator A 151(1), 81–86 (2009). [CrossRef]
23. J. Gan, G. Y. Chong, and C. S. Tan, “Study of hydrophilic Si direct bonding with ultraviolet ozone activation for 3D integration,” ECS J. Solid State Sci. Technol. 1(6), 291–296 (2012). [CrossRef]
24. Q.-Y. Tong and U. Gosele, Semiconductor Wafer Bonding: Science and Technology, 1st ed. (John Wiley & Sons, 1998).
25. Y. Mori and N. Watanabe, “A new etching solution system, H3PO4-H2O2-H2O, for GaAs and its kinetics,” J. Electrochem. Soc. 125(9), 1510–1514 (1978). [CrossRef]
26. G. C. DeSalvo, W. F. Tseng, and J. Comas, “Etch rates and selectivities of citric acid/hydrogen peroxide on GaAs, Al0.3Ga0.7As, In0.2Ga0.8As, In0.53Ga0.47As, In0.52Al0.48As, and InP,” J. Electrochem. Soc. 139(3), 831–835 (1992). [CrossRef]
27. C. Carter-Coman, R. Bicknell-Tassius, R. G. Benz, A. S. Brown, and N. M. Jokerst, “Analysis of GaAs substrate removal etching with citric acid:H2O2 and NH4OH:H2O2 for application to compliant substrates,” J. Electrochem. Soc. 144(2), L29–L31 (1997). [CrossRef]
28. J. L. Merz and R. A. Logan, “GaAs double heterostructure lasers fabricated by wet chemical etching,” J. Appl. Phys. 47(8), 3503–3509 (1976). [CrossRef]