Hybrid silicon evanescent devices

Hybrid silicon evanescent devices Si photonics as an integration platform has recently been a focus of optoelectronics research because of the promise of low-cost manufacturing based on the ubiquitous electronics fabrication infrastructure. The key challenge for Si photonic systems is the realization of compact, electrically driven optical gain elements. We review our recent developments in hybrid Si evanescent devices. We have demonstrated electrically pumped lasers, amplifiers, and photodetectors that can provide a low-cost, scalable solution for hybrid integration on a Si platform by using a novel hybrid waveguide architecture, consisting of III-V quantum wells bonded to Si waveguides. Alexander W. Fang 1*, Hyundai Park 1, Ying-hao Kuo 1, Richard Jones 2, Oded Cohen 3, Di Liang 1, Omri Raday 3, Matthew N. Sysak 1, Mario J. Paniccia 2, and John E. Bowers 1 1 Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA 2 Intel Corporation, 2200 Mission College Blvd, SC12-326, Santa Clara, CA 95054, USA 3 Intel Corporation, S.B.I. Park Har Hotzvim, Jerusalem, 91031, Israel *E-mail: awfang@ece.ucsb.edu The indirect bandgap of Si has been a key hurdle in the achievement of optical gain elements. Raman lasers and amplifiers 1-3 have been demonstrated, and optical gain in nanopatterned Si 4 has also been observed, but an electrically pumped all-si gain element has yet to be realized. An alternative to creating an electrically pumped all-si gain mechanism is to take prefabricated lasers and couple them to Si waveguides. However, because of the tight alignment tolerances of the optical modes and the need to align each laser individually, this method has a limited scalability and it is difficult to envision die attaching more than a few lasers to each chip without prohibitive costs. We are developing wafer-scale approaches that could result in the simultaneous fabrication of thousands of lasers on a single Si wafer. Recently, we demonstrated an electrically driven laser 5 and amplifier 6 based on a hybrid waveguide structure that uses III-V quantum wells bonded to Si waveguides to achieve optical gain. In addition, under reverse bias operation, the same structure acts as a photodetector 7. The lateral homogeneous nature of the III-V quantum layer structure allows the optical mode to be defined by the Si waveguide, leading to an alignment-free bonding process. Moreover, the mode lies primarily in the Si region, leading to low coupling losses from the active hybrid waveguide to passive Si waveguide regions. This architecture allows for 28 ISSN:1369 7021 Elsevier Ltd 2007

Hybrid silicon evanescent devices REVIEW of the wafer could be designed to have wide waveguide widths to increase the saturation power of the amplifier. Fig. 1 Cross section of the hybrid Si evanescent device. (Reprinted with permission from 8. 2007 Optical Society of America.) thousands of lasers, amplifiers, and photodetectors to be fabricated in a single bonding step. Device structure and optical mode characteristics The cross section of the hybrid Si evanescent waveguide device is shown in Fig. 1. It consists of a III-V multiple quantum well epitaxial layer structure bonded to a Si-on-insulator (SOI) rib waveguide. The device fabrication process can be divided into three major parts. First, the Si waveguides and any other desired Si devices are fabricated in a complementary metal-oxide-semiconductor (CMOS) fabrication facility. Next, the III-V epitaxial layer structure is transferred to the Si waveguides through an O 2 plasma-assisted, low-temperature bonding process. Finally, post-processing of the III-V layers is done after bonding to control the flow of current through the structure to ensure efficient optical gain to the waveguide mode. The details of the transferred epitaxial structure are shown in Table 1. As stated above, the optical mode characteristics are determined by the Si rib waveguide dimensions. Fig. 2 shows the Beamprop calculated optical mode with a fixed waveguide height for various waveguide widths. It can be seen that as the Si waveguide becomes wider, the mode is pulled more into the Si region, with the same trend being seen for variation of the waveguide height. This feature can be used to tailor each device s optical gain characteristics. For example, lasers could be designed with narrower waveguides, which increase high modal gains to achieve lower thresholds, while amplifiers in an adjacent section Plasma-assisted low-temperature wafer bonding A key step in the fabrication of this device platform is the bonding of the InP-based epitaxial layer structure to Si. Wafer bonding follows the Si waveguide processing on an SOI substrate and the growth of the III-V epitaxial layer structure on an InP substrate. The wafers undergo a rigorous surface cleaning that involves a solvent clean and a rinse with Tergitol, a mild detergent. The surface is inspected for particles under a Nomarski microscope at 20x magnification and the cleaning process is repeated until no particles are present. After cleaning, the surface oxides of the Si and InP are removed with buffered HF and NH 4 OH, respectively. After oxide removal, the samples are inspected, cleaned if necessary, and then undergo an ozone cleaning treatment. Next the surfaces are treated with an O 2 plasma. The samples are then dipped in deionized (DI) water, dried with N 2, and the top surfaces are placed in physical contact. At this point, a weak spontaneous bonding occurs. To strengthen the bond, the wafers are held together under vacuum at a pressure of 2 MPa and a temperature of 300 C for 12 hours. After bonding, the InP substrate is removed in a HCl solution. Because of the thermal expansion coefficient mismatch of Si and InP (α Si = 2.6 x 10-6 K -1, α InP = 4.8 x 10-6 K -1 ), a low-temperature, O 2 plasma-assisted bonding (<300 C) approach is preferred over the conventional direct wafer bonding to InP done at 600 C with other material systems such as GaAs 9. The O 2 plasma surface treatment prior to the contact of SOI and InP enhances the bonding strength in both physical and chemical ways. By careful control of the discharge conditions (radio frequency or RF power, chamber pressure, gas flow rate, etc.), O 2 energetic ion bombardment can remove hydrocarbon and water-related species from the sample surface very efficiently. An ultrathin layer of oxide (<5 nm) grown by plasma oxidation 10 turns the hydrophobic sample surfaces into very smooth (root mean square, rms <5 Å) and extremely hydrophilic surfaces, which are less Table 1 The III-V epitaxial layer structure 5. Name Composition Doping concentration Thickness P contact layer P-type In 0.53 Ga 0.47 As 1 x 10 19 cm -3 0.1 μm Cladding P-type InP 1 x 10 18 cm -3 1.5 μm Separate confinement heterostructure P-type Al 0.131 Ga 0.34 In 0.528 As, 1.3 μm 1 x 10 17 cm -3 0.25 μm Quantum wells Al 0.089 Ga 0.461 In 0.45 As, 1.3 μm (9x) Al 0.055 Ga 0.292 In 0.653 As, 1.7 μm (8x) undoped undoped 10 nm 7 nm N layer N-type InP 1 x 10 18 cm -3 110 nm Superlattice N-type In 0.85 Ga 0.15 As 0.327 P 0.673 (2x) N-type InP (2x) 1 x 10 18 cm -3 7.5 nm 1 x 10 18 cm -3 7.5 nm N bonding layer N-type InP 1 x 10 18 cm -3 10 nm 29

REVIEW Hybrid silicon evanescent devices Fig. 2 Calculated optical mode for waveguide widths of 2.5 μm and 3 μm. sensitive to microroughness than hydrophobic surface 11. The Si-O-Si bonds of the oxide (SOI side) are also found to be more strained than conventional oxides formed in the standard RCA-1 cleaning process or other hydrophilic wet-chemical treatments, indicating a greater readiness to break and form new bonds 12. The post-o 2 plasma surface treatment DI water dip further terminates the oxide surface with polar hydroxyl groups, OH -, which form bridges between mating surfaces resulting in spontaneous bonding at room temperature. The final 300 C annealing process enhances out-diffusion of interface-trapped molecules and desorption of chemisorbed surface atoms, such as hydrogen, and activates the formation of covalent bonds to achieve higher bonding energies 11. Fig. 3 shows a Nomarski photograph of the top surface of the epitaxial layer structure transferred to a SOI substrate at 600 C and 250 C. Crosshatching can be seen for the direct wafer bonding sample, which can lead to material quality degradation and scalability issues because of the accumulation of stress over larger samples sizes. In contrast, the low-temperature sample shows a very smooth surface morphology. In addition, since the bonded interface intersects the optical mode, it must be transparent at wavelengths longer than 1.1 μm and have a smooth morphology to minimize absorption and scattering losses. Full details of the hybrid device processing can be found elsewhere 5. Hybrid Si evanescent laser The first electrically driven hybrid Si evanescent device demonstrated was the hybrid Si evanescent laser 5. These devices had a waveguide height, width, and rib-etch depth of 0.76 μm, 2.5 μm, and 0.76 μm, respectively. The cavity for these lasers was made by dicing the ends of a hybrid waveguide and polishing them to a mirror finish, resulting in a cavity length of 850 μm. The cross-sectional scanning electronic microscope (SEM) image of the fabricated device is shown in Fig. 4a. Fig. 4b shows seven Si evanescent lasers operating simultaneously. The chip shown has 36 lasers fabricated with a single bond step. The number of lasers shown operating at a single time is limited by the current experimental set up (the number of probes), and it is feasible to operate all the lasers simultaneously if the devices are wire bonded instead of relying on probing. Fig. 3 Nomarski microscope images, showing the surface roughness of the transferred III-V surface at bonding temperatures of 600 C and 250 C. Fig. 4c shows the power out of the laser versus drive current (LI curve). The laser light is collected through one side of the laser through a lensed fiber while being driven at various currents. It can be seen that the laser operates at temperatures up to 40 C. Taking into account the measured 6 db fiber coupling loss and the fact that light is only collected out of one of the two laser facets, the maximum output power and differential efficiency are estimated to be ~14 mw and 12.7%, respectively. Fig. 4d shows the laser spectrum for two drive currents. At 70 ma, the laser is right above lasing threshold. The standard Fabry-Perot fringes can be seen with a main lasing peak in the 1577 nm regime. At 100 ma, other wavelengths also begin to lase, as expected in a Fabry- Perot laser. A 15 cm -1 modal loss is measured in the long wavelength regime using the Hakki-Paoli technique. 30

Hybrid silicon evanescent devices REVIEW (c) (d) Fig. 4 Si evanescent laser device cross-sectional SEM image, seven Si evanescent lasers operating simultaneously, (c) laser power out versus the drive current (LI curve), (d) laser spectrum for drive currents of 70 ma and 100 ma.(reprinted with permission from 5. 2006 Optical Society of America.) Hybrid Si evanescent amplifier We have reported 6 hybrid Si evanescent amplifiers that consist of 1.36 mm long hybrid waveguides with a waveguide height, width, and rib-etch depth of 0.76 μm, 2 μm, and 0.76 μm, respectively. The facets were diced and polished at a 7 angle and coated with an antireflection single quarter-wavelength layer of Ta 2 O 5 to minimize cavity effects caused by the facets. The quantum well confinement factor was calculated to be 3.4%. The amplifiers were tested by launching a laser signal into one side of the device through a lensed fiber and collecting the amplified light with a subsequent lensed fiber on the opposite side of the device. Fig. 5a shows the transverse electric (TE) fiber-to-fiber gain as a function of current. Taking into consideration the measured 5 db coupling loss for these waveguide dimensions, the chip gain is given on the secondary y-axis. The maximum chip gain for this length is ~13 db. It can be seen that at drive currents greater than 100 ma, the gain saturates, which can be attributed to thermal effects. The inset shows the net modal gain and material gain as a function of current density. Fig. 5b shows the TE fiber-to-fiber gain spectrum as a function of various drive currents. It can be seen that the peak gain occurs in the 1575 nm range with a full-width half-maximum of 62 nm at a 200 ma drive current. The 3 db output saturation power from the chip is measured to be 11 dbm, as shown in Fig. 5c. The 3 db output saturation power can be written as: P0, SAT G0log 2 wd hυ = (1) G 2 Γ ( dg / dn) τ 0 where G 0 is the unsaturated chip gain, w is the optical mode width at the quantum well region, d is the total thickness of the active material, hυ is the photon energy, dg/dn is the differential gain, and τ is the carrier lifetime. Fig. 5d shows the calculated 3 db output saturation power with different confinement factors (Γ) and optical mode widths (w). The evanescent coupling scheme of the device structure typically provides 2% to 3% of the quantum well (QW) confinement factor, resulting in higher output saturation powers than amplifiers with centered quantum wells whose typical confinement factor is around 5% to 15%. Device performance for digital data transmission is typically characterized with the use of eye diagrams. Eye diagrams consist of the superposition of transitions between one and zero bits forming an eye such that the transient behavior between the bits can be analyzed. Eye closure as a result of carrier dynamics in semiconductor optical amplifiers (SOAs) is a major issue in the design of SOAs. Injecting a 31

REVIEW Hybrid silicon evanescent devices short, high-power optical pulse into a SOA will cause gain saturation instantaneously 13. The slow gain-recovery process associated with carrier injection requires several hundred picoseconds to restore the unsaturated gain. In the case of pseudo-random bit sequences (PRBS), if the data rate is close to the gain-recovery time, the amplified output can be strongly degraded by pattern effects. Fig. 6a shows measured 10 Gbps non return to zero (NRZ) eye diagrams with three different input powers. The measured data agrees qualitatively with simulated eye diagrams, which are calculated using the rate equation model for multiple quantum wells 14. A carrier lifetime of 1.1 ns results in the best agreement with the measured eye diagrams. The degradation of the Q factor of the signal can be observed with input power above -4 dbm. Bit error rate (BER) measurements are used to characterize digital performance when a device is used in a digital communications system. The BER measurements at three different data rates, 2.5 Gbps NRZ, 10 Gbps NRZ, and 40 Gbps return to zero (RZ), have been performed to investigate the power penalties imposed by the amplifier. For 10 Gbps and 40 Gbps, PRBS of 2 31-1 was used to carry out the BER test. A shorter sequence of 2 10-1 was chosen for 2.5 Gbps because of low-frequency cut off in the measurement system. The average input power is set to -16 dbm to keep the device unsaturated. A variable optical attenuator (VOA) is used between the output of the amplifier and the receiver (Rx) to adjust the received power. The power penalty of the amplifier is extracted by comparing the BER performances of the Tx-amplifier-Rx link with the back-to-back transmitter (Tx). As shown in Fig. 6b, a power penalty of 0.5 db for all three data rates is achieved. This penalty comes largely from the amplified spontaneous emission (ASE) of the amplifier, which is not related to the data rate or the pulse duration. To compare the distortion arising from the pattern effect, another BER curve with a higher input power of 2 dbm is also plotted in Fig. 6b. An additional power penalty of 0.5 db can be seen when pattern effects start to reduce the Q factor of the signal. Hybrid Si evanescent photodetector Si evanescent photodetectors have been demonstrated using the same epitaxial structure operated in reverse bias 7. The devices consist of a (c) (d) Fig. 5 Si evanescent amplifier gain versus current. Inset: net modal gain extracted from the chip gain versus current density at 1575 nm. Gain versus wavelength at different current levels. (c) Gain versus output power at 1575 nm. (d) 3 db saturation output power versus confinement factor and different optical mode widths. (Reprinted with permission from 6. 2007 IEEE.) 32

Hybrid silicon evanescent devices REVIEW Fig. 6 Digital performance of a Si evanescent amplifier. Simulated and measured eye diagrams. From left to right, input powers are -7 dbm, 2 dbm, and 5 dbm, respectively. The simulation agrees well with the measurement of a fitted carrier life time of 1.1ns, which is typical for quantum wells. BER and eye diagrams. From left to right: 2.5 G back-to-back (B2B), 2.5 G amplified (Amp), 10 G B2B, 10 G Amp (low input, -16 dbm), 10 G Amp (high input, 2 dbm), 40 G RZ B2B, 40 G RZ Amp. ~100 μm long passive Si waveguide coupled to a 400 μm long hybrid can be seen from Fig. 7d that the responsivity is relatively flat from photodetector region (Fig. 7a). The waveguide height, width, and 1500 nm to 1600 nm under 3 V reverse bias. rib-etch depth are 0.69 μm, 0.19 μm, and 0.5 μm, respectively. The passive to active junction is shown in Fig. 7b. The photodetector responsivity was measured by launching light From measurements of the output power from the Si output waveguide, the TE material absorption is estimated to be ~1594 cm-1. The measured TM responsivity was 0.23 A/W, which into the passive waveguide through a lensed fiber. The TE responsivity is, as expected, substantially lower than the TE responsivity because versus reverse bias is shown in Fig. 7c. With a 5.5 db measured of the compressively strained quantum wells. The 1 db saturation coupling loss from the fiber, the device responsivity is ~1.13 A/W. The input powers are 1.8 mw and 8.8 mw for 0 V and 1 V reverse bias, quantum efficiency is ~90%, as shown on the right axis of Fig. 7c. It respectively. No output current saturation is observed beyond a reverse MT1007_p28_35.indd 33 33 06/06/2007 15:05:46

REVIEW Hybrid silicon evanescent devices (c) (d) Fig. 7 Performance of a Si evanescent photodetector. SEM image of a fabricated device. Close-up view of a junction between the input Si waveguide and the hybrid photodetector. (c) TE responsivity with different biases at 1550 nm. (d) Spectral response for TE polarization. (Reprinted with permission from 7. 2007 Optical Society of America.) bias of 4 V for the available 14 mw of fiber coupled power. The dark current is typically 50 na to 200 na with a reverse bias range of 1 V to 4 V. The measured bandwidth of the photodetector was 470 MHz at a reverse bias of 4 V with a 50 Ω termination. This agrees with an RC-limited bandwidth of 480 MHz calculated from the measured series resistance R and capacitance C of 11 Ω and 5.5 pf, respectively. The capacitance of the device can be minimized by reducing the width and length of the III-V mesa and changing the SiN x insulation layer to a benzocyclobutene layer several microns thick. Higher bandwidth can be achieved by minimizing the capacitance of the device and incorporating traveling wave electrode designs. Integrated hybrid Si evanescent racetrack laser and photodetector An integrated Si evanescent racetrack laser and photodetector based on this platform has also been developed 8. Unlike the previous demonstration of Si evanescent lasers mentioned above, this laser does not rely on facet dicing or polishing and can be tested on-chip with simple probing of the laser and photodetectors. The waveguide height, width, and rib-etch depth are 0.69 μm, 1.5 μm, and 0.5 μm, respectively. The laser layout and an SEM image are shown in Fig. 8. It consists of a racetrack ring resonator with a straight waveguide length of 700 μm and ring radii of 200 μm and 100 μm. A directional coupler is formed on the bottom arm by placing a bus waveguide 0.5 μm away from the racetrack. Multiple coupling lengths of 100 μm, 200 μm, 300 μm, 400 μm, 500 μm, and 600 μm were made. The laser power is collected in the two 440 μm long photodetectors. These photodetectors have the same waveguide architecture as the hybrid laser, the only difference being that they are reverse biased to collect photogenerated carriers. The laser output power versus drive current (LI) curves are shown as a function of temperature in Fig. 8c for a laser with a ring radii of 200 μm and a coupler interaction length of 400 μm. The laser has a total output power of 29 mw with a maximum lasing temperature of 60 C. The differential efficiency and laser threshold at 15 C is 17% and 175 ma, respectively. The laser spectrum is shown in Fig. 8d, with its lasing peak in the range of 1592.5 nm. Concluding remarks The hybrid Si evanescent waveguide architecture has been used to demonstrate amplifiers, photodetectors, and lasers. The amplifiers operate with maximum on-chip gains of ~13 db and an output 3 db gain compression of 11 dbm saturation power. The photodetectors operate from 1500 nm to 1600 nm with quantum efficiencies in the 34

Hybrid silicon evanescent devices REVIEW (c) (d) Fig. 8 The layout of the racetrack resonator and photodetectors. Top view SEM image of two racetrack resonator lasers. The racetrack resonator lasers on the top and bottom have radii of 200 μm and 100 μm, respectively. (c) The LI curve for a laser with bend radius, R, of 200 μm, and directional coupler coupling length, L interaction, of 400 μm for various temperatures. (d) The spectrum for a laser with R = 100 and L interaction = 400. (Reprinted with permission from 8. 2007 Optical Society of America.) 90% range. The lasers operate up to 60 C with 29 mw output power. In addition, the demonstration of an integrated laser and photodetector give a glimpse of the possible photonic integrated circuits that could be conceived with this platform when used in conjunction with Si multiplexers/demultiplexers, add-drops, and high-speed modulators without suffering the high coupling losses between active and passive sections. Looking into the future, we believe that this device platform is the answer for achieving very large scale integration (VLSI) photonic circuits. Acknowledgments This work was supported by the Defense Advanced Research Projects Agency (DARPA) through contracts W911NF-05-1-0175 and W911NF-04-9-0001, and by Intel. The authors thank Jag Shah and Mike Haney for useful discussions and K. Callegari and G. Zeng for sample preparation. REFERENCES 1. Rong, H., et al., Nature (2005) 433, 725 2. Boyraz, O., and Jalali, B., Opt. Express (2004) 12, 5269 3. Espinola, R., et al., Opt. Express (2004) 12, 3713 4. Cloutier, S. G., et al., Nat. Mater. (2005) 4, 887 5. Fang, A. W., et al., Opt. Express (2006) 14, 9203 6. Park, H., et al., IEEE Photon. Technol. Lett. (2007) 19, 230 7. Park, H., et al., Opt. Express (2007) 15, 6044 8. Fang, A. W., et al., Opt. Express (2007) 15, 2315 9. Black, A., et al., IEEE JSTQE (1997) 3, 943 10. Liu, Y. C., et al., J. Appl. Phys. (1999) 85, 1911 11. Pasquariello, D., and Hjort, K., IEEE JSTQE (2002) 8, 118 12. Tong, Q.-Y., and Gosele, U., Semiconductor Wafer Bonding: Science and Technology, Wiley, New York, USA (1999) 13. Girardin, F., et al., IEEE Photon. Technol. Lett. (1998) 10, 784 14. Ishikawa, M., et al., IEEE J. Quantum Electron. (1992) 28, 2230 35