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A new way to make high speed modulators using Si waveguides is demonstrated. The hybrid silicon evanescent electroabsorption modulator with offset AlGaInAs quantum wells has an extinction ratio over 10dB and modulation bandwidth of 10GHz. The modulator has a clean open eye at 10Gb/s with sub-volt drive.
©2008 Optical Society of America
1. Introduction
Silicon-based modulators have attracted much attention with devices reported using free carrier plasma dispersion in Mach-Zehnder interferometric (MZI) form [1, 2] or with a ring resonator structure [3,4] to increase the interaction of light with the active material. MZI type modulators are generally less efficient and require longer interaction length and larger driving voltage. While ring resonator type modulators reduce the size, they also increase the sensitivity to temperature and reduce the operational optical bandwidth. InP based electroabsorption modulators have shown small footprint and decent modulation at low driving voltage and had been widely used to make electroabsorption modulated lasers (EML) for high speed data communication. Recently it has been reported that strained silicon also exhibits linear electro-optic refractive index modulation [5] and electroabsorption modulator on silicon had been demonstrated based on the Franz-Keldysh effect in strained SiGe [6]. However the absorption coefficient in SiGe MQW is still lower than InP based MQW. The additional absorption caused by indirect bandgap of Ge also introduces higher propagation loss at zero bias. Here we present a new approach of integration of modulators into silicon using wafer bonded hybrid silicon evanescent platform. The modulator described here can be integrated with lasers [7], amplifiers, and photodetectors [8] using quantum well intermixing [9]. High performance integrated high speed transmitters should be possible on the demonstrated silicon evanescent platform.
2. Device description
The silicon evanescent electroabsorption modulator (EAM) is a hybrid structure that consists of an offset multiple quantum well (MQW) region bonded to a silicon waveguide fabricated on a silicon-on-insulator (SOI) wafer as shown in Fig. 1. The III–V epitaxial structure, shown in Table 1, is grown on an InP substrate. InGaAlAs is chosen as the MQW material because it has larger conduction band offset, ΔEc, which provides a stronger carrier confinement and produces a strong quantum confined stark effect (QCSE) with higher extinction ratio [10,11]. The MQW section contains 10 wells and 11 barriers with photoluminance (PL) peak at 1478nm. The silicon waveguide was fabricated with a height of 0.6µm and buried oxide thickness of 1.0µm. The silicon waveguide has a width of 1.5µm for passive segments, and width of 1.0µm under III–V mesa for higher optical confinement. The width of the III–V mesa for the absorber is 4µm at the top InP cladding layer and 3µm at the SCH and MQW layers to reduce the capacitance of the device [11, 12]. Figure 1b shows the transition between the passive silicon waveguide and the hybrid waveguide of the EAM. At both ends of the device, the widths of the III–V mesa and silicon waveguides are laterally tapered over a length of 70µm to adiabatically transform the optical mode between the passive and hybrid structure and to minimize reflection. The contact electrode pads are designed to be 100µm apart from center to center to use a standard GSG RF probe for high speed testing. Several devices were fabricated at once with different lengths. The fabricated hybrid devices all contain two 70µm long tapers but one absorber length of 100µm and 250µm, respectively.
Table 1. III–V epitaxial layer structure.
Fig. 1. (a). Schematic diagram of device cross section and (b) SEM picture of the tapered hybrid waveguide. The picture was taken before the spin-coating of SU8 polymer. All structure elements are formed except SU8 and probe metal.
The III–V epitaxial layers are transferred to the patterned SOI wafer through low temperature oxygen plasma assisted wafer bonding at a 300 °C annealing temperature under vacuum. In this work, we used a faster bonding process utilizing vertical outgassing channels (VOC) [13]. The array of 6×6µm VOCs with 50 µm spacing assists in quenching H2 produced at bonding interface. This allows for shorter bond times of 4 hours over the standard 12 hours as reported in Ref. [7,8]. The InP substrate is removed using a mixture of HCl/H2O. The details of silicon waveguide fabrication and wafer bonding process are described in Ref. 7. The III–V back end processing starts with blanket deposition of Pd/Ti/Pd/Au p-contacts. The p metal serves as a hard mask for the self aligned process to form the III–V mesa [8]. The undercut profile is created by wet etch SCH and MQW layers. Ni/Au/Ge/Ni/Au alloy are deposited onto the exposed n-type InP layer to form ohmic contacts. The n-InP layer is selectively removed by RIE dry etch to expose the silicon input and output waveguides. A 4µm thick SU-8 polymer layer is used to minimize the parasitic capacitance between the p-probe pad and the n-type InP layer and to provide additional mechanical support to the thin n-InP layer over the silicon waveguide air gap. 2 µm Ti/Au probe pads are deposited and connected to contact metals for final electrical contact. The waveguide output is designed to be tilted 7° related to the diced and polished facets. An anti-reflection (AR) coating is deposited on both facets of the finished device to minimize the reflection and increase coupling efficiency.
3. Device characteristics
The absorption shift due to QCSE can be observed by measuring photocurrent at different wavelengths as shown in Fig. 2. The device under test has a 100µm long absorber and the optical input power is kept at 0.5mW across all wavelengths. The absorption edge shifts about 20nm for each additional volt applied on the device. Figure 3 shows the relative extinction at wavelength of 1550nm under various reverse biases. More than 10dB extinction can be achieved with less than 4V bias for 100µm long devices. For a longer device with 250µm long absorber, it only takes 2.5V to achieve 10dB extinction. The propagation loss of the hybrid waveguide is measured to be 2.6 to 3.6dB/mm by measuring devices loss with different absorber lengths. A pair of cascade EAMs reversely biased at 5V were used to measure the on-chip loss of the fabricated 100µm EAM. The on-chip loss is around 3dB mainly due to the excess loss from both tapers.
The tested 100µm device has a series resistance around 30Ω and capacitance of 0.2pF, which will results a cut-off frequency around 10GHz. For the 250µm device, the serial resistance and capacitance are 10Ω and 0.45pF respectively. It matches with the measured small signal modulation response as shown in Fig. 4. To investigate the performance of large signal modulation, we drive our modulator with a 231-1 pseudorandom bit sequence (PRBS). The devices are biased at -2.2V and -1.8V for 100µm and 250µm devices, respectively. Peak to peak drive voltages of 0.82V and 1.63V are used to measure the performance. The modulated light is collected with lensed fiber and amplified with an EDFA. A 100GHz filter is used to reduce the ASE noise before the signal is detected by a 30GHz photodetector and sampled using an Agilent digital communication analyzer (DCA). Figure 5 shows eye diagrams measured at non-return-to-zero (NRZ) 10 Gb/s. The 10 Gb/s signal has an extinction ratio at least 5dB for 100µm device at 0.82V swing and up to 17dB for 250µm long device at 1.63V swing. The measured ER values are slightly lower than the DC extinction due to microwave voltage drop at cladding and ohmic contacts. The measurements show clear open eyes even at sub-volt driving voltage. The 250um device has a bandwidth around 7GHz, but is still capable of 10Gb/s modulation.
The tested devices are limited to 10GHz because of conservative undercut of the MQW and SCH region. By reducing the MQW width down to 2µm, the measure capacitance drops to 0.1pF and increase the modulation bandwidth over 16GHz as shown in Fig. 6. However the aggressive selective etch is capable of over-etching under the taper section which leads to higher insertion loss and low yields. To further optimize the modulation performance and the insertion loss, careful control of the wet-etching process for the under-cut is essential to prevent the damage of tapers and reduce the parasitic capacitance. The modulation speed can be future improved by employing traveling-wave electrodes on top of EAM with proper termination of the device.
Fig. 6. Modulation response of a 100µm device made with 2µm-wide MQW region. (inset) 10Gb/s NRZ eye diagram coming out of the EAM with 3.2V swing.
4. Conclusion
We have demonstrated the first hybrid silicon evanescent electroabsorption modulator with offset AlGaInAs quantum well. The fabricated device has DC extinction ratio over 10dB at 4V and 2.5V bias for 100µm and 250µm long absorber respectively. The on-chip loss for a device with one 100µm absorber and 2 tapers is around 3dB. The small signal modulation bandwidths are 9.5GHz and 7.0GHz respectively. We showed a clear eye opening at 10Gb/s with peak to peak drive voltage of 0.82V at either length. The current device is RC limited to 10GHz bandwidth and has been demonstrated that it can be improved to 16GHz with reduced capacitance. By employing a traveling wave electrode with a longer absorber, we can further improve the bandwidth and extinction ratio. The approach developed here is not limited by the relatively weak, relatively slow response of carrier injection Si modulators.
Acknowledgments
The authors thank Alex Fang and Hyundai Park for useful discussions on fabrication and Di Liang for help with bonding process. The authors acknowledge financial support from DARPA and the ARL through contract W911NF-05-1-0175
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