A high speed and low loss silicon optical modulator based on carrier depletion has been made using an original structure consisting of a p-doped slit embedded in the intrinsic region of a lateral pin diode. This design allows a good overlap between the optical mode and carrier density variations. Insertion loss of 5 dB has been measured with a contrast ratio of 14 dB for a 3 dB bandwidth of 10 GHz.
©2008 Optical Society of America
Silicon microphotonics is widely considered to enhance the performances of integrated circuits, for either optical telecommunications  or optical interconnects [2–3]. One of the main components to make a high speed optical link is the optical modulator. Although several ways are investigated to achieve intensity modulation in silicon compatible devices (electro-optical effects in strained silicon  or SiGe superlattices , quantum confined Stark effect in silicon-germanium/germanium quantum wells [6–7], Franz Keldysh effect in GeSi diode, ) most studies are based on free carrier concentration variations [9–16]. These variations are responsible for local refractive index variations and then phase modulation of a guided wave travelling through the active region. An interferometric structure is then used to convert phase modulation into intensity modulation.
To obtain carrier concentration variations, injection in PIN diodes has been widely used. The operating speed of these devices is usually limited by the carrier lifetime in the junction. However data transmission up to 12.5 Gbit/s has been measured, thanks to both a specific complex driving scheme to overcome recombination mechanisms and the use of a ring resonator as the interferometric structure . Indeed, the non linear transfer function of the resonator allows the optical response time to be much shorter than the electrical response time, however this device suffers from the resonator drawbacks, such as a limited spectral bandwidth and temperature sensitivity.
Carrier concentration variations can be reached by carrier accumulation near the gate dielectric of a MOS capacitor. Data transmission at 10 Gbit/s has been demonstrated with on-chip insertion loss of 10 dB . Another way to achieve high frequency operation is to use carrier depletion in a reverse biased diode [12–16]. 15 dB extinction ratio and 7 dB insertion loss have been demonstrated using a Mach Zehnder interferometer with 3 mm-long phase shifter. 20 GHz roll-off frequency has been demonstrated using a shorter phase shifter (1mm-long), with a fairly smaller extinction ratio .
The modulator presented hereafter is based on carrier depletion in a lateral pin diode integrated on a SOI substrate. Mach Zehnder interferometer with 4 mm-long phase shifter is used, and the device exhibits simultaneously large extinction ratio, low insertion loss and high speed operation. In comparison with vertical diodes [12–16] the technological process does not require any epitaxial step and is clearly more straightforward. The design and the fabrication of such an optical modulator are presented, and experimental results are reported.
2. Device design
Schematic views of the device cross section are shown in Fig. 1(a–b). The silicon rib waveguide width is 660 nm, the rib height is 400 nm and the etching depth is 100 nm, leading to a single mode propagation of the guided mode at 1.55 µm wavelength (Fig. 1(c)). A p-doped slit, with a doping concentration of 1018 cm-3 is inserted in the intrinsic region of the lateral pin diode and acts as a source of holes. Doping concentration in the p and n- part of the diode are 1018 cm-3. The n-doped region slightly overlaps the guided mode, to ensure an efficient depletion of the thin p-doped slit. However, optical loss is reduced as a large part of the waveguide does not include any doped regions and metallic contacts are deposited on both sides of the waveguide, a few microns apart. Moreover, this design is quite favorable for high speed operation as the capacitance and the access resistances are reduced in comparison with the usually considered vertical structures.
At equilibrium, holes are confined in the doped slit inside the rib. When a reverse bias is applied to the diode, holes are swept out. The hole concentration variation is responsible for effective index variation , which creates a phase shift of the guided mode. A good overlap between the carrier density variation zone and the guided mode is obtained, leading to high effective index change. The silicon modulator is based on an asymmetric Mach-Zehnder interferometer (Fig. 1(d)). The phase shifter is inserted in both arms over a length of 4 mm, and electrodes are used to bias one arm. Waveguide splitters are star couplers with a reduced area (10×2 µm2) .
At first order, the small signal response of the device can be modeled as a capacitance and a serial resistance which issues from the resistances of silicon doped layers and from the contact resistances. To ensure high frequency operation, RC time constant has been minimized. The serial resistance of the device is reduced using silicide at the metallic – doped region interfaces. The capacitance of the device was evaluated using small-signal simulations . The diode capacitance per unit length varies from 0.23 to 0.18 fF/µm for reverse biases from 0V to -10 V. In comparison with vertical diodes or MOS capacitor [11, 14–16] the capacitance of the diode is reduced by at least one order of magnitude, which is a main advantage for high speed operation and low electrical power dissipation.
Coplanar waveguide electrodes have been designed to obtain a characteristic impedance around 50 ohms taking into account the capacitance of the pin diode. The width of the signal electrode is 5 µm and the gap between the signal and ground electrodes is 25µm.
The optical modulator was fabricated on an undoped 8 inch SOI substrate with a 1 µm-thick buried oxide (BOX) layer and a 400 nm-thick crystalline silicon film. A silicon dioxide (SiO2) cap layer was first deposited onto the wafer using low pressure chemical vapor deposition (LPCVD). A 100 nm-wide slit is defined using 193 nm deep-UV lithographic patterning followed by a reactive ion plasma etching of silicon dioxide. Double ion implantation is performed to obtain a thin doped layer on the whole Si layer thickness. Waveguides are then patterned, using classical technological processes, i.e. DUV optical lithography, reactive ion plasma etching. P and N doped regions are obtained by ion implantation, followed by thermal annealing. Finally, Ti/TiN/AlCu/Ti/TiN metal stack is deposited onto the wafer and electrodes are patterned and etched down to the SiO2 cap layer. The used process is fully compatible with SOI CMOS technology and could be transferred in high-volume microelectronic manufacturing.
The experimental set-up uses a tunable laser centered at 1550 nm. A linearly TE polarized light beam is coupled into the waveguide using a polarization-maintaining lensed-fiber. The output light is collected by an objective and focused on an IR detector. Electrical probes are used to bias the diode. Very low values of the reverse current (-2 µA at -10 V) have been measured that ensures low electrical power dissipation in DC configurations.
The output spectra of the modulator are recorded for 0 and -10 V bias (Fig. 2). The measured spectra have been normalized with the transmission of straight waveguides without phase shifters. The effective index variation in the phase shifter due to hole depletion creates a red-shift of the spectrum. Due to the very small overlap of the guided mode with doped regions, the measured insertion loss is as low as 5 dB. DC extinction ratio is around 14 dB from 0 to -10 V.
The phase variation is deduced from Fig. 2, using equation 1, where Δλ is the wavelength shift induced by the reverse bias, and FSR the Free Spectral Range of the asymmetric Mach Zehnder interferometer (~16 nm).
At -10 V, Δλ is 6.4 nm. The phase shift is then of about 0.8×π for a 4 mm-long phase shifter. To evaluate the modulation phase efficiency, a figure of merit is usually defined as the product VπLπ, where Vπ and Lπ are the applied voltage and the length required to obtain a π phase shift of the guided wave, respectively. The obtained value is VπLπ=5 V.cm. This experimental result is consistent with this device simulation results. Waveguide optimization would increase the optical mode confinement in the rib and the overlap between the guided mode and the doped slit. VπLπ values as low as 2 V.cm are predicted.
The frequency response of the modulator is measured using an AC signal generated by a synthesized sweeper (Hewlett Packard 8341B 10 MHz-20 GHz) coupled to DC bias, using a bias tee. The RF signal is coupled to the coplanar electrodes from the optical input side and a 50 Ω termination load is added at the output side. The modulated optical signal is coupled into a high speed photodiode with a flat response up to 12 GHz. The electrical output power of the photodiode is recorded with a spectrum analyzer (Hewlett Packard 8563E) as a function of the frequency.
The normalized optical response of the modulator is plotted in Fig. 3 for a DC bias of -5 V. The amplitude of the AC signal used for the measurement was 2.8 V peak to peak. A 3 dB cut-off frequency of ~ 10 GHz is measured.
Theoretical studies showed that intrinsic time constants lower than 2 ps are obtained with this device . The device speed limitation is not due to carrier transport mechanisms, but rather to electrical supply. The optimization of the phase modulation efficiency will allow shorter phase shifter, leading to an increase of the modulation bandwidth. The device rapidity can also be increased by an optimization of the RF travelling wave coplanar electrodes so that both electrical and optical signals propagate along the phase shifter with similar speeds.
In summary, an experimental demonstration of a new kind of all-silicon optical modulator based on carrier depletion has been reported. The proposed modulator is fabricated with a simpler technological process than the previously published structures. It exhibits simultaneously a large contrast ratio about 14 dB, insertion loss as low as 5 dB, and high frequency operation with a 3 dB optical bandwidth up to 10 GHz.
Further improvements can be considered. Progress in modulation efficiency is possible: a reduction of VπLπ product down to 2V.cm is theoretically predicted thanks to a good overlap between the optical mode and the doped region in the middle of the waveguide where carrier depletion occurs. This will open the possibility to decrease the phase shifter length, while keeping a large extinction ratio, which is favorable for high speed operation. Improved design of the RF travelling wave is also required. After optimizations, 3dB-cut off frequency of several tens of GHz can be reached.
The proposed structure offers a good potential for the realization of high performances integrated high speed modulators. Insertion loss is reduced as the high doped regions for contacts can be pushed away from the rib waveguide. The reduced capacitance is a good point for high speed and low electrical power consumption, and data transmission up to 40 Gbit/s can be forecast with such a device.
The authors thank Paul Crozat and Juliette Mangeney from IEF for fruitful discussions. They also acknowledge the staff of the 200 mm clean rooms of the LETI for the fabrication of high quality optical structures.
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