A very simple and fast MachZehnder electro-optic modulator based on a p-i-n configuration, operating at λ = 1.55 μm, has been fabricated at 170°C using the low cost technology of hydrogenated amorphous silicon (a-Si:H). In spite of the device simplicity, refractive index modulation was achieved through the free carrier dispersion effect resulting in characteristic rise and fall times of ~2.5 ns. By reverse biasing the p-i-n device, the voltage-length product was estimated to be Vπ∙Lπ = 40 V⋅cm both from static and dynamic measurements. Such bandwidth performance in as-deposited a-Si:H demonstrates the potential of this material for the fabrication of fast active photonic devices integrated on standard microelectronic substrates.
© 2012 OSA
Silicon Photonics is a rapidly growing technology candidate to complement the superiority of the CMOS technology in microelectronics [1–3]. In fact, with a constantly increasing number of functionalities on a single microchip, and the outstanding characteristics of photonics in data communication, a merge of the two worlds seems already a fixed choice for the next generation of devices.
One critical device for on-chip photonics circuits (PC) is a high-speed waveguide-integrated electro-optic modulator, which converts data from the electrical to the optical domain. Beside the tens of Gbps devices , necessary for addressing the data exchange bottleneck at chip level, plenty of opportunities also exist for Gbps ones, such as for fiber-to-the-home (FTTH) terminal units and routers, or remote sensors.
To date, a definitively prevailing technology for the integration of photonic components on a CMOS platform is still to emerge. Crystalline silicon (c-Si)-based photonic devices require a high thermal budget and not truly standard fabrication processes. In contrast, non-crystalline forms of silicon, such as polycrystalline (poly-Si) and amorphous silicon (a-Si), can be deposited yielding low cost and flexible fabrication. Recently, experimental results have been reported of direct electrical and all-optical modulation [5–7], at the communication wavelengths, in poly-Si-based thin-film devices. However, poly-Si waveguides introduce high optical losses from scattering at the grain boundaries and the involved thermal crystallization temperature (~1100°C), necessary to maximize the optical and electronic material properties, is above the suggested limit of 450°C set for back-end integration. Near-UV laser pulses were therefore used to locally melt and crystallize a top layer of poly-Si at 400°C , but no active devices have been realized with it yet. On the other hand, hydrogenated amorphous silicon (a-Si:H), deposited using low temperature (120-300°C) Plasma Enhanced Chemical Vapour Deposition (PECVD) technique, seems useful for realizing minimally-invasive on-chip passive [8–11] and active [9,12–14] devices by depositing vertically stacked layers. Broadband all-optical modulation has recently been reported in as-deposited a-Si:H , exploiting the free carrier absorption effect (Δα) by a pump-probe technique, demonstrating carrier recombination time, in a-Si:H nano-sized waveguides, comparable to that of c-Si.
In a more recent paper , we have shown the first experimental results of an effective refractive index variation (Δneff) obtained by an electrically induced carrier depletion in a-Si:H-based p-i-n waveguiding device deposited at a maximum temperature of 170°C. Characteristic switch-on and switch-off times of ~14 ns were measured in Fabry-Perot (FP) resonators allowing a modulation rate of ~30 MHz, the highest value ever reported for waveguide-integrated electrically driven a-Si:H-based optical devices.In this paper we report experimental results obtained in an improved device, namely a MachZehnder interferometer (MZI), where phase shift is induced by carrier depletion in an optimized p-i-n diode embedded in the a-Si:H rib waveguides forming the arms of the MZI. By reverse biasing the device, we induce a very fast modulation of the space charge volume that, in turn, modifies the refractive index profile of the waveguide and therefore the optical phase of the 1.55 μm wavelength light passing through it.
2. Device structure and fabrication
The schematic layout of the realized modulator is shown in Fig. 1(a) together with its geometrical dimensions. The device consists of a W = 6 μm input rib-waveguide that is in-tapered over a distance of 1 mm to a W = 4 μm single-mode rib-waveguide. Input-output S-bent curved waveguides, 2050 μm-long, were used to split and combine the optical beam. The S-bent radius of curvature and the intersecting angle of 1.4° result from parametrical optical simulation  in order to achieve a trade-off between the need for a compact device and low insertion losses. A scanning electron microscope (SEM) image of the splitting region is shown in Fig. 1(b).
The distance between the two parallel waveguides, 1.3 cm-long, is 50 μm while the overall device length is about 1.86 cm. In Fig. 2(a) we report the schematic cross section of the rib-waveguide (W = 4 μm), which consists of a p-i-n structure made of a 2 μm-thick a-Si:H undoped core region between a p-doped a-SiC:H bottom cladding (2 μm-thick) and an n-doped a-SiC:H top layer (300 nm-thick). The substrate is a p-doped Silicon wafer, 300 μm thick.
The simulated fundamental TE0 and TM0 mode-field profiles supported by the structure are shown in Fig. 2(c) and Fig. 2(d). The simulations show that it is birefringence free and single mode for wavelengths between 1.51 and 1.6 μm. The calculated losses are 3.66 dB/cm and 4.06 dB/cm respectively, with and without an aluminum (Al) top contact (Al refractive index = 1.57 + i15.6 at λ = 1.55 μm), in agreement with the experimental results.
To ensure an easy back-end integration with CMOS, we decided to avoid an SiO2 under-cladding layer, and therefore the p-i-n waveguides were fabricated at low temperature (TMAX = 170°C) directly on the p-doped c-Si substrate by Plasma Enhanced Chemical Vapour Deposition (PECVD) in an MVSystems reactor. The measured fundamental optical and electrical material parameters are listed in Table 1 .
More details about the process were given in a previous work .
3. Experimental results and discussion
The W = 4 μm (6 μm)-wide rib waveguides were patterned by photolithography and etched to a depth of 520 nm by a reactive ion etching (RIE). One arm of the MZ modulator was covered by a 100 nm-thick evaporated Al film to ensure a good ohmic contact.
We have tested the behavior of the waveguide-integrated MZI by performing both DC and AC measurements. In our set-up a TE-polarized light beam from a 25 mW tunable laser-diode was coupled via a lensed fiber focused onto the input facet of the tapered waveguide, and the output light signal was collected by a single-mode fiber and detected by a high speed InGaAs photodiode. Light intensity is modulated by the phase shift induced in one arm of the interferometer by free carrier depletion in the p-i-n junction. The optical transmission of the MZI was scanned over a wavelength range of 2 nm between 1549 nm and 1551 nm. At each wavelength, a DC reverse bias between 0 V and 60 V, in steps of 1 V, was applied to the phase shifter between the top Al contact and the substrate. We measured modulation depths, defined as (Imax-Imin)/Imax, where Imax and Imin are the maximum and minimum transmitted signals respectively, of 14%. The figure of merit Vπ∙Lπ (the voltage required to achieve a π phase shift for a given length) was extracted from the transmission vs. reverse voltage plot. Figure 3 shows the normalized optical transmission as a function of the bias level from which we calculated a static Vπ∙Lπ product of ~40 V∙cm.
The on-chip insertion loss was ~18 ± 1 dB when the MZI is in the “ON” state, including ~4 dB of coupling losses at the tapered input. However, it is common to place contact metal aside of the rib, far from the optical field, in order to cut these high absorption losses . We also measured the fiber-to-fiber losses of a straight p-i-n rib waveguide (W = 4 μm) to evaluate the propagation losses, estimating a value of about 4.3 ± 0.2 dB/cm. For the considered branch angle (1.4°), the loss due to leakage from the splitting action is about 1.2 dB as estimated by optical simulations.
The speed performance of the reported vertical p-i-n depletion modulator was analyzed by applying to one arm of the MZI a pulsed signal (Agilent 8114A) with frequency f = 10 MHz, rise (fall) time of ~2 ns, and adjustable peak amplitude Vp. High-frequency electrical micro probe-heads were used to deliver the pulses to the device.
The detected modulated signal was sent to an electrical spectrum analyzer, for an easy tracking of the modulation amplitude, and to a 2 GHz bandwidth oscilloscope, allowing for the measurement of the characteristic rise and fall times which set the limit to the maximum modulation speed. In Fig. 4 we show an optical pattern obtained for pulses with an amplitude of Vp = 31 V, applied to the 1.3 cm-long MZI arm. The maximum observed modulation depth is again approximately 14%, as already observed in DC measurements.
The measured rise and fall times are ~2.3 ns, a result which excludes the potential impact of thermo-optic effects on our experiments. As discussed for previous experiments , the dynamic behavior is still limited by the high specific resistance of the top contact. We expect that sub-ns operations are possible in a device with optimized contacts.
It is worth noting that the large pulse amplitude (~31 V) required for achieving a Δϕ = π is strongly dependent on the waveguide thickness (H = 4.3 μm), dictated in turn by the target of limiting the coupling losses. In an optimized setup, the thickness could be easily reduced allowing a reduction of the pulse amplitude (Vp) that we estimated of a factor of 10 in a 400 nm waveguide . Moreover, higher doping concentration can lead to higher index change, but with higher loss due to the free carrier absorption. Therefore careful engineering of the material parameters is critical to enable acceptable optical loss while improving the figure of merit Vπ∙Lπ.
Polarization-dependence measurements demonstrated comparable phase modulation efficiency for TM polarization.
We have demonstrated a fast and effective electro-optical modulation at λ = 1.55 μm in a p-i-n waveguiding, birefringence-free structure, based on the CMOS-compatible technology of a-Si:H. The MZ interferometric device was easily fabricated on a c-Si wafer without the deposition of a thick oxide layer at the bottom for shielding the waveguide from the substrate, and the maximum temperature during the PECVD process was as low as 170°C. The effective refractive index variation was measured in both static and dynamic conditions. The voltage-length product for inducing a phase variation Δϕ = π in one arm of the MZI was measured to be Vπ⋅Lπ = 40 V∙cm. Switching times of 2.3 ns have been measured.
The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 224312 HELIOS.
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