We demonstrate a strained SiGe variable optical attenuator (VOA) with a lateral pin junction which exhibits record-low injection-current for 20-dB attenuation. We optimize the distance between the highly doped p + and n + regions in the lateral pin junction to effectively inject electrons and holes, taking into account the propagation loss. In conjunction with the enhanced free-carrier absorption in strained SiGe, the SiGe VOA with the optimized lateral pin junction exhibits 20-dB attenuation by 20-mA/mm injection current, which is 1.5 times lower current density than that of the Si VOA. The SiGe VOA also shows better RF response than the Si VOA due to the short carrier lifetime in SiGe, allowing us to achieve efficient and fast attenuation modulation simultaneously. Furthermore, 2-GHz switching and error-free transmission of 4 × 12.5 Gbps WDM signal have been also achieved.
© 2015 Optical Society of America
Since the strong demand of data centers is growing rapidly, an electronic-photonic integrated circuit (EPIC) is going to be a promising solution to provide broadband, low-power, and high-density optical interconnections for the data center network . Among various platforms for EPICs, Si photonics is a key technology, by which photonic components including lasers, optical modulators, photodetectors, and passive waveguide devices have been dramatically developed in the past ten years [2–7]. Si-based variable optical attenuators (VOAs) are also necessary components for controlling optical power levels in EPICs. Si VOAs based on the free-carrier absorption have been mostly investigated and demonstrated using carrier injection through a forward-biased lateral pin junction [8–14]. However, the efficiency in absorption modulation is still low due to the small free-carrier absorption in Si , resulting in high injection current and large power consumption. Therefore, it is indispensable to enhance the free-carrier effect for achieving low-power VOAs with a low injection current. To obtain the enhancement in the free-carrier effect, we have proposed to use strained SiGe grown on Si, which is applicable to SiGe optical modulators, switches, and VOAs. The enhancement of the free-carrier effects, the plasma dispersion effect and the free-carrier absorption, is expected by reduction in the conductivity effective mass of holes in strained SiGe [15, 16]. Recently, we have demonstrated strain-induced enhancements in the plasma dispersion effect and free-carrier absorption for optical modulator and VOA applications by using the carrier-injection strained SiGe waveguide device with a lateral pin junction . However, the first demonstration of the SiGe VOA in  was not fully optimized in terms of the distance between the heavily doped p + and n + regions, resulting in the low carrier injection efficiency. The diffusion length of injected carriers is limited various recombination processes. Therefore it is necessary to shorten the distance between the heavily doped regions as shown in Fig. 1.
In this paper, we demonstrate strained SiGe VOAs with high carrier injection efficiency through the optimization of the distance between the highly doped p + and n + regions by using electron beam (EB) lithography, taking into account the tradeoff between attenuator loss and carrier-injection efficiency. As a result, we have achieved the record-low injection current of 20 mA for 20-dB attenuation in the SiGe VOA. Furthermore, we have obtained 2-GHz switching and error-free transmission of 50-Gbps WDM signal.
2. Optimization of the distance between highly doped p + and n + regions
To effectively inject carriers into a waveguide core through a lateral pin junction, it is necessary to make the distance between highly doped p + and n + regions as short as possible due to the small diffusion length of carriers limited by recombination. However, the propagation loss caused by the free-carrier absorption in the highly-doped regions inevitably increases as the distance is reduced.
First, we have investigated the propagation loss of the waveguide mode in terms of the distance between the edge of the waveguide mesa and the highly-doped region by the method of numerical analysis using by technology computer-aided design (TCAD) simulation and finite-difference optical mode analysis [16, 17]. The propagation loss is calculated by the overlap between the free-carrier distribution with 0 V bias and the fundamental transverse electric (TE) mode of the waveguide. Figure 2 shows the simulation result of propagation loss as a function of the distance between the edge of the waveguide mesa and the highly-doped region. When the distance is smaller than 400 nm, the propagation loss dramatically increases due to the free-carrier absorption in the heavily doped regions. As shown in Fig. 2, the propagation loss can be ignored when the distance is larger than 500 nm. In this study, we design the distance to be 650 nm, considering the process margin of EB lithography.
3. Device fabrication
We have fabricated strained SiGe VOAs using a (001) Si-on-insulator (SOI) wafer with a 250-nm-thick top Si layer and 2-μm-thick buried oxide (BOX) layer. To obtain the designed structure of Si/SiGe/SOI as shown in Fig. 3(a), we thinned the top Si layer down to 95 nm by thermal oxidation, then a 30-nm-thick fully strained Si0.77Ge0.23 layer and a 75-nm-thick Si layer were grown by chemical vapor deposition (CVD). The 600-nm-wide waveguide mesa was formed by EB lithography and dry etching as shown in Fig. 3(b). To form a lateral pin junction, we implemented the ion implantation of boron and phosphorus for the highly doped p + and n + regions, respectively, followed by the activation annealing in nitrogen atmosphere at 1000 °C. Due to the high thermal process, Ge diffusion occurs, which affect reduction in Ge fraction in the SiGe layer. After device fabrication, the Ge fraction is reduced to 14% . Finally, the Al contact pads for the p + and n + regions were formed by thermal evaporation, followed by forming gas annealing at 400 °C to decrease contact resistances. Si VOAs are also fabricated at the same time.
We observed the fabricated SiGe device by scanning electron microscopy (SEM) as shown in Fig. 4. The tilted SEM image in Fig. 4 shows the well-defined Si/SiGe/Si waveguide and Al electrodes formed by EB lithography.
4. Static characteristics
We measured the optical attenuation by injecting current to evaluate the device performance of the SiGe VOAs, including Si VOAs as reference devices. Continuous-wave (CW) light, which was polarized to a TE-like mode by a polarization controller, was coupled to the waveguide through a lensed fiber, which was in the wavelength range from 1.34 to 1.64 μm. Then, the output power was monitored using an InGaAs photodetector while changing the injection current through the pin junction. For calibrating the total coupling loss of approximately −30 dB, the insertion loss without injection current was subtracted from the measured output power. In terms of the thermal effect in the attenuation, the free-carrier absorption theoretically increases linearly with an increase in temperature. However, the temperature change is not significant when the injection current is below 50 mA/mm. We have analyzed the thermal change in the Si phase-shifter as a function of power consumption by COMSOL. Even when the power consumption is as high as 40 mW, the change is less than 10 K, which may result in a few percent changes in the free-carrier absorption. Furthermore, we used the thermal controller maintaining the constant temperature of sample stage to make the thermal effect as small as possible. Thus, the thermal effect is not significant in our experiments.
Figure 5(a) shows the measured optical attenuation versus the injected current density of the 1-mm-long SiGe and Si VOAs at a wavelength of 1.55 μm. As increasing injection current by forward-bias voltage, the optical attenuation increases due to the free-carrier absorption. The higher attenuation is shown for the SiGe VOA than the Si VOA at the same current density due to the enhanced free-carrier absorption in strained SiGe . By optimizing the lateral pin junction structure, the injection current for 20-dB attenuation in the Si device as well as the SiGe device is reduced. For the SiGe VOA, the injection current density is as low as 20 mA/mm for 20-dB attenuation. The benchmark of injection current for 20-dB attenuation as a function of the distance between the p + and n + regions in the lateral pin junction is shown in Fig. 5(b). It is notable that the current required for 20-dB attenuation decreases with decreasing the junction distance due to improved carrier injection efficiency. Therefore, the optical attenuation property is improved by shortening the distance between p + and n + regions. In the SiGe VOA, we achieved record-low injection current of 20 mA/mm for 20-dB attenuation thanks to the optimization and strain effect in SiGe  as compared with Si VOAs reported so far [12, 13].
We show the wavelength dependence of the SiGe VOA in Fig. 6. Figure 6(a) shows the transmission spectra of the SiGe VOA from 1.34 to 1.64 μm with 0 and 30 mA/mm injection current. We achieved the wide optical bandwidth despite bandgap shrinkage in strained SiGe [16, 17]. Figure 6(b) shows the optical attenuation at injection currents of 5, 15, and 25 mA/mm as a function of the square of the wavelength. It is shown that the attenuation and the square of the wavelength have clear linear relationship, indicating the optical attenuation is from the free-carrier absorption.
Figure 7 shows current density vs. voltage (I-V) characteristics of the lateral pin junctions in the SiGe and Si VOAs. It is found that the threshold voltage of the SiGe VOA is smaller than that of the Si VOA due to the reduction in built-in potential by strained SiGe. It is interesting to note that the reduction in the threshold voltage can contribute low-power operation especially for optical modulators and switches. In terms of the power consumption between the SiGe and Si VOAs, the SiGe and Si VOAs have approximately 22 mW and 36 mW for 20-dB attenuation, respectively. The power consumption of the SiGe device is reduced by an half of the reported value  due to the pin junction optimization for the distance.
5. Dynamic characteristics
We carried out the RF measurement for the dynamic characteristics. We investigated the small-signal frequency response of Si and SiGe VOAs as shown in Fig. 8(a). Although the response is limited by the minority carrier lifetime in case of forward-biased pin junctions , the SiGe VOA shows the better frequency response than the Si VOA. The 3-dB bandwidth in the SiGe VOA is approximately improved by 1.5 times. Also, we have obtained 2-GHz switching operation at a bias voltage of 0.77 V with the pre-emphasis method  as shown in Fig. 8(b).
To clarify the higher RF response of the SiGe VOA, we have numerically analyzed the effective lifetime using TCAD simulation and finite-difference optical mode analysis . To reproduce the experimentally obtained attenuation characteristics by the simulation, we used the fitting parameters of Shockley-Read-Hall (SRH) recombination, τSi for Si layers and τSiGe for SiGe layer . First, the attenuation result of the Si VOA is fitted by choosing 2.8 ns for τSi as shown in Fig. 9(a). Then, we simulated the attenuation characteristics of the SiGe VOA with the same parameter of 2.8 ns in Si and SiGe layers. However, it is shown in the grey solid line of Fig. 9(a) that the simulation result for the SiGe VOA is not in agreement with the experiment result when τSi and τSiGe are assumed to be 2.8 ns. The simulated attenuation is larger than the experimental result, indicating the number of the injected carriers are lower than expected due to the larger recombination rate in the SiGe layer. Thus, we assume that the SiGe layer has a larger recombination rate probably caused by defects. As shown in the blue solid line of Fig. 9(a), the simulation result is in good agreement with the experiment result of the SiGe VOA when τSiGe is assumed to be 2.0 ns. From the simulation results, we calculated the effective lifetime of carriers  as shown in Fig. 9(b). As increasing bias voltage, the carrier concentrations exponentially increase by carrier-injection, causing a decrease in the effective lifetime. The SRH recombination and Auger recombination mainly affect the lifetime less than and more than around 1 V, respectively. Therefore, even assuming the same carrier lifetime of 2.8 ns for Si and SiGe, the effective carrier lifetime in the SiGe device becomes smaller than in the Si device because the injected carriers are confined in the SiGe when the bias voltage is larger than approximately 1 V. When τSiGe and τSi are assumed to be 2.0 ns and 2.8 ns, respectively, it is found that the effective lifetime of the SiGe device is shorter than that of the Si device even with a bias voltage of less than 1 V. At the bias voltage of 0.8 V, the effective lifetime of Si and SiGe is 6.4 ns and 4.6 ns, respectively. As a result, the 1.5 times higher RF response of the SiGe VOA can be explained by 1.4 times shorter effective lifetime, which is obtained probably by more crystal defects in the SiGe layer. Even though the crystal defects simultaneously degrade the attenuation property through low carrier injection efficiency , the attenuation in the SiGe VOA is still larger than in the Si VOA with maintaining higher RF response owing to the enhanced free-carrier absorption in SiGe.
The transmission characteristics for 50 Gb/s wavelength division multiplexing (WDM) signal were also evaluated under the measurement setup as shown in Fig. 10. 4 × 12.5 Gb/s non-return-to-zero (NRZ) 231-1 pseudo random bit sequence (PRBS) optical signals was fed into the SiGe VOA. The total input power was approximately 18 dBm. The output signal was demultiplexed and evaluated by a digital communication analyzer (DCA) and a bit-error tester (BERT). Figure 11 shows the bit error rate (BER) curves of all the wavelength channels. Although Ch. 4 exhibits relatively large power penalty which might be attributable to the nonlinear crosstalk in SiGe , error-free transmission for all the channels with clear eye openings were successfully obtained.
We have demonstrated record-low injection-current SiGe VOAs by optimizing the distance between the p + and n + regions in the lateral pin junction. The injection current for 20-dB attenuation of the SiGe VOA is as low as 20 mA/mm thanks to the optimization and strain-induced enhancement of the free-carrier absorption in strained SiGe. We have also found 1.5 times improvement in the RF response of the SiGe VOA comparing to the Si VOA, and 2-GHz switching was achieved. Error-free transmission of 4 × 12.5 Gb/s WDM signal was also demonstrated.
This work was partly supported by the Strategic Information and Communications R&D Promotion Programme of the Ministry of Internal Affairs and Communications and NEDO in “Integrated Photonics-Electronics Convergence System Technology (PECST)”. The authors would like to thank T. Osada and M. Hata, Sumitomo Chemical Company Ltd., Tsukuba, Japan, for their technical supports.
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