(Invited) High-Performance Si Optical Modulator and Ge Photodetector and Their Application to Silicon Photonics Integrated Circuit

Abstract

We designed the optimum Si optical modulator (Si-MOD) structure with applying p-type-doped strained SiGe. By applying the strained Si70Ge30 layer, about twice larger optical modulation efficiency in VπL can be obtained in the simulation. In case of a relaxed Si70Ge30 layer, enhancement factor in VπL decrease to about 50%, but optical loss is comparable to that without a SiGe layer. We demonstrated a high modulation efficiency of 0.67 Vcm, which is about 50% more efficient than that of Si-MODs with just a lateral pn junction. From the Raman spectra and SIMS analyses, Ge composition of SiGe was estimated to be around 35%. Crystalline strain of SiGe was estimated to be 1.7% on the 0.04 mm2 square pattern, which is comparable to the theoretical value. On the other hand, it was estimated to be 1.1% for the selective growth of SiGe on the Si WG with a lateral pn junction. Therefore, biaxial strain of SiGe was a little relaxed, because the selective growth of SiGe pattern has a large aspect ratio with 400 nm width and 800 μm length. The Si-MOD showed 27.3 GHz bandwidth at -3 Vdc with 200μm-long phase-shifter. We demonstrated eye diagrams of NRZ (non-return-to-zero) 40 Gbps of 231-1 PRBS (pseudorandom binary sequence) and PAM (pulse amplitude modulation)-4 40Gbps at 1.3 μm wavelength under the condition of differential RF drive with 1.6-2.0 Vpp at -2Vdc. The optical loss of the Si-MOD is estimated to be 1.0-1.5 dB/mm, which is comparable to that of simulation. With increase in temperature up to 80℃, optical loss increased by about 0.3-0.5 dB, which would originate from band-gap shrinkage of a strained-SiGe layer. We obtained the clear eye diagram at 25 Gbps with 27-1 PRBS from the optical transceiver chip when the CMOS-driver output voltage was 0.9 Vpp and Vdc was -0.3V. ER (extinction ratio) of about 2 dB was obtained in case of 800μm-long phase shifter with 4-divided electrodes. We also studied a high-speed and high-efficiency of surface-illumination type Ge photodetector (Ge-PD) with a 1800 nm thick Ge layer. By optimizing the anti-reflection coating stack structure, high responsivity of 0.8-0.9 A/W was uniformly obtained within the wafer. At 3 Vdc, about 15 GHz bandwidth was obtained. In case of the Ge-PD with thick Ge-layer, photo-carrier transit time mainly limits the frequency bandwidth. Therefore, we optimized the layered structure of Ge thickness on SOI to satisfy the high-responsivity and high-bandwidth. We demonstrated an output waveform from integrated CMOS-TIA (trans-impedance amplifier) at 25 Gbps with 27-1 PRBS at 1.3 μm wavelength at 3.3 Vdc. Clear eye opening was obtained, which would contribute to the efficient optical interconnect.