III–V/Si Hybrid MOS Optical Phase Shifter for Si Photonic Integrated Circuits

We present a novel optical phase modulation scheme on a Si photonic platform that uses a III–V/Si hybrid metal–oxide–semiconductor (MOS) capacitor formed by bonding an n-type InGaAsP membrane on a p-type Si waveguide. We numerically revealed that the phase modulation efficiency was improved by a factor of 7–8 owing to electron accumulation at the InGaAsP MOS interface when the n-type Si layer in a Si MOS optical phase shifter was replaced by an n-type InGaAsP layer. To realize the III–V/Si hybrid MOS capacitor, we developed an Al₂O₃ bonding interface deposited by atomic layer deposition that enabled a low interface trap density of <10¹² cm^–2·eV^–1 at an InGaAsP MOS interface, which was essential for electron accumulation. We demonstrated a modulation efficiency of 0.047 V·cm at 1.55-<italic>μ</italic>m wavelength owing to the electron-induced change in the refractive index of InGaAsP. Since no holes were induced in the III–V layer of the III–V/Si hybrid MOS capacitor, we avoided large hole-induced absorption in InGaAsP. As a result, when we had a π phase shift, we obtained optical absorption of 0.23 dB, approximately ten times smaller than that of a Si MOS optical phase shifter. We found by numerical analysis that the efficient low-loss III–V/Si hybrid MOS optical phase shifter improved markedly the optical modulation amplitude, indicating its suitability for high-speed modulation beyond 100 Gb/s. We also demonstrated a Mach–Zehnder interferometer optical switch using the proposed optical phase shifter with a switching time of less than 20 ns. We achieved an extremely low switching power of approximately 1 nW, enabling a large-scale optical switch and universal photonic integrated circuits. We also discuss the feasibility of a photonic neural network for deep learning.