(A) study of applications of metal-oxide materials for integration in the silicon photonic circuits

Abstract

Recently, silicon photonics has attracted a lot of interest in the miniaturization and integration of elec-tro-optic devices, such as amplitude/phase modulators and switches for optical signal processing, because of its high modal confinement and low absorption losses at near-infrared wavelengths. However, intrinsic limita-tions of the silicon materials possess a fundamental trade-off between the modulation efficiency and the operational speed. To overcome the disadvantages of silicon materials, silicon-waveguide-based hybrid modulators with high-efficient electro-optic materials have been proposed. In particular, indium-tin-oxide (ITO) and vanadium oxides are important candidates for such applications due to its unique feature of electrically tunable permittivities. First, an ultra-compact phase modulator which consists of a silicon slot waveguide with a thin ITO film in the slot region is theoretically proposed. In the near-infrared regime, bias-voltage-dependent free-carrier accumulation at the dielectric-ITO interface induces an epsilon-near-zero (ENZ) effect, and contributes to the strong phase modulation of the guided electromagnetic wave. With a voltage swing of 2 V, the device experiences a large variation of the effective modal index, resulting in a $\pi$ radian phase shift within the device length of <$5\mu m$ at 210 THz. According to the computer simulations, a high modulation efficiency of $V_\pi L_{\pi}$ ~ $0.0071 V \cdot cm$ and a large device bandwidth of ~70 GHz is demonstrated. Moreover, a phase shift property of the ultra-compact ITO modulators might be applied to beam steering device for the novel integrated photonics circuits. Finite difference time domain (FDTD) simulations based on an ITO phase array show that considerably high accuracy of beam steering can be achieved compared to the ideal phase array antenna. Secondly, a vanadium oxide integration method for the silicon photonic platform is experimentally demonstrated. In general, the silicon waveguide-based optical switches or modulators with vanadium dioxide show higher modulation efficiency and smaller device dimension. However, deposition of the pure $VO_2$ films requires high temperature over $500^\circ C$$ that is not compatible to the CMOS back-end process. In this thesis, I suggest an alternative method to locally produce vanadium oxides on the silicon waveguide by using electri-cally-induced oxidation at relatively low temperature. Based on Raman spectrums and an R-V curve, it is found that the produced vanadium oxides have a semiconductor-metallic phase transition effect. Moreover, I theoretically suggest one of the applications as an electro-optic modulator using the vanadium oxidation method. I believe that the simple oxidation method for vanadium can be easily applied to the integrated silicon photonic circuits using CMOS technologies. The above-mentioned silicon hybrid modulators with ITO and vanadium oxide are expected to have distinguished merits for future chip-based optical communication modules and optoelectronic integrated cir-cuits in terms of ultra-compactness, low power, and large bandwidth.