Facile integration of electro-optic SiO<sub>2</sub>/ITO heterointerfaces in MIS structures for CMOS-compatible plasmonic waveguide modulation

By taking advantage of the absence of diffraction limit restrictions in plasmonic structures, strong modal confinement is made possible, paving the way for improved optical processes and miniaturized photonic circuit integration. Indium tin oxide (ITO) has emerged as a promising plasmonic material that serves as a relatively low-carrier density Drude metal by its electro-optic tunability and versatility as an integrative oxide. We herein demonstrate the facile integration of SiO2/ITO heterointerfaces into metal–insulator–semiconductor (MIS) electro-optic structures. The first MIS device employs a SiO2/ITO heterostructure grown on thin polycrystalline titanium nitride (poly-TiN) and capped at the ITO side with thin aluminum (Al) film contact electrode. The TiN interlayer acts as a bottom electrode, forming a metal–insulator–semiconductor-metal (MISM) heterojunction device, and grows directly on (100)-oriented silicon (Si). This MISM device enables one to examine the electrical properties of semiconductive ITO layers. The second MIS device incorporates a semiconductive ITO layer with a SiO2 dielectric spacer implemented on a silicon-on-insulator (SOI) platform, forming a graded-index coupled hybrid plasmonic waveguide (CHPW) modulator. This device architecture represents a crucial step towards realizing plasmonic modulation using oxide materials. The CHPW device performance presented herein provides a proof-of-concept that demonstrates the advantages offered by such device topology to perform optical modulation via charge carrier dispersion. The graded-index CHPW can be dynamically reconfigured for amplitude, phase, or 4-quadrature amplitude modulation utilizing a triode-like biasing strategy. It exhibited extinction ratio (ER) and insertion loss (IL) levels of around 1 dB/μm and 0.128 dB/μm, respectively, for a 10 μm waveguide length.