Design of an ultra-compact electro-absorption modulator comprised of a deposited TiN/HfO₂/ITO/Cu stack for CMOS backend integration.

An ultra-compact electro-absorption (EA) modulator operating around 1.55-μm telecom wavelengths is proposed and theoretically investigated. The modulator is comprised of a stack of TiN/HfO2</ITO/Cu conformally deposited on a single-mode stripe waveguide to form a hybrid plasmonic waveguide (HPW). Since the thin ITO layer can behave as a semiconductor, the stack itself forms a MOS capacitor. A voltage is applied between the Cu and TiN layers to change the electron concentration of ITO (NITO), which in turn changes its permittivity as well as the propagation loss of HPW. For a HPW comprised of a Cu/3-nm-ITO/5-nm-HfO2/5-nm-TiN stack on a 400-nm × 340-nm-Si stripe waveguide, the propagation loss for the 1.55-μm TE (TM) mode increases from 1.6 (1.4) to 23.2 (23.9) dB/μm when the average NITO in the 3-nm ITO layer increases from 2 × 10(20) to 7 × 10(20) cm(-3), which is achieved by varying the voltage from -2 to 4 V if the initial NITO is 3.5 × 10(20) cm(-3). As a result, a 1-μm-long EA modulator inserted in the 400-nm × 340-nm-Si stripe waveguide exhibits insertion loss of 2.9 (3.2) dB and modulation depth of 19.9 (15.2) dB for the TE (TM) mode. The modulation speed is ~11 GHz, limited by the RC delay, and the energy consumption is ~0.4 pJ/bit. The stack can also be deposited on a low-index-contrast waveguide such as Si3N4. For example, a 4-μm-long EA modulator inserted in an 800-nm × 600-nm-Si3N4 stripe waveguide exhibits insertion loss of 6.3 (3.5) dB and modulation depth of 16.5 (15.8) dB for the TE (TM) mode. The influences of the ITO, TiN, HfO2 layers and the beneath dielectric core, as well as the processing tolerance, on the performance of the proposed EA modulator are systematically investigated.