  By combining the large Purcell effect of photonic crystal nanocavity and the
strong plasma dispersion effect of the transparent conductive oxides,
ultra-compact silicon modulators with heterogeneously integrated
indium-tin-oxide (ITO) can potentially achieve unprecedented energy efficiency
to atto-joule per bit. In this article, we report the first high-speed silicon
nanocavity modulator driven by an ITO gate, achieving 2.2 GHz bandwidth.
On-off-key modulation is measured up to 5 Gb/s with only 2 V voltage swing and
18.3 fJ/bit energy efficiency. In addition, we perform in-depth analysis of the
energy efficiency and high frequency simulation of the nanocavity modulator,
revealing the critical role played by the semiconductor conduction path and the
overlapping factor between the accumulated free carriers and the cavity
resonant mode. Based on our analysis, we propose a strategy to further improve
the modulation bandwidth to 23.5 GHz by node-matched doping and reduce the
energy consumption to the range of hundreds of atto-joule per bit.
