Design and fabrication of indium tin oxide for high performance electro-optic modulators at the communication wavelength

In recent years, transparent conductive oxides(TCO) based electro-optic modulators has garnered significant attention due to its epsilon-near-zero(ENZ) effect to enhance the local optical field and produce a large refractive index change in the vicinity of ENZ wavelength, thereby enabling substantial optical field modulation at micrometer size. The modulators manipulate the light field by altering the carrier concentration in TCO through an electric field. Therefore, the modulator's performance is closely related to the TCO's material behavior. In this paper, the effect of carrier concentration and mobility of indium tin oxide(ITO) on the modulation efficiency, depth and bandwidth was analyzed and it is found that ITO with the carrier concentration near 1 x 10(20) cm(-3) and mobility as high as 80 cm(2)/Vs is beneficial to the electro-optic modulators with large modulation depth and bandwidth. Specifically, our research shows that the carrier concentration and mobility of ITO films can be tunned over a wide range from 2.2 x 10(19)cm(-3) to 7.7 x 10(20)cm(-3) and 7.2 cm(2)/Vs to 82.3 cm(2)/Vs respectively, by changing doping concentration, deposition conditions and annealing parameters. As a result, ITO film integrated on a 3.5-mu m-long silicon waveguide with an MOS structure can achieves a high modulation bandwidth of over 40 GHz at the optical communication wavelength.