Electro-optic modulation is achieved in devices consisting of single-crystalline thin films of N-benzyl-2-methyl- 4-nitroaniline grown from the melt in the slots of phase modulators based on horizontally slotted silicon waveguides. To the best of our knowledge, this is the first experimental realization of an electro-optically active horizontally slotted silicon waveguide and also the first demonstration of organic crystalline materials being implemented into the slotted silicon photonics technology. The experimentally determined half-wave voltage times length product and the losses are estimated to be $V_{\pi }\times L=14.7\pm 2\mathrm{V}·\mathrm{cm}$ and $10\pm 2.4\mathrm{dB}/\mathrm{cm}$ , respectively. The fabrication concept employed here circumvents technological issues present in the context of conventional vertically slotted waveguide structures, since the slots with cross-sectional dimensions of about $1000\mathrm{nm}\times 160\mathrm{nm}$ have been patterned with standard optical photolithography into thermally grown oxide sandwiched between two fusion bonded device silicon layers. In contrast to previously reported vertically slotted silicon waveguides with polymeric slot materials, organic crystalline based devices do not require high-electric-field poling prior to operation and feature an excellent long-term stability of dipole orientation in addition to superior photochemical stability.