Robust Ferroelectricity in Silicon Dioxide upon Intercalation of Ammonia

The nanoelectronic applications of current ferroelectrics have been greatly impeded by their incompatibility with silicon. In this paper, we propose a way to induce ferroelectricity in silicon dioxide (SiO₂), which is still the most widely used dielectric material in silicon-based chips. We show first-principles evidence that the intercalation of NH₃ molecules into crystalline SiO₂ is exothermic (ΔE = -0.327 eV/molecule), where NH₃ molecules form quasi-bonds with SiO₂, giving rise to large and robust polarizations. In general, such polarization can be reversed via the reformation of N-Si bondings, which is multiaxial, so vertical ferroelectricity may emerge in their thin films of any facets. When the applied external electric field is large enough, however, the system may exhibit unconventional quantized ferroelectricity of unprecedented magnitude, where NH₃ may migrate for multiple lattice constants like mobile ions in ion conductors. Compared with ion conductors with charged mobile ions and ion vacancies that may lead to current leakage, herein the intercalated systems can be denoted as "neutral ion conductors" where both pristine SiO₂ and SiO₂ filled with NH₃ are insulating. Similar ferroelectricity may exist in various SiO₂ crystalline polymorphs, its amorphous phase, and other porous structures intercalated by NH₃. Our findings may not only resolve the bottleneck issues for the compatibility of ferroelectrics and silicon but also develop unconventional mechanisms of ferroelectricity.