Super High-<i>k</i> Unit-Cell-Thick α-CaCr₂O₄ Crystals

High-dielectric-constant (high-<i>k</i>) insulators are indispensable components to integrate semiconductors into metal-oxide-semiconductor field-effect transistors with sub-10 nm channel length, where the equivalent oxide thickness (EOT) of high-<i>k</i> insulator needs to be decreased to subnanometer scale. The traditional insulators, including Al₂O₃, SiO₂, and HfO₂, fit well with the existing silicon industry but suffer from serious degeneration of insulating properties, such as large leakage currents caused by high-density borders and interface traps, when their thicknesses are reduced to a few nanometers. Here, we synthesize a high-quality nonlayered ultrathin α-CaCr₂O₄ crystal down to unit-cell thickness (∼1.2 nm) by an elements slow-supply chemical vapor deposition (CVD) method. The unit-cell-thick α-CaCr₂O₄ crystals show a super high dielectric constant of 87.34, which is over 20 times higher than that of well-known layered insulator <i>h</i>-BN and corresponds to an EOT below 1 nm. Furthermore, it has a high breaking strength (39 GPa) and excellent stability. This strategy can also be used to fabricate other ultrathin ternary oxides, such as high-<i>k</i> ultrathin FeNb₂O₆ crystals, demonstrating the universality of the CVD method.