Multi-layer graphene induced orthorhombic zirconia stabilized by ZrN interlayer for superior heat transfer and electrocaloric cooling

A multi-layer graphene (MLG) nanosheet with extremely high thermal conductivity was implemented as the top electrode (TE) of ZrO₂ capacitors and their ferroelectric properties were stabilized by inserting a ZrN interlayer (IL) at the MLG and ZrO₂ interface to achieve superior electrocaloric characteristics for future in-chip cooling. By replacing the traditional TiN TE with an MLG nanosheet, a transition from tetragonal to orthorhombic ZrO₂ thin films was realized, contributing to the modified characteristics from antiferroelectricity to ferroelectricity. The phase transition can be ascribed to the strain-induced lattice transformation as confirmed by grazing-incidence X-ray diffraction, high-angle annular bright-field and annular dark-field micrographs, and nano-beam electron diffraction. Hence, a high remnant polarization of 14.8 µC/cm² and an excellent electrocaloric temperature change (ΔT) of 22.3 K at 298 K were obtained for the ferroelectric ZrO₂ capacitors with a high thermal conductivity of the MLG TE and superior phase stabilization in crystallinity of the ZrN IL with nine atomic layer deposition cycles, promising to serve as a cooling unit in monolithic high-density integrated circuits (ICs).