Interfacial Charge-Injection-Induced Ferroelectric Phase Instability in Epitaxial Yttrium-Doped Hafnium Oxide Films

In hafnia-based ferroelectric capacitors, it is believed that the interface between hafnia and the electrode material can profoundly influence the phase composition of the hafnia films. Nevertheless, the exact physical origin of this matter remains elusive. Epitaxial hafnia films provide a suitable platform to study the ferroelectric-electrode interface effect. Here, by designing Y-doped HfO₂ (YHO) epitaxial capacitors with varied architectures, we demonstrate that the interfacial charge transfer plays a decisive role in dictating the phase structure of YHO, instead of the interface chemistry. X-ray absorption spectroscopy measurements reveal a more prominent charge transfer at the LaNiO₃ (LNO)/YHO interface compared to the La_2/3Sr_1/3MnO₃ (LSMO)/YHO interface, in line with their difference in work functions calculated by density functional theory. The excessive hole injection from LNO is suggested to destabilize the ferroelectric phase in YHO and suppresses the ferroelectricity. It is further found that the LNO capping layer mainly affects the phase composition in the near-surface region, manifested by a more evident suppression of polarization in thinner YHO films. By contrast, as a bottom electrode, LNO transfers excessive holes to the initial YHO seed layers, leading to a nonpolar-phase-dominated film with diminished polarization. Our results offer important insights into the engineering of ferroelectricity in hafnia-based thin films by interface design.