Effect of Polarization Reversal in Ferroelectric TiN/Hf_0.5Zr_0.5O₂/TiN Devices on Electronic Conditions at Interfaces...

Because of their compatibility with modern Si-based technology, HfO₂-based ferroelectric films have recently attracted attention as strong candidates for applications in memory devices, in particular, ferroelectric field-effect transistors or ferroelectric tunnel junctions. A key property defining the functionality of these devices is the polarization dependent change of the electronic band alignment at the metal/ferroelectric interface. Here, we report on the effect of polarization reversal in functional ferroelectric TiN/Hf_0.5Zr_0.5O₂/TiN capacitors on the potential distribution across the stack and the electronic band line-up at the interfaces studied in operando by hard X-ray photoemission spectroscopy. By tracking changes in the position of Hf_0.5Zr_0.5O₂ core-level lines with respect to those of the TiN electrode in both short- and open-circuit configurations following in situ polarization reversal, we derive the conduction band offset to be 0.7 (1.0) eV at the top and 1.7 (1.0) eV at the bottom interfaces for polarization, pointing up (down), respectively. Energy dispersive X-ray spectroscopy profiling of the sample cross-section in combination with the laboratory X-ray photoelectron spectroscopy reveal the presence of a TiO_x/TiON layer at  both interfaces. The observed asymmetry in the band line-up changes in the TiN/Hf_0.5Zr_0.5O₂/TiN memory stack is explained by different origin of these oxidized layers and effective pinning of polarization at the top interface. The described methodology and first experimental results are useful for the optimization of HfO₂-based ferroelectric memory devices under development.