Hf_0.5Zr_0.5O₂/Al₂O₃ Bilayer Ferroelectric Tunnel Junctions: Investigations of The Chemistry of Interfaces and Films by HAXPES

The discovery of ferroelectricity in hafnium dioxide-based ferroelectrics has revived the research on ferroelectric devices. In particular, the H_f0.5Zr_0.5O₂ (HZO) solid solution compound provides an opportunity for back-end-of-line integrable devices due to its low crystallization temperature (&lt; 450°C). Among the different ferroelectric-based devices, ferroelectric tunnel junction (FTJ) memories are well suited for emerging neuromorphic applications due to their low power consumption, non-volatile nature, and potential to achieve multiple resistance states through domain partial switching of domains. As the conventional metal-ferroelectric-metal FTJ stack requires an ultra-thin ferroelectric layer (1-3 nm), it is quite challenging to fabricate with a polycrystalline Hf_0.5Zr_0.5O₂ (HZO) layer. With a bilayer stack architecture based on HZO/Al₂O₃, a thicker HZO can be used (10 nm) and can still provide a high ON current as the tunneling occurs through the thin dielectric layer (~3 nm) [1]. The performance of such devices rely on the ability to stabilize a large switchable ferroelectric polarization in HZO, which is interfaced - on one side - with a dielectric and not a metal. The remanent polarization and the overall device performance also depends strongly on the nature of the bottom and top electrodes, on the presence of interfacial layers (whether intentional or not), and on the oxygen vacancies’ content and distribution.<br/>In this study, we discuss the role of processing conditions on the ferroelectric polarization and device performance of TiN / 10 nm HZO / 3 nm Al₂O₃ / W FTJs. The HZO is crystallized at 400[endif]--&gt;C for full CMOS back-end of-line compatibility. As observed in the electrical characterization, the annealing conditions of the ultrathin Al₂O₃ tunneling barrier plays a key role in the ability to stabilize and switch the polarization in the HZO layer. Hard X-ray photoelectron spectroscopy (HAXPES) was used to probe the bonds and chemistry of the different buried interfaces and layers in the stack and to quantify the oxygen vacancy content. Measurements at different synchrotron beam angles were performed to probe various depths of the HZO layer. The top Al₂O₃/W layers were investigated by combining laboratory XPS with low energy Argon ion sputtering. Complementary information was gained from scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). A strong difference in the oxygen vacancy profiles in HZO is found depending on the stack fabrication conditions. The dependence of the remnant polarization with the voltage pulse width points to the major role of charge traps in minimizing the depolarizing field. We will discuss the influence of the processing conditions on providing the needed traps for screening charges, which is essential in stabilizing the ferroelectric polarization in bilayer HZO-based FTJs.<br/><br/>[1] V. Deshpande <i>et al.</i>, Solid State Electronics 186, 108054 (2021).