Memory Window Improvement in Ferroelectric Thin-Film Transistors with HfZrO2/AlN Bilayers Grown by Atomic Layer Deposition

This study demonstrates proximity-induced ferroelectric switching in a stacked ferroelectric film composed of fluorite-structured HfZrO2 (HZO) and wurtzite-structured AlN, which are grown by thermal atomic layer deposition (ALD) and plasma-enhanced ALD (PEALD), respectively. The stacked film shows remanent polarization close to that of HZO but with a much higher coercive field, desirable for enhancing the memory window of ferroelectric thin-film transistors (FeTFTs) for storage applications. The FeTFT with a 5 nm-thick amorphous In–Ga–Zn–O (a-IGZO) channel layer, also grown by ALD, shows a memory window of ∼6.1 V, and data retention over 10 years. It also shows stable endurance cycles of ∼104. These enhancements over the single-layer HZO or stacked HZO/Al2O3 films, which show a minimal memory window of ∼0.3 V and ∼−0.3 V, respectively, are ascribed to the proximity ferroelectric effects between the soft ferroelectric HZO and hard ferroelectric AlN. These results, together with the conformal deposition of the HZO/AlN stack in high-aspect-ratio structures, demonstrate the feasibility of extending the ALD/PEALD-based Prox-FE materials platform toward three-dimensional (3D) process integration.