Investigation of the role of in-plane stress behavior on ferroelectric properties of scaled-up hafnium zirconium oxide superlattices

Hafnium zirconium oxide (HZO) based ferroelectric (FE) devices are promising candidates for next-generation low-power memory applications. Recently, there has been a growing interest in implementing HZO-based superlattice structures to improve FE response, resonator applications, and for physical analysis needed for in-depth understanding of scaled-down HZO, as it is challenging to characterize sub-10 nanometer-thin films. However, it has not been explored whether these superlattices retain identical FE properties as they are scaled up. In this work, we have looked into the role of in-plane stress on FE properties of superlattices, by designing superlattices that consist of different numbers of alternating layers of atomic layer deposited 9 nm thick lanthanum doped HZO (La:HZO) and 0.5 nm thick Al₂O₃ interlayers, sandwiched between a top and bottom electrode (TE and BE). Here we show that after annealing the stacks, for a given BE/TE, in-plane stress becomes independent of the number of HZO layer repeats in the superlattice, thereby retaining a similar FE/non-FE phase composition, which results in an identical FE response. Discrepancies in FE properties between the single-layer HZO stack and the superlattices are attributed to differences in phase composition, emphasizing the impact of in-plane stress and interfaces. Therefore, this suggests that it is possible to scale up these superlattices while preserving identical FE properties by tuning the in-plane stress of the HZO layers and their interfaces, making them suitable for characterization purposes and applications that require thicker FE HZO films.