Combining Ferroelectric Nitrides and Topological Insulators for Ferroelectric Control of Spin-Charge Interconversion

Ferroelectric nitrides such as (Al, Sc)N possess exceptionally high polarisation and Curie temperatures. Additionally, they can be synthesised using techniques compatible with back-end-of-line (BEOL) integration, making them a promising class of ferroelectrics for microelectronic and CMOS-compatible applications¹. While most research has primarily focused on charge-based devices² (ferroelectric field-effect transistors, ferroelectric diodes, etc.), we aim to investigate how these remarkable properties can be exploited for spintronic applications. One potential avenue is the combination of ferroelectric materials with topological insulators, which exhibit appealing spin textures.
In this work, we explore the integration of (Al, Sc)N with BiSb, a topological insulator characterised by chiral spin textures in k-space and highly efficient spin-charge interconversion³. We hypothesise that by coupling these two materials, the spin-to-charge interconversion in BiSb can be modulated by the ferroelectric state of the nitride layer. This offers exciting possibilities for advancing spintronics-based devices.
We have optimised the epitaxial growth of (Al, Sc)N/BiSb heterostructures via sputtering at BEOL-compatible temperatures. The crystalline quality of these heterostructures was confirmed through x-ray diffraction and scanning transmission electron microscopy (STEM), validating their high-quality epitaxial nature. The ferroelectric properties of the (Al, Sc)N layer were assessed using ferroelectric switching measurements and piezoresponse force microscopy (PFM). Additionally, the spin-charge interconversion in the BiSb layer was characterised through terahertz emission spectroscopy.
This presentation will detail the growth, structural, and functional properties of these bilayer heterostructures. Our findings pave the way for the development of spintronic devices with the potential to reduce the growing power consumption of modern electronics while enabling nonvolatile memory and in-memory computing.

[1] S. Fichtner, et al., AlScN: A III-V semiconductor based ferroelectric, Journal of Applied Physics, 125, 11 (2019).
[2] K.-H. Kim, et al., Scalable CMOS back-end-of-line-compatible AlScN/two-dimensional channel ferroelectric field-effect transistors. Nature Nanotechnology 18, 9 (2023).
[3] E. Rongione, et al., Spin-momentum locking and ultrafast spin-charge conversion in ultrathin epitaxial Bi_1-xSb_x topological insulator. Advanced Science 10, 19 (2023).