Exploring Cooperative Effects of Multi-Element Substitution in AlN

Novel ferroelectric materials with improved properties such as stability and defect-tolerance are key to enabling more energy-efficient computing. Cation-substituted AlN-based materials with wurtzite-derived structures have a range of exceptional electronic, electromechanical, and dielectric properties, and Al_1-xSc_xN was recently found to be a good ferroelectric.[1] In addition, recent work has showed that many other metals can be successfully incorporated into AlN, opening the possibility of adding additional functionalities. Motivated by the desire to add magnetic or light emission properties to AlN, we have explored the possibility of substituting AlN with rare earth cations. For example, Al_1-xGd_xN can be stabilized in the wurtzite structure up to x ≈ 0.25,[2] and Al_1-xGd_xN and Al_1-xGd_xN can incorporate at least up to x ≈ 0.2.[3] Co-substitution of multiple cations into AlN may therefore be possible and yield intriguing multifunctional materials.
To investigate this possibility, we employ a joint computational and experimental approach to explore the quaternary phase diagram of (Al,Sc,Gd)N. We use thermodynamic stability calculations across this pseudo-ternary heterogeneous alloy space as a function of effective growth temperature to map the phase diagram. To demonstrate this experimentally, we perform a high throughput exploration of this phase space using combinatorial radio-frequency sputtering combined with spatially-resolved characterization. Using this computationally-guided approach, we explore the possible cooperative effects of co-substitution upon the ferroelectric properties of (Al,Sc,Gd)N. This high-throughput approach allows us to begin to understand the interplay between composition, structure, and properties in a complex phase space, which is crucial for designing and synthesizing novel functional materials.

[1] S. Fichtner et al., J. Appl. Phys. 125, 114103 (2019).
[2] R.W. Smaha et al., Chemistry of Materials 34, 10639 (2022).
[3] B. Paudel et al., arXiv:2408.07848 (2024).