Phase control in polymorphic transformation-based nonvolatile memory devices for reliable data reading in X-point structures.

The MnTe polymorph, which exhibits reversible and nonvolatile phase transformations between NiAs- and wurtzite-type structures, is a promising candidate for next-generation phase-change memory technologies. These melting-free polymorphic transformations realize low-energy and high-speed operation. However, the wurtzite-type β' phase-the metastable, highly resistive state in MnTe-based memory devices -remains fundamentally underexplored. In this work, we investigate the effects of the resistivity (ρβ') and volume fraction (fβ') of the metastable β' phase on the memory performance. A percolation model relating fβ' to the resistive contrast (ΔR) suggests that ρβ' ≈ 500 Ω cm is a plausible value. Based on this model, we fabricated several memory cells with varying ΔR to evaluate their threshold characteristics from the β' to the α phase. The I-V curves reveal that the threshold voltage (Vth) increases with increasing ΔR. These results demonstrate that threshold parameters can be systematically modulated by controlling fβ' and the MnTe layer thickness. This tunability is crucial for bridging the Vth gap between MnTe-based phase-change materials and Ovonic threshold switch materials, offering valuable insights into the design of polymorphic transformation-based memory devices compatible with three-dimensional X-point architectures.