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 (ρ_β') and volume fraction (f_β') of the metastable β' phase on the memory performance. A percolation model relating f_β' to the resistive contrast (ΔR) suggests that ρ_β' ≈ 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 (V_th) increases with increasing ΔR. These results demonstrate that threshold parameters can be systematically modulated by controlling f_β' and the MnTe layer thickness. This tunability is crucial for bridging the V_th 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.