d- and f-Electron Type Chalcogenides Realizing Unique Resistive Change Behavior

The unique electrical properties of chalcogenides make them functional in semiconductor devices. Chalcogenides, which exhibit reversible phase transitions between amorphous and crystalline phases, are known as phase change materials (PCMs). These PCMs are utilized as recording layers in non-volatile memory and for synaptic functions in neuromorphic computing. Most PCMs are developed from combinations of chalcogen and p-block elements (groups 14-16). The valence electrons in p-orbitals are considered crucial for material design, while d- and f-orbital electrons are traditionally regarded less significant. Although there is potential within the 3-12 group elements, material design has primarily focused on the p-block. To overcome this limitation, the authors propose d- and f-electron type chalcogenides, which exhibit superior or unique properties compared to conventional p-electron chalcogenides.<br/>Typically, PCMs exhibit phase transitions between high-resistance amorphous states and low-resistance crystalline states. The large resistive contrast upon phase transition enables the non-volatile recording function of PCMs. Ge-Sb-Te (GST) is the most developed PCM, offering high resistance contrast and fast phase change speed, leading to reliable operation. However, the high energy required for amorphization in GST needs improvement. Increasing the resistivity of the crystalline phase is an effective strategy to reduce amorphization energy, but achieving a substantial increase in resistivity in p-electron PCMs is challenging.<br/>Cr₂Ge₂Te₆ (CrGT) has been developed as a d-electron-type PCM and demonstrates an inverse resistance change with high-resistive crystalline and low-resistive amorphous phases. Due to the high resistivity of the crystalline phase, the amorphization energy of CrGT-based memory devices is significantly lower, approximately 1/300th of that for GST-based devices [1]. The authors have revealed the unique phase change mechanism of CrGT [2,3] and demonstrated the superior device properties of CrGT-based devices [4,5]. Furthermore, recent studies highlight the potential of f-electron-type chalcogenide, such as SmTe [6]. SmTe films exhibit over four orders of magnitude in resistive contrast between as-deposited and annealed states. Despite the large resistive contrast, no structural transition like the amorphous-to-crystalline transition occurs. This isomorphic transition is driven by the valence state change of Sm, suggesting a new principle for non-volatile recording.<br/>The exploration of d- and f-electron-type chalcogenides opens new avenues in the designing new chalcogenides. By leveraging the high resistivity of the crystalline phase in d-electron chalcogenides, such as CrGT, it is possible to significantly reduce the energy required for phase transitions. Additionally, the discovery of isomorphic transitions in f-electron chalcogenides like SmTe, driven by valence state changes, introduces novel mechanisms for data storage. These findings pave the way for high-performance non-volatile recording applications, advancing the field of semiconductor technology.<br/><br/>[1] S. Hatayama et al. ACS Appl. Mater. Interfaces 10, 2725 (2018).<br/>[2] S. Hatayama et al. ACS Appl. Mater. Interfaces 11, 43320 (2019).<br/>[3] S. Hatayama et al. Phys. Rev. Mater. 5, 085601 (2021).<br/>[4] S. Hatayama et al. Mater. Sci. Semicond. Processing 133, 105961 (2021).<br/>[5] S. Hatayama et al. ACS Appl. Mater. Interfaces 14, 44604 (2022).<br/>[6] S. Hatayama et al. ACS Nano 18, 2972 (2024).