Atomic and electronic origin of robust off-state insulation properties in Al-rich Al_xTe_y glass for ovonic threshold switching applications

Ovonic threshold switching (OTS) selectors play a critical role in suppressing the sneak-path current of three-dimensional crossbar integration circuits. Compared to conventional nonmetal-telluride OTS selectors, selectors based on AlxTey glass are found to have both satisfactory on-state current and selectivity. However, it is unclear why the Al-rich AlxTey glass-based OTS selectors have robust insulation properties for reducing the off-state current. This work reveals the structure-property correlations of amorphous AlxTey at the atomic scale by first-principles calculations. It is found that the stoichiometric Al2Te3 glass tends to have a clean bandgap owing to the covalent and dative bonds formed by non-equivalent sp(3)-hybridized Al orbitals and the lone-pair electrons of Te. Unexpectedly, for Al-rich AlxTey glass (Al2.21Te2.79), the Al-Al bonds formed by redundant Al-atoms have an integrated crystal orbital bond index (ICOBI) of 0.8-0.9, which is much larger than that of Al-Al bonds in pure metals (0.227), indicating they are covalent. It is the covalent Al-Al bonds that ensure the robust insulation characteristics of Al-rich AlxTey glass, while the Te-Te interaction in the Al-poor AlxTey glass (Al1.79Te3.21) produces midgap states, thereby reducing the insulativity. The presented atomic and electronic pictures here will provide useful theoretical insights for designing OTS selectors with improved performances.