Origins of midgap states in Te-based Ovonic threshold switch materials.

The non-linear threshold-type current-voltage behavior that characterizes selector devices for three-dimensional (3D) crossbar-type nonvolatile memory devices relies upon a phenomenon known as Ovonic threshold switching (OTS). Because current practical OTS materials are based on toxic elements, such as Se and As, Te-based OTS materials are expected to offer a more environmentally friendly option. However, the electronic structure that determines the OTS behavior of Te-based OTS materials is not well understood. In this paper, the electronic structure of amorphous Si 0.29 Te 0.71 , has been explored using hard X-ray photoelectron spectroscopy (HAXPES) in conjunction with density functional theory (DFT) calculations. The HAXPES results show that the Si 0.29 Te 0.71 amorphous network of the simulated amorphous structure is based upon Te-Te, Te-Si, and Si-Si bonding. DFT calculations revealed that Si3p and Te5p states contribute to bonding, whereas occupied non-bonding Te5p states form the top of the valence state. A projected local density of states analysis shows that the Si site forms conduction-tail states, whereas the Te site forms both conduction- and valence-tail states. Furthermore, Te-Te dimers contribute significantly to the midgap states that characterize the OTS behavior. Finally, the valence-tail state extension within the mobility gap of Si 0.29 Te 0.71 was experimentally demonstrated. [Display omitted] [ABSTRACT FROM AUTHOR]