Impact of Local Atomic Arrangements on Ovonic Threshold Switching of Amorphous Ge-As-Se Thin Films

Amorphous Ge-As-Se film features a good ovonic threshold switching (OTS) performance. However, both endurance of OTS operation and crystallization temperature become dramatically deteriorated at specific compositions. We employ EXAFS technique to elucidate local atomic environments in cosputter-deposited amorphous (Ge50Se50)x-(Ge30As70)1-x films. At low Se contents, Se atoms are preferentially bonded to Ge atoms, and therefore Ge-Se fraction is conspicuously higher than As-Se counterpart. A covalent network structure typical of amorphous chalcogenides is supposed to form. At high Se contents, however, As-Se pair starts to abruptly increase at the expense of As-Ge fraction. Now verified in these high-Se compositions is appearance of molecular units composed of As and Se atoms, i.e., As4Se4 and/or As4Se3. The sudden deterioration is attributed to presence of the molecular units which lower connectivity of the covalent network via introducing inhomogeneity in a medium-range scale. The present finding exemplifies impact of local atomic arrangements on OTS phenomenon.