Ovonic threshold switching induced local atomic displacements in amorphous Ge60Se40 & nbsp;film probed via in situ EXAFS under DC electric field

Abstract

In an effort to verify whether the Ovonic threshold switching in amorphous chalcogenide film is concomitant with any noticeable atomic rearrangements, we analyze EXAFS spectra of amorphous chalcogenide Ge60Se40 (at %) film measured in situ under DC electric field. The in situ EXAFS measurements are performed with specifically prepared specimens, in which 100-nm-thick amorphous Ge60Se40 layer is sandwiched between two metallic electrode layers, all the while a constant DC electric bias being applied. Our EXAFS analysis indicates that only Debye-Waller factor undergoes a noticeable change upon the Ovonic threshold switching, keeping both coordination number and interatomic distance unaltered. The changes in Debye-Waller factors of Ge-Se and Ge-Ge pairs appear to be reversible during the ON/OFF cycle; however, heteropolar Ge-Se pair experiences a conspicuously greater change in its Debye-Waller factor than homopolar Ge-Ge counterpart. Increase of thermal disorder due to Joule heating is insufficient to support this observation, which instead is caused by a preferential increase of static disorder associated with heteropolar Ge-Se bonds in the ON state. The heteropolar Ge-Se pair is supposed to feel the electrical wind force (momentum transfer by conducting electrons) as well as the Coulombic force more strongly than the homopolar Ge-Ge pair.