EXAFS study of local order in the amorphous chalcogenide semiconductor Ge 2Sb 2Te 5

Abstract

Studies of amorphous ( a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently switching of the a-chalcogenide Ge 2Sb 2Te 5 has been applied quite successfully to DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching provides researchers today with an active arena of technological as well as fundamental study. On the theoretical front, bond constraint theory — or BCT — provides a powerful framework for understanding the structure and properties of a-materials. Applications of BCT to switching in Ge 2Sb 2Te 5 holds the promise of finding the best composition suited for switching applications. This work presents EXAFS data that describe local bonding configurations in as-deposited Ge 2Sb 2Te 5. The data show that Ge 2Sb 2Te 5 may best be viewed as a random array of Ge 2Te 3 and Sb 2Te 3 structural units imbedded in a tissue of a-Te, 17% of which is over coordinated. In addition, a valence alternation pair defect is introduced to the model to satisfy charge conservation constraints.