EXAFS study of local order in the amorphous chalcogenide semiconductor Ge2Sb2Te5

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 Ge2Sb2Te5 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 Ge2Sb2Te5 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 Ge2Sb2Te5. The data show that Ge2Sb2Te5 may best be viewed as a random array of Ge2Te3 and Sb2Te3 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.