Education:

1. Amorphous Semiconductors for Phase-Change MemoriesP. Craig Taylor, Colorado School of Mines, DMR 0702351 Semiconductors that can be switched between their crystalline and amorphous phases by a voltage pulse are of intense interest due to their potential use in data storage technologies. The most important composition is Ge2Sb2Te5 for which only the basic structure of the amorphous phase is known. We have utilized nuclear magnetic resonance experiments on Sb and Te to confirm that the Ge, Sb, and Te atoms have 4, 3, and 2 1/2 nearest neighbors, respectively. In addition we have discovered that two distinct Sb and Te sites exist - one axially symmetric and one highly asymmetric Sb site and one two-fold and one three-fold coordinated Te site. This structure is very different from crystalline Ge-Sb-Te alloys, where all three elements have six nearest neighbors, but similar to crystalline Sb2Se3 where the Sb is 3-fold and the Se 2-fold coordinated. Understanding the structure of the amorphous phase is the first step toward understanding the phase-change mechanism. Te Ge Sb Te Sb Ge Sb Te Model of the structure of amorphous Ge2Sb2Te6. The green, yellow and blue atoms are Ge, Sb, and Te, respectively. [courtesy of A. H. Edwards, AFRL Kirtland]

2. Amorphous Semiconductorsfor Phase-Change MemoriesP. Craig Taylor, Colorado School of Mines, DMR 0702351 Societal Impact: Understanding the phase-change mechanism will lead to electrically switched memories using Ge2Sb2Te5 and other chalcogenide alloys. Education: Several undergraduate students, five graduate students, and one postdoctoral research associate contributed to this work. Three graduate students completed their PhD degrees this year, two of whom are women. Service: The principal investigator helped to organize and edit the proceedings of a symposium entitled “Phase Change Materials for Reconfigurable Electronics and Memories” Graduate students, David Bobela (left) Summer Montgomery (center), and Brian Simonds (right) preparing for a measurement of the optical properties of amorphous Ge2Sb2Te5.