AVS 53rd International Symposium
    Thin Film Tuesday Sessions
       Session TF-TuM

Paper TF-TuM10
Local Bonding Arrangements in Ge-Sb-Te Alloys for Optical Memory Applications: Correlations between Device Performance and Te and Sb Bonding

Tuesday, November 14, 2006, 11:00 am, Room 2022

Session: Materials for Flexible Substrates, Displays, and Optoelectronics
Presenter: D.A. Baker, NC State University
Authors: D.A. Baker, NC State University
M.A. Paesler, NC State University
G. Lucovsky, NC State University
P.C. Taylor, Colorado School of Mines
Correspondent: Click to Email
EXAFS studies of as-deposited Ge@sub 2@Sb@sub 2@Te@sub 5@ films yield Ge K@sub 1@ spectra essentially the same as previously reported. The earlier studies assumed only Te nearest neighbors, but the more comprehensive analysis of this paper indicates significant concentrations of both Ge-Ge and Ge-Te bonds. Bond-lengths and coordinations determined from EXAFS, combined with relative bond enthalpies yield the following molecular structure: G@sub 2@Sb@sub 2@Te@sub 5@ = Ge@sub 2@Te@sub 3@ + Sb@sub 2@Te@sub 3@ with 17% of the Te-atoms 3-, rather than 2-fold coordinated. The average bond coordination, , and number of valence bonding constraints/atom, , have been determined from an extension of bond-constraint theory (BCT) that includes broken bond-bending constraints for Ge-Ge bonds. This paper addresses additional compositions to better understand the role of broken bond-bending constraints for Sb-Sb bonds that are present in alloys with Sb@sub 2@Te@sub 2@ molecular structures. Broken constraints reduce for Ge from 7 to 4.33, and Ge@sub 2@Sb@sub 2@Te@sub 5@, and optical memory alloys on the Sb@sub 2@Te@sub 3@-GeTe tie-line have ideal values of ~3.05±0.05. Similar decreases in occur for Sb-Sb bonds, and are important in identifying network idealicity in other GST alloys. The fraction of 3-fold coordinated Te increases from 7.1 to 25% along the Sb@sub 2@Te@sub 3@-GeTe tie-line, and the amorphous to crystalline optical transmittivity transition occurs over a narrow temperature range, <10°C. The transmittivity changes scale linearly with the fraction of 3-fold Te. This paper provides a basis for understanding optical transitions in Sb-rich alloys along a different tie-line between Ge@sub 2@Sb@sub 2@Te@sub 4@ and Sb, where transmittivity changes occurs over a wider temperature range >30°C, and with a different bond scaling relationship.