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Erschienen in:
01.10.2007
Local bonding arrangements in amorphous Ge2Sb2Te5: the importance of Ge and Te bonding
Erschienen in: Journal of Materials Science: Materials in Electronics | Sonderheft 1/2007
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Abstract
Extended X-ray absorption fine structure (EXAFS) studies of as-deposited Ge2Sb2Te5 films yield Ge K edge spectra essentially the same as previously reported; however, these earlier studies assumed only Te nearest neighbors, but the comprehensive analysis of this paper indicates significant concentrations of both Ge–Ge and Ge–Te bonds. Average bond coordination and number of valence bonding constraints/atom have been determined from an extension of bond-constraint theory that includes broken bond-bending constraints for Ge–Ge bonds, and optical memory alloys on the Sb2Te3–GeTe tie-line ∼3.05 ± 0.05 bonding constraints/atom, consistent with formation of an intermediate phase. The fraction of threefold or over-coordinated Te increases from 7.1 to 25% along this tie-line, and the amorphous to crystalline optical transmissivity transition occurs over a narrow temperature range, <10 °C. Over-coordination and Te leads to formation of positively charged defects, which must be balanced by under-coordinated of negatively charged Sb or Ge defect bonding arrangements.