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The as-deposited (AD) amorphous structure of the prototype phase change material GeSbTe (GST-225) has been studied by density functional calculations for a 648-atom sample generated by computer-aided deposition at 300 K. The AD sample differs from a melt-quenched (MQ) sample in essential ways: (1) Ge atoms are predominantly tetrahedrally coordinated, and (2) homopolar and Ge-Sb bonds are more common and reduce the number of squares (, Sb; ), the characteristic building blocks of the material. The first observation resolves the contradiction between measured (EXAFS) and calculated Ge-Te bond lengths, and the latter explains the very different crystallization speeds. Sb and Te have higher chemical coordination than suggested by the “8- rule” of covalent networks ( is the number of valence electrons). The EXAFS signal calculated for AD agrees much better with …