Amorphous phase of GeTe-based phase-change memory alloys: Polyvalency of GeTe bonding and polyamorphism

The structure of the amorphous phase of GeTe-based phase-change materials is discussed. Comparison of the Ge(4):Te(2) and Ge(3):Te(3) configurations present in the amorphous phase suggests that the former is locally more stable while the latter can lower its energy due to ‘resonance’ interactions in structures within more extended order. We further demonstrate that polyvalency of the GeTe bonds can lead to the formation of negative-U defects accounting for the high resistivity of the amorphous phase. Finally, polyamorphysm of the amorphous phase of Ge2Sb2Te5 is discussed.