Penn gap rule in phase-change memory materials: No clear evidence for resonance bonds

Although a proposal of resonance bonds in crystalline phase-change materials based on the GeSbTe system has been provided, we do not find any clear evidence in favor of the proposal. The ellipsometric study demonstrates that a change in the high frequency dielectric constant ε∞ between the amorphous and crystalline phases is only scaled by the average bandgap (the Penn gap rule). Even for a pure antimony film, regarded as a prototype resonance bonding material, ε∞ was found to follow the Penn gap rule. Experimentally, we did not find any evidence of a significant change in the optical transition matrix element during the phase change, which is necessary to support the idea of resonance bonds.