Germanium: the good, the bad, and the ugly, howd-orbitalscan ruin materials or create new opportunities

We examine from a high-level perspective the chemical basis that has limited use of Ge in modern technology to niche applications. From the measured optical properties [n,k] of c-Ge, GeTe, and Ge2Sb2Te5 (GST-225), we examine their sum-rule and superconvergence (SC) properties (to effectively measure the total Ne/atom) and thereby demonstrate the significance of d-orbitals. We find that while the crystalline form (fcc) of GST-225 only makes use of s- and p-orbitals, the amorphous form has a greater overall Ne/atom ratio, which can only occur if d-orbitals also participate. Similar results are found for GeTe. We interpret these Ne/atom densities by a simple valence model, which places rather strict limitations on the type and concentrations of different local atomic environments. Moreover, we show that since the SC is closely related to electric susceptibility, quantitative changes in the SC between the crystalline and amorphous forms imply removal (suppression) of Te’s lone pair orbitals (existing in the crystalline form) by the d-orbitals (existing in the amorphous form). With this, it now appears that we have a more comprehensive understanding of how d-orbitals play important roles in the switching behaviors in both optical and electronic devices. We propose that this understanding can eventually be extended to the ultimate “single-atom” man-made electronic devices.