Electronic and optical properties of phase change alloys studied with ab initio methods

In this work the correlation between structural, electronic and optical properties of so-called phase-change materials is studied. Employing methods of computational physics, in particular density functional theory and many-body perturbation theory this work reveals the correlation between the change of the local atomic structure upon amorphization and the change of the electronic properties. In the phase change alloy Ge1Sb2Te4 the germanium atoms, which are octahedrally coordinated in the crystalline phase, switch to tetrahedrally coordinated positions in the amorphous phase. Calculations with density functional theory show, that such a local arrangement is energetically favorable while it correctly reproduces the experimentally observed density change upon amorphization. This structural change leads to pronounced changes in the electronic levels. In particular electrons of the tellurium atoms sharply decrease in energy. While they significantly contribute to the density of states at the Fermi energy in the crystalline phase, they are well below the Fermi level in the amorphous state. This decrease in energy results in an opening of the band gap in the amorphous state. This is in contrast to conventional semiconductors, which exhibit a reduction of the band gap energy in the amorphous phase. This widening of the band gap in the amorphous phase is of great importance for the future application of phase change materials in electronic data storage. Besides the electronic properties the optical properties are of great importance for phase change materials. The large difference of the optical absorption in the amorphous and the crystalline phase, which is not observed in conventional covalent semiconductors such as Si or GaAs, is not yet understood. In order to study this optical contrast between the two structural states, calculations of the optical absorption of GeTe and Ge1Sb2Te4 within time-dependent density functional theory and many-body perturbation theory have been performed within this work. The results reveal the effect of the change in structural and electronic properties upon amorphization on the optical properties. The optical absorption becomes broader and flatter in the amorphous state, which is confirmed by spectroscopic measurements using Fourier-transform infrared spectroscopy and ellipsometry. Thus the calculations reveal for the first time the correlation between the local structure and the optical absorption. Finally a detailed analysis of the calculated data leads to an explanation for the unusual change in the absorption upon amorphization. Usually it is assumed, that the amorphization of covalent semiconductors leads to the creation of defect states in the gap and thus to a smearing of the electronic states which in turn results in a moderate change of the optical properties. The calculations presented in this work show that in GeTe as well as in Ge1Sb2Te4 the optical contrast cannot solely be explained by this effect. In both alloys the matrix elements of the optical transitions significantly change upon amorphization, providing an important contribution to the large optical contrast. The fact, that the strength of the matrix elements and of their change upon amorphization is different for the two alloys, indicates that this important property of phase change materials can be adjusted and controlled by a systematic selection of the stoichiometry. Thus this result represents an important contribution to a systematic material optimization of phase change alloys