Ge Containing Compounds

As seen in Sect. 18.2.1, the structure of Sb2Tc3 and Sb2Te, which are building blocks for most of current PCMs, is intrinsically close to that of their crystalline phase. The other building block for GST PCMs is GeTe, and its amorphous structure, as well as that of GST amorphous materials has been shown to be more complex. 

It is actually possible to use DFT calculations to compute XANES for amorphous and crystalline PCMs. To compute the XANES spectra, we use a new implementation by one of the present authors (V.L.), employing the PAW formalism [44] (as implemented in the VASP code [45, 46]) and including a self-consistent treatment of the core hole effects on the unoccupied states. 

The XANES spectrum is calculated using Fermi s golden rule as 

For the C phase, we considered a series of possible crystal structures for GST124, including both ordered and disordered RS structures. In Fig. 18.9 we show the computed Ge-X edge XANES signatures for these structures, compared to those for a-GeTe and a spinel structure with the GST124 composition. 

Spectra beyond the WL region can be related to the neglect of shake-off excitations, reported by Lucovsky et al. [48], that are not included in our DFT single excitation 

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Interest in the fundamental processes involved in formation of amorphous Ge-containing compounds by chemical vapor deposition processes assisted by radiolysis has been responsible for a number of studies dedicated to the reactions of Ge ions with O2, NH3, PH3, SiFLj and simple alkanes and alkenes131,145-150. 

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