Tetrahedral Complexity in Amorphous Networks: A Possible Clue for the Unique Properties of Phase‐Change Materials

Abstract

A typical binary amorphous telluride GeTe2 is investigated from first‐principles molecular dynamics simulations. After a comparison with chemical analogs from neutron or X‐ray diffraction experiments, such as GeO2 or GeSe2, the structure of this material is studied by examining real and reciprocal space properties. It is found that the base geometrical motifs of the germanium atom can be either in tetrahedral or in defected coordinations involving pyramidal units. A review of previous results for other compositions reveals that such binary Ge tellurides contain soft tetrahedra, at variance with lighter chalcogenides, such as GeS and GeSe, and are characterized by an increased angular bending motion (typically 20°) as compared with, e.g., GeS (5°). In addition, for amorphous Ge‐rich materials, GeTe2 and GeTe, a secondary tetrahedral geometry appears, related to the presence of GeGe bonds, having a larger mean angle of about 125°. These typical features not only relate to characteristics observed from scattering experiments but may also be a crucial feature for the understanding of the phase‐change phenomena.

Evolution of Short‐Range Order of Amorphous GeTe Upon Structural Relaxation Obtained by TEM Diffractometry and RMC Methods

“…An increased occurrence of these intermediate geometries is accompanied by a decreased extent of resistance drift. [67] Thus, the increase in intermediate (PD-like) geometries observed here is in line with the observation of resistance drift observed in experiments. [11,12] An additional observation is that the main contribution is best described by an effective potential minimum at 𝜃 1 ≈ 103.2°t hat shifts toward larger angles upon structural relaxation (cf.…”

“…Further analysis of the Ge motifs is accomplished by computing the Te‐Ge‐Te bond angle distribution (BAD) for each RMC simulation run (four per annealing state). To reveal minor shifts in the BADs of different annealing states each BAD can be fitted to a Boltzmann probability distribution based on effective bond angle potentials $$U_{\text{eff}}^{(i)}(\theta )$$ [ 66,67 ] : $$$$\begin{align} P(\cos \theta )&\propto \exp {\left(-U_{\text{eff}}^{(i)}(\theta )/k_{\text{B}}T\right)}\end{align}$$$$ $$$$\begin{align} U_{\text{eff}}^{(i)}(\theta )&=\dfrac{k_2^{(i)}}{2}{\left(\theta -\theta _i\right)}^2 \end{align}$$$$where the Boltzmann constant is denoted by k B and T is the temperature fixed at 300 K. θ i is the effective potential minimum and $$k_2^{(i)}$$ the corresponding rigidity parameter, which gives a measure for the tolerance of a bond angle to deviate from the potential minimum. Thus, a large rigidity suggests that the tolerance is small and most motifs in the systems possess angles close to the mean value θ i .…”

“…A larger stiffness $$k_2^{(\theta _1)}$$ can be associated with a lower drift coefficient, i. e ., a slower relaxation. [ 67 ] Thus, it is expected that upon resistance drift, which slows down exponentially with time, the stiffness $$k_2^{(\theta _1)}$$ must increase. This behavior of the stiffness is indeed what is observed here upon structural relaxation of amorphous GeTe.…”

“…Further analysis of the Ge motifs is accomplished by computing the Te-Ge-Te bond angle distribution (BAD) for each RMC simulation run (four per annealing state). To reveal minor shifts in the BADs of different annealing states each BAD can be fitted to a Boltzmann probability distribution based on effective bond angle potentials U (i) eff (𝜃) [66,67] :…”

Impact of structural short‐range order and tetrahedral germanium on electronic and dielectric properties in phase‐change materials

“…The equilibrated trajectories of similar compositions have led to an excellent agreement of measured structure functions from neutron and X-ray scattering in the liquid and amorphous state 25,28,29 (Figure 2). The same strategy has been employed for GST225, 5 and the resulting structure at 300 K used for the present purpose was obtained from four independent quenches, starting from equilibrated configurations analyzed previously in the liquid state, and, again, successfully compared to experiments.…”