The potential of chemical bonding to design crystallization and vitrification kinetics

Abstract

Controlling a state of material between its crystalline and glassy phase has fostered many real-world applications. Nevertheless, design rules for crystallization and vitrification kinetics still lack predictive power. Here, we identify stoichiometry trends for these processes in phase change materials, i.e. along the GeTe-GeSe, GeTe-SnTe, and GeTe-Sb2Te3 pseudo-binary lines employing a pump-probe laser setup and calorimetry. We discover a clear stoichiometry dependence of crystallization speed along a line connecting regions characterized by two fundamental bonding types, metallic and covalent bonding. Increasing covalency slows down crystallization by six orders of magnitude and promotes vitrification. The stoichiometry dependence is correlated with material properties, such as the optical properties of the crystalline phase and a bond indicator, the number of electrons shared between adjacent atoms. A quantum-chemical map explains these trends and provides a blueprint to design crystallization kinetics.

Fragile‐to‐Strong Transition in Phase‐Change Material Ge3Sb6Te5

“…This value is smaller than that of Ge 2 Sb 2 Te 5 ; 80 nm and GeTe; 600 ns reported in Ref. [11] and hence indicates advantageous crystallization kinetics. It must be noted, that these values include the period of crystal nucleation, namely the time until the first grain forms.…”

“…We performed additional minimum crystallization time measurements similar to Ref. [11] which indicates the minimum time to crystallize a completely amorphous PCM film at optimal conditions. This measurement resulted in a value of ≈50 ns, see Figure S6, Supporting Information.…”

“…The distinct physical properties of the amorphous and crystalline solid phase enable various schemes for encoding information in a robust nonvolatile form, [4] while the rapid increase in viscosity with temperature of the undercooled liquid (UCL) phase enables fast crystallization. [5] Among these three phases the crystalline state has received the most attention and involves a chemical bonding type (namely metavalent bonding [6][7][8][9][10][11] ) associated with a unique set of physical properties distinct from its metallic and covalent counterparts. Its high reflectivity and conductivity provide the necessary property contrast to the amorphous state for information storage and processing.…”

Tailoring Crystallization Kinetics of Chalcogenides for Photonic Applications

“…The results of those alloys that can be crystallized and form a single crystalline phase are presented in such as GeTe or SnTe, which crystallized systematically faster than covalent bonded compounds such as GeSe. [25] This stoichiometry dependence can even be presented more quantitatively by using the quantum mechanical map of bonding introduced above. For this purpose, the coordinates of this map, the electrons shared and electrons transferred, are calculated as described in.…”

“…It also increases the ease of glass formation and hence the stability of the amorphous, i.e., glassy state characterized by T ro , the reduced onset temperature of glass transition, from 0.47 (GeTe) to 0.58 (GeSe). [25] The key challenge for metavalently bonded materials is thus the realization of pronounced optical contrast for photon energies of 1 eV and above, to enable a significant optical response in the visible range. Since the crystalline chalcogenides in the green region of Figure 1 all have rather small band gaps, typically below 1 eV, it is difficult to realize photonic switches, which can also be employed in this range.…”