Mechanical stresses upon crystallization in phase change materials

Abstract

Crystallization processes in different Te alloys, employed in phase change materials for optical data storage, have been investigated by in situ mechanical stress measurements. Upon crystallization a considerable stress buildup is observed, which scales with the volume change upon crystallization. Nevertheless the observed stress change only corresponds to approximately 9% of the stress estimated for a purely elastic transformation. Further evidence of stress relief phenomena comes from the temperature dependence of the stress in the crystalline and amorphous states. Ultrathin dielectric layers have a profound influence on the crystallization process as evidenced by simultaneous optical reflectance and mechanical stress measurements. This observation can be explained by heterogeneous nucleation of crystallites at the interface between the dielectric layer and the phase change film.

Atomistic origins of the phase transition mechanism in Ge2Sb2Te5

“…Using this procedure, we will be able to take the volume changes in amorphization as a fingerprint in which to compare our multiple configurations and potential pathways against experimental results. 10,36,62 First, we compared the a-GST structures generated using cubic and hexagonal unit cells from high temperature MD simulations. The pair correlation functions for all pairs of atoms ͑Ge-Te, Ge-Sb, Sb-Te, Ge-Ge, Sb-Sb, and Te-Te͒ were obtained by averaging the pair correlation functions over about ten structures for both unit cells.…”

“…61 Ge-Ge and Ge-Sb bonds are found in those motifs, which is assumed to be due to disorder effects, since they are not present in the crystalline phases. Furthermore, a-GST shows volume expansion of 6%-7% compared with the m-GST phase, 10,36,62 and it has a higher energy ͑28-40 meV/at.͒ with respect to m-GST. 63 Based on EXAFS studies and Ge coordination change, Kolobov et al 54 suggested that the m-GST to a-GST transition is related to an umbrella flip of Ge atoms from octahedral to tetrahedral sites without the rupture of strong covalent bonds.…”

Reversing the Resistivity Contrast in the Phase‐Change Memory Material GeSb2Te4 Using High Pressure

“…The understanding of atomic and electronic behaviors of PCMs under pressure is important for memory devices, which work under fairly high stresses due to the mismatch of crystalline and amorphous densities. [31] Moreover, our work offers a new method to engineer the band structure by tuning the voids and the local distortion with P, paving the way for the development of novel devices that are driven by elastic strains or even hydrostatic pressures. [32] …”

Determination of the Anisotropic Elastic Properties of Rocksalt Ge2Sb2Te5 by XRD, Residual Stress, and DFT

“…Published results are generally limited to the bulk modulus (K) 7,[19][20][21] , with a few mentions of the biaxial (Y)and Young's moduli (E) 8,11,22 . Recent simulations have indicated that the closely related material Ge 1 Sb 2 Te 4 (GST124) exhibits significant elastic anisotropy with a Zener ratio (A) of 0.24 23 .…”

“…The importance of these chalcogenide alloys for advanced memory applications makes the determination of material properties, such as elastic ones, a critical issue. For example, thermal expansion during the transformation from crystalline to amorphous state can lead to an onset of pressures in the gigapascals range in the amorphous bits of PCRAMs [6][7][8][9][10][11] . This can have an important effect on device performance and reliability.…”