Electron diffraction and high-resolution transmission electron microscopy of the high temperature crystal structures of GexSb2Te3+x (x=1,2,3) phase change material

Abstract

Electron diffraction and high-resolution transmission electron microscopy of the high temperature crystal structures of GexSb2Te3+x (x=1,2,3) phase change material Kooi, B. J.; de Hosson, J. Th. M.

Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

“…An impressive achievement has been accomplished when it was realized that PCM memory cells based on superlattices (SLs), structures made of alternating GeTe and Sb 2 Te 3 layers, showed dramatically improved performance in terms of reduced switching energies with ultra-low energy consumption, enhanced write-erase cycle lifetimes, and faster switching speeds. [5][6][7] However, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) investigations carried out on molecular beam epitaxy (MBE) grown Sb 2 Te 3 /GeTe SLs 8 have shown that the constituting materials intermix at the interfaces, forming alternating layers of Sb 2 Te 3 and natural Ge x Sb 2 Te 3+x (GST) 9 blocks with 7-to 15-atomic layers randomly distributed along the growth direction. Such phenomenon, explained in terms of the different bonding dimensionality of GeTe and Sb 2 Te 3 , is thermodynamically driven and therefore it cannot be easily controlled during the growth.…”

Atomistic origins of the phase transition mechanism in Ge2Sb2Te5

“…One distinction is that in s-GST, the intrinsic vacancies do not occupy lattice sites, but form vacancy planes perpendicular to the hexagonal ͓0001͔ direction and separate the building GST blocks. [23][24][25][26][27][28][29][30][31] Thus, the intrinsic vacancies are not randomly distributed in the lowest energy configurations. Calculations for a random distribution of the intrinsic vacancies in m-GST yield configurations with high energies ͑180-720 meV/f.u.͒, which will be more easily obtained from MD simulations compared to the ordered lowest energy configuration.…”

“…Thus, three states can be distinguished in the writing and erasing processes, namely, crystalline, liquidlike, and amorphous ͑a-GST͒ phases. The GST compound, as well as other compositions, crystallizes in two well known phases, namely, the stable hexagonal [23][24][25][26][27][28][29][30][31] ͑s-GST͒ and metastable rocksalt ͑RS͒ ͑m-GST͒ phases. Experimental studies indicate that only the m-GST phase takes part in optical storage applications; 3,5,[10][11][12][13][14] however, both structures have very similar structure features as discussed by Da Silva et al 32 employing first-principles calculations.…”

“…32 In this work, we will restrict ourselves to the m-GST phase due to the key role that it plays in optical storage applications. 3,5,[10][11][12][13][14] Experimental techniques such as high-resolution transmission electron microscopy ͑HRTEM͒, x-ray diffraction study ͑XRD͒, extended x-ray absorption fine-structure spectroscopy ͑EXAFS͒, and theoretical studies for the m-GST [27][28][29][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] phase have reported that the Te atoms occupy the anion sites and Ge, Sb, and the intrinsic vacancies ͑20% in GST͒ occupy the cation sites in the RS structure. Despite the seeming simplicity, the structural details of the m-GST structure are still under intense debate.…”

Interband characterization and electronic transport control of nanoscaledGeTe/Sb2Te3superlattices

“…[17] Density functional theory calculations showed that the insulating state is caused by the localization of charge carriers in vacancy-rich areas, and the transition to the metallic state happens when vacancies reconfigure into ordered layers [18]. The presence of vacancy layers in the high temperature annealed phase of GSTs was proven by high-resolution TEM and electron diffraction [25].…”