Abstract
Ge-Sb-Te alloys have been widely used in optical/electrical memory storage. Because of the extremely fast crystalline-amorphous transition, they are also expected to play a vital role in next generation nonvolatile microelectronic memory devices. However, the distribution and structural properties of vacancies have been one of the key issues in determining the speed of melting (or amorphization), phase-stability, and heat-dissipation of rock-salt GeSbTe, which is crucial for its technological breakthrough in memory devices. Using spherical aberration-aberration corrected scanning transmission electron microscopy and atomic scale energy-dispersive X-ray mapping, we observe a new rock-salt structure with high-degree vacancy ordering (or layered-like ordering) at an elevated temperature, which is a result of phase transition from the rock-salt phase with randomly distributed vacancies. First-principles calculations reveal that the phase transition is an energetically favored process. Moreover, molecular dynamics studies suggest that the melting of the cubic rock-salt phases is initiated at the vacancies, which propagate to nearby regions. The observation of multi-rock-salt phases suggests another route for multi-level data storage using GeSbTe.
Highlights
The important technological material GeSbTe (GST), especially on the GeTe-Sb2Te3 pseudo-binary line, shows an extraordinary potential in optical/electrical applications due to the outstanding switching behavior[1,2,3,4]
There is controversies with respect to the vacancy distribution, which was reported as a random distribution based on the X-ray diffraction (XRD) data[15,16], but some theoretical studies predicted that the vacancies could be highly ordered and even form layers on the (111) planes[8,14] or be more complicated[18]
Crystallization of the pulse-laser deposited (PLD) amorphous GST during in-situ annealing was studied by transmission electron microscopy (TEM) [see Fig. S1, Supplementary Information (SI)]
Summary
Behavior in Rock-Salt GeSbTe received: 04 February 2016 accepted: 15 April 2016 Published: 03 May 2016. The distribution and structural properties of vacancies have been one of the key issues in determining the speed of melting (or amorphization), phase-stability, and heat-dissipation of rock-salt GeSbTe, which is crucial for its technological breakthrough in memory devices. Molecular dynamics studies suggest that the melting of the cubic rock-salt phases is initiated at the vacancies, which propagate to nearby regions. The vacancy distribution affects the structural stability and the phase transition behavior[8,14], and the physical properties, such as the electronic[3] and thermal[19] transports. We report a combined experimental and theoretical study on the vacancy ordering in cubic GST, in connection with its role in the cubic-to-hexagonal phase transition and amorphization, and the possibility of multi-level cubic and amorphous GST phases for PCM. Melting (or amorphization) in the cubic phases occurs at the vacancies and propagates into nearby regions
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