Phase change materials in non-volatile storage

Since their discovery in the 1960s, metallic glasses have continuously attracted much interest across the physics and materials science communities. In the forefront are their unique properties, which hold the alluring promise of broad application in fields as diverse as medicine, environmental science and engineering. However, a major obstacle to their wide-spread commercial use is their inherent temporal instability arising from underlying relaxation processes that can dramatically alter their physical properties. The result is a physical aging process which can bring about degradation of mechanical properties, namely through embrittlement and catastrophic mechanical failure. Understanding and controlling the effects of aging will play a decisive role in our on-going endeavor to advance the use of metallic glasses as structural materials, as well as in the more general comprehension of out-of-equilibrium dynamics in complex systems. This review presents an overview of the current state of the art in the experimental advances probing physical aging and relaxation processes in metallic glasses. Similarities and differences between other hard and soft matter glasses are highlighted. The topic is discussed in a multiscale approach, first presenting the key features obtained in macroscopic studies, then connecting them to recent novel microscopic investigations. Particular emphasis is put on the occurrence of distinct relaxation processes beyond the main structural process in viscous metallic melts and their fate upon entering the glassy state, trying to disentangle results and formalisms employed by the different groups of the glass-science community. A microscopic viewpoint is presented, in which physical aging manifests itself in irreversible atomic-scale processes such as avalanches and intermittent dynamics, ascribed to the existence of a plethora of metastable glassy states across a complex energy landscape. Future experimental challenges and the comparison with recent theoretical and numerical simulations are discussed as well.

https://iopscience.iop.org/article/10.1088/1361-648X/aa9964/pdf

Relaxation processes and physical aging in metallic glasses.

The structural relaxation (SR) process in an amorphous chalcogenide material (Ge2Sb2Te5) is studied by electrical measurements on phase-change memory devices. SR induces a change in the conduction regime from Poole to Poole–Frenkel transport, evidencing a temperature accelerated defect-annihilation process. Based on an Arrhenius kinetics with distributed activation energies, a temperature-acceleration law is shown, relating the time to reach a specific relaxed state to the temperature during isothermal experiments. This law is demonstrated comparing the time evolution of resistance for different temperatures. These results allow for a significant time reduction in reliability testing of devices and materials affected by SR.

Temperature acceleration of structural relaxation in amorphous Ge2Sb2Te5

Phase-change memory (PCM) cells are affected by structural relaxation (SR), which is the atomic-scale rearrangement of the amorphous phase of the chalcogenide material. Since SR affects the stability of electrical parameters of the PCM cell, such as resistance and threshold voltage, physics-based models for SR are necessary to analyze and predict the device reliability. This paper presents a physical model for SR in amorphous chalcogenide materials, linking the defect annihilation dynamics to the conduction behavior of the cell, hence to the electrical characteristics of the PCM cell. The model is able to predict the PCM cell characteristics as a function of annealing time, temperature, and read conditions.

Reliability Impact of Chalcogenide-Structure Relaxation in Phase-Change Memory (PCM) Cells—Part II: Physics-Based Modeling

Effect of annealing below the crystallization temperature on the corrosion behavior of Al–Ni–Y metallic glasses

Structural relaxation in several transition metal based metallic glasses has been studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), electrical resistivity, acoustic emission and Mossbauer spectroscopy techniques Annealing of metallic glass below the crystallization temperature gives rise to discontinuous changes in electrical resistance These changes are accompanied by acoustic emissions The latter process provides an indication for the viscous flow in the matrix In diverse alloy compositions, it has been observed that changes in electrical resistivity which begin instantly with the onset on isothermal annealing do not follow diffusion kinetics Mossbauer experiments also show that neither long range diffusion nor the formation of a new phase occurs as a result of annealing It is concluded that viscous flow is the mechanism for structural relaxation induced by isothermal annealing and short range ordering during this process is also a consequence of the viscous flow process

Structural relaxation in metallic glasses

In the present paper, the relaxation phenomena in a multicomponent metallic glass, Ni 45 Fe 5 Co 20 Cr 10 Mo 4 B 16 , has been investigated through electrical resistance, acoustic emission and Mossbauer spectroscopy to understand the mechanism of structural relaxation in the amorphous matrix. Isothermal electrical resistance measurements clearly show that the structural relaxation occurs discontinuously through a number of discrete steps. There exists a good deal of correspondence between the variations of the acoustic emissions and the electrical resistance with time and temperature. It is shown that the stepwise change in the electrical resistance arises due to the viscous flow in the matrix which is associated with the acoustic emissions. Fourier deconvolution analysis of the Mossbauer spectra recorded at room temperature was carried out for the as-received sample as well as those relaxed at various annealing temperatures. The QS-distributions and QS-IS correlation, so obtained, clearly show that the structural relaxation induces a small but definite change in the short-range order. Present results indicate that the viscoelastic flow is the primary unit process responsible for structural relaxations in the matrix.

G.L. Goswami

Papers

Structural relaxation in metallic glasses

Mechanism of structural relaxation in a metallic glass