Abstract
Aging is a ubiquitous phenomenon in glasses. In the case of phase-change materials, it leads to a drift in the electrical resistance, which hinders the development of ultrahigh density storage devices. Here we elucidate the aging process in amorphous GeTe, a prototypical phase-change material, by advanced numerical simulations, photothermal deflection spectroscopy and impedance spectroscopy experiments. We show that aging is accompanied by a progressive change of the local chemical order towards the crystalline one. Yet, the glass evolves towards a covalent amorphous network with increasing Peierls distortion, whose structural and electronic properties drift away from those of the resonantly bonded crystal. This behaviour sets phase-change materials apart from conventional glass-forming systems, which display the same local structure and bonding in both phases.
Highlights
Aging is a ubiquitous phenomenon in glasses
We carry out simulations based on standard density functional theory (DFT) functionals, as well as on non-local ‘van der Waals’ functionals, to achieve the required accurate description of the atomic and electronic structure in the amorphous phase
Using a new approximate technique to calculate the stability of these different local environments, we identify the driving forces leading to the resistivity drift, and assess our results by comparing computed characteristic properties with experimental photothermal deflection spectroscopy (PDS) data, as well as impedance spectroscopy data
Summary
Aging is a ubiquitous phenomenon in glasses. In the case of phase-change materials, it leads to a drift in the electrical resistance, which hinders the development of ultrahigh density storage devices. For GexSbyTe1 þ x þ y alloys, DFT-based atomistic computer simulations[18,19,20] report a significant proportion of tetrahedrally bonded Ge atoms in the amorphous phase, while no crystalline phase with such Ge environment exists These tetrahedrally coordinated Ge atoms, denoted GeT, are obtained by quenching liquid configurations, mimicking the actual experimental process. Experimental cooling rates have to be very high to prevent crystallization (PCMs are poor glass formers21); simulated quenches are even several orders of magnitude faster[22], due to the computational cost of DFT-based molecular dynamics This raises the question whether the tetrahedral structures observed in the DFT models are relevant to experiments or originate from the too high quench rates. Using a new approximate technique to calculate the stability of these different local environments, we identify the driving forces leading to the resistivity drift, and assess our results by comparing computed characteristic properties with experimental photothermal deflection spectroscopy (PDS) data, as well as impedance spectroscopy data
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