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Abstract
Cu2GeTe3 (CGT) phase-change material, a promising candidate for advanced fast nonvolatile random-access-memory devices, has a chalcopyritelike structure with sp3 bonding in the crystalline phase; thus, the phase-change (PC) mechanism is considered to be essentially different from that of the standard PC materials (e.g., Ge-Sb-Te) with threefold to sixfold p-like bonding. In order to reveal the PC mechanism of CGT, the electronic structure change due to PC has been investigated by laboratory hard x-ray photoelectron spectroscopy and combined first-principles density-functional theory molecular-dynamics simulations. The valence-band spectra, in both crystalline and amorphous phases, are well simulated by the calculations. An inherent tendency of Te 5s lone-pair formation and an enhanced participation of Cu 3d orbitals in the bonding are found to play dominant roles in the PC mechanism. The electrical conductivity of as-deposited films and its change during the PC process is investigated in connection with valence-band spectral changes near the Fermi level. The results are successfully analyzed, based on a model proposed by Davis and Mott for chalcogenide amorphous semiconductors. The results suggest that robustness of the defect-band states against thermal stress is a key to the practical application of this material for memory devices.
Original language | English |
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Article number | 195105 |
Journal | Physical Review B |
Volume | 97 |
Issue number | 19 |
Early online date | 3 May 2018 |
DOIs | |
Publication status | Published - 15 May 2018 |
Funding
This work was supported by KAKENHI (Grant No. 15H04113) and the Kato Foundation for Promotion of Science. J.M.S. is grateful for the support of an Internal Graduate Studentship award from Trinity College, Cambridge, and for funding from the UK Engineering and Physical Sciences Research Council (EPSRC, Grant No. EP/K004956/1). The calculations were performed using the UK Archer HPC facility, accessed through the UK Materials Chemistry Consortium (MCC), which is funded by the EPSRC (Grant No. EP/L000202). A subset of the calculations was also performed on the Balena HPC facility at the University of Bath, which is maintained by Bath University Computing Services. Y.S. and S.S. acknowledge Prof. Junichi Koike for valuable discussions.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
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Applying Long-Lived Metastable States in Switchable Functionality via Kinetic Control of Molecular Assembly
Raithby, P. (PI), Burrows, A. (CoI), Lewis, D. (CoI), Marken, F. (CoI), Parker, S. (CoI), Walsh, A. (CoI) & Wilson, C. (CoI)
Engineering and Physical Sciences Research Council
1/11/12 → 30/04/18
Project: Research council