• Fons, Paul
• Kobata, Masaaki
• Elliott, Stephen
• Ando, Daisuke
• Kobayashi, Keisuke
• Kolobov, Alexander V.
• Skelton, Jonathan M.
• Sutou, Yuji
• Saito, Yuta
• Shindo, Satoshi

Abstract

<p>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.</p>

Topics

• density
• impedance spectroscopy
• amorphous
• theory
• x-ray photoelectron spectroscopy
• simulation
• crystalline phase
• semiconductor
• defect
• random
• electrical conductivity