Atomistic Modeling of Charge‐Trapping Defects in Amorphous Ge‐Sb‐Te Phase‐Change Memory Materials

Abstract

Phase-change memory materials based on Ge-Sb-Te alloys encode stored digital binary data as metastable structural states of the chalcogenide material with contrast in optoelectronic properties, and they correspond to a contender for next-generation, nonvolatile electronic-memory technology. [1] Moreover, they are also promising candidates for neuromorphic and in-memory computing applications, as well as for new storage-class memory devices. [2] The function of phase-change random-access electronic-(optical-)memory (PCRAM) devices is governed by the application of voltage (laser) pulses, which switch the chalcogenide memory material due to Joule heating, very rapidly (%ns) and reversibly, between a degenerate semiconducting, electrically conductive crystalline state (1-bit) and a semiconducting, electrically resistive amorphous (glassy) state (0-bit). [3] The existence of defect-related electronic states in the chalcogenide materials has been considered to be influential in the operation of PCRAM devices. [4][5][6] The resistance contrast between the crystalline and amorphous states has been associated with defects that affect the position of the Fermi level in the two structural phases. [7,8] The electric-field-assisted threshold switching in chalcogenide glasses has been attributed to localized states in the bandgap and also to the electron-trapping kinetics associated with these defect electronic states. [9][10][11] The electrical resistance of the amorphous phase in a PCRAM cell increases ("drifts") logarithmically with time, hindering the development of multibit storage, multilevel programming in PCRAM devices. [2] The time-dependent, resistance-drift phenomenon has been ascribed to structural relaxation of the glassy state, [12][13][14][15] which is strongly correlated to the annihilation of localized mid-gap defect states from the bandgap of the glass. [16][17][18][19][20][21] The intrinsic charge-trapping processes at the localized electronic states of the amorphous material have also been considered as an alternative (electronic) explanation of the resistance drift. [22][23][24] Atomistic simulations have reported the existence of several localized unoccupied and occupied electronic states in the vicinity of the bandgap in amorphous phase-change materials. Zipoli et al. associated the localization of defects in the bandgap of models of glassy GeTe with clusters of Ge atoms, in which at least one Ge atom is over-or undercoordinated. [18] In some cases, Ge-Ge bonds are present, whereas in some other cases, the Ge atoms are not bonded to each other. [18] In addition, GeTe cubes, not properly aligned and sharing a Ge atom, were also identified as possible structural motifs that can host defect states in their model structures. [18] Gabardi et al. reported that the mid-gap states in glassy GeTe originate from a kind of Ge-Ge chain-like structure, where most Ge atoms are 4-coordinated in a defective-octahedral geometry. [17] Raty et al. correlated the emergence of in-gap states in amorphous GeTe with the Ge-Ge ...

Toward Memristive Phase‐Change Neural Network with High‐Quality Ultra‐Effective Highly‐Self‐Adjustable Online Learning

“…Melting kinetics studies have revealed a melting process for the T material state, wherein a crystallized cluster splits into different structural entities with a medium-range order, i.e., cubes and planes of atoms, which then divide into separate four-fold rings that finally disintegrate as the PCM layer melts. [61][62][63] This mechanism can be expanded to the continuous melting induced in a sequence of targeted-amplitude partial-reset pulses. At the glass-crystal interface, heating the PCM layer beyond the melting point results in a substantial population of structural defects, e.g., dangling bonds, a "thermal shock" in the crystallized PCM layer and bond breaking.…”