Table 1 The classification of redox-based resistive switching mechanisms and operation principles of redox-based RRAM
From: Conductance Quantization in Resistive Random Access Memory
Switching mechanism | Electrochemical metallization (ECM) | Valence change mechanism (VCM) | Thermochemical mechanism (TCM) |
---|---|---|---|
Dominating charged species | Metal cations | O anions or oxygen vacancies (Vo) | O anions or oxygen vacancies (Vo) |
Intrinsic nature of CF | Metal CF | Vo-CF (bipolar) | Vo-CF (unipolar) |
Dominating driving force | External electric field | External electric field | Thermal gradient |
Primary operation principle | SET process: (1) The active TE material (Ag, Cu, Ni) in the interface is oxidized under positive electric field; (2) the cations (Ag+, Cu+ or Cu2+, Ni+) drift into the RS layer; (3) the cations are reduced back from the BE/RS-layer interface or from the bulk RS layer or even from the TE/RS layer, depending on the difference between the drift velocity of cations and electrons; (4) metal CF is formed to connect BE and TE, with the reduction process continuing. | SET process: (1) Under positive electric field, TE material in the TE/RS-layer interface is oxidized and O2−/Vo is generated; (2) O ions drift to TE or O vacancies drift to BE through the RS layer to form Vo-CF. The valence states of corresponding cations are changed. | SET (antifuse) process: O vacancies are generated, diffused, and redistributed to form Vo-CF in the bulk RS layer under the thermal gradient induced by electric field. The valence states of corresponding cations are changed. |
RESET process: Under the opposite electric field, the metal atoms in the CF are oxidized and drift away, thus CF is partially ruptured. | RESET process: Under the opposite electric field, O ions migrate back to the bulk RS layer to recombine with O vacancies in the CF, thus CF is partially ruptured. | RESET (fuse) process: CF is ruptured or fused as a result of joule heating along the CF through the thermal diffusion process of O vacancies. | |
Typical RS materials | Ion-conducting solid electrolyte (sulfides, selenides, or telluride of Ge, As, Sb, or Ga) such as Ag2S [53], GeSe [224], Cu2S [172, 225], Ag2Se [226], Ag-Ge-Se [227], (AgI)0.5(AgPO3)0.5 [228], etc.; Binary or complex oxides such as HfO2 [215, 222, 229, 230], ZrO2 [231, 232], SiO2 [233], WO3 [234], TaOx [235], GdOx [236], etc. | Transition metal oxides (TMOs) such as TiO2 [51, 87], HfO2 [106], ZrO2 [112], SrTiO3 [5, 61], TaOx [102], WO3 [111], etc.; doped SiO2 [104, 107–109]; amorphous C [103, 105, 110], etc. | Transition metal oxides (TMOs) such as HfO2 [69, 70, 216, 237], NiO [22, 238–244], CoO [245], CuO [246], Fe2O3 [247], etc. |
Typical electrode materials | (1) Top electrode (TE): an electrochemically active metal such as Ag, Cu, and Ni. | (1) Top electrode (TE): a low work function metal not easily reduced back after oxidation, such as Ti, Al, and Nb. | (1) Top electrode (TE): inert electrodes such as Pt, Pd, Ir, Ru, W, Au, etc. |
(2) Bottom electrode (BE): an electrochemically inert counter electrode such as Pt, Pd, Ir, Ru, W, Au, etc. | (2) Bottom electrode (BE): inert electrodes, such as Pt, Pd, Ir, Ru, W, Au, etc. | (2) Bottom electrode (BE): inert electrodes such as Pt, Pd, Ir, Ru, W, Au, etc. | |
Dominating material | Electrode | RS layer and electrode | RS layer |
Typical I–V curve | (Cu/HfO2/Pt) | (Ti/HfO2/Pt) | (Pt/HfO2/Pt) |
Operation polarity | Bipolar | Bipolar | Unipolar |
RS type | Localized | Localized | Localized |