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(a) Schematic diagram of the experimental setup and (b) typical I–V curve of the Ti/Hf2−x/Co devices. (c), (d) Schemas pointing out the radically different behavior observed in our devices (c) with respect to what is expected in standard bipolar RS devices (d). It implies a discontinuity in the resistance state when the polarity of the applied voltage changes.
Hysteresis switching loops obtained with opposite sensing bias on the same device. (a) When the sensing bias is +4 V, it reveals the behavior of the Ti/HfO2−x interface. (b) Instead, when the bias is −4 V, it evidences the behavior of the Co/HfO2−x interface.
(a) Schematic illustration of the structure of our devices. (b) Energy band diagram at thermal equilibrium of the system, where EFm,L and EFm,R are the Fermi energies of the metal contacts (CL, CR), WL and WR are the widths of the depletion layers, Eg is the energy band gap of the semiconductor, qψbi is the built-in potential, EV and EC are the valence and conduction band, respectively, and qΦBn,L and qΦBn,R are the energy heights of the Schottky barriers. (c–e) Different possible configuration of the system; with high concentration of oxygen vacancies (spheres) at the left interface (c), right (d), and in both interfaces (e). For the sake of simplicity, in this plot, we represented the injection barriers alike and not changing after the introduction of the oxygen vacancies. Actually, in our model, we assume that the injection barriers are strongly modified by the oxygen vacancies, turning the metal-semiconductor interfaces into ohmic contacts. We also account for unlike junctions.
Experimental and simulated I–V curves for the devices Co/HfO2−x/Ti (a) and Au/HfO2−x/Ti (e). (b–d) Evolution of the simulated parameters in time-steps, corresponding to the numerical integration presented in (a). For positive bias (voltage is presented in panel (b)), the HfO2−x/Co interface do not limit the injection of carriers into the device, the current is governed by the Ti/HfO2−x interface and the bulk; for negative bias it is limited by the HfO2−x/Co interface and the bulk (see panel (c)). The sequence of the different states of the system is presented in panel (d) together with the voltage at the interfaces. This sequence, numbered (1) to (6) in accordance with panel (a), is: (1) the Ti/HfO2−x interface is in HRS and the HfO2−x/Co interface is in LRS, (2) SET of the Ti/HfO2−x interface, (3) RESET of the HfO2−x/Co interface, (4) state complementary to (1), (5) the SET of the HfO2−x/Co interface, and finally (6) RESET of the Ti/HfO2−x interface.
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