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TiN/Ti/HfO2/Pt resistive switching devices have been fabricated, measured, and modeled. After programming the devices in the low resistance state, the current–voltage characteristic below the reset switching voltage was measured at different temperatures (from 90 to 350 K). A weak but complex temperature dependence was obtained for several voltage regimes. These memristors belong to a wider set known as valence change memories, whose conductance is determined by the formation of conductive filaments (CFs) linked to a high density of oxygen vacancies in a dielectric sandwiched between two metal electrodes. This usually leads to ohmic conduction in the low resistance state. However, a non-linear current dependence has been also observed in the measured devices, in addition to symmetric current–voltage curves for positive and negative biases in the 0–0.6 V voltage range. Three different thermal dependences have been considered for explaining the whole set of experimental data. Two of them are linked to ohmic filamentary conduction; the CF shows a conductivity enhancement due to thermally activated mechanisms at low temperatures; on the contrary, a CF conductivity degradation is observed at the higher temperatures. Finally, an additional slightly higher value for the non-linear current component as the temperature rises has also been taken into account. A semiempirical compact model has been implemented including these conduction mechanisms and their corresponding temperature dependences, the device has been simulated in LT-Spice and the experimental currents have been correctly reproduced. [ABSTRACT FROM AUTHOR]

Memristive devices have shown a great potential for non-volatile memory circuits and neuromorphic computing. For both applications it is essential to know the physical mechanisms behind resistive switching; in particular, the time response to external voltage signals. To shed light in these issues we have studied the role played by the applied voltage ramp rate in the electrical properties of TiN/Ti/HfO2/W metal–insulator–metal resistive switching devices. Using an ad hoc experimental set-up, the current–voltage characteristics were measured for ramp rates ranging from 100 mV s−1–1 MV s−1. These measurements were used to investigate in detail the set and reset transitions. It is shown that the highest ramp rates allow controlling the resistance values corresponding to the intermediate states at the very beginning of the reset process, which is not possible by means of standard quasistatic techniques. Both the set and reset voltages increase with the ramp rate because the oxygen vacancies movement is frequency dependent so that, when the ramp rate is high enough, the conductive filaments neither fully form nor dissolve. In agreement with Chua's theory of memristive devices, this effect causes the device resistance window to decrease as the ramp rate increases, and even to vanish for very high ramp rates. Remarkably, we demonstrate that the voltage ramp rate can be straightforwardly used to control the conductance change of the switching devices, which opens up a new way to program the synaptic weights when using these devices to mimic synapses for neuromorphic engineering applications. Moreover, the data obtained have been compared with the predictions of the dynamic memdiode model. [ABSTRACT FROM AUTHOR]

HfO 2 and Fe 2 O 3 thin films and laminated stacks were grown by atomic layer deposition at 350 °C from hafnium tetrachloride, ferrocene, and ozone. Nonlinear, saturating, and hysteretic magnetization was recorded in the films. Magnetization was expectedly dominated by increasing the content of Fe 2 O 3 . However, coercive force could also be enhanced by the choice of appropriate ratios of HfO 2 and Fe 2 O 3 in nanolaminated structures. Saturation magnetization was observed in the measurement temperature range of 5-350 K, decreasing towards higher temperatures and increasing with the films' thicknesses and crystal growth. Coercive force tended to increase with a decrease in the thickness of crystallized layers. The films containing insulating HfO 2 layers grown alternately with magnetic Fe 2 O 3 exhibited abilities to both switch resistively and magnetize at room temperature. Resistive switching was unipolar in all the oxides mounted between Ti and TiN electrodes.

Crystal structure and electrical properties of hafnium-praseodymium oxide thin films grown by atomic layer deposition on ruthenium substrate electrodes were characterized and compared with those of undoped HfO 2 films. The HfO 2 reference films crystallized in the stable monoclinic phase of HfO 2 . Mixing HfO 2 and PrO x resulted in the growth of nanocrystalline metastable tetragonal HfO 2 . The highest relative permittivities reaching 37-40 were measured for the films with tetragonal structures that were grown using HfO 2 :PrO x cycle ratio of 5:1 and possessed Pr/(Pr + Hf) atomic ratios of 0.09-0.10. All the HfO 2 :PrO x films exhibited resistive switching behavior. Lower commutation voltages and current values, promising in terms of reduced power consumption, were achieved for the films grown with HfO 2 :PrO x cycle ratios of 3:1 and 2:1 and showing Pr/(Pr + Hf) atomic ratios of 0.16-0.23. Differently from the undoped HfO 2 films, the Pr-doped films showed low variability of resistance state currents and stable endurance behavior, extending over 10 4 switching cycles.