Figure 1(a) shows the DC swept current–voltage (I–V) curves of the SL-RRAMs and the DL-RRAM. Forming-free and bipolar resistive switching (RS) behaviors are observed from the multi-cycled I–V curves. The DL-RRAM switches from an initial high resistance state (HRS) to a low resistance state (LRS) with a compliance current ( ) of ∼ 2 μA to avoid hard breakdown, and the set process is thus completed by a low power of ∼ 14 μW. For the reset process to complete the transformation from an LRS to an HRS, ∼ 0.8 μW (100 nA, −8 V) is required. Consequently, the HfO_x/ZnO double-layer suppresses switching current to μA from mA. Another obvious feature of the DL-RRAM is the asymmetric I–V curves. Considering the symmetric I–V curves of the SL-RRAMs [Figs. 1(a),1(c), and 1(d)], the asymmetric I–V curves may be ascribed to the barrier at the HfO_x/ZnO interface.^[29,30] Additionally, uniformity of set voltage is improved by the HfO_x/ZnO double-layer with suppressed fluctuations. To evaluate the thermal stability of the switching properties, we investigated the temperature dependence of the set voltage and the set current from room temperature to 180 °C. As shown in Fig. 2, increasing ambient temperatures decreases the set voltages to less than 5 V for both the DL-RRAM and the SL-RRAM. As for the set current to complete the transformation from an HRS to an LRS, the DL-RRAM sustains a low switching current (∼ 2 μA) with neglectful fluctuations at ambient temperatures up to 180 °C, while the set current of the SL-RRAM abruptly increases at temperatures over 120 °C [see Fig. 2(b)].

Fig. 2.

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	Fig. 2. (color online) Temperature-dependence of (a) set voltage and (b) set current of the DL-RRAM and the 30-nm HfOx-based SL-RRAM.

As shown in Fig. 3, the currents at both the HRS and the LRS increase with temperature rise, which excludes electrons transportation through metallic conductive filaments but suggests a semiconductor conduction behavior.^[31] Several mechanisms have been proposed to explain conductions during switches, the fittings of the measured I–V curves using different conduction mechanisms such as Schottky emission, Poole–Frenkel, Fowler–Nordheim, and space charge limited current (SCLC) behavior, have been carried out.^[32] The well linear fittings of the ln( and the ln( shown in Fig. 1(b) respectively suggest a Schottky emission dominant conduction in the HRS and a Poole–Frenkel emission governed conduction in the LRS. However, the double logarithmic scale I–V curves of the SL-RRAMs shown in Figs. 1(c) and 1(d) indicate SCLC-dominant conductions for the HRS due to the integration of an Ohmic region (I–V), a Child’s square-law region (I– , and even a region with exponentially increased current (I– ). As for the LRS of the SL-RRAMs, the presented Ohmic behaviors suggest a filament-assisted conduction. Therefore, the applied electric field in an initial HRS SL-RRAM usually drives oxygen ions out of the lattice matrix and leaves V_os to form CFs to complete the normally observed transformation from an HRS to an LRS.^[31]

Fig. 3.

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	Fig. 3. (color online) Temperature-dependent currents @0.5 V for the HRS and LRS of the DL-RRAMs.

To highlight the switching mechanism in the DL-RRAM, oxygen migration should be taken into account. Once the HfO_x layer is deposited on the ZnO layer, the oxygen ions in the ZnO thin film would migrate into the HfO_x thin film due to a difference in Gibbs free energy of oxide formation ( kJ/mol and kJ/mol),^[20,22] which results in a more conductive ZnO layer with many more V_os and a more electrically insulative HfO_x layer with more stoichiometric properties. To confirm this hypothesis, we prepared thick ( nm) HfO_x films on glass substrate, thick (∼ 30 nm) HfO_x films on the ZnO layer, and thin (∼ 5 nm) HfO_x films on the ZnO layer. O 1s XPS spectra of the three samples were then acquired as shown in Fig. 4. An ignorable peak shifting can be observed for the two thick HfO_x films, while an obvious O 1s peak shifting to lower values appears for the thin HfO_x films on the ZnO layer, suggesting oxygen release from the ZnO layer creates a less oxygen-deficient HfO_x layer and leaves the ZnO layer with many more V_os.^[20] Therefore, the ZnO layer in an as-prepared DL-RRAM is indeed highly conductive which also agreed with what was observed in Figs. 1(a) and 1(d). To complete electron transport from the BE to the TE, conduction band offset (CBO) acting as barrier (2.35±0.02 eV) at the HfO_x/ZnO interface should thus be overcome,^[28] exhibiting a Schottky emission dominant conduction in the HRS as shown in Fig. 1(b).^[28,30]

Fig. 4.

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	Fig. 4. (color online) O 1s XPS spectra of the HfOx thin films and those deposited on the ZnO.

If further increased electric field is applied to the memory cell to complete the set process, some V_os in the HfO_x thin film would be generated to serve as trapping centers for electron transportation.^[28] Unlike the V_os in the ZnO with a high concentration to form conductive channels, V_os in the HfO_x thin film are still insufficient for electron direct transport between them. Considering the increased electric field to reduce Coulomb potential energies of the electrons, the probability of an electron getting into the conduction band of the HfO_x thin film by thermal excitation out of the trapping centers (V_os in the HfO_x thin film) would be increased.^[32] Therefore, we can observe the Poole–Frenkel emission conduction in the LRS, whose current density ( is thus given by^[33] 1 where T is the temperature, is the Boltzmann constant, q is the electric charge, E is the electric field, is the permittivity of free space, is the relative dielectric permittivity, is the barrier height for electron emission from the trapping center, C is constant, and represents the ordinates intercepts of the extension line in the graph of ln( against [see Fig. 5(a)]. The emission barrier height ( could thus be acquired according to the slope of versus as shown in Fig. 5(b), which is agreed with the trapping energy of V_o in α-HfO_x.^[34] The conduction in the LRS could thus be governed by a Poole–Frenkel emission mechanism with the V_os assisting electron transport in the HfO_x.

Fig. 5.

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	Fig. 5. (color online) (a) – curves of the LRS of the DL-RRAM at different temperatures, and (b) the intercepts obtained from the curves in panel (a) as a function of .

Thermal stability of the switching current is very important for maintaining normal working status at high temperatures. The Schottky emission dominant conduction in the HRS suggests a close relationship between the current and the Schottky barrier height . We therefore investigated the temperature-dependence of Schottky barrier height of the DL-RRAMs.

Increased temperature may facilitate oxygen ions getting out of lattice matrix by leaving V_os in the oxides. For the SL-RRAM under an electric field, more V_os would be generated at high temperatures to favor the formation of V_o assisted conductive filament. Consequently, we observe a drastically increased current at high temperatures as shown in Fig. 2(b). However, situations would be totally different for the DL-RRAMs.^[28] As the oxide-deficient HfO_x exhibits p-type conduction behavior and the presence of V_os normally causes n-type ZnO,^[35,36] a p–n junction forms at the HfO_x/ZnO interface. According to the band diagram of the DL-RRAMs,^[28] a more p-type HfO_x with a more upward band bending and a more n-type ZnO with a more downward band bending might be formed at the HfO_x/ZnO interface due to the generation of V_os under high temperature. Conduction band offset (CBO) acting as at the HfO_x/ZnO interface should thus be increased to sustain ultralow switching current at even 180 °C as what was observed in Fig. 2(b). Apart from the above discussion, experimental results also confirm the hypothesis. The current density at the HRS can be determined by the Schottky emission:^[37] 2 where is the Richardson constant, is the Schottky barrier height, and the other parameters are previously defined. At a certain temperature, can be expressed as where means the current density at the temperature if the electric field is zero. We could thus plot – curves for the HRS at different temperatures as shown in Fig. 6, according to which the changes less than 10 upon the temperature variations from 298 K to 393 K. Considering the room temperature CBO (∼ 2.35 eV) of the HfO_x/ZnO layer,^[28] the barrier heights at the HfO_x/ZnO interface would thus be reduced to ∼ 2.43, ∼ 2.61, , and 2.91 eV respectively for 323, 348, 368 and 393 K, indicating the actually increases with the rising of temperature.

Fig. 6.

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	Fig. 6. (color online) – curves of the HRS of the DL-RRAM at different temperatures.

Data retention is also crucial for the practical applications of the RRAMs. To investigate the retention reliability of the RRAMs, the thermal accelerating measurement for resistance switching under various temperatures was carried out. Figure 7 shows the data retentions of the DL-RRAMs and the SL-RRAMs at different temperatures. The retention failure time was defined as the time for the resistance increasing to twice of its original value (@LRS) or decreasing to half of its original value (@HRS). According to the retention failure times present in Fig. 7, the HfO_x/ZnO layers extend the HRS retention failure time at all temperatures, suggesting desirable HRS retention characteristic with the HfO_x/ZnO double layers. For the LRS, the HfO_x/ZnO layers shorten the retention failure time to 600 s from 2500 s at 180 °C, but prolong it to 9800 s from 8900 s at lower temperature of 160 °C, indicating the HfO_x/ZnO double layers might favor LRS data retention at lower temperatures. To further estimate the retention characteristic, we plotted the mean retention failure time versus the reciprocal temperature as shown in Fig. 8, which presents the data retention ability of resistance states by using the Arrhenius equation. This equation is expressed as , where is the retention time, is the activation energy for the oxygen ions diffusion or V_o migration, and T are Boltzmann’s constant and the ambient temperatures, respectively.^[38,39] The retention lifetime of the RRAM could be obtained by an extraction result of the Arrhenius equation. Therefore, the HfO_x/ZnO double layers stabilize the 85-°C HRS retention time to be ∼ 10 years instead of 159 hours for the SL-RRAM. Additionally, the 85-°C LRS retention of the DL-RRAM could also be extended to be more than 10 years from 78 hours of the SL-RRAM.

Fig. 7.

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	Fig. 7. (color online) Data retention at different temperatures for [(a) and (c)] the DL-RRAM and [(b) and (d)] the SL-RRAM.

Fig. 8.

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	Fig. 8. (color online) Temperature-dependence of the retention failure times and their fitting lines following the Arrhenius equation for the DL-RRAM- and the 30-nm HfOx-based SL-RRAMs.

Retention failure should be associated with redistribution of V_os to affect electron transport and device resistance. As previously discussed, the V_os assisted CFs would be responsible for the LRS conduction in an HfO_x-based SL-RRAM, instead of the V_os sparsely distributing to assist electron transport governed by a Poole–Frenkel emission conduction in a DL-RRAM. Driven by the concentration gradient, oxygen ions easily move to CFs and combine with V_os to narrow or even cut off CFs. Therefore, the redistribution of the V_os in an SL-RRAM usually exhibits obvious resistance changes. As for the DL-RRAMs, redistribution of V_os might also be affected by temperature rise. However, since the V_os in the HfO_x are sparsely and topologically distributed, slight stochastic movement of the V_os is able to secure electron transport with neglectful variations in resistance. Consequently, resistance variations of the LRS SL-RRAM would be more sensitive to temperature rise than that for an LRS DL-RRAM as shown in Fig. 8.

When it comes to the HRS data retention, the formation of V_os is obviously necessary to be responsible for the retention failure. A rise in temperature produces V_os in both the DL-RRAMs and the SL-RRAMs, which favors the formation of a conductive filament in the SL-RRAMs and results in an HRS retention failure. However, once V_os generate in the HfO_x thin films of an HRS DL-RRAM, it might be filled with some oxygen ions migrating from the ZnO layer, as ZnO has a greater Gibbs free energy of oxide formation than HfO . The HfO_x/ZnO double layers could thus hold a better HRS data retention than a single HfO_x layer.