Influence factors to the bipolar resistive switching

This section is like section 1.3.2 that a variety of factors will influence the switching properties. The switching mechanism of the bipolar resistive switching is much complicated than that of unipolar one, therefore, a little change on the microstructure and composition may dominate the switching properties to different switching mechanism, as we discussed above the bi-switching mechanism. The basic stacked structure M-I-M has been investigated in detail to find out the relationship

between each two parameters. A similar description like unipolar one will be discussed in the following steps: the buffer layer or metal thin layer, which inserted at the top electrode (TE)/insulator, at the middle insulator, or at the insulator/bottom electrode (BE), stacked structure, doping impurities with additional atoms by implanted or annealing, embedded metal nanoparticles inside TMO film, and special structure like nanowires or nanorods, have been demonstrated. A correlation between this influence factors and valence change memory will be linked.

(a) Electrode

We have reported above that the metal thin film has its specific work-function (WF), electron affinity, and Gibbs free energy for oxygen, as well as the different contact properties when in contact to different insulator films. Since the interaction may occur at the interface, the resistive switching characteristics can be greatly influenced and determined by the properties of this IL. So, we may discuss the electrode effect first and then the separate them into different categories.

(i) Schottky barrier modulation

The effect of the Schottky barrier formed between the metal/insulator is regard as one of the critical factors on switching properties. The Schottky barrier can influence the conducting current by regulating the injected carriers into the TMO thin films.

Schottky barrier height modulation by trapping and detrapping the injected carriers are discussed as shown in last section. Here, some groups based on the migrated oxygen ions and vacancies to generate a plausible model for the Schottky junction modulation. Choi et al. [138] reported on Pt/PbZr0.3Ti0.7O3/LSCO structure that the contact potential decreases as the positive bias was applied on Pt electrode, and the generated oxygen vacancies can form tunneling paths for electron conduction. By

applying a negative bias on the Pt top electrode, large amounts of oxygen are accumulated in oxygen vacancies, thus preventing electrons tunneling. Similar explanation was also proposed by Pt/SrTiO3 by Kim et al. [139]. Lee et al. [140]

observed the NiO film by using a scanning probe as the top electrode. The modulation of the potential barrier width and resulting tunneling current through the interface may be attributed to accumulation/depletion of injected holes or oxygen vacancies caused by the electrochemical reaction.

Shang et al. [141] studied the Schottky junction of Au/SrTiO3 structure by using photovoltaic effects under different applied electric pulses. They observed the corresponding Schottky barrier, which was deduced from the photorespo nse data about 1.5 eV, is independent of junction resistance. The observed results reveal the invariance of the interfacial Schottky barrier during resistance switching. We suggested that the resistive switching may occur at the localized region, therefore , the macroscopic investigation of photovoltaic analysis may detect insensitively.

(ii) Gibbs free energy

We have previously reported that the effects of electrodes on the unipolar switching properties must be considered as a critical factor in pursuit of the appropriate metal materials as electrodes [37]. For the bipolar operation, the analysis of the bipolar I-V curves strongly depend on the Gibbs free energy of oxidation of the TEs are investigated on PCMO thin film by Liao et al. [142]. From the measurement of virgin resistance (VR, i.e. the contact resistance) between the TE and PCMO film, the observations suggest that the interface is not a typical Schottky barrier formed at the interface, but is somehow associated with the presence of the interfacial metal oxide. The VR information, I-V curves, and its related microstructure are shown in Fig. 1-26 and Fig. 1-27. The metal/PCMO/Pt devices can be categorized into two

groups, which based on the Gibbs free energy for oxidation of the TE is larger or less than PCMO oxidation free energy. When nonreactive metal group (Pt, Ag, Au or Cu) was used as electrodes, no RS properties are observed, while the RS properties occurs with forming-free character when reactive metal is used as top electrode. The reactive metal with lower free energy for oxidation than PCMO film can easily induce an interfacial metal oxide at the interface and modulate the oxygen vacancy concentrations in the vicinity of insulator film. Under the applied positive bias, the oxygen ions migrated from the PCMO film to the reactive metal and oxidize with reactive metal to form a thicker interfacial layer, thus the resistance state switched to HRS. As the negative bias was applied, the dissociation of the metal oxide layer by the redox reaction at the metal/PCMO interface. Li et al. [143,144] reported the similar concepts to explain the free energy effect on switching properties of PCMO films. Shono et al. [145] and Kawano et al. [146] further observed the microstructure change of the interfacial metal oxide by cross-section TEM investigation under the applied bias. Yang et al. [147] investigated the switching behaviors of M/LCMO/Pt junctions and found that the switching polarity of the junctions for M=Pt, Ag, and Cu was opposite to those for M=Al, Ti, and W based on Gibbs free energy for oxidation.

A little difference from the PCMO film [142] is that the opposite switching direction can be observed on the metal whose free energy is larger than LCMO film. Though the low free energy for oxidation metals can attract oxygen ions easily during the interfacial metal oxide formation, but the reduction of their oxides require more power and longer switch time since the formation and dissociation of the interface oxide determine the switching speed. Hasan et al. [148] improved the switching speed of LCMO/Pt films by substituting the less reactive metal Al for the Sm metal. Liu et al. [149] used the less reactive metal W as electrode and obtained a low power reset switch of only a maximum current of about 4 μA, which is much lower than the

Al/PCMO/LaNiO3 structure by Harada et al. [150] and Al/PCMO/Pt by Jo et al. [151].

Although the reactive metal can easily modulate the distribution of the oxygen contents and vacancies inside the TMO film and results in RS behaviors, the switching mechanism does not all the same. As we followed the description above [142], Al/TiO2/Al by Jeong et al. [152,153], Al/PCMO/Pt by Seong et al. [154], and Ti/PCMO/Pt by Liu et al. [155] reported that the reactive metal can form an interfacial metal oxide at the TE/TMO interface, and the switching mechanism is dominated by this metal oxide layer under applied bias. The I-V sweeping curves exhibit the clockwise loop in semi-logarithm plot. However, an opposite sweeping curves, i.e. the counter-clockwise loop, was observed when the reactive metal contacts on the following oxide film, such as CeOx, CuxO, HfO2, ZrO2, and Al2O3 films. There are several explanations on their respective material, including (i) external oxygen reservoir, (ii) oxygen vacancy-riched layer. This is why they are categorized into the valence change memory, because a stacked structure composed of different oxygen contents will be formed. The stacked oxide structure can provide defects, traps, or space for the oxygen ions or vacancies migration under the applied bias, thus the switching behaviors can be obtained. (i) The external oxygen reservoir is used to differentiate from intrinsic oxygen reservoir TiN because this kind of reservoir is attributed to the formation of the interfacial metal oxide. Ti/ZrO2/Pt by Lee et al.

[156,157,131,132,133], and Al/Al2O3/Pt by Wu et al. [158,134] based on this concept and explained that this metal oxide can change the switching from unipolar (noble metal) to bipolar operation. Oxygen ions diffuse in and out of the interfacial metal oxide is used to explain their switching mechanism. The switching involves the thermal Joule heat may also observed in Al/CuxO/Cu structure by Lv et al. [159]. (ii) The reactive metal will induce an oxygen vacancy-riched layer, as reported by Al/CeOx/Pt by Sun et al. [160]. The switching properties are dominated by aligning or

breaking the filamentary paths composing of oxygen vacancy both inside the vacancy-riched layer and vacancy-deficient layer. Although both the Al and Ti are reactive metals and can oxidize by attracting oxygen ions from the below layer, the characters between them is not totally the same. Because of the self-stop oxidation nature character of AlOx, the Al metal layer is more suitable as an oxygen reservoir than that of Ti metal one. The Ti metal can easily over-extract the oxygen ions from the underlying TMO films, thus causing the RS failure easily [159]. This problem can be ameliorated in the following section – by controlling the Ti thickness.

(b) Metal thin layer inserted at M/O interface

Lee et al. [161] compared the switching characteristics of AlCu/HfO2/TiN structure with and without the inserting layer TiN/Ti at the AlCu/HfO2 interface. They suggest that the Ti metal can induce more oxygen vacancies as the trapping centers than that of AlCu electrode does in the HfOx layer. By well controlling the Ti effect, a forming-free device [162] and high yield device [163] can be achieved. However, some authors may regard the Ti as TiOx metal oxide during fabrication because of its oxygen getter nature, as we discussed last section.

(c) The buffer layer inserted at M/O interface (stacked structure)

Because the switching characteristics can be easily modulated by the electrode based on the model we summarized, some groups modifies the switching properties by inserting the buffer layer or metal thin layer at the M/O interface. The purpose of the additional process is for the improvement of the switching behaviors, as well as the switching parameters dispersion. Chen et al. [164] reported that the stacked TaN/Al2O3/NbAlO/Al2O3/Pt structure exhibits better uniformity than TaN/NbAlO/Pt device. The inserted layer can induce the similar effect like we discussed above: an

external oxygen reservoir (Pt/Cr:SZO/TiON/Pt reported by Park et al. [165]). The oxygen vacancy-deficient/vacancy-riched stacked structure was deliberately fabricated and oxygen ions or vacancies can be triggered by the applied bias, causing the formation and rupture of filaments at the interface of the stacked structure, as Pt/TiO2/TiO2-x/Pt by Do et al. [166], Pt/ZrOx/HfOx/TiN by Lee et al. [167], Ti/TiOx/LCMO/Pt by Liu et al. [168], Pt/Ta2O5/TaOx/Pt by Hur et al. [169], and Al/SZO(OR)/SZO(OD)/LNO/Pt by Lin et al. [170]. When the top portion of the stacked oxide layer is quite thin (TiOx in [168] and Ta2O5 in [169]), the authors suggest that the whole top portion oxide takes part in the resistive switching, i.e. the formation and dissolution of filaments occurs at the top portion oxide layer.

(d) Metal thin layer inserted in the middle of insulator film

Inserting a metal layer within the oxide layer followed by post annealing is another beneficial way to improve the switching stability. The inserted metal layer can induce many defects inside TMO thin film and produce a space charge region for oxygen ions migration, as reported in the Al/SZO/Cr/SZO/LNO structure by Lin et al.

[171], Ti/ZrO2/Mo/ZrO2/Pt by Wang et al. [157], and TiN/HfO2:Al layer/Pt by Yu et al.

[172].

(e) Embedded metal nanocrystal (NC)

The embedded metal NCs are reduced from the metal thin layer under high temperature post annealing process. This additional NCs can provide the benefits of improving the fluctuation of switching parameters, which may be attributed to the local enhancement or concentration in electric field induced by the embedded NCs.

The structures of Au/ZrO2:nc-Au/n⁺-Si by Zuo et al. [173], Pt/TiO2:nc-Pt/Pt by Chang et al. [174], and Al/SrZrO3:E-Pt/Pt by Lin et al. [175] all observed an improvement on

their switching characteristics. Reduction on the effective oxide thickness may also strengthen the electric field on some localized spots points.

(f) Post annealing

Post annealing can modulate the thin film morphology, surface roughness, crystallinity, electrical characteristics, atomic composition and binding energy, and the resistive switching properties. The treatment process is performed to improve the resistive switching stability. Au/PCMO/Pt by Kim et al. [118,119], Pt/Ti/Al2O3/Pt by Lin et al. [176], Pt/TiO2/Pt by Kim et al. [177], and Cu/BFO/Pt by Yin et al. [178]

observed that the crystallinity of TMO thin film increases with the annealing temperature increasing. Since crystallinity increases, then the grain size increases [178], followed by the reduction on root mean square [177] and operation voltage [176,178]. Yang et al. [179] observed the opposite properties that the grain size of Cu2S thin film in Cu/Cu2S/Pt structure decreases with increasing annealing temperature. However, they all have a larger high to low resistance ratio after annealing. Liu et al. [180] reported an opposite result because oxygen annealing can lead to the decrease of oxygen vacancies in the LCMO surface, thus causing the ratio decreasing.