Binary oxides

Binary oxides were first investigated as resistive switching memory in 1960’s. Various oxides such as TiO221,22 , 23, NiO24, Al2O325, ZrO226, MoOx27, CuOx28, CoO29, WO330, Cr2O331and SiO232 ,33 have been discovered to exhibit resistive switching behavior. In recent years, most of the research has been focused on TiO2, NiO, and ZrO2 in the family of transition metal oxides. Oxygen vacancy migration can induce oxidation/reduction of metal cations in these transition metal oxides. The reduced (nonstochiometry) phases usually have a higher conductivity, which is the main reason for resistance switching in these materials. For these materials, both bipolar and unipolar switching can occur between electrodes of Pt, Ag, Au and Cu, and this phenomenon was mostly explained by the filamentary effect34,35,36,37,38. Illustration of filaments induced resisance switching is

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shown in Fig. 6. They have been observed using conducting atomic force microscopy (C- AFM): conducting spots appear in C-AFM mapping when the cell is switched into the LR state, indicating conducting paths have formed during this process34,35. Further characterization of TEM and EDX results also showed diffusion of a node ions into active layer36.

Fig. 6 Filamentary effect of a RRAM device.

For NiO, Son & Shin directly probed the conducting filaments by using a Hg top electrode35. The Hg electrodes were removed after switching two cells, one to HR and the

other to LR. Then, C-AFM mapping reveals high leakage current in several regions when the cell is in the LR state, whereas minor leakage current was observed when the cell is in the HR state. The size and density of the filaments were then simulated by Monte Carlo calculations. The calculations were done using very small electrodes, namely nanospheres39. The simulated electrode diameter is as small as 10nm and switching probability was found to decreases significantly as the cell size decreases. Yoshida et al. suggested that these leakage sites were Ni deficient NiO phases40. A time- of- flight-secondary ion mass spectrometry (TOF-SIMS) mapping of a HR region and another LR region shows different ratio of 16O and 18O. The voltage-driven oxygen

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and another non stoichiometry Ni1-δO phase (semiconducting, LR state), as shown in Fig. 7. A study of different electrode materials effecting NiO switching performance also suggested that switching voltage is related to the contact potential between the metal electrode and NiO interface41.

Fig. 7 Indication of oxygen migration in NiO RRAM.

In the TiO2 RRAM device, the redox mechanism has also been proposed. Oxidation/reduction reactions near the cations site are suggested to occur at the interfaces with active metals electrodes or near the filaments inside the active layer. Insulators, in this case TiOx18,21,42, serve as oxygen reservoirs, and the HR state is obtained when an anode- metal-oxide layer forms between the metal and the insulator. Under an opposite bias or a lower voltage, the layer is reduced back to the metal, so the LR state is recovered. Recently, the formation and disruption of a TinO2n-1 oxygen-deficient phase of a nanofilament form extending throughout the film was identified using high- resolution transmission electron microscopy (HRTEM) during in-situ current-voltage measurements37. The TEM image (Fig. 8) also revealed another crystalline phase, a

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direct measurement performed on these two phases shows that the Magneli phase is more conducting than the original TiO2 phase. Jeong et al.43 states that the oxygen migrates to the interface between the top electrode (TE) and TiO2, and such resistive switching can be enhanced by adding an inert metal above the TE to prevent oxygen out-diffusion. It was further confirmed by electron energy loss spectroscopy (EELS) t hat the oxygen atoms accumulate at the first couple layers near the interface.

Fig. 8 Blown-out region on a Pt/TiO2 RRAM and its HRTEM image.

A memristive switching device was reported in anisotropic TiO221, as illustrated in Fig. 9. The device consists of two active layers: a n oxygen rich region and an oxygen poor region, connected in series. Strukov et al. demonstrated that the resistance of t he device is a function of a state variable w controlled by the boundary of the two regions; i.e., v/i = R(w), and dw/dt ~ i. The state can be modulated by electric current to achieve atomic rearrangement, such as oxygen migration, which triggers resistance switching.

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Fig. 9 Illustration of a memristive RRAM device.

Recent attention to HfO44,45 and TaOx46,47 RRAM devices has also been paid in view of their outstanding endurance limit. Switching of 109 cycles was achieved in HfO by Lee et al.45 and a TaOx RRAM device investigated by Yang et al.47 has successfully switched over 1010 cycles. The switching mechanism proposed for HfO and TaOx is similar to that for TiO2: oxygen vacancies migration creating filaments with highly conducting non- stochiometric HfO and TaOx phases. HfO and TaOx are known to exhibits better thermal stability47 compare to other transition metal oxide RRAM and this is believed to be the main reason of their excellent endurance property.

In ZrO2 films, Zr+ ions, serving as trap sites distributed across the active layer, have been implanted to investigate the space-charge- limit-conduction (SCLC) mechanism.48

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When the set voltage is reached, electrons are injected and they occupy the trap sites, switching the device to the LR state. A reverse bias de-traps the electrons from the trap sites, returning the state to the HR state. A structural study using X-ray photoelectron spectroscopy (XPS) on ZrO2 supports this mechanism49: near the top electrode, the film is stoichiometric and has a high resistance, whereas near the bottom electrode, the film is oxygen deficient and conducting.

Another redox resistance switching material is Al2O325,50,51. However, since Al2O3 is not a transition metal oxide, the redox reaction is believed to only occur at the interface, usually with the top electrode, such as Ti forming TiOx that accompanies Al3+ reduction.

Another binary oxide that is not a transition metal oxide is SiOx. SiOx-based RRAM devices have drawn attention since 1960’s16

,32,33,38, 52 , 53 because of its CMOS compatibility in the silicon based semiconductor industry. The earliest report was that of Simmons et al.16 using gold electrode on top of SiO2. Resistance switching can be triggered after forming the device. It is believed that Au ions are driven into the SiO2 film, creating localized states that facilitate charge trapping. Schindler et al.32,38 and Jo et al.52,53 reported filamentary switching in pure SiO2 or SiOx using low melting metal electrodes. Again, resistance switching requires a forming process in order to drive metal ions from the electrode into the dielectric layer. In the work of Yao et al.33, cross-section transmission electron microscopy revealed Si nanocrystal formation in the SiOx matrix. It is determined that switching takes place through the voltage-driven formation and modification of silicon nanocrystals embedded, with SiOx serving as the source of Si along this pathway.

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