Interface‐engineered resistive memory using plasma‐modified electrode on polyimide substrate

In this study, we report a low power Ni/GeO_x /TiO_y /TaN resistive random access memory (RRAM) using plasma‐modified electrode. The low sub‐mA switching current, highly uniform switching cycles (only 4% variation for the set) and good high‐temperature current distribution at 125 °C are simultaneously achieved in this RRAM device. Such good performance can be ascribed to interface plasma treatment on TaN electrode where the resulting Ta–N ionic bond increases the oxidation resistance and reduces the oxygen vacancy concentration near TaN interface that is favorable to lower switching power and improve high‐temperature current distribution.

     

The influence of interfacial barrier engineering on the resistance switching of In2O3：SnO2/TiO2/In2O3：SnO2 device

The I-V characteristics of In2O3：SnO2/TiO2/In2O3：SnO2 junctions with different interracial barriers are inves- tigated by comparing experiments. A two-step resistance switching process is found for samples with two interfacial barriers produced by specific thermal treatment on the interfaces. The nonsynchronous occurrence of conducting filament formation through the oxide bulk and the reduction in the interracial barrier due to the migration of oxygen vacancies under the electric field is supposed to explain the two-step resistive switching process. The unique switching properties of the device, based on interracial barrier engineering, could be exploited for novel applications in nonvolatile memory devices.     

The effect of substrate material on the properties of tantalum oxide films

The effect of substrate material on the electrical characteristics of Ta _x O _y films produced by high-frequency magnetron sputtering of a tantalum oxide target is studied. The effect of oxygen plasma on leakage currents, dielectric permittivity, and dielectric dissipation factor of thin (300–400 nm) Ta _x O _y layers is found. It is proposed to process tantalum oxide films in oxygen plasma to control their electrical and dielectric properties.     

Resistive switching behaviour of highly epitaxial CeO2 thin film for memory application

We report on the remarkable potential of highly epitaxial and pure (001)‐oriented CeO₂ thin films grown on conducting Nb‐doped SrTiO₃ (NSTO) substrates by laser molecular beam epitaxy for nonvolatile memory application. Resistive switching (RS) devices with the structure of Au/epi‐CeO₂/NSTO exhibit reversible and steady bipolar RS behaviour with large high/low resistance ratio and a narrow dispersion of the resistance values. Detailed analysis of the conduction mechanisms reveals that the trapping/detrapping processes and oxygen vacancies migration play important roles in the switching behaviour. In the light of XPS measurement results, the CeO₂/NSTO interface with oxygen vacancies or defects is responsible for the RS effect. Furthermore, a model is proposed to explain this resistance switching behaviour. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Composition of tantalum nitride thin films grown by low-energy nitrogen implantation: a factor analysis study of the Ta 4 f XPS core level

Tantalum nitride thin films have been grown by in situ nitrogen implantation of metallic tantalum at room temperature over the energy range of 0.5–5 keV. X-ray photoelectron spectroscopy and factor analysis (FA) have been used to characterize the chemical composition of the films. The number of the different Ta–N phases formed during nitrogen implantation, as well as their spectral shapes and concentrations, have been obtained using principal component analysis and iterative target transformation factor analysis, without any prior assumptions. According to FA results, the composition of the tantalum nitride films depends on both the ion dose and the ion energy, and is mainly formed by a mixture of metallic tantalum, β-TaN_0.05 , γ-Ta₂N and cubic/hexagonal TaN phases. The kinetics of tantalum nitridation is characterized by two stages. In the first stage, the formation of β-TaN_0.05 species leads to a strong attenuation of the metallic tantalum signal. During the second stage, β-TaN_0.05 transforms into γ-Ta₂N and cubic/hexagonal TaN species. For intermediate ion doses, the concentration of γ-Ta₂N reaches a maximum, subsequently decreasing because of its transformation into cubic/hexagonal TaN phases with increasing ion dose up to saturation. At saturation, the films are mainly composed of a mixture of γ-Ta₂N and cubic/hexagonal TaN phases, but small Ta⁰ and β-TaN_0.05 signals are also observed. They should be attributed to preferential sputtering of nitrogen and/or to the limited thickness of the film. Comparison of the experimental nitrogen concentration with that obtained using TRIDYN simulations suggests that, in addition to nitrogen implantation and atomic mixing, other mechanisms, like ion beam enhanced diffusion or the chemical reactivity of the tantalum substrate towards nitrogen, should also be taken into account at higher ion-beam energies. PACS 68.49.Uv; 68.55.Nq; 81.05.Je; 81.70.Jb     

Bipolar resistive switching effect and mechanism of solution-processed orthorhombic Bi2SiO5 thin films

Orthorhombic Bi₂SiO₅ thin films with dense surface were synthesized by using a chemical solution deposition method. The crystallized films were first utilized to implement resistive memory cells with Pt/Bi₂SiO₅/Pt sandwich architecture. It exhibited outstanding switching parameters including concentrated distributions of low and high resistance states, uniform switching voltages, cycling endurance, and long retention. Furthermore, the model of formation and rupture of conductive filaments consisted of oxygen vacancies was used to well explain resistive switching behavior. The results revealed that the solution-processed Bi₂SiO₅ thin film devices have great potential for forefront application in nonvolatile memory.     

Fully room‐temperature‐fabricated TiN/TaOx/Pt nonvolatile memory devices

The reliable resistive switching properties of TiN/TaO_x/Pt structures fabricated with a fully room‐temperature process are demonstrated in this letter. The devices exhibited a low operation voltage of 0.6 V as well as good endurance up to 10⁵ cycles. No data loss was reported upon continuous readout for more than 10⁴ s at 125 °C. Multilevel storage is feasible due to the dependence of the low resistance state (LRS) on the initial “SET” (switch from high to low RS) compliance current. The values of LRS showed no dependence on the size of the device, which correlated with the localized conductive filament mechanism. This nonvolatile multilevel memory effect and the fully room‐temperature fabrication process make the TiN/TaO_x/Pt memory devices promising for future nonvolatile memory application. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical properties and carrier transport mechanisms of nonvolatile memory devices based on randomly oriented ZnO nanowire networks

We report reproducible bipolar resistive memory devices based on Au/ZnO nanowire networks (ZnO NWNs)/ITO, which show a high R_on/R_off ratio (～10⁴) and narrow dispersion of set and reset threshold voltages. The dominant conduction mechanisms of low resistance state and high resistance state were verified by Ohmic behavior and space charge limited current, respectively. The switching mechanism is confirmed in terms of the formation and rupture of conductive filaments, with oxygen vacancies localized on the ZnO NWNs surface involved in. The Au/ZnO NWNs/ITO devices presented in our work show potential applications in the next generation nonvolatile memory field. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface redox induced bipolar switching of transition metal oxide films examined by scanning probe microscopy

The bipolar resistive switching mechanisms of a p-type NiO film and n-type TiO₂ film were examined using local probe-based measurements. Scanning probe-based current–voltage (I–V) sweeps and surface potential/current maps obtained after the application of dc bias suggested that resistive switching is caused mainly by the surface redox reactions involving oxygen ions at the tip/oxide interface. This explanation can be applied generally to both p-type and n-type conducting resistive switching films. The contribution of oxygen migration to resistive switching was also observed indirectly, but only in the cases where the tip was in (quasi-) Ohmic contact with the oxide.     

Investigating the composition of periodic nano-size column structures of anodic titanium oxide by the method of IR spectroscopy

Periodic column structures of anodic tantalum oxide (ATO), formed by anodizing in different regimes a Ta—Al two-layer thin-film composition in an oxalic-acid solution as well as the films of dense anodic oxide and thermal tantalum oxide, are investigated by the methods of scanning electron microscopy and reflection IR spectroscopy. It is established that the ATO nano-size columns are composed of at least three known oxide phases from a Ta—O system: TaO, TaO₂ and Ta₂O₅. The structure of tantalum oxide compounds is amorphous with the near order (inclusions) of TaO₂ rutile, rhombic Ta₂O₅, and α- and β-modifications of tantalum pentoxide. The dense-ATO films are distinguished by a more disordered structure of the oxygen compounds of tantalum and an increased content of amorphous tantalum pentoxide. The thermal-ATO films have the most ordered structure of all the oxide phases that enter in their composition and increased relative contents of α- and β-Ta₂O₃. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 850–856, November–December, 1998.     

Atomic and Electronic Structures of Intrinsic Defects in Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>: Ab Initio Simulation

The atomic and electronic structure of intrinsic point defects in orthorhombic tantalum oxide has been studied by numerical simulation within the density functional theory. It has been shown that all defects responsible for metal enrichment of Ta₂O₅ serve as electron and hole traps. Under conditions of strong oxygen depletion and at a metal–insulator interface, which are characteristic of resistive memory elements, interstitial tantalum atoms compete with an oxygen vacancy in the formation of a conducting filament. Interstitial oxygen atoms are not involved in charge transport. Tantalum substituting oxygen can be considered as a combination of the oxygen vacancy and interstitial tantalum. The analysis of the calculated thermal and optical energies of trap ionization shows that the oxygen vacancy is a key defect for charge transport in Ta₂O₅.     

Combined SIMS,AES, and XPS investigations of tantalum oxide layers

Thick layers of tantalum oxide prepared by thermal and anodic oxidation have been studied by combined SIMS, AES, and XPS during depth profiling by 3keV Ar⁺ ion sputtering. The chemical composition of these films is revealed by the OKLL and O 1s signals and by the “lattice valence” parameter determined from the TaO _n ^± intensities. Thus the anodic film consists of a contamination layer, an oxygen-rich reactive interface and a thick homogeneous oxide layer followed by an interface to the Ta metal. The thermal oxide shows an oxygen concentration decreasing with depth and a broad oxide-metal interface. In both cases, carbon contamination (carbide) prevents the application of the valence model to the clean Ta substrate. The sputtering yield of the oxides was found to be 0.6 Ta₂O₅/ion.     

Structural and electrical properties of tantalum nitride thin films fabricated by using reactive radio-frequency magnetron sputtering

TaN thin film is an attractive interlayer as well as a diffusion barrier layer in [FeN/TaN]_n multilayers for the application as potential write-head materials in high-density magnetic recording. We synthesized two series of TaN films on glass and Si substrates by using reactive radio-frequency sputtering under 5-mtorr Ar/N₂ processing pressure with varied N₂ partial pressure, and carried out systematic characterization analyses of the films. We observed clear changes of phases in the films from metallic bcc Ta to a mixture of bcc Ta(N) and hexagonal Ta₂N, then sequentially to fcc TaN and a mixture of TaN with N-rich phases when the N₂ partial pressure increased from 0.0% to 30%. The changes were associated with changes in the grain shapes as well as in the preferred crystalline orientation of the films from bcc Ta [100] to [110], then to random and finally to fcc TaN [111], correspondingly. They were also associated with a change in film resistivity from metallic to semiconductor-like behavior in the range of 77–295 K. The films showed a typical polycrystalline textured structure with small, crystallized domains and irregular grain shapes. Clear preferred (111) stacks parallel to the substrate surface with embedded amorphous regions were observed in the film. TaN film with [111]-preferred orientation and a resistivity of 6.0 mΩ cm was obtained at 25% N₂ partial pressure, which may be suitable for the interlayer in [FeN/TaN]_n multilayers. Received: 6 December 1999 / Accepted: 24 July 2000 / Published online: 9 November 2000     

Top electrode material related bipolar memory and unipolar threshold resistance switching in amorphous Ta2O5 films

Tantalum oxide (Ta₂O₅) is one of the most studied materials for its stable resistance switching and potential application in nonvolatile memory devices. Top electrode and essential switching material are two critical points dominating its switching characteristics. Here, Ta₂O₅ films of amorphous nature (a-Ta₂O₅) with tunable thicknesses were made by changing the applied voltage during anodic oxidation of Ta-metal foils. The resistance-switching behavior of an a-Ta₂O₅ film in a metal/a-Ta₂O₅/Ta configuration was investigated by using a sputtered W or Ag metal film as the top electrode. The unipolar threshold switching phenomenon was observed using W as top electrode (W_TE), while bipolar switching behaviors were achieved using active Ag metal as top electrode (Ag_TE). The thickness of the a-Ta₂O₅ film shows an obvious effect on the SET voltage in a W_TE/a-Ta₂O₅/Ta device. The interfacial redox reaction induced formation of more conductive Ta-rich suboxide and the Joule heating effect are proposed to contribute to the unipolar threshold switching behavior. It is also suggested that the bipolar switching could have resulted from the electrochemical reaction-induced dissolution and growth of Ag conducting channels inside the Ta₂O₅ films.     

On a current mechanism in Ta2O5 thin films

Electrical conduction in the temperature range of 120–370 K has been studied in sandwiched structures of Al/Ta₂O₅/Si. The tantalum oxide films were prepared by evaporation of tantalum on a p-Si crystal substrate, followed by oxidation at a temperature of 600°C. The temperature-dependent current-voltage (I–V) characteristics are explained on the basis of a phonon-assisted tunnelling model. The same explanation is given for I–V data measured on Ta₂O₅ films by other investigators. From the comparison of experimental data with theory the density of states in the interface layer is derived and the electron-phonon interaction constant is assessed.      

Performance improvement of amorphous indium–gallium–zinc oxide ReRAM with SiO2 inserting layer

In this study, the resistive switching performance of amorphous indium–gallium–zinc oxide (a-IGZO) resistive switching random-access memory (ReRAM) was improved by inserting a thin silicon oxide layer between silver (Ag) top electrode and a-IGZO resistive switching layer. Compared with the single a-IGZO layer structure, the SiO₂/a-IGZO bi-layer structure exhibits the higher On/Off resistance ratio larger than 10³, and the lower operation power using a smaller SET compliance current. In addition, good endurance and excellent retention characteristics were achieved. Furthermore, multilevel resistance states are obtained through adjusting SET compliance current and RESET stop voltage, which shows a promise for high-performance nonvolatile multilevel memory application.     

Colossal resistive switching behavior and its physical mechanism of Pt/<Emphasis Type="Italic">p</Emphasis>-NiO/<Emphasis Type="Italic">n</Emphasis>-Mg<Subscript>0.6</Subscript>Zn<Subscript>0.4</Subscript>O/Pt thin films

A heterojunction structure of p-NiO/n-Mg_0.6Zn_0.4O with an aim to tuning or improving the resistive switching properties was fabricated on Pt/TiO₂/SiO₂/Si substrates by the sol-gel spin-coating technique. The Pt/NiO/Mg_0.6Zn_0.4O/Pt heterojunction thin-film device shows excellent resistive switching properties, such as a reduced threshold current of 1 μA for device initiation, a small dispersion of reset voltage ranging from 0.54 to 0.62 V, long retention time and a high resistance ratio of high-resistance state to low-resistance state about six orders of magnitude. These results indicate that the resistive switching properties can be greatly improved by constructing the p-NiO/n-Mg_0.6Zn_0.4O heterojunction for nonvolatile memory applications. The physical mechanism responsible for colossal resistive switching properties of the heterojunction was analyzed based on interfacial defect effect and formation and rapture of conductive filaments.     

Dynamic moderation of an electric field using a SiO2 switching layer in TaOx ‐based ReRAM

ReRAMs using oxygen vacancy drift in their resistive switching are promising candidates as next generation memory devices. One remaining issue is degradation of the on/off ratio down to 10² or less with an increased number of switching cycles. Such degradation is caused by a local hard breakdown in a set process due to a very high electric field formed just before the completion of a conductive filament formation. We found that introducing an ultra‐thin SiO₂ layer prevents the hard breakdown by dynamical moderation of the electric field formed in the TaO_x matrix, resulting in repeated switching while retaining a higher on/off ratio of about 10⁵. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Scanning transmission X‐ray microscopy probe for in situ mechanism study of graphene‐oxide‐based resistive random access memory

Here, an in situ probe for scanning transmission X‐ray microscopy (STXM) has been developed and applied to the study of the bipolar resistive switching (BRS) mechanism in an Al/graphene oxide (GO)/Al resistive random access memory (RRAM) device. To perform in situ STXM studies at the C K‐ and O K‐edges, both the RRAM junctions and the I₀ junction were fabricated on a single Si₃N₄ membrane to obtain local XANES spectra at these absorption edges with more delicate I₀ normalization. Using this probe combined with the synchrotron‐based STXM technique, it was possible to observe unique chemical changes involved in the BRS process of the Al/GO/Al RRAM device. Reversible oxidation and reduction of GO induced by the externally applied bias voltages were observed at the O K‐edge XANES feature located at 538.2 eV, which strongly supported the oxygen ion drift model that was recently proposed from ex situ transmission electron microscope studies.     

Trap-assisted tunneling hole injection in SiO<Subscript>2</Subscript>: Experiment and theory

The injection of holes from silicon through silicon oxide (SiO₂) in a tantalum nitride-aluminum oxide-silicon nitride-silicon oxide-silicon (TANOS) structure has been studied experimentally. Using the high-permittivity Al₂O₃ insulator as a blocking one suppresses the parasitic injection of electrons from the conducting TaN contact. This allows the injection of holes from the substrate into nitride to be studied up to comparatively high electric fields. The experimental data are not described by the standard Fowler-Nordheim law with reasonable physical parameters. At the same time, these data are in good agreement with the model of trap-assisted tunneling hole injection in SiO₂. The developed theory shows that the traps in a narrow energy band make a major contribution to this process, i.e., this injection is resonant in nature.