Fabrication and characterization of Pt intermediate transparent and conducting ITO/Pt/ITO multilayer films

Platinum intermediate transparent and conducting ITO/metal/ITO (IMI) multilayered films were deposited by RF and DC magnetron sputtering on polycarbonate substrates without intentional substrate heating. Changes in the microstructure and optoelectrical properties of the films were investigated with respect to the thickness of the intermediate Pt layer in the IMI films. The thickness of Pt film was varied from 5 to 20 nm.In XRD measurements, neither ITO single-layer films nor IMI multilayer films showed any characteristic diffraction peaks for In₂O₃ or SnO₂. Only a weak diffraction peak for Pt (1 1 1) was obtained in the XRD spectra. Thus, it can be concluded that the Pt-intermediated films in the IMI films did not affect the crystallinity of the ITO films. However, equivalent resistivity was dependent on the presence and thickness of the Pt-intermediated layer. It decreased as low as 3.3×10⁻⁴ Ω cm for ITO 50 nm/Pt 20 nm/ITO 30 nm films. Optical transmittance was also strongly influenced by the Pt-intermediated layer. As Pt thickness in the IMI films increased, optical transmittance decreased to as low as 30% for ITO 50 nm/Pt 20 nm/ITO 30 nm films.     

In situ TEM study of reversible and irreversible electroforming in Pt/Ti:NiO/Pt heterostructures

Experimental verification of the microscopic origin of resistance switching in metal/oxide/metal heterostructures is needed for applications in non‐volatile memory and neuromorphic computing. Numerous reports suggest that resistance switching in NiO is caused by local reduction of the oxide layer into nanoscale conducting filaments, but few reports have shown experimental evidence correlating electroforming with site‐specific changes in composition. We have investigated the mechanisms of reversible and irreversible electroforming in 250–500 nm wide pillars patterned from a single Ta/Ti/Pt/Ti‐doped NiO/Pt/Ta heterostructure and have shown that these can coexist within a single sample. We performed in situ transmission electron microscopy (TEM) electroform‐ ing and switching on each pillar to correlate the local electron transport behavior with microstructure and composition in each pillar. DFT calculations fitted to electron energy loss spectroscopy data showed that the Ti‐doped NiO layer is partially reduced after reversible electroforming, with the formation of oxygen vacancies ordered into lines in the 〈110〉 direction. However, under the same probing conditions, adjacent pillars show irreversible electroforming caused by electromigration of metallic Ta to form a single bridge across the oxide layer. We propose that the different electroforming behaviors are related to microstructural variations across the sample and may lead to switching variability. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Nanoscale resistive switching in SrTiO3 thin films

The local conductivity of SrTiO₃ thin films epitaxially grown on SrRuO₃‐buffered SrTiO₃ single crystals has been investigated in detail with an atomic force microscope equipped with a conducting tip (LC‐AFM). These experiments demonstrate that the conductivity of SrTiO₃ thin films originates from nanoscale well‐conducting filaments connecting the surface to the SrRuO₃ bottom electrode. The electrical conduction of the filaments is shown to be reversibly modulated over several orders of magnitude by application of an appropriate electrical field. We analyze the resistive switching by addressing individual filaments with the AFM tip as well as by scanning areas up to the µm scale. Temperature dependent measurements reveal that resistive switching on a macroscopic scale can be traced down to the insulator‐to‐metal transition of the independently switchable filaments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation between crystallinity and resistive switching behavior of sputtered WO3 thin films

The as-deposited WO₃ thin films were post-annealed at different temperatures (300 °C and 600 °C) in air to investigate a correlation between crystallinity and switching behavior of WO₃ thin films. Associating the results of XRD, FTIR, XPS and FESEM measurements, the annealing-caused crystallinity change contributes to the variation of the switching behaviors of the WO₃ thin films. The as-deposited WO₃ films with low crystalline structure are preferred for random Ag conducting path, resulting in large switching ratio but fluctuating I–V hysteresis, whereas the annealed WO₃ films with crystallized compact structure limits Ag conducting path, favoring the stable I–V hysteresis but small switching ratio. It is therefore concluded that electrochemical redox reaction-controlled resistance switching depends not only on electrode materials (inert and reactive electrodes) but also on crystallinity of host oxide.     

Effect of thickness on the stability of transparent conducting impurity‐doped ZnO thin films in a high humidity environment

The resistivity of transparent conducting Al‐ and Ga‐doped ZnO (AZO and GZO) thin films prepared with a thickness in the range from 20 to 200 nm on glass substrates at a temperature below 200 °C was found to increase with exposure time when tested in a high humidity environment (air at 90% relative humidity and 60 °C). The resistivity stability (resistivity increase) was considerably affected by the thin film thickness. In particular, thin films with a thickness below about 50 nm were very unstable. The increase in resistivity is interpreted as carrier transport being dominated by grain boundary scattering resulting from the trapping of free electrons due to oxygen adsorption on the grain boundary surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultra‐thin nanocrystalline diamond films (<100 nm) with high electrical resistivity

Thick diamond films are known to exhibit remarkably high electrical resistivity and thermal conductivity. However, on thin films, difficulties are often observed to achieve such performances. In this study, the synthesis of ultra‐thin diamond films was optimized towards the possibility to maintain high dielectric performances on layers compatible with today requirements for Silicon‐On‐Diamond technology, and namely aiming at films with thicknesses equal or below 150 nm. The nucleation of diamond nanocrystals is crucial to obtain films with thickness lower than 100 nm. A Bias Enhanced Nucleation step (BEN) was improved to achieve nucleation densities above 10¹¹ cm^–2 although the process was also tuned to limit the size of the nanocrystals during this step. The control of the carbonization of the silicon substrate is also essential to reach such a density with a high reproducibility. The BEN is followed by a growth step with optimized conditions. The films were characterized by SEM and Spectroscopic Ellipsometry. Electrical conductivity measurements were conducted on thin diamond films and values obtained on layers below 100 nm were as high as 5 × 10¹³ Ω cm; a value significantly higher than the state of the art for such thin films. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pulsed laser deposition of PZT/Pt composite thin films with high dielectric constants

PbZr_0.53Ti_0.47O₃ (PZT) thin films containing nanoparticles of Pt (3–10 nm) were produced using pulsed laser deposition (PLD). The Pt content can be tuned by varying the energy density of the laser beam. Phase and microstructure analysis of the thin films was performed using XRD, SEM, TEM and AFM. The electrical properties were investigated by C–V and I–V measurements. The effective dielectric constant of the composite films increased substantially through the Pt dispersion. These films are promising candidates, for instance, for high-density dynamic random access memory (DRAM) devices. PACS 77.22.Ch; 77.84.Lf; 81.15.Fg     

Growth, coalescence, and electrical resistivity of thin Pt films grown by dc magnetron sputtering on SiO2

Ultra thin platinum films were grown by dc magnetron sputtering on thermally oxidized Si (1 0 0) substrates. The electrical resistance of the films was monitored in situ during growth. The coalescence thickness was determined for various growth temperatures and found to increase from 1.1 nm for films grown at room temperature to 3.3 nm for films grown at 400 °C. A continuous film was formed at a thickness of 2.9 nm at room temperature and 7.5 nm at 400 °C. The room temperature electrical resistivity decreases with increased growth temperature, while the in-plain grain size and the surface roughness, measured with a scanning tunneling microscope (STM), increase. Furthermore, the temperature dependence of the film electrical resistance was explored at various stages during growth.     

Effects of the compliance current on the resistive switching behavior of TiO2 thin films

Nanocrystalline TiO₂ thin films were fabricated on Pt(111)/Ti/SiO₂/Si substrates by the thermal oxidation of evaporated Ti films. Effects of the compliance current on the resistive switching behavior of the Pt/TiO₂/Pt sandwich structures were studied in detail. The reset current increased when the compliance current increased from 10 mA to 20 mA. When the compliance current exceeded 20 mA, the switching behavior disappeared, which could be attributed to the change of the conducting behavior in the low-resistance state. A resistance change ratio of as high as 10² was obtained between the high-resistance state and the low-resistance state. The study of the effect of compliance current contributes to obtaining stable and reliable resistive switching behavior for nonvolatile memory applications.     

Correlation between diode polarization and resistive switching polarity in Pt/TiO2/Pt memristive device

We have investigated the correlation between diode polarization and switching polarity in electroformed Pt/TiO₂/Pt memristive device. Before forming, the diode direction of the Pt/TiO₂/Pt device is reversible under the current pulses with varying current amplitude. The diode polarization arises from oxygen vacancy migration in fully depleted Pt/TiO₂/Pt films. The measurement results indicated that only the polarized diode can be electroformed and the metallic suboxide filament is created in parallel to the diode with a switching polarity dependent on the polarization of stack prior to forming. The non‐polar state inhibits field concentration at either end of the device at the specified current, preventing the electroforming. On and off state currents are measured at 0.2 V for 5 × 10⁴ s showing good retention, which is promising for non‐volatile memory application. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Thermal Stability of Reliable Polycrystalline Zirconium Oxide for Nonvolatile Memory Application

Thermal stability of resistive switching of stoichiometric zirconium oxide thin films is investigated for high yielding nonvolatile memory application. The A1/ZrO2/AI cell fabricated in the conventional device process shows highly reliable switching behaviour between two distinct stable resistance states. The retention capabilities are also tested under various conditions and temperatures. The excellent performance of Ai/ZrO2/AI ceil can be explained by assuming that anode/ZrO2 interface exists and by conducting filament forming/rupture mechanism. The device failure is illustrated in terms of permanent conducting filaments formation.     

Effect of thickness on structural, optical and electrical properties of nanostructured ZnO thin films by spray pyrolysis

Transparent conducting zinc oxide thin films were prepared by spray pyrolytic decomposition of zinc acetate onto glass substrates with different thickness. The crystallographic structure of the films was studied by X-ray diffraction (XRD). XRD measurement showed that the films were crystallized in the wurtzite phase type. The grain size, lattice constants and strain in films were calculated. The grain size increases with thickness. The studies on the optical properties show that the direct band gap value increases from 3.15 to 3.24 eV when the thickness varies from 600 to 2350 nm. The temperature dependence of the electrical conductivity during the heat treatment was studied. It was observed that heat treatment improve the electrical conductivity of the ZnO thin films. The conductivity was found to increase with film thickness.     

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)     

Nonvolatile unipolar resistive switching in ultrathin films of graphene and carbon nanotubes

We report unipolar resistive switching in ultrathin films of chemically produced graphene (reduced graphene oxide) and multiwalled carbon nanotubes. The two-terminal devices with yield >99% are made at room temperature by forming continuous films of graphene of thickness ∼20 nm on indium tin oxide coated glass electrode, followed by metal (Au or Al) deposition on the film. These memory devices are nonvolatile, rewritable with ON/OFF ratios up to ∼ 10⁵ and switching times up to 10 μs. The devices made of MWNT films are rewritable with ON/OFF ratios up to ∼400. The resistive switching mechanism is proposed to be nanogap formation and filamentary conduction paths.     

Conductive Atomic Force Microscopy (C-AFM) observation of conducting nanofilaments formation in GeSbTe phase change materials

GST (GeSbTe) thin films were deposited on glass substrates by electron beam evaporation; Ni was used as the top and bottom electrodes. The I–V (current–voltage) characteristic of the phase change memory (PCM) cell was measured; results showed an electrical threshold switching characteristic for the sample with a threshold voltage of 3.08 V. The threshold switching is attributed to the formation of conductive filaments in the amorphous matrix. Current-voltage spectra which were obtained by C-AFM show that the GST thin film switching from amorphous to the crystalline phase occurs at 1.51 V. C-AFM was used to fabricate crystalline nanoarrays on the sample surface and examine the electrical properties of arrays. In the I–V measurements by C-AFM, when the applied voltage is higher than threshold voltage, conducting nanofilaments with average sizes of 15–60 nm were formed and crystallized spots with current signals were observed. Different times of I–V spectroscopies were applied on thin films to investigate the electrical properties of films during the phase change process. C-AFM results show that as the times of I–V spectroscopies increased, the morphology of crystallized spots changed from bump to pit; the sizes of conductive nanofilaments and detected current signals increased. These results can be attributed to the energy induced by Joule heating dissipated to surrounding films increases with the increasing times of I–V spectroscopies.     

Epitaxial growth and structural characterization of Pb(Fe1/2Nb1/2)O3 thin films

We have grown lead iron niobate thin films with composition Pb(Fe_1/2Nb_1/2)O₃ (PFN) on (0 0 1) SrTiO₃ substrates by pulsed laser deposition. The influence of the deposition conditions on the phase purity was studied. Due to similar thermodynamic stability spaces, a pyrochlore phase often coexists with the PFN perovskite phase. By optimizing the kinetic parameters, we succeeded in identifying a deposition window which resulted in epitaxial perovskite-phase PFN thin films with no identifiable trace of impurity phases appearing in the X-ray diffractograms. PFN films having thicknesses between 20 and 200 nm were smooth and epitaxially oriented with the substrate and as demonstrated by RHEED streaks which were aligned with the substrate axes. X-ray diffraction showed that the films were completely c-axis oriented and of excellent crystalline quality with low mosaicity (X-ray rocking curve FWHM?0.09°). The surface roughness of thin films was also investigated by atomic force microscopy. The root-mean-square roughness varies between 0.9 nm for 50-nm-thick films to 16 nm for 100-nm-thick films. We also observe a correlation between grain size, surface roughness and film thickness.     

Redox driven conductance changes for resistive memory

The relationship between bias-induced redox reactions and resistance switching is considered for memory devices containing TiO₂ or a conducting polymer in “molecular heterojunctions” consisting of thin (2–25 nm) films of covalently bonded molecules, polymers, and oxides. Raman spectroscopy was used to monitor changes in the oxidation state of polythiophene in Au/P3HT/SiO₂/Au devices, and it was possible to directly determine the formation and stability of the conducting polaron state of P3HT by applied bias pulses [P3HT = poly(3-hexyl thiophene)]. Polaron formation was strongly dependent on junction composition, particularly on the interfaces between the polymer, oxide, and electrodes. In all cases, trace water was required for polaron formation, leading to the proposal that water reduction acts as a redox counter-reaction to polymer oxidation. Polaron stability was longest for the case of a direct contact between Au and SiO₂, implying that catalytic water reduction at the Au surface generated hydroxide ions which stabilized the cationic polaron. The spectroscopic information about the dependence of polaron stability on device composition will be useful for designing and monitoring resistive switching memory based on conducting polymers, with or without TiO₂ present.     

Switching process in hard Co–Pt films

The switching process of electrodeposited Co-rich Co–Pt thin films with perpendicular magnetic anisotropy is investigated by out-of-plane angle-dependent hysteresis loop measurements. The switching field angular dependence is discussed in terms of basic reversal mechanisms. A model is proposed, based on a two-step switching process, to evaluate the variations of the intensity and orientation of the internal field as the modulus of external magnetic field is varied at each angle φ. Several experimentally observed salient features are well-understood, indicating that switching is due to inverse domain propagation.     

As-grown preparation of FePt-ordered alloy thin films from Pt/Fe bi-layered structure

A novel method to fabricate FePt-ordered alloy thin films with (0 0 1) preferential orientation from Pt/Fe bi-layered films has been developed. It is able to omit post-annealing with a furnace, and the whole process can be conducted in a chamber without exposing samples to air. In this method, the inter-diffusion and the hetero-epitaxial relation between Fe and Pt are key factors to attain an FePt-ordered alloy thin film with (0 0 1) preferential orientation. An FePt-ordered alloy thin film with (0 0 1) preferential orientation fabricated from Pt[3 nm]/Fe[3 nm] bi-layer employing this method exhibited a relatively large coercivity of 5.6 kOe and high squareness ratio of 0.94 on perpendicular magnetization property. This novel method has a potential to fabricate FePt-based perpendicular magnetic recording media more easily than several conventional methods.     

Enhancement of L10 ordered FePt by Pt buffer layer

L₁₀-ordered FePt thin films prepared by molecular-beam epitaxy on MgO (0 0 1) substrate at 320 °C with different thickness of Pt buffer layer have been investigated. The out-of-plane coercivity increases with increasing thickness of Pt buffer. The maximum values of the long-range order parameter and uniaxial magnetic anisotropy energy are 0.72 and 1.78×10⁷ erg/cm³, respectively, for films with 12 nm thick Pt buffer layer, where the c/a ratio (0.976) shows the minimum value. The reason for the enhancement in ordering is due to the proper lattice strains Pt buffer bestows on FePt layer, these strains are equal to the contraction in lattice parameter c and the expansion in a. Studies of angular-dependent coercivity revealed that the magnetization reversal behaviour shifts from a domain-wall motion dominated case towards a near rotational mode with increasing thickness of Pt buffer layer.