Non-lattice Oxygen Research Articles

An influence of bottom electrode material on electrical conduction and resistance switching of TiOxthin films

We investigated the electrical conduction and resistance switching mechanisms of TiOx thin films grown on three kinds of bottom electrode at room temperature (an inert Pt, an active Ti and fluorine tin oxide FTO electrodes). The bottom electrode materials strongly affect the I-V characteristics and switching parameters. The I-V characteristic is explained through the presence of interface states in the metal electrode devices (Pt and Ti) and the work function in the metal oxide device (FTO). The Pt device has the smallest VSET and largest switching ratio, while the Ti device shows the largest VSET and smallest switching ratio. XPS data shows non-lattice oxygen in TiOx films. Therefore, the proposed bipolar resistance switching arises from formation and rupture of filament paths, generated by the movement of oxygen vacancies. All devices depict the same electrical conductions, trap-controlled space-charge-limited, FN tunneling and Ohmic conductions for a high resistance state and a low resistance state, respectively. In this study, the rarely reported FN tunneling conduction in published TiOx-based ReRAM device was found, which can be attributed to an influence of the bottom electrode on the electronic distribution in devices.

Bipolar resistive switching properties of Ti-CuO/(hexafluoro-hexa-peri-hexabenzocoronene)-Cu hybrid interface device: Influence of electronic nature of organic layer

This study reports the change in the structural and junction properties of Ti-CuO-Cu structure on incorporation of a 2-dimensional (2D) organic layer comprising of n-type hexafluoro-hexa-peri-hexabenzocoronene (6F-HBC). A bipolar resistive switching is observed in the device having interface between sputter deposited copper oxide (CuO) and vacuum sublimated 6F-HBC hybrid interface. The CuO/6F-HBC hybrid interface exhibits rectifying I-V characteristics in complete contrast to the ohmic and rectifying characteristics of junctions based on individual 6F-HBC and CuO layers. Large change in resistive switching property from unipolar resistive switching in CuO/HBC to bipolar resistive switching in CuO/6F-HBC interface was observed. At the CuO/6F-HBC interface, C1s peak corresponding to fluorinated carbon is shifted by 0.68 eV towards higher binding energy (BE) side and O1s peak due to non-lattice oxygen is shifted by 0.6 eV towards lower BE, confirming the interaction of O2− ion in CuO with fluorinated carbon atoms in 6F-HBC at the hybrid interface. Correlation between conductive atomic force microscopy images and atomic force microscopy topography images, I-V characteristics in conducting, non-conducting, and pristine regions along with x-ray photoelectron spectroscopy results establishes the important role of hybrid interface to determining the resistive switching properties. This study demonstrates that the resistive switching and interface properties of a hybrid device based on inorganic and organic 2D materials can be modified by changing the electronic properties of organic layer by attaching suitable functional groups.

In-situ post-annealing technique for improving piezoelectricity and ferroelectricity of Li-doped ZnO thin films prepared by radio frequency magnetron sputtering system

Li-doped zinc oxide (L0.03Z0.97O) thin films are deposited onto Pt/Ti/SiO2/Si substrates via the radio frequency magnetron sputtering method. The structure evolution with annealing temperature of the predominantly (002)-oriented Li-doped ZnO (LZO) films after in-situ post-annealing process is determined. The largest values of the piezoelectric coefficient (d33) and the remnant polarization (Pr) (22.85 pm/V and 0.655 μC/cm2, respectively) are obtained for LZO films post-annealed at 600 °C, which can be attributed to the predominant (002)-oriented crystalline structure, the release of intrinsic residual compressive stress, and less non-lattice oxygen.

Effects of Crystallization and Non-Lattice Oxygen Atoms on Cu<SUB><I>x</I></SUB>O-Based Resistive Switching Memory

In this work, the effects of crystallization and non-lattice oxygen atoms on the Cu(x)O-based memory device are investigated. The 150 degrees C-deposited Cu(x)O film possesses a larger amount of non-lattice oxygen atoms than those deposited at the higher temperatures, leading to the formation of AIOy interface layer during the sputtering deposition of Al top electrode. Resistive switching occurring within the interface layer is easily controlled, so the set and reset voltages are decreased. In addition, it is demonstrated that the set and reset processes agree with the formation and rupture of a conductive filament in the Cu(x)O film. The 150 degrees C-deposited Cu(x)O-based memory device with good non-volatility is possibly used in the next-generation non-volatile memory.

Resistive Switching Characteristics of Tm$_{2}$O $_{3}$, Yb$_{2}$O $_{3}$, and Lu$_{2}$O$_{3}$-Based Metal–Insulator–Metal Memory Devices

In this paper, we investigated the electroforming-free resistive switching (RS) behavior in the Ru/RE2O3/TaN (rare-earth, RE, RE = Tm, Yb, and Lu) memory device fabricated with full room temperature process. The conduction mechanism of RE2O3-based memory devices in the low-resistance state is ohmic emission, whereas Tm2O3, Yb2O3, and Lu2O3 memory devices in the high-resistance state are space charge limited conduction (SCLC), ohmic behavior, and SCLC, respectively. The Ru/Lu2O3/TaN device showed a high-resistance ratio of ~ 104, a high device yield of ~70%, a good data retention as long as 105s measured at 85°C, and a reliable endurance for up to 100 cycles, suggesting the optimal chemical defects (metallic Lu and nonlattice oxygen ion) in Lu2O3 film. All of these results suggest that Ru/Lu2O3/TaN structure memory is a good candidate for future nonvolatile RS memory applications.

Improvement of oxygen vacancy migration through Nb doping on Ba0.7Sr0.3TiO3 thin films for resistance switching random access memory application

Undoped and Nb-doped Ba0.7Sr0.3TiO3 (BST) thin films were fabricated by RF magnetron sputtering. The bipolar resistance switching behaviors of both thin films were observed with the stable endurance by DC voltage sweep. Nb doping in BST influenced the defect distribution and improved the uniformity of resistance switching random access memory (ReRAM) properties. The defect distribution was strongly related to the resistance switching properties and the decrease in the grain size caused by Nb doping made the oxygen migration more efficient. The oxygen migration in BST was assisted by Nb dopants which increased the concentration of the non-lattice oxygen in BST layer during ReRAM operation.

Resistive switching behavior of sol–gel deposited TiO2 thin films under different heating ambience

We prepared anatase TiO2 films by sol–gel method under three thermal firing conditions to investigate the bipolar resistive switching (BRS) and unipolar resistive switching (URS) in Ag/TiO2/Pt structure. The devices are URS in an air atmosphere at 760Torr, while those in an oxygen ambience at 1Torr show BRS accompanying with forming-free and self-compliance. By examining the X-ray photoelectron spectroscopy (XPS) spectrum, different non-lattice oxygen content is observed. High concentration of oxygen vacancy is expected under oxygen-deficient treatment, and that would determine the electrode/oxide interface property and induce switching mode of polarity dependent or not. An improved performance of operation voltage dispersion down to 0.5V and endurance up to 3000cycles is obtained for those in reducing Ar.

Effect of non-lattice oxygen on ZrO2-based resistive switching memory

ZrO2-based resistive switching memory has attracted much attention according to its possible application in the next-generation nonvolatile memory. The Al/ZrO2/Pt resistive switching memory with bipolar resistive switching behavior is revealed in this work. The thickness of the ZrO2 film is only 20 nm. The device yield improved by the non-lattice oxygen existing in the ZrO2 film deposited at room temperature is firstly proposed. The stable resistive switching behavior and the long retention time with a large current ratio are also observed. Furthermore, it is demonstrated that the resistive switching mechanism agrees with the formation and rupture of a conductive filament in the ZrO2 film. In addition, the Al/ZrO2/Pt resistive switching memory is also possible for application in flexible electronic equipment because it can be fully fabricated at room temperature.

Non-lattice surface oxygen species implicated in the catalytic partial oxidation of decane to oxygenated aromatics

The one-step transformation of C(7)-C(12) linear alkanes into more valuable oxygenates provides heterogeneous catalysis with a major challenge. In evaluating the potential of a classic mixed-metal-oxide catalyst, we demonstrate new insights into the reactivity of adsorbed oxygen species. During the aerobic gas-phase conversion of n-decane over iron molybdate, the product distribution correlates with the condition of the catalyst. Selectivity to oxygenated aromatics peaks at 350°C while the catalyst is in a fully oxidized state, whereas decene and aromatic hydrocarbons dominate at higher temperatures. The high-temperature performance is consistent with an underlying redox mechanism in which lattice oxide ions abstract hydrogen from decane. At lower temperatures, the formation of oxygenated aromatics competes with the formation of CO(2), implying that electrophilic adsorbed oxygen is involved in both reactions. We suggest, therefore, that so-called non-selective oxygen is capable of insertion into carbon-rich surface intermediates to generate aromatic partial oxidation products.

All Solution-Processed, Fully Transparent Resistive Memory Devices

We fabricated all-solution processed, fully transparent resistive random access memory (sol-TRRAM) with a configuration of ITO/GaZnO(GZO)/ITO. All layers, including an active layer and top and bottom ITO electrodes, were deposited on a glass substrate by either spin coating or inkjet printing using a sol-gel solution. Our sol-TRRAM was transparent, with 86.5% transmittance at 550 nm. An initial forming process is unnecessary for the production of transparent memory due to the presence of sufficient inherent nonlattice oxygen ions in the solution-processed GZO layer. The sol-TRRAM also showed reasonable bipolar resistance switching with a low operation current (<100 μA) and excellent cycle endurance properties (>300 cycles). The main conduction mechanism during the set process can be explained by the trap-controlled space-charge limited conduction, and the resistance change occurred by the modification of the potential barrier height because of the charge injection by Fowler-Nordheim tunneling.

Investigation of oxygen states and reactivities on a nanostructured cupric oxide surface

Nanostructured copper (II) oxide was formed on clean copper foil at room temperature using activated oxygen produced by RF discharge. CuO particles of approximately 10–20 nm were observed on the surface by Scanning Tunneling Microscopy (STM). The copper states and oxygen species of the model cupric oxide were studied by means of X-ray Photoelectron Spectroscopy (XPS). These oxide particles demonstrated abnormally high reactivity with carbon monoxide (CO) at temperatures below 100 °C. The XPS data showed that the interaction of CO with the nanostructured cupric oxide resulted in reduction of the CuO particles to Cu 2O species. The reactivity of the nanostructured cupric oxide to CO was studied at 80 °C using XPS in step-by-step mode. The initial reactivity was estimated to be 5 × 10 −5 and was steadily reduced down to 5 × 10 −9 as the exposure was increased. O1s spectral analysis allowed us to propose that the high initial reactivity was caused by the presence of non-lattice oxygen states on the surface of the nanostructured CuO. We established that reoxidation of the partially reduced nanostructured cupric oxide by molecular oxygen O 2 restored the highly reactive oxygen form on the surface. These results allowed us to propose that the nanostructured cupric oxide could be used for low temperature catalytic CO oxidation. Some hypotheses concerning the nature of the non-lattice oxygen species with high reactivity are also discussed.

Anion-Migration-Induced Bipolar Resistance Switching in Electrochemically Deposited TiOx Films

films were electrochemically deposited on TiN and Pt electrodes with a subsequent annealing under or ambient. The resistive switching characteristics of such films were investigated. The electrodeposited films showed bipolar resistive switching and the operation voltages such as electroforming voltage, and reset/set voltages showed a dependence on the annealing ambient, displaying higher operation voltages for the annealing under than for the annealing under . The compositions of the different chemical states of species in films were observed to be modulated by the condition of electrochemical deposition and the subsequent annealing. The films annealed under the ambient showed a less concentration of nonlattice oxygen ions than under ambient. The reduction of nonlattice oxygen for annealing under was accompanied by the increase of in films and the increased reset/set voltages. It was concluded that the bipolar resistive switching of the electrochemically deposited film was strongly related to the nonlattice oxygen anions in the films.

Modification of surface layer of magnesium oxide via partial dissolution and re-growth of crystallites

A procedure to modify surface layer of metal oxide is presented. By way of partial dissolution and re-growth of crystallites, a new MgO surface layer on the “core” of the original MgO particles was formed. XRD analyses indicate that the new surface layer is different from the original MgO particles in crystallinity. Thus a higher reducibility of surface non-lattice oxygen species is generated. As the extent of dissolution and re-growth of crystallites increased, reducible surface non-lattice oxygen species increased, which led to a lowering of surface non-lattice oxygen concentration on the X%-MgO catalysts in the OCM reaction atmosphere. This is considered to be the major reason for decreasing of CO2 formation.

Atomic-Layer-Deposited HfLaO-Based Resistive Switching Memories With Superior Performance

The excellent resistive switching characteristics of atomic-layer-deposited HfLaO-based devices are investigated for nonvolatile memory applications. With the help of nonlattice oxygen ions which is designed to incorporate into the film by decomposition of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> during process, highly uniform and reproducible resistance switching cycles could be observed with the resistive ratio as high as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> for more than 10 000 cycles. In addition, the fast operation speed (10 ns) has been demonstrated. Conduction of the off state is dominated by the space-charge-limited hopping, while the ohmic behavior dictates the on state, which suggests a filamentary conduction mechanism. Moreover, the estimated readout characteristics under different read voltages from 0.3 to 1 V were sufficiently stable to fulfill the requirement for memory application. Considering the excellent memory switching behavior, a resistance switch device composed of a promising high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> HfLaO dielectric film is a possible candidate to be integrated into future memory processes.

Bipolar resistive switching behavior in Ti/MnO2/Pt structure for nonvolatile memory devices

This study examined the electrical properties of Ti/MnO2/Pt devices with stable and reproducible bipolar resistive switching behavior. The dependency of the memory behavior on the cell area and operating temperature suggest that the conducting mechanism in the low resistance states is due to the locally conducting filaments formed. X-ray photoelectron spectroscopy showed that nonlattice oxygen ions form at the MnO2 surface. The mechanism of resistance switching in the system examined involves the generation and recovery of oxygen vacancies with the nonlattice oxygen ions.

Resistance switching behaviors of V-doped La0.67Ca0.33MnO3 thin films on F-doped SnO2 conducting glass

V-doped La0.67Ca0.33MnO3 (LCMO) thin films were prepared on fluorine-doped SnO2 (FTO) conducting glass substrates with a sol-gel technique. The resistance switching properties of Au/V:LCMO/FTO heterostructures investigated by electrochemical workstation showed reproducible resistive switching behaviors at room temperature. The interactions between nonlattice (mobile) oxygen and oxygen vacancies and/or the cationic vacancies contributed to the carrier transport of the LCMO layer sandwiched systems. With proper doping concentration (3% V-doped LCMO), the resistive switching behaviors could be well improved and stabilized. The maximum resistance ratio obtained could be reached up to 700%. The experimental results show a high potential for nonvolatile memory application on amorphous substrates.

The Effect of Oxygen Annealing on the Resistance Switching Properties of the La&lt;sub&gt;0.7&lt;/sub&gt;Ca&lt;sub&gt;0.3&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; Films

A series of La0.7Ca0.3MnO3 films are fabricated by pulsed laser deposition under controlling the oxygen partial pressure or adding an oxygen post-annealing process. The film after oxygen post-annealing shows an orientation change and has more non-lattice oxygen near the surface than those of the films without annealing. Moreover, only the structure of Ag/La0.7Ca0.3MnO3/Pt with the oxygen annealed film behaves a big current-voltage hysteresis and stable resistance switching properties by applying voltage pulse. Experimental results indicate that the increase of non-lattice oxygen content induced by oxygen annealing in the vicinity of the Ag/La0.7Ca0.3MnO3 interface contributes to the occurrence of resistance switching. And with the accurate controlling of the oxygen annealing parameters, the improvement of resistance switching property of the La0.7Ca0.3MnO3 film can be expected.

Similarity and differences in the oxidative dehydrogenation of C 2–C 4 alkanes over nano-sized VO x species using N 2O and O 2

The effect of O 2 and N 2O on alkane reactivity and olefin selectivity in the oxidative dehydrogenation of ethane, propane, n-butane, and iso-butane over highly dispersed VO x species (0.79 V/nm 2) supported on MCM-41 has been systematically investigated. For all the reactions studied, olefin selectivity was significantly improved upon replacing O 2 with N 2O. This is due to suppressing CO x formation in the presence of N 2O. The most significant improving effect of N 2O was observed for iso-butane dehydrogenation: S(iso-butene) was ca. 67% at X(iso-butane) of 25%. Possible origins of the superior performance of N 2O were derived from transient experiments using 18O 2 traces. 18O 16O species were detected in 18O 2 and 18O 2–C 3H 8 transient experiments indicating reversible oxygen chemisorption. In the presence of alkanes, the isotopic heteroexchange of O 2 strongly increased. Based on the distribution of labeled oxygen in CO x and in O 2 as well as on the increased CO x formation in sequential O 2–C 3H 8 experiments, it is suggested that non-lattice oxygen species (possibly of a bi-atomic nature) originating from O 2 are non-selective ones and responsible for CO x formation. These species are not formed from N 2O.

Characteristics and mechanism of conduction/set process in TiN∕ZnO∕Pt resistance switching random-access memories

The characteristics and mechanism of conduction/set process in TiN∕ZnO∕Pt-based resistance random access memory devices with stable and reproducible nanosecond bipolar switching behavior were studied. The dependencies of memory behavior on cell area, operating temperature, and frequency indicate that the conduction mechanism in low-resistance states is due to electrons hopping through filament paths. We also identify that the set process is essentially equivalent to a soft dielectric breakdown associated with a polarization effect caused by the migration of space charges under a low electric field stress. The generation/recovery of oxygen vacancies and nonlattice oxygen ions play a critical role in resistance switching.

Catalytic wet air oxidation of acetic acid over different ruthenium catalysts

Ruthenium catalysts were prepared by impregnation of different supports: ZrO2, CeO2, TiO2, ZrO2–CeO2 and TiO2–CeO2. Their activities for acetic acid oxidation in aqueous solution were investigated in a stirred reactor at a reaction temperature of 200 °C and total pressure of 4 MPa. The order of the catalyst activity obtained was RuO2/ZrO2–CeO2 > RuO2/CeO2 > RuO2/TiO2–CeO2 > RuO2/ZrO2 > RuO2/TiO2, which corresponds to surface concentration of non-lattice oxygen (defect-oxide or hydroxyl-like group) of these catalysts. The non-lattice oxygen on the catalyst surface plays an important role in the catalytic activity.