Metal Oxide Junctions Research Articles

Electrical Scanning Probe Microscopy Investigation of Schottky and Metal-Oxide Junctions on Hetero-Epitaxial 3C-SiС on Silicon

This paper presents a macro-and nanoscale electrical investigation of Schottky and metal-oxide junctions with hetero-epitaxial 3C-SiC layers grown on Si. Statistical current-density-voltage (J-V) characterization of Pt/3C-SiC Schottky diodes showed an increase of the reverse leakage current with increasing the devices diameters. Furthermore, C-V and J-V analyses of SiO2/3C-SiC capacitors revealed non-idealities of the thermal oxide, such as a high trapped positive charge (3×1012 cm−2) and a reduced breakdown field (EBD=6.5 MV/cm) compared to ideal SiO2. Nanoscale electrical characterizations by conductive atomic force microscopy (CAFM) and scanning capacitance microscopy (SCM) allowed to shed light on the origin of non-ideal behavior of Schottky and thermal oxide junctions, by correlating the morphological features associated to 3C-SiC crystalline defects with local current transport and carrier density.

Novel preparation of functional β-SiC fiber based In2O3 nanocomposite and controlling of influence factors for the chemical gas sensing

The gas sensing ability of a pure β-SiC fiber is limited due to its low-sensitivity and selectivity with poor recovery time during a gas sensing test. The combination of functional β-SiC fibers with metal-oxide (MO) can lead to excellent electronic conductivity, boosted chemical activity, and high reaction activity with the target gas and β-SiC–In2O3 sensor material. Influence factors such as amounts of MO, current collectors, and gas species (CO2, O2 and without gas) for the gas sensing ability of β-SiC–In2O3 nanocomposite were determined at standard room temperature (25 °C) and high temperature (350 °C) conditions. The gas sensing ability of the functional β-SiC fiber was significantly enhanced by the loading of In2O3 metal-oxide. In addition, the MO junction on the β-SiC fiber was mainly subjected to the Si–C–O–In bond sensor layer with an effective electron-transfer ability. The gas sensing mechanism was based on the transfer of charges, in which the sensing material acted as an absorber or a donor of charges. The sensor material could use different current- collectors to support the electron transfer and gas sensing ability of the material. A 1:0.5M SiC–In2O3 coated Ni-foil current collector sensor showed better sensing ability for CO2 and O2 gases than other gas sensors at room temperature and high temperature conditions. The sensing result of the electrode was obtained with different current density values without or with gas purging conditions because CO2 and O2 gases had electron acceptor properties. During the gas sensing test, the sensor material donated electrons to target gases. The current value on the CV graph then significantly changed. Our obtained sample analysis data and the gas sensing test adequately demonstrated that MO junctions on functional β-SiC fibers could improve the sensitivity of a sensor material and particularly upgrade the sensor material for gas sensing.

Enhanced gas sensing response for 2D α-MoO3 layers: Thickness-dependent changes in defect concentration, surface oxygen adsorption, and metal-metal oxide contact

• Synthesis of α- MoO 3 2D layers and thin films via pulsed laser deposition technique. • 2D layers show high gas sensing performance towards NO 2 as compared to the thin film. • p-type gas sensing behavior observed in 2D layers in contrast to n-type gas sensing behavior in thin film. • Sensor behavior is explained based on enhanced surface activity, oxygen vacancies, and metal- metal oxide contacts. In this study, α-MoO 3 two-dimensional (2D) layers and thin films were synthesized using pulsed laser deposition technique. X-ray diffraction and Raman spectroscopy confirm the formation of anisotropic α-MoO 3 . Atomic Force Microscopy images show the evolution of morphology from 2D layers to oriented crystallite growth with increase in film thickness. Temperature and gas concentration dependent, gas sensing response has been observed to be quite different in 2D layers in comparison to thin film sample. 2D α-MoO 3 layers (∼ 6 nm) show higher response of about 25 % at a lower temperature (100 °C); They exhibit lower detection limits (up to 100 ppb) and selectivity towards NO 2 gas. Also, 2D layers show p-type gas sensing response and nonlinear current-voltage (I–V) characteristics of metal- metal oxide junctions, in complete contrast to thin film sample, which shows an n-type gas sensing response and linear I–V characteristics. The results have been explained on the basis of lower oxygen defect concentration, enhanced oxygen adsorption at the surface, and metal- α-MoO 3 contact favoring hole conduction.

Transparent Stacked Photoanodes with Efficient Light Management for Solar-Driven Photoelectrochemical Cells.

Solar-driven hydrogen generation is one of the most promising approaches for building a sustainable energy system. Photovoltaic-assisted photoanodes can help to reduce the overpotential of water splitting in photoelectrochemical (PEC) cells. Transparent photoanodes can improve light-conversion efficiency by absorbing high-energy photons while transmitting lower energy photons to the photocathode for hydrogen production. In this work, transparent photoanodes were implemented by forming metal-oxide junctions of NiO/TiO2 heterostructures for creating the photovoltaic effect. The photovoltaic-induced transparent photoelectrode (PTPE) provides the photovoltage (0.7 V), which efficiently reduces the onset potential voltage by -0.38 V versus the reversible hydrogen electrode (RHE), as compared to 0.17 V versus RHE for a single-TiO2 photoanode. The PEC cell has a high photocurrent of 1.68 mA at 1.23 V with respect to the RHE. The chemical endurance of metal-oxides maintains the stability of the PTPE for over 100 h in an alkaline electrolyte of 0.1 M KOH. The results of this study reveal that combining multiple PTPE cells to create a stacked photoanode enhances the photocurrent roughly in proportion to the number of PTPE cells. This design scheme for optimizing the light-conversion efficiency in a PTPE-photoanode system is promising for creating robust systems for on-site energy producers.

High On/Off Ratio Memristive Switching of Manganite/Cuprate Bilayer by Interfacial Magnetoelectricity

Memristive switching serves as the basis for a new generation of electronic devices. Conventional memristors are two‐terminal devices in which the current is turned on and off by redistributing point defects, e.g., vacancies. Memristors based on alternative mechanisms have been explored, but achieving both high on/off ratio and low switching energy, as needed in applications, remains a challenge. This study reports memristive switching in La0.7Ca0.3MnO3/PrBa2Cu3O7 bilayers with an on/off ratio greater than 103 and results of density functional theory calculations in terms of which it is concluded that the phenomenon is likely the result of a new type of interfacial magnetoelectricity. More specifically, this study shows that an external electric field induces subtle displacements of the interfacial Mn ions, which switches on/off an interfacial magnetic “dead layer”, resulting in memristive behavior for spin‐polarized electron transport across the bilayer. The interfacial nature of the switching entails low energy cost, about of a tenth of atto Joule for writing/erasing a “bit”. The results indicate new opportunities for manganite/cuprate systems and other transition metal oxide junctions in memristive applications.

Gas-filling dynamics of a hollow-core photonic bandgap fiber for nonvacuum conditions

This paper introduces an analytical model for the pressure-driven delivery of gas mixtures consisting of multiple components of gases with varied viscosity to hollow-core photonic bandgap fibers. Photonic bandgap fibers have been used for all-fiber spectroscopy configurations for gases like methane and acetylene that are relevant to the development of all-fiber chemical sensors. The unique structure of these fibers permits the formation of a gas cell with micrometer-scaled volumes to promote strong interaction with the guided optical field supported by increased interaction lengths and low optical losses. The influence of the effective viscosity of the delivered gas is significant to the fiber-filling process and is emphasized in this discussion. Unlike previous work, this investigation is not limited to closed-loop configurations with low gas flows, single-gas delivery systems, or vacuum conditions. The findings of this study are relevant to applications employing spectroscopy principles including manufacturing process monitoring for integrated fibers possessing metal–oxide junctions, antiresonant fibers, and suspended-core-type fiber evanescent-field Raman spectroscopy applications. The experimental results agree with the predictions for the case of hydrodynamic flow under nonvacuum conditions. The dynamics of the gas delivery to the perforated optical fiber were predicted and demonstrated for a 0.365 m fiber length with a 12.5 μm core. This core region was occupied by a preliminary gas, which affected the filling rates. The effective viscosities for the pure gas (acetylene) produced filling rates of 6 s (at 10 psi) and 11 s (at 5 psi). However, mixed gases (acetylene balance nitrogen) presented viscosity-dependent delays of 0.3 and 0.5 s.

Modeling of memristive and memcapacitive behaviors in metal-oxide junctions.

Memristive behavior has been clearly addressed through growth and shrinkage of thin filaments in metal-oxide junctions. Capacitance change has also been observed, raising the possibility of using them as memcapacitors. Therefore, this paper proves that metal-oxide junctions can behave as a memcapacitor element by analyzing its characteristics and modeling its memristive and memcapacitive behaviors. We develop two behavioral modeling techniques: charge-dependent memcapacitor model and voltage-dependent memcapacitor model. A new physical model for metal-oxide junctions is presented based on conducting filaments variations, and its effect on device capacitance and resistance. In this model, we apply the exponential nature of growth and shrinkage of thin filaments and use Simmons' tunneling equation to calculate the tunneling current. Simulation results show how the variations of practical device parameters can change the device behavior. They clarify the basic conditions for building a memcapacitor device with negligible change in resistance.

Separate assessment of chemoresistivity and Schottky-type gas sensitivity in M–metal oxide–M′ structures

Maximum response levels reported for chemoresistive sensors span from less than 10 to over 10 8. These differences are attributed to either the different micro- and nano-structured oxide pallets used or the properties of the metal–metal oxide junctions provided. Here, we report separate measurements and model-based estimations of the chemical responses arising from these different origins. The results quantitatively connect the observed responses to the parameters of the metal and metal oxide components of the device. It is shown that while the peak chemoresistive response is microstructure-dependent, the highest attainable Schottky-type gas sensitivity is almost microstructure-independent and is determined by the intrinsic properties of the materials involved. Measurements carried out on different Ag–TiO 2–Ti, Au–TiO 2–Ti and Ti–TiO 2–Ti structures verified the estimations: While chemoresistive responses in TiO 2 can hardly rise over 10 2, atmosphere-sensitive noble metal–TiO 2 junctions can cause responses as high as ∼10 7.

Experimental study of resistive bistability in metal oxide junctions

We have studied resistive bistability (memory) effects in junctions based on metal oxides, with a focus on sample-to-sample reproducibility which is necessary for the use of such junctions as crosspoint devices of hybrid CMOS/nanoelectronic circuits. Few-nm-thick layers of NbOx, CuOx and TiOx have been formed by thermal and plasma oxidation, at various deposition and oxidation conditions, both with or without rapid thermal post-annealing (RTA). The resistive bistability effect has been observed for all these materials, with particularly high endurance (over 1000 switching cycles) obtained for single-layer TiO2 junctions, and the best reproducibility reached for multi-layer junctions of the same material. Fabrication optimization has allowed us to improve the OFF/ON resistance ratio to about 1000, but the sample-to-sample reproducibility is so far lower than that required for large scale integration.

Consideration of switching mechanism of binary metal oxide resistive junctions using a thermal reaction model

The authors investigated the resistive switching of transition metal oxide (TMO) junctions by applying a short voltage pulse and found that the response time of the “reset” process was dependent on the resistance in the low resistive state. By using a thermal conductive equation to calculate the temperature of the filamentary conductive path in the TMO film, the temperature in the reset process was estimated to reach the same temperature grade in each reset. On this basis, the previous experimental relation is well explained by assuming a general thermal chemical reaction model for the reset process.

Experimental evidence and consequences of rare events in quantum tunneling

Tiny spatial fluctuations of tunnel barrier parameters are shown to have dramatic consequences on the statistical properties of quantum tunneling. A direct experimental evidence is provided that the tunnel current through metal-oxide junctions, imaged at a nanometric scale, exhibits broad statistical distributions extending over more than 4 orders of magnitude. Striking effects of broad current distributions are shown: the total tunnel transmission is dominated by few highly transmitting sites and the typical current density varies strongly with the size of the junction. Moreover, self-averaging of the tunnel current fluctuations occurs only for unexpectedly large junction areas.

Interface Stability in Hybrid Transition Metal-Oxide Magnetic Junctions

AbstractRecent experiments revealed an apparently bias-dependent tunneling magnetoresistance between a transition metal (such as Fe, Co) and an oxide barrier such as SrTiO3. We examine the materials issues involved in this type of hybrid transition metal-oxide junctions. The junction interface is shown to be unstable against thermal treatment or high-bias current stress. We conclude that the junction magnetoresistance is largely determined by the formation of an interface oxide layer different from the barrier or the transition metal electrode themselves.

Nanometric cartography of tunnel current in metal–oxide junctions

Recently, ferromagnet/insulator/ferromagnet trilayer junctions were shown to exhibit large magnetoresistance effects. However, these effects proved to be poorly reproducible from sample to sample. To get a nanoscopic insight on the origin of these fluctuations, we have used a combined atomic force microscope/scanning tunneling microscope setup to map the tunnel current that flows through metal/oxide stacks. The current histogram, which extends over 1–2 orders of magnitude, is found to be well described by a simple model which takes into account tiny spatial fluctuations of tunnel barrier thickness (typically, 0.1 nm). Moreover, our analysis shows that the total conductance of imperfect metal/oxide junctions tends to be dominated by very few sites. This result allows us to relate the sample-to-sample conductance fluctuations to slight local variations of the tunnel barrier parameters.

Metal oxide coated p–n junction silicon electrodes for photoelectrochemical solar energy conversion

Photoelectrochemical properties of p+–n junction silicon electrodes, abbreviated as p+n–Si, coated with thin oxide films of titanium, tungsten, or iron have been investigated in aqueous solutions of hydrogen iodide and iodine. The films were deposited by electron-beam evaporation of the metal oxide sinters. The photocurrent in a p+n–Si electrode coated with TiO2 increased dramatically when it was heated at temperatures higher than 500 °C, though the O/Ti atomic ratio as determined by ESCA analysis was unchanged. The fill factor of the photovoltaic cell composed of this electrode, a platinum counter electrode and an aqueous solution of 7.6 M HI and 0.03 M I2 was improved by coating with ruthenium or its oxide. The photovoltaic cell attained a high solar to electrical energy conversion efficiency of 9.3% (AM1), with an open circuit photovoltage of 0.58 V, a short circuit photocurrent of 26.6 mA cm−2 and a fill factor of 0.61. The photocurrent was stable for 350 h. For the cases of electrodes coated with a Fc2O3 or WO3 film, high fill factors were obtained even without ruthenium coating. The effect of the nature of Si – metal oxide junctions, the donor density in the oxide, the relations between the flat band potential of the oxide and the redox potential of the solution, etc., on the conversion efficiency was discussed.

Electrical Conduction in Ferromagnetic Metal Oxide Junctions

The electrical conduction from 4.2°K to room temperature in thin film Ni–NiOx-metal junctions was studied. Tunneling is the dominant conduction mechanism at low temperature; at high temperature, Schottky thermionic emission dominates. The effective barrier thickness determines at what temperature the conductivity changes from tunneling to Schottky emission. Atomic penetration into the oxide is important, and there appear to be broad transition regions at the metal-oxide interfaces. Using the conventional models, the barrier heights are approximately 0.2 eV.

Zero-Bias Anomalies in Normal Metal Tunnel Junctions

We have investigated the current flow through thin chromium-oxide layers from 1\ifmmode^\circ\else\textdegree\fi{}K to 290\ifmmode^\circ\else\textdegree\fi{}K. We believe that current flows by means of a tunneling mechanism, but the dependence of the dynamic resistance of the junction on voltage and temperature is completely anomalous in terms of expected tunneling behavior. Some new results on other metal-oxide junctions strongly suggest that properties of the oxide layer are responsible for the anomaly observed by Wyatt in tantalum oxide junctions.