Colossal resistance switching in Pt/BiFeO3/Nb:SrTiO3 memristor

We report reversible resistance switching behaviors in Pt/BiFeO₃/Nb:SrTiO₃ memristor. The resistance of the junctions can be tuned up to about five orders of magnitude by applying voltage pulses at room temperature, which exhibits excellent retention and anti-fatigue characteristics. The high performances are promising for employing ferroelectric junctions in nonvolatile memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the formation and annihilation of trap centers in the BFO films, resulting in Poole–Frenkel emission for low resistance state and the thermionic emission for high resistance state, respectively.     

Metal and annealing atmospheres dependence of resistive switching in metal/Nb0.7wt%-SrTiO3 interfaces

We investigated the resistance switching (RS) effect of metal/Nb-doped SrTiO₃ interfaces under different treating conditions. Two types of I–V characteristics appeared due to the modification of Schottky-like barrier and the formation of insulating layer. According to X-ray photoelectron spectroscopy analysis, the change in interface potential barrier was contributed to the migration of oxygen vacancies and electrons trapping/detrapping of carriers in the vicinity of interface. Nonlinear fitting is applied to the curves to study the conduction mechanism of metal/NSTO. For “barrier height” style, Schottky emission and Poole–Frenkel (P–F) emission are dominating; for “insulating layer” style, space-charge-limited current, controls the conduction.     

Analysis of electrical characteristics of Er/p-InP Schottky diode at high temperature range

We report on the temperature-dependent electrical characteristics of Er/p-InP Schottky barrier diodes. The current–voltage (I–V) and capacitance–voltage (C–V) measurements have been carried out in the temperature range of 300–400 K. Using thermionic emission (TE) theory, the zero-bias barrier height (Φ_bo) and ideality factor (n) are estimated from I–V characteristics. It is observed that there is a decrease in n and an increase in the Φ_bo with an increase in temperature. The barrier height inhomogenity at the metal/semiconductor (MS) interface resulted in Gaussian distribution of Φ_bo and n. The laterally homogeneous Schottky barrier height value of approximately 1.008 eV for the Er/p-InP Schottky barrier diodes is extracted from the linear relationship between the experimental zero-bias barrier heights and ideality factors. The series resistance (R_s) is calculated by Chenug's method and it is found that it increases with the decrease in temperature. The reverse-bias leakage current mechanism of Er/p-InP Schottky diode is investigated. Both Poole–Frenkel and Schottky emissions are described and discussed. Furthermore, capacitance–voltage (C–V) measurements of the Er/p-InP Schottky contacts are also carried out at room temperature in dark at different frequencies of 10, 100 and 1000 kHz. Using Terman's method, the interface state density is calculated for Er/p-InP Schottky diode at different temperatures.     

Electrical parameters of Schottky contacts in CaCu3Ti4O12 thin film capacitors

CaCu₃T_i4O₁₂ (CCTO) thin films were grown by pulsed laser deposition on Pt and La_0.9Sr_1.1NiO₄ (LSNO) bottom electrodes. The electrical characteristics of the CCTO/Pt and CCTO/LSNO Schottky junctions have been analyzed by impedance spectroscopy, capacitance–voltage (C–V) and current–voltage (I–V) measurements as a function of frequency (40 Hz–1 MHz) and temperature (300–475 K). Similar results were obtained for the two Schottky diodes. The conduction mechanism through the Schottky junctions was described using a thermionic emission model and the electrical parameters were determined. The strong deviation from the ideal I–V characteristics and the increase in capacitance at low frequency for ?0.5 V bias are in agreement with the presence of traps near the interfaces. Results point toward the important effect of defects generated at the interface by deposition of CCTO.     

Preparation, structural and electrical properties of zinc oxide grown on silicon nanoporous pillar array

Polycrystalline thick film of zinc oxide (ZnO) is grown on a unique silicon substrate with a hierarchical structure, silicon nanoporous pillar array (Si-NPA), by using a vapour phase transport method. It is found that as-grown ZnO film is composed of closely packed ZnO crystallites with an average size of $\sim$10\,\mu$m. The film resistivity of ZnO/Si-NPA is measured to be $\sim$8.9\Omega\cdot$\,cm by the standard four probe method. The lengthwise $I$-$V$ curve of ZnO/Si-NPA heterostructure is measured. Theoretical analysis shows that the carrier transport across ZnO/Si-NPA heterojunction is dominated by two mechanisms, i.e. a thermionic process at high voltages and a quantum tunnelling process at low voltages.     

Electrical conduction mechanism of polyvinylidenefluoride (PVDF) – polysulfone (PSF) blend film

The electrical conductivity of polyvinylidenefluoride (PVDF) – polysulfone (PSF) blend films have been measured by studying the I–V characteristics in the temperature range of 298–398 K. The results are shown by measuring the dependence of current on field, temperature, and blending compositions in the form of I–V characteristics and analysis has been made by interpretation of Poole–Frenkel, Schottky ln (J) vs. T plots, Richardson and Arrhenius plots. For individual polymers, the conduction mechanism observed to be a Poole–Frenkel type. On blending, the charge conduction appears to be the Schottky emission at lower temperature while Poole–Frenkel mechanism at higher temperature. The analysis of these results suggests that Pool–Frenkel mechanism is mainly responsible for the observed conduction. The conductivity was found to increase with an increase in the polysulfone concentration in the blend; it could be justified in terms of mobility of charge carriers. It is found that mobility of charge carriers increases with the increase in polysulfone concentration in the blend.     

Spin currents and magnetoresistance of graphene-based magnetic junctions

Using the tight-binding approximation and the nonequilibrium Green’s function approach, we investigate the coherent spin-dependent transport in planar magnetic junctions consisting of two ferromagnetic (FM) electrodes separated by a graphene flake (GF) with zigzag or armchair interfaces. It is found that the electron conduction strongly depends on the geometry of contact between the GF and the FM electrodes. In the case of zigzag interfaces, the junction demonstrates a spin-valve effect with high magnetoresistance (MR) ratios and shows negative differential resistance features for a single spin channel at positive gate voltage. In the case of armchair interfaces, the current-voltage characteristics behave linearly at low bias voltages and hence, both spin channels are in on state with low MR ratios.     

异质结界面电荷对突变InP/InGaAs异质结双极晶体管热场发射影响研究

异质结界面电荷的存在改变了异质结的内建势, 这引起了界面势垒尖峰高度和形状的扰动, 从而使异质结界面载流子的输运产生相应的变化, 最终导致异质结双极晶体管 (HBT) 性能的改变. 基于热场发射-扩散模型, 对异质结界面电荷对InP/InGaAs HBT性能的改变做了研究, 得到结论是正极性的界面电荷有利于InP/InGaAs HBT的直流和高频特性的改善, 而负极性的界面电荷则使器件的直流和高频特性变差. 关键词： InP/InGaAs异质结双极晶体管 界面电荷 内建势 热场发射     

Barrier formation at organic interfaces in a Cu(100)-benzenethiolate-pentacene heterostructure

The energy level alignment at the metal-organic and organic-organic interfaces of the Cu(100)/benzenethiolate/pentacene heterostructure is studied by photoemission spectroscopy and discussed theoretically using a model that includes, in a consistent way, charge transfer, Pauli repulsion, intrinsic molecular dipoles, and interface screening as a function of coverage. Despite the different nature of the two interfaces, our model provides a unified explanation for the work-function changes at both junctions and enables us to determine the benzenethiolate orientation as a function of coverage.     

Mechanism of current leakage through metal/n-GaN interfaces

Detailed current–voltage–temperature (I–V–T) measurements were performed on the Schottky diodes fabricated on MOVPE-grown n-GaN layers. A large deviation from the thermionic emission (TE) transport was observed in the reverse I–V curves with a large excess leakage. From the calculation based on the thermionic-field emission (TFE) model, it was found that the tunneling plays an important role in the carrier transport across the GaN Schottky barrier even for doping densities as low as 1×10¹⁷ cm⁻³. A novel barrier-modified TFE model based on presence of near-surface fixed charges or surface states is proposed to explain the observed large reverse leakage currents.     

Mechanism of Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures via laser annealing

The physical mechanisms of Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures by laser annealing and rapid thermal annealing are systematically investigated. The microstructures indicate that a better surface morphology and an intact contact interface are formed after laser annealing. None of the TiN alloy spikes are formed at the interface of the laser annealing sample. The experimental results show that the current transport mechanism through the ohmic contact after laser annealing is different from the conventional spike mechanism, and it is dominated by thermionic field emission.     

Spin-polarized edge and magnetoresistance in graphene flake

We examine spin-polarized edge and magnetoresistance (MR) in Fe/graphene flake (GF)/Fe junctions. For simulating various edge designs, we use the tight-binding approximation, mean-field scheme for the Hubbard model and the Landauer-Büttiker formalism. The Fe electrodes strongly affect electronic states and magnetic properties of the GF and induce magnetism in the edge atoms even in case of armchair interfaces. Also, the edge magnetic moments of the zigzag interface rotate and couple antiferromagnetically with the Fe electrodes. The conductivity of the junctions strongly depend on the relative magnetic orientation of the Fe electrodes, so, the junctions show high MR ratios. Moreover, edge geometry and localized edge state in the GF alter the MR ratios and produce large (small) variation in the MR at low (high) bias voltages.     

Electrical and piezoresistive properties of ion beam deposited DLC films

In present study diamond like carbon (DLC) films were deposited by closed drift ion source from the acetylene gas. The electrical and piezoresistive properties of ion beam synthesized DLC films were investigated. Diode-like current–voltage characteristics were observed both for DLC/nSi and DLC/pSi heterostructures. This fact was explained by high density of the irradiation-induced defects at the DLC/Si interface. Ohmic conductivity was observed for DLC/nSi heterostructure and metal/DLC/metal structure at low electric fields. At higher electric fields forward current transport was explained by Schottky emission and Poole–Frenkel emission for the DLC/nSi heterostructures and by Schottky emission and/or space charge limited currents for the DLC/pSi heterostructures. Strong dependence of the diamond like carbon film resistivity on temperature has been observed. Variable range hopping current transport mechanism at low electric field was revealed. Diamond like carbon piezoresistive elements with a gauge factor in 12–19 range were fabricated.     

Sub-micro watt resistive memories using nano-crystallized aluminum oxynitride dielectric

Using nano-crystallized aluminum oxynitride (nc-AlO_xN_y) dielectric, the Al/nc-AlO_xN_y/AlN/n⁺-Si resistive random access memory (RRAM) with ultralow sub-micro watt power is reported in this study. The RRAM devices exhibit excellent memory characteristics, including reproducible bipolar resistive switching under >100 times memory window, very low set and reset current of ~10 nA, high voltage distributions and good data retention. It is demonstrated that the reset current decreases as the compliance current decreases, which provides an approach to lower the power consumption. The conduction mechanisms for high- and low-resistance states are dominated by Frenkel–Poole conduction and space-charge-limited current, respectively. These good memory characteristics in this RRAM show great potential in future high-performance memory applications.     

Transient photovoltage and photoluminescence study of exciton dissociation at indium tin oxide/pentacene interface

The exciton dissociation at ITO/pentacene interface is studied by means of transient photovoltage measurement.Opposite to ITO/NPB,ITO/CuPc or ITO/C60 interface where polarity change of transient photovoltage is observed,no interfacial dissociation is found at room temperature,which indicates a lack of Frenkel excitons in pentacene.Temperature-dependent photoluminescence (PL) is investigated.More like the behavior of inorganic semiconductors,the integrated PL intensity exhibits monotonic decrease with increa...     

Scanning probe microscopy study of exfoliated oxidized graphene sheets

Exfoliated oxidized graphene (OG) sheets, suspended in an aqueous solution, were deposited on freshly cleaved HOPG and studied by ambient AFM and UHV STM. The AFM images revealed oxidized graphene sheets with a lateral dimension of 5–10 μm. The oxidized graphene sheets exhibited different thicknesses and were found to conformally coat the HOPG substrate. Wrinkles and folds induced by the deposition process were clearly observed. Phase imaging and lateral force microscopy showed distinct contrast between the oxidized graphene and the underlying HOPG substrate. The UHV STM studies of oxidized graphene revealed atomic scale periodicity showing a (0.273 ± 0.008) nm × (0.406 ± 0.013) nm unit cell over distances spanning few nanometers. This periodicity is identified with oxygen atoms bound to the oxidized graphene sheet. I(V) data were taken from oxidized graphene sheets and compared to similar data obtained from bulk HOPG. The dI/dV data from oxidized graphene reveals a reduction in the local density of states for bias voltages in the range of ±0.1 V.     

Ultrafast exciton relaxation in microcrystalline pentacene films

The exciton dynamics in microcrystalline pentacene films is investigated by transient absorption measurements with 30 fs time resolution. It is found that the emission from photoexcited Frenkel excitons decays within 70 fs due to the ultrafast formation of an excitonic species with a strongly reduced transition dipole to the ground state and an absorption dipole in the plane of the film. We propose that an excimer exciton is formed and stabilized by changes of the local crystal structure. The subsequent dynamics is dominated by diffusion controlled annihilation and trapping.     

Leakage currents in ferroelectric thin films

The main mechanisms of leakage currents in thin lead zirconate titanate (PZT) ferroelectric films prepared by the sol–gel method are discussed. Four specific regions are determined in I–V dependencies. At very weak fields (10–20 kV/cm), the current falls with the voltage increase as a result of depolarization. In the low fields region (about 70–100 kV/cm), the leakage current decreases with the decrease of voltage ramp speed and its components are the ohmic and displacement currents. In the high fields region (≥130 kV/cm), the leakage current increases with the decrease of step voltage ramp in contrast to the previous case. Possible conductivity mechanisms are the Poole– Frenkel emission and hopping conduction. In the transition region between above-mentioned ones (from 80–90 to ～130 kV/cm), an abrupt unstable increase of current is observed caused by breakdown of reverse bias Schottky barrier. Depolarization currents are studied for sol–gel PZT films prepared at different preparation conditions.     

Exciton dynamics at interfaces of organic semiconductors

We review recent progress of using time-resolved two-photon photoelectron spectroscopy (2PPE) to study the energetics and dynamics of excitons at surfaces and interfaces of two prototypical organic semiconductors: C₆₀ and pentacene. For C₆₀ thin films epitaxially grown on Au(1 1 1) and Cu(1 1 1) surfaces, we observe both charge transfer and exciton states. For excitons in C₆₀, the proximity of a metal surface leads to rapid, exciton band-mediated quenching. At the surface of pentacene thin films we observe a series of charge-transfer excitons where the electron and the photohole are bound across the interface. The ability of 2PPE to measure and directly relate exciton levels to single-electron levels is illustrated.     

Injection of holes at indium tin oxide/dendrimer interface: An explanation with new theory of thermionic emission at metal/organic interfaces

The traditional theory of thermionic emission at metal/inorganic crystalline semiconductor interfaces is no longer applicable for the interface between a metal and an organic semiconductor. Under the assumption of thermalization of hot carriers in the organic semiconductor near the interface, a theory for thermionic emission of charge carriers at metal/organic semiconductor interfaces is developed. This theory is used to explain the experimental result from Samuel group [J.P.J. Markham, D.W. Samuel, S.-C. Lo, P.L. Burn, M. Weiter, H. Baessler, J. Appl. Phys. 95 (2004) 438] for the injection of holes from indium tin oxide into the dendrimer based on fac-tris(2-phenylpyridyl) iridium(III).