Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the ‘memristor’ (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron–ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance. Nanoscale metal/oxide/metal devices that are capable of fast non-volatile switching have been built from platinum and titanium dioxide. The devices could have applications in ultrahigh density memory cells and novel forms of computing.

Memristive switching mechanism for metal/oxide/metal nanodevices.

Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

Cobalt ion mediated self-assembly of genetically engineered bacteriophage for biomimetic Co-Pt hybrid material.

The distribution of Schottky barrier heights over the contact area in Au/III-V semiconductor (GaAs, InP, AlxGa1-xAs, InxGa1-xAs) diodes was determined using ballistic electron emission microscopy. Samples which received a chemical pretreatment in aqueous HF or HCl solutions showed changes in the barrier height distribution. In some cases, short rinses in deionized water could remove these effects. Additional XPS measurements and our former work on Si enabled us to propose a model wherein negatively charged species containing F or Cl at the interface are assumed to be responsible for these changes in barrier height distribution. However, in some cases, these effects were shadowed by more drastic influences due to the chemical processing such as changes in the stoichiometry of the surface region.

http://iopscience.iop.org/article/10.1088/0268-1242/14/9/321/pdf

A ballistic electron emission microscopy study of barrier height inhomogeneities introduced in Au/III-V semiconductor Schottky barrier contacts by chemical pretreatments

Nanosphere lithography has been used to prepare a series of ordered, periodic arrays of low barrier height nanometer-scale n-Si/Ni contacts interspersed among high barrier height n-Si/liquid contacts To form the arrays, ordered bilayers of close-packed polystyrene spheres were deposited onto (100)-oriented n-type single crystal Si surfaces The spheres formed a physical mask through which Ni was evaporated to produce regularly spaced and regularly sized Si/Ni contacts By varying the diameter of the latex spheres from 174 to 1530 nm, geometrically self-similar Si/Ni structures were produced having triangular Si/Ni regions with edge dimensions of 100−800 nm The resulting Si surfaces were used as electrodes in contact with a methanolic solution of LiClO_4 and 1,1‘-dimethylferrocene^(+/0) The current−voltage and photoresponse properties of these mixed barrier height contacts were strongly dependent on the size of the Ni regions, even though the fraction of the Si surface covered by Ni remained constant Electrodes formed from large-dimension Si/Ni and Si/electrolyte contacts behaved as expected for two area-weighted Schottky diodes operating independently and in parallel, whereas electrodes having nanoscale Si/Ni regions surrounded by Si/liquid contacts behaved in accord with effective barrier height theories that predict a “pinch-off” effect for mixed barrier height systems of sufficiently small physical dimensions

Investigation of the size-scaling behavior of spatially nonuniform barrier height contacts to semiconductor surfaces using ordered nanometer-scale nickel arrays on silicon electrodes

Polycrystalline and epitaxial (100)-, (110)-, and (111)-oriented ${\mathrm{CoPt}}_{3}$ and ${\mathrm{Co}}_{0.35}{\mathrm{Pt}}_{0.65}$ films were deposited at various growth rates and over a range of growth temperatures from -50 to 800 \ifmmode^\circ\else\textdegree\fi{}C. Films grown at moderate temperatures (200--400 \ifmmode^\circ\else\textdegree\fi{}C) exhibit remarkable growth-induced properties: perpendicular magnetic anisotropy and large coercivity, as well as enhanced Curie temperature and low-temperature saturation magnetization. Magnetic measurements indicate significant Co clustering in these epitaxial fcc films. These properties are independent of crystallographic orientation, increase with increasing growth temperature, and vanish with annealing. We propose that the correlation between magnetic inhomogeneity, magnetic anisotropy, and enhanced moment is explained by clustering of Co into thin platelets in a Pt-rich lattice. This clustering occurs at the growth surface and is trapped into the growing film by low bulk atomic mobility.

Growth-induced magnetic anisotropy and clustering in vapor-deposited Co-Pt alloy films

Nanometer-resolved lateral variations in the Schottky barrier height (SBH) formed at a chemically prepared Au/n-type GaAs interface were measured using ballistic-electron-emission microscopy (BEEM). The spatial profile and the statistical distribution of the SBH's thus obtained were compared to current-voltage (IV) and capacitance-voltage (CV) characteristics of the same metal-semiconductor contact. This comparison showed that the macroscopic SBH obtained from the IV measurements can be successfully interpreted using the parallel conduction model applied to the BEEM-derived distribution, if the effect of thermionic field emission is included. The SBH obtained from the CV measurements is greater than the mean value obtained from BEEM measurements by nearly the image-force lowering expected for a Au/GaAs diode.

Nanometer-resolved spatial variations in the Schottky barrier height of a Au/n-type GaAs diode.

The development of ferroelectric properties in barium titanate (BaTiO3) polycrystalline films has been investigated as a function of substrate type. The films were deposited by physical vapor deposition (PVD) onto different, Pt-coated substrates (magnesium oxide, thermally oxidized silicon, and sapphire) and annealed at temperatures from 725 to 1050 °C. Grain sizes from 50 to 200 nm were produced, with structural and dielectric properties that showed a marked transition from nonferroelectric to ferroelectric behavior across this range. Anneal temperatures below 950 °C result in films with grain sizes less than 150 nm, and ferroelectric properties that are strongly suppressed, regardless of the substrate. Observations for this temperature range include low dielectric constant (e), no polarization hysteresis, and no peaks in the temperature dependence of the dielectric constant. The onset of ferroelectric behavior occurs for anneal temperatures above 950 °C, coinciding with the appearance of strong substrat...

Substrate effects on the ferroelectric properties of fine-grained BaTiO3 films

The lateral variation in the Schottky barrier height (SBH) formed at UHV prepared Au/PtSi/(100)Si (n=4.5×1014) diodes was measured on length scales ranging from a few to several hundred nanometers using ballistic electron emission microscopy (BEEM). The spatial profile and the statistical distribution of the SBHs thus obtained were compared to broad area current–voltage (I–V) and capacitance–voltage (C–V) characteristics of these metal–semiconductor contacts. This comparison showed that the macroscopic SBHs obtained from I–V and C–V measurements can be successfully interpreted using the parallel conduction model applied to the BEEM derived barrier height distribution. In addition, we found that the variations in the SBH were strongly correlated, with an autocovariance length of ∼20 nm at short wavelengths and with a strong peak in the spectral density at a spatial frequency of ∼(225 nm)−1.

Lateral variation in the Schottky barrier height of Au/PtSi/(100)Si diodes

Self-aligned silicidation is a well-known process to reduce the source, drain, and gate parasitic resistances of submicron metal-oxide-semiconductor devices. This process is particularly useful for devices built on very thin Si layer (∼1000 A or less) on insulators. Since the amount of Si available for silicidation is limited by the thickness of the Si layer, once the Si in the source and drain region is fully consumed during silicidation, excessive silicide formation could lead to void formation near the silicide/silicon interface beneath the oxide edge. In this article, we study the effects of different metals (Ti, Ni, Co, and Co/Ti bilayer) with varying thickness on the formation of voids. A change in the moving species during lateral silicide formation was found to be the likely cause for the voids, even if the metals are the moving species during silicidation in the thin film case.

Structural investigation of self-aligned silicidation on separation by implantation oxygen

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