Positive Tip Bias Research Articles

Nanoscale Resistive Switching in Ultrathin PbZr0.2Ti0.8O3–La0.7Sr0.3MnO3 Bilayer

Electric switching of ultrathin ferroelectric films is vital for electronic elements such as multiferroic tunnel junctions, but the large required field may induce ionic motions as addressed here. Unipolar current–voltage (I–V) characteristics of ultrathin (3 nm) PbZr0.2Ti0.8O3 (PZT) grown on La0.7Sr0.3MnO3 (LSMO) by pulsed laser deposition have been investigated in an ambient‐condition force microscope. Topographic changes have been recorded after the I–V measurements. No ferroelectric switching is found, but resistive switching with features in agreement with earlier work on similar perovskite titanates. The onset of hysteretical I–V loops is correlated with a clear height increase in the contacted area under the Pt‐covered tip. The onset voltage of 4.5 V (−2.5 V) for positive (negative) tip bias is clearly different for the two voltage polarities. The observed height changes after applying electrical voltage reveal a volume expansion that must be attributed to chemical changes in both the PZT and the LSMO layers. A fast and a slow process of ionic motion or electrochemical reaction contributing to the observed features is identified. The slow one occurs at negative voltage, causes very large height increase, and is hypothetically attributed to processes following the electro‐splitting of the adsorbed surface water.

Contact electrification and charge decay on polyester fibres: A KPFM study

Surface contact electrification experiments have been performed on insulating polyester fibres in contact with a conductive fibre using biased atomic force microscopy tips. With positive tip bias, charge spots have been deposited in a reproducible manner. The density of deposited charges on the fibre surface was observed by Kelvin probe force microscopy as a function of time, charging position and relative humidity. Two main charge dissipation mechanisms were identified with different importance depending on the relative humidity value.

Field-induced doping-mediated tunability in work function of Al-doped ZnO: Kelvin probe force microscopy and first-principle theory

We demonstrate that the work function of Al-doped ZnO (AZO) can be tuned externally by applying an electric field. Our experimental investigations using Kelvin probe force microscopy show that by applying a positive or negative tip bias, the work function of AZO film can be enhanced or reduced, which corroborates well with the observed charge transport using conductive atomic force microscopy. These findings are further confirmed by calculations based on first-principles theory. Tuning the work function of AZO by applying an external electric field is not only important to control the charge transport across it, but also to design an Ohmic contact for advanced functional devices.

Spurious phenomena occurring during current measurement on ultra-thin dielectric layers: From electro-thermal effects to surface damage

In this paper, the conduction properties of dielectric ultra-thin layers are studied using atomic force microscopy. Especially, the conductive-atomic force microscope allows to measure the leakage current at the nanoscale and to study the degradation mechanisms locally. Nonetheless, the dielectric layer seems to be damaged by a technique's specific phenomenon: hillocks appear when a positive tip bias is applied on different dielectrics. In this paper, the formation of these hillocks is studied. Contrary to what is observed during the dielectric breakdown, the conductivity is reduced after hillocks formation which occurs after the dielectric breakdown. Moreover, we have observed the formation of cavities in the silicon substrate linked to the formation of hillocks, which is not compatible with a swelling process (as dielectric breakdown induced epitaxy). We propose that these results may be explained by an electro-thermal effect due to the large dissipated energy, maybe combined with the oxidation of the substrate. Finally, the interdependence of measurements is demonstrated during serial acquisition.

Tunable atomic force microscopy bias lithography on electron beam induced carbonaceous platforms

Tunable local electrochemical and physical modifications on the carbonaceous platforms are achieved using Atomic force microscope (AFM) bias lithography. These carbonaceous platforms are produced on Si substrate by the technique called electron beam induced carbonaceous deposition (EBICD). EBICD is composed of functionalized carbon species, confirmed through X-ray photoelectron spectroscopy (XPS) analysis. AFM bias lithography in tapping mode with a positive tip bias resulted in the nucleation of attoliter water on the EBICD surface under moderate humidity conditions (45%). While the lithography in the contact mode with a negative tip bias caused the electrochemical modifications such as anodic oxidation and etching of the EBICD under moderate (45%) and higher (60%) humidity conditions respectively. Finally, reversible charge patterns are created on these EBICD surfaces under low (30%) humidity conditions and investigated by means of electrostatic force microscopy (EFM).

Bias polarity dependent local electrical conduction in resistive switching TiO2 thin films

AbstractThe localized electrical conduction behaviour of a resistive switching TiO2 film was examined by conductive atomic force microscopy (CAFM). Localized filamentary electrical conduction was well resolved with a negative tip bias with respect to the Pt bottom electrode whereas almost uniform electrical conduction was observed with a positive bias. The bias dependence of the CAFM result was attributed to local electrochemical reactions mediated by the high electric field near the CAFM tip. A positive tip bias extended the electrically conducting area much more than the actual tip–surface contact area presumably due to facilitated oxygen loss by the high electric field. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electric charging and nanostructure formation in polymeric films using combined amplitude-modulated atomic force microscopy-assisted electrostatic nanolithography and electric force microscopy

A hybrid technique, combining lithography which exploits atomic force microscope tip manipulation with modified electric force microscopy was used to study surface electric charging (deposition and evolution) of polymethyl methacryalate and polystyrene films. Upon charging the films past a threshold voltage, two distinct regimes were observed: (1) stable feature formation related to electric breakdown and mass transport resulting in stable film deformation due to the negative surface charging (negative tip bias) and (2) no stable feature formation regime attributed to viscoelastic deformation of polymer surface followed by the surface relaxation in the case of positive surface charging (positive tip bias).

Enhanced probe nano-oxidation by charge pump effect in swept tip voltage cycles

A probe-based nano-oxidation method for enhanced vertical oxide growth on silicon is presented. The technique involves cycling the tip biases between positive and negative polarities to produce high-aspect-ratio nano-oxides. Enhanced oxidation was observed to take place in the positive tip bias region as opposed to the negative tip voltage required for anodic oxidation. A model based on interface space charge accumulation and neutralization, OH− reactant mobilization, and diffusion is proposed to account for the oxidation enhancement observed under positive tip voltages. The proposed model is analogous to the transient charge pump effect which is at work when the polarity of a capacitor is switched. The results reveal the dynamical behavior of nano-oxidation under nonstatic fields which can be harnessed for fabricating oxide nanostructures with improved aspect ratios.

Mechanisms of Electron Transfer in Two Decaheme Cytochromes from a Metal-Reducing Bacterium

In this report, we analyze and interpret single-molecule current-voltage (I-V) tunneling spectra collected for two decaheme c-type cytochromes using a scanning tunneling microscope. The cytochromes (OmcA and MtrC) are outer-membrane proteins from the metal-reducing bacterium Shewanella oneidensis and function as metal-reducing enzymes. Although the two cytochromes are similar in heme count, charge-carrying amino acid content, and molecular mass, their I-V spectra are significantly different. The I-V spectra for OmcA show smoothly varying symmetric exponential behavior. These spectra are well fit by a coherent tunneling model that is based on a simple square barrier description of the tunneling junction. In contrast, the I-V spectra for MtrC have significant breaks in slope in the positive tip bias range. Two large peaks in the normalized differential conductance spectra of MtrC were fit to a tunneling model that accounts for the possibility of transient population of empty states stabilized by vibrational relaxation. Reorganization energies deduced for the two features are similar to those normally assigned to metal centers in other metalloproteins. Work function measurements of the cytochrome films were used to convert the energies of these two spectral features to the normal hydrogen electrode (NHE) scale for comparison with the redox potential domain previously measured by protein film voltammetry, which showed good correspondence. We conclude that MtrC mediates tunneling current by discretely resolved heme orbital participation at -81 and -365 mV versus NHE. The difference in tunneling behavior between OmcA and MtrC suggests distinct physiological functions for the two cytochromes; in contrast to OmcA, MtrC appears to be tuned to a specific operating potential.

Scanning gate microscopy of InAs nanowires

Scanning gate microscopy, in which a conductive probe tip in an atomic force microscope is employed as a local, nanoscale top gate contact, has been used to characterize local carrier and current modulation effects in a 45nm diameter InAs semiconductor nanowire grown by metal organic chemical vapor deposition. Measurement of current flow in the nanowire as a function of tip position reveals that for both positive and negative tip bias voltages, carrier and current modulation is strongest when the probe tip is near the source and drain nanowire contacts, reaching a global maximum approximately 100–200nm distant from the source contact and a secondary maximum a similar distance from the drain contact and decreasing at greater tip-contact distances. This effect is explained, with verification by numerical simulation, as a consequence of the capacitance between the tip and the source and drain contacts as a function of tip location. Measurement of transconductance as a function of tip position reveals that the transconductance is approximately 80%–90% greater near the source contact than at the center of the nanowire.

Reversible creation and annihilation of a local leakage path in HfO2∕GeOx stacked gate dielectrics: A direct observation by ultrahigh vacuum conducting atomic force microscopy

By direct observation using ultrahigh vacuum conducting atomic force microscopy, it is found that a local leakage path in HfO2∕GeOx stacks created by an electrical stress with a positive tip bias annihilates after applying a reverse tip bias. The creation and annihilation of these paths are repeatable without noticeable degradation. The fact that these paths annihilate by a very small reverse bias suggests that this behavior is caused by local reduction and oxidation in the HfO2 layer.

Nanostructures on oxidized Si surfaces fabricated with the scanning tunneling microscope tip under electron-beam irradiation

Atom transfer from an oxidized Si surface to the tip of a scanning tunneling microscope (STM) was achieved for samples at room temperature, when the tip-sample interaction area is irradiated with an external electron beam. The transfer proceeded stably with use of a Si apex of the STM tip and of a thin Ge coverage on the oxidized Si surface. The extraction of atoms from the sample occurred at negative tip bias voltages, producing nanosized Si windows in the Si oxide film. Atoms accumulated on the tip apex during the extraction can be redeposited back to the windows at positive tip bias voltages. The mechanism of extraction is related to chemically assisted field evaporation, which was realized due to relatively strong bonding strength of extracted atoms to the surface of the Si tip apex.

Surface Modifications of CMR Manganite Thin Films Using Atomic Force Microscope

ABSTRACTReproducible nanoscale surface lithography have been produced on La00.66Ba0.33MnO3 thin films using Atomic Force Microscopy. Using an anodic oxidation technique, structures were produced using both positive tip bias voltages as well as negative tip voltages. The electric field produced by applying a positive tip bias acts to reduce the region near the surface, whereas negative tip bias causes an oxidation reaction. Using a positive tip bias, stable structures were consistently achieved which demonstrated a linear dependence of height and width on tip voltage and writing speed. Heights ranged from 5 – 50 nm and widths from 400 – 2000 nm. Negative bias voltages produced a rapid oxidation of the surface, resulting in uncontrollable writing. After etching with a 50% HCL solution it was found that controlled writing was possible with negative bias voltages.

Linear chains of styrene and methylstyrene molecules and their heterojunctions on silicon: Theory and experiment

We report on the synthesis, STM imaging and theoretical studies of the structure, electronic structure and transport properties of linear chains of styrene and methyl-styrene molecules and their heterojunctions on hydrogen-terminated dimerized silicon (001) surfaces. The theory presented here accounts for the essential features of the experimental STM data including the nature of the corrugation observed along the molecular chains and the pronounced changes in the contrast between the styrene and methyl-styrene parts of the molecular chains that are observed as the applied bias is varied. The observed evolution with applied bias of the STM profiles near the ends of the molecular chains is also explained. Calculations are also presented of electron transport along styrene linear chains adsorbed on the silicon surface at energies in the vicinity of the molecular HOMO and LUMO levels. For short styrene chains this lateral transport is found to be due primarily to direct electron transmission from molecule to molecule rather than through the silicon substrate, especially in the molecular LUMO band. Differences between the calculated position-dependences of the STM current around a junction of styrene and methyl-styrene molecular chains under positive and negative tip bias are related to the nature of lateral electron transmission along the molecular chains and to the formation in the LUMO band of an electronic state localized around the heterojunction.

STM patterning of SnO 2 nanocrystalline surfaces

In this work, we report on the ability to write features less than 15 nm in size on nanocrystalline SnO 2 by applying negative voltage pulses onto an STM tip. The fact that these features can be erased by scanning with a positive tip bias, and the strong dependence of the apparent height of the features with scanning bias after writing seems to indicate that a degree of charge confinement within the 8 nm nano-crystals is involved.

Oxide growth and its dielectrical properties on alkylsilated native-SiO 2/Si surface

The kinetics and mechanism of probe-induced oxide growth on an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) passivated native-SiO 2/Si surface have been investigated using atomic force microscopy (AFM). It was found that oxidation strongly depends on tip polarity and the local chemistry of OTS molecules. At negative tip voltage, oxide grows and the oxidation involves a decomposition and depletion of OTS SAM. While under positive tip bias, OTS molecules are partially degraded. The dielectrical properties of the oxide were characterized by I– V measurements using current sensing AFM. The barrier height of AFM grown oxide, in comparison with thermal oxide and OTS SAM, was determined. Current spikes relating to the release of single charge were also observed on the I– V curves of AFM oxide.

Structure-dependent charge transport and storage in self-assembled monolayers of compounds of interest in molecular electronics: effects of tip material, headgroup, and surface concentration.

The electrical properties of self-assembled monolayers (SAMs) on a gold surface have been explored to address the relation between the conductance of a molecule and its electronic structure. We probe interfacial electron transfer processes, particularly those involving electroactive groups, of SAMs of thiolates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. Peak-shaped i-V curves were obtained for the nitro- and amino-based SAMs studied here. Peak-shaped cathodic i-V curves for nitro-based SAMs were observed at negative potentials in both forward and reverse scans and were used to define the threshold tip bias, V(TH), for electric conduction. For a SAM of 2',5'-dinitro-4,4'-bis(phenylethynyl)-1-benzenethiolate, VII, V(TH) was nearly independent of the tip material [Ir, Pt, Ir-Pt (20-80%), Pd, Ni, Au, Ag, In]. For all of the SAMs studied, the current decreased exponentially with increasing distance, d, between tip and substrate. The exponential attenuation factors (beta values) were lower for the nitro-based SAMs studied here, as compared with alkylthiol-based SAMs. Both V(TH) and beta of the nitro-based SAMs also depended strongly on the molecular headgroup on the end benzene ring addressed by the tip. Finally, we confirmed the "memory" effect observed for nitro-based SAMs. For mixed SAMs of VII and hexadecanethiol, I, the fraction of the charge collected in the negative tip bias region that can be read out at a positive tip bias on reverse scan (up to 38%) depended on the film composition and decreased with an increasing fraction of I, suggesting that lateral electron hopping among molecules of VII occurs in the vicinity of the tip.

Molecule/Metal Surface Interactions Evidenced Quantum Mechanically via Tip-Induced CS2 Interaction with Friedel Oscillations on Au{111}

Submonolayer coverages of CS2 adsorbed on Au{111} at 4 K were studied using scanning tunneling microscopy. The molecule forms well-ordered islands on the terraces and molecular chains at the bottoms of the steps. The adsorption of the CS2 molecule at specific surface sites is explained in terms of the substrate electron density. Strong tip/molecule interactions are shown to be prevalent in this system at negative tip biases and yield images showing reversed corrugation. At low positive tip bias, the tip again perturbs the molecules, but in this regime, the tip/molecule interaction is comparable to the molecule/surface interaction and higher residence times at certain surface sites are observed. This effect is explained fully in terms of the CS2 molecule having increased interactions with the areas of high electron density on the peaks of standing waves arising from electrons close to the Fermi energy. The importance of this result is discussed in terms of the fundamental surface physics of adsorbate/metal bonding.

Scanning spreading resistance microscopy current transport studies on doped III–V semiconductors

Two-dimensional (2D) carrier concentration profiling using scanning spreading resistance microscopy (SSRM) has been carried out on molecular beam epitaxy-grown GaAs and InP dopant calibration samples. The current transport mechanisms between the diamond-coated SSRM tip and the III–V semiconductor cleaved surface (110) was investigated as a function of semiconductor dopant concentration via current–voltage (I–V) measurement. A positive or negative tip bias was applied while scanning over each dopant concentration region (1016–1019 cm−3). The results were compared to simulated I–V curves based on thermionic emission theory. The best fits to the data obtained under forward bias indicated that the contact barrier heights, φb, were much lower than expected from conventional large area planar contacts to GaAs or InP. The effect increases with increasing doping concentration, as a result of a combination of barrier height lowering mechanisms such as image forces, thermionic field emission and minority carrier injection. Under reverse bias, the thermionic emission theory only holds for highly doped semiconductors while for low doping, surface and bulk generation currents determine the overall detectable SSRM current.

STM tip-induced diffusion of In atoms on the Si(111)3×3-In surface

Scanning tunneling microscopy (STM) tip-induced diffusion of In atoms was observed on the $\mathrm{Si}(111)\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{-}\mathrm{I}\mathrm{n}$ surface. We found that In migration to the region beneath the tunneling tip results in the In coverage increase and spontaneous $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{-}\mathrm{I}\mathrm{n}\ensuremath{\rightarrow}2\ifmmode\times\else\texttimes\fi{}2\ensuremath{-}\mathrm{I}\mathrm{n}$ structural transformation at negative tip bias voltages. At positive tip bias voltage In diffusion from the region beneath the tunneling tip results in the In coverage decrease and $2\ifmmode\times\else\texttimes\fi{}2\ensuremath{-}\mathrm{I}\mathrm{n}\ensuremath{\rightarrow}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{-}\mathrm{I}\mathrm{n}$ structural transformation. Indium adatoms in our experiments show up as being positively charged. STM tip field-induced In-atom migration occurs at room temperature, at bias voltages typical for the STM observations, without additional tip movement toward the surface.