Barrier Height Imaging Research Articles

Electrical characterization of Al/SnO2/PbO/Si double layer MOS under the moderate radiation effect

The metal-oxide semiconductor-based structures are among the most prominent optoelectronic devices that can be used in several application fields. The performance of these structures is strongly dependent on the local radiation dose. In this study, we examined the sample structures with and without radiation. The electrical parameters of the studied samples such as ideality factor, series resistance, saturation current, barrier height, and zero-bias barrier height were obtained by performing the forward bias I–V data analyses. The values of n indicate that the irradiation emission mechanism is dominated. Also, this value depends on the evaluation method of the device. According to the Cheung method, while the ideality factor was found to be 5.72 before irradiation, it was found to be 7.04 after irradiation with 30 kGy. According to the TE method, on the other hand, the value of the ideality factor was found to be 6.337 before irradiation while it was found to be 5.016 after irradiation. The high figures of n were considered to be the result of the interface states of the metal/oxide/semiconductor, i.e. the device, and the barrier height's image force lowering. The obtained dielectric parameters do not have to be absolutely correct, especially when there is radiation on the device. It was found that the value of Rs extremely depended on the particular current transport mechanism's contributions to the total current. The non-ideality factor n increases with the 30 kGy irradiation according to the Cheung method while it does not according to the TE theory. At the same time, the forward bias I–V characteristics and their logarithmic plots reveal two distinct linear sections in the entire linearly increasing voltage.

Gate defects analysis in AlGaN/GaN devices by mean of accurate extraction of the Schottky Barrier Height, electrical modelling, T-CAD simulations and TEM imaging

This paper proposes an investigation focused on the Schottky diode related electrical behaviors on GaN high frequency technologies. As the Schottky diode represents the electrical input terminal (the command) of High Electron Mobility Transistors (HEMTs), this study also correlates with some first order degradation in the active channel (current IDS). Non-invasive methods and related models have been used to determine the accurate Schottky Barrier Height (SBH) of the diode in terms of mean value and dispersion; this approach is convenient to evidence different failure mechanisms on virgin and stressed devices that can be correlated with DC or transient electrical parameters. It is shown that according to given temperature windows and IGS ranges, linear relationships can be extracted between the mean SBH and the inhomogeneities of the SBH that appear in forward-biased diode mode. This original approach permits to determine an increase or a decrease of the global SBH after a stress period. Electrical behaviors issued from the proposed non-destructive technique and from electrical modelling of the diode at different temperatures are found to be consistent with Transmission Electron Microscope (TEM) investigations. T-CAD models have also been used and tuned to account for the impact of interface fixed charge density changes on the electrical signatures of the HEMTs.

Semiquantitative Submolecular Barrier Height Measurements of 4-Aza-8-fluorotryptanthrin Monolayers on HOPG: Orbital-Mediated Tunneling

4-Aza-8-fluoroindolo[2,1-b]quinazoline-6,12-dione (4-aza-8-fluorotryptanthrin) was imaged via scanning tunneling microscopy and barrier height tunneling spectroscopy at the graphite–solution interface. Submolecular resolution was achieved for the apparent barrier height (ϕap) images and revealed that ϕap displays significant variation over the area of the molecule. Topographic effects and electrostatic potential effects were found to be insufficient to explain the behavior of the measured ϕap variations. Specifically, where barrier height images were collected over regions of large molecular orbital (MO) wave function amplitudes, the measured ϕap did not agree with either affecting phenomenon. The authors attribute the disparity to the fact that electron tunneling is orbital-mediated for these systems. Thus, the ϕap is dominated by the spatial disposition of the mediating MO in locations of large wave function amplitude. This hypothesis is corroborated by excellent agreement with a computational treatment...

Semiquantitative Electron Tunneling Barrier Height Measurements of Molecular Monolayers at the Solution–Graphite Interface: Nonorbital-Mediated Tunneling

Semiquantitative apparent tunneling barrier measurements were collected of stearic acid adsorbed onto highly oriented pyrolytic graphite from 1-phenyloctane. A detailed investigation of the topography revealed an interesting geometric phenomenon, namely, the observation of different conformers within a single monolayer; this phenomenon has not been previously reported, despite the large volume of work on these and similar systems in the literature. Apparent barrier height images showed two separate effects. The apparent barrier height map is dominated by the distribution of surface charge (as tracked by electrostatic potential) in regions where significant polarity exists (i.e., near the carboxyl groups). Conversely, in regions where there is little variation in electrostatic potential (i.e., near the interlaced alkane chains), the manifestation of the topography dominates the apparent barrier height. The electron tunneling involved in imaging these types of monolayers is not mediated by adlayer molecular...

Cu(111)表面上のグラフェンのLocal Barrier Height測定

Cu(111)表面上のグラフェンのLocal Barrier Height測定

Barrier height imaging of magnetic films: Use for studying the initial growth of Co films and the surface structure of FePt films

Barrier-height (BH) imaging using scanning tunneling microscopy (STM) was used to study the growth of Co films on Au(001) surfaces. We have observed BH of metastable bcc Co film (>1ML) for the first time, and that showed a large BH value (~6eV), whereas the observed BH of the Au(001) surface (~3.5eV) was consistent with the previous results. The origin of the large BH was qualitatively understood by considering that 3d electrons for tunneling are dominant for the Co(001) surface. We have observed numerous islands with different sizes and heights after 0.15ML Co coverage and successfully obtained, from the BH imaging, an element-specific contrast, i.e. recognizing aggregated Au islands and Co islands, and information about inhomogenities of BH with proper consideration of the artifacts near the step edges. The height modification by the large BH difference is discussed.STM/BH studies of FePt films revealed two kinds of monolayer heights, the sum which was equal to the c axis lattice constant of L10 FePt. Two different dI/dz signal levels were observed on atomically flat terraces.

Unveiling Molecular Adsorption Geometry in Cyclohexanethiolate Self-Assembled Monolayers with Local Barrier Height Imaging

The Au–S interface of thiol-based, self-assembled monolayers grown on Au(111) surfaces is far less understood than it should be. Local barrier height imaging, which has recently resolved work function variations in the buried interface of alkanethiolate monolayers, is now used to study self-assembled monolayers on Au(111) formed by cyclohexanethiol, a molecule with a flexible aliphatic ring structure. Multiple ordered phases are observed, consistent with its conformational flexibility which makes multiple interadsorbate interactions possible. In one particular phase, the appearance of two readily distinguishable features in local barrier height images implies at least two types of adsorption sites. Correlation of simultaneously acquired topographic and local barrier height data allows for a preliminary structural model that accounts for adsorption geometry.

Multiparameter Imaging and Understanding the Role of the Tip - Atomic Resolution Images of Rutile TiO2 (110)

ABSTRACTA major challenge for scanned probe microscopy is to identify structures and chemical species on a surface, which have not already been inferred from other analytical techniques. Progress is impeded by the fact that in general the structure and composition of the tip atom is not known. To illustrate some of the issues involved, we report simultaneous scanning tunneling microscopy/atomic force microscopy (STM/AFM) of the TiO2 (110) surface. The use of small amplitudes enabled the simultaneous acquisition of force gradient and barrier height images during standard STM imaging. Surprisingly, we find most STM images exhibit a corrugation contrast inverse to that usually reported in the literature. However, regardless of the contrast in STM, force gradient images always showed greater attraction over O rows. Barrier height images also show this consistency, always being greater over O rows. This supports the theoretical model of the electronic structure of the surface, but shows that the tip structure and interaction cannot be ignored in modeling STM images. We conclude that there is a fine balance between topography and local density of states (LDOS) in STM imaging of this surface; which of them dominates the STM image is determined by the tip. Simultaneous multi-parameter imaging is useful in interpreting images reliably, particularly on multi-component surfaces.

Local Tunneling Barrier Height at and around Subsurface Dopant Sites on p-GaAs(110)

The local tunneling barrier height at and around subsurface Zn acceptor atoms on GaAs(110) has been investigated by scanning tunneling microscopy (STM) and local barrier height (LBH) imaging. The measured LBH showed the bias dependence, which was in good agreement with the theoretical simulation that takes into account the effect of tip-induced band bending (TIBB). Above the acceptor sites, the bias dependence of LBH shifted to the positive-bias side as compared with that of the area distant from acceptors. We explained the observed shift by the additional local band bending that is induced by ionized acceptors.

Molecular Rotors on Au(111): Rotator Orientation from IR Spectroscopy

Gold surfaces carrying altitudinal molecular rotors firmly attached through sulfur and/or mercury atoms have been examined by IR spectroscopy. The presence of an intact rotator has been confirmed and its average orientation with respect to the gold surface determined by single reflection thin-layer attenuated total reflection (ATR) spectroscopy, using metal surface selection rules. The requisite IR polarization directions were obtained from IR linear dichroism of a model rotator oriented in stretched polyethylene. The results are compatible with those of prior differential barrier height imaging measurements and molecular mechanics calculations.

Local tunneling barrier height observations of NiAl(1 1 0)

Local work functions at individual atoms on a compound metal surface have been experimentally examined. The simultaneously obtained STM and local tunneling barrier height (LBH) images of NiAl(1 1 0) show that LBH at geometrically lower Ni sites is much higher than that at geometrically higher Al sites, indicating that each individual atom on the NiAl(1 1 0) surface has a specific potential barrier for relevant electrons.

Barrier height imaging of Si(1 1 1)3 × 1–Ag reconstructed surfaces

We investigated the bias voltage polarity dependence of atomically resolved barrier height (BH) images on Si(111)3×1–Ag surfaces. The BH images were very similar to scanning tunneling microscopy (STM) images in both the empty and filled states. This similarity strongly supports the interpretation that the BH image reflects the vertical decay rate of the surface local density of states (LDOS). Differences in contrast and protrusion shapes between BH and STM images were observed. We attributed these differences to the geometric contribution to the STM image and to the improved spatial resolution of the BH image due to the lock-in technique.

Scanning Tunneling Microscopy and Barrier-Height Imaging of Subsurface Dopants on GaAs(110)

Subsurface dopant atoms are investigated by scanning tunneling microscopy (STM) and barrier height (BH) imaging. The obtained BH image shows a local reduction in BH -(1–2) eV at positively charged donor sites while it shows a local increase in BH (1–5) eV at negatively charged surface Ga vacancies. Our results indicate that the charge-sensitive imaging can be accomplished by utilizing the STM–BH technique. A simple one-dimensional simulation of tunneling current through a STM junction on the GaAs(110) surface indicates that the results of BH imaging well reflect the charge state of the subsurface dopant or charged surface defects under appreciable electron accumulation conditions.

Relative Conductances of Alkaneselenolate and Alkanethiolate Monolayers on Au{111}

The electronic properties of alkanethiolate [CH3(CH2)nS-, n = 9 and 11] and alkaneselenolate [CH3(CH2)nSe-, n = 9 and 11] self-assembled monolayers on Au{111} have been quantitatively compared. Simultaneously acquired apparent tunneling barrier height (ATBH) and scanning tunneling microscopy (STM) images reveal that alkanethiolate molecules have a lower barrier to tunneling, and therefore a higher conductance than alkaneselenolates of the same alkyl chain length. Molecular and contact conductance differences were elucidated by using observed STM topographic tunneling height differences between the analogous species. This apparent topographic difference combined with comparative ATBH data indicate that the observed decrease in conductance for alkaneselenolates compared to alkanethiolates originates exclusively from the Au-chalcogenide physical, chemical, and electronic contact.

Local-barrier-height images ofTiO2(110)surfaces

Local barrier height (LBH) and constant current mode (CCM) images of nonreconstructed (110) single-crystalline titanium dioxide surfaces are determined under ultrahigh vacuum conditions. The barrier height measurements are performed with the tip-oscillation technique in the constant current mode (CCM) of a beetle-type scanning tunneling microscope with tunneling currents between 1 and $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nA}$. The mean apparent height $\ensuremath{\phi}$ of the surface is derived as a function of the tunneling gap width. The data $\ensuremath{\phi}$ are in accordance with the results from $I\ensuremath{-}z$ spectroscopy. A simple electrostatic space-charge layer model explains the observed decrease of the apparent height for small tunneling gaps. The LBH technique is applied to investigate the structures of point defects on $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ and their effects on the work function on an atomic scale. We find small concentrations of vacancies of the protruding bridging oxygen atoms, giving rise to bright atomic-scale spots between the Ti rows. The barrier height images indicate a localized and reduced apparent barrier height $\ensuremath{\phi}$ at these sites, in correspondence to the overall decrease in the work function $\ensuremath{\Phi}$ that has been observed earlier in ultraviolet photoelectron spectra of ion-sputtered and reduced $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surfaces. Besides the oxygen vacancies, the surface reveals the presence of distinct other types of atomic-scale features, both in topographic and barrier height images. Along the Ti rows, depressions with a concentration of approximately 0.02 monolayers can be identified in the constant current as well as in the barrier height images extending over one and (to a significantly lower extent) two atomic distances along [001]. They are attributed to the so-called type-B defects and they exhibit a local increase of the work function $\ensuremath{\Phi}$.

Local tunneling barrier height imaging of a reconstructed Pt(1 0 0) surface

The effect of surface reconstruction on work function of a quasi-hexagonal reconstructed Pt(100) surface has been studied by local tunneling barrier height (LBH) imaging with scanning tunneling microscopy (STM). Relatively low LBH is observed at the areas providing the strongly-corrugated geometric structure, where the bright contrast is observed in the STM image. This reduction in the local work function has been interpreted in terms of the dipoles induced by the Smoluchowski smoothing effect. This suggests the reason why the macroscopic work function of the reconstructed Pt(100) surface is lower than that of the Pt(111) surface, despite the similar hexagonal surface structures.

Scanning Tunneling Microscopy Barrier Height Imaging of Subsurface Dopant Atoms on Hydrogen-Terminated Si(111)

Dopant atoms beneath a hydrogen-terminated Si(111) 1×1 surface are investigated by scanning tunneling microscopy (STM) and barrier height (BH) imaging. Because of the weak screening effect of the hydrogen-terminated Si(111) surface, subsurface donors and acceptors can be imaged by STM. From BH images, we can deduce a local change in BH at subsurface dopant sites. Both the sign and magnitude of the observed BH change are in good agreement with calculations assuming a screened Coulomb potential for a dopant potential.

Local Tunneling Barrier Height Studies of Thermally Treated CO- and O-covered Pt(100) Surfaces

CO- and O-covered Pt(100) surfaces after thermal treatment have been studied by scanning tunneling microscopy (STM) and local tunneling barrier height (LBH) imaging. The coexistence of the reconstructed (hex) and reconstruction-deconstructed (1×1) domains with similar well-defined shapes is observed for both surfaces. However, the LBH images show that the local work function of the (1×1) domains is lower than that of the hex domains in the case of the CO-covered surface, but higher in the case of the O-covered surface. This feature is considered due to the electric dipole moment induced by O2 adsorption being higher than that induced by CO adsorption.

Dipolar and nonpolar altitudinal molecular rotors mounted on an Au(111) surface.

We describe the preparation of a compound whose molecules consist of two metal sandwich stands carrying tentacles with affinity to metal surfaces and holding an axle that carries a dipolar or a nonpolar rotator. The dipolar rotor exists as three pairs of enantiomers, rapidly interconverting at room temperature. When mounted on a gold surface, each molecule represents a chiral altitudinal rotor, with the rotator axle parallel to the surface. The surface-mounted rotor molecules are characterized by several spectroscopic and imaging techniques. At any one time, in about one-third of the dipolar rotors the rotator is free to turn and the direction of its dipole can be flipped by the electric field applied by an STM tip, as revealed by differential barrier height imaging. Molecular dynamics calculations suggest that electric field normal to the surface causes members of one pair of enantiomers to rotate unidirectionally.

A Search for Subsurface Dopants on Hydrogen-Terminated Si(111) Surfaces

Dopant atoms located beneath the hydrogen terminated Si(111)1×1 surface have been observed by scanning tunneling microscopy (STM) and barrier-height (BH) imaging. Since the hydrogen terminated Si(111)1×1 surface has no significant surface states within the band gap, the screening effect of this surface on subsurface dopants is expected to be weak, compared to that on clean silicon surfaces. This enables us to observe electrically charged dopants located near the surface by STM. Simultaneously obtained BH images show a local reduction of surface potential above the dopant sites. The average reduction of the tunneling barrier height is ∼-1.5 eV.