Tungsten Adatoms Research Articles

Two-Dimensional Tunneling Memtransistor with Thin-Film Heterostructure for Low-Power Logic-in-Memory Complementary Metal-Oxide Semiconductor.

With the demand for high-performance and miniaturized semiconductor devices continuously rising, the development of innovative tunneling transistors via efficient stacking methods using two-dimensional (2D) building blocks has paramount importance in the electronic industry. Hence, 2D semiconductors with atomically thin geometries hold significant promise for advancements in electronics. In this study, we introduced tunneling memtransistors with a thin-film heterostructure composed of 2D semiconducting MoS2 and WSe2. Devices with the dual function of tuning and memory operation were realized by the gate-regulated modulation of the barrier height at the heterojunction and manipulation of intrinsic defects within the exfoliated nanoflakes using solution processes. Further, our investigation revealed extensive edge defects and four distinct defect types, namely monoselenium vacancies, diselenium vacancies, tungsten vacancies, and tungsten adatoms, in the interior of electrochemically exfoliated WSe2 nanoflakes. Additionally, we constructed complementary metal-oxide semiconductor-based logic-in-memory devices with a small static power in the range of picowatts using the developed tunneling memtransistors, demonstrating a promising approach for next-generation low-power nanoelectronics.

Electronic transport and scattering times in tungsten-decorated graphene

The electronic transport properties of monolayer graphene have been studied before and after the deposition of a dilute coating of tungsten adatoms on the surface. For coverages up to 2.5\% of a monolayer, we find tungsten adatoms simultaneously donate electrons to graphene and reduce the carrier mobility, impacting the zero- and finite-field transport properties. Two independent transport analyses suggest the adatoms lie nearly 1 nm above the surface. The presence of adatoms is also seen to impact the low field magnetoresistance, altering the signatures of weak localization.

Surface diffusion of tungsten adatoms induced by low-energy bombardment with He atoms

ABSTRACTUsing low-temperature field ion microscope techniques, we studied at the atomic level the elementary events of radiation-induced surface diffusion of tungsten adatoms on the ideally pristine surface. The experiments on surface-damage formation and adatom displacements have been performed in situ with a source of neutral helium atoms with an energy of 5 keV. It was demonstrated that the low-energy He atomic bombardment at grazing incidence was able to induce a substantial surface mobility of adatoms. Most of the radiation-induced adatom displacements were oriented along the direction of irradiation. The long impact-induced jumps of adatoms, spanning more than a nearest-neighbor distance, were revealed. Performed molecular dynamics simulations reproduce the general experimental trends and conclusions.

Surface self-diffusion behavior of individual tungsten adatoms on rhombohedral clusters

The diffusion of single tungsten adatoms on the surfaces of rhombohedral clusters is studied by means of molecular dynamics and the embedded atom method. The energy barriers for the adatom diffusing across and along the step edge between a {110} facet and a neighboring {110} facet are calculated using the nudged elastic band method. We notice that the tungsten adatom diffusion across the step edge has a much higher barrier than that for face-centered cubic metal clusters. The result shows that diffusion from the {110} facet to a neighboring {110} facet could not take place at low temperatures. In addition, the calculated energy barrier for an adatom diffusing along the step edge is lower than that for an adatom on the flat (110) surface. The results show that the adatom could diffuse easily along the step edge, and could be trapped by the facet corner. Taking all of this evidence together, we infer that the {110} facet starts to grow from the facet corner, and then along the step edge, and finally toward the {110} facet center. So the tungsten rhombohedron can grow epitaxially along the {110} facet one facet at a time and the rhombohedron should be the stable structure for both large and small tungsten clusters.

Atom Movement on a Dislocated Surface

The standard picture of growth at a screw dislocation assumes that the movement of adatoms on a dislocation loop is the same as on an ideal plane. We have examined this proposition by investigating the movement of a single tungsten adatom on a W(110) plane intersected by a screw dislocation. Surprisingly enough, adatom movement was entirely different than on a normal (110) plane: the overall diffusivity was higher, and the mobility varied with the location of the adatom relative to the dislocation core. This study demonstrates that surface transport is strongly affected in the vicinity of dislocations.

Direct minimization to generate electronic states with proper occupation numbers

We carry out the direct minimization of the energy functional proposed by Mauri, Galli, and Car [Phys. Rev. B $47,$ 9973 (1993)] to derive the correct self-consistent ground-state solution of the Kohn-Sham equation. Since this method completely avoids the instability caused by the level crossing, one can determine the electronic structure of metallic systems to a high degree of accuracy without the aid of level broadening of the Fermi-distribution function. The efficiency of the method is illustrated by calculating the ground-state energy of ${\mathrm{C}}_{2}$ and ${\mathrm{Si}}_{2}$ molecules and the $W(110)$ surface to which a tungsten adatom is adsorbed.

A study of local density of states of atom clusters on the W(110) surface

The kinetic energy distributions of field desorbed He ions from tungsten clusters of one to five atoms on a W(110) surface are measured using a high resolution pulsed-laser time-of-flight atom probe. The He field ion energy distribution from the single W adatom shows an extra peak-like feature centered at 2.7 eV above the Fermi level. It has a full width at half maximum (fwhm) of 2.3 eV. The data from two tungsten adatoms separated by two lattice constants have nearly the same feature with the extra peak located at 2.5 eV above the Fermi level. These peaks arise from resonance tunneling with the adatom local density of states (LDOS). The He ion energy distribution of a tungsten dimer has an extra peak centered at 1.5 eV above the Fermi level. The fwhm is about 4 eV. The spectra from four and five tungsten adatom clusters show only one peak each, similar to that from a flat plane.

Long jumps in surface diffusion: One-dimensional migration of isolated adatoms.

Diffusion of individual metal atoms on a metal is usually viewed as a random walk between adjacent sites. We have now examined the actual jump processes participating in atomic migration by measuring the distribution of displacements for a single atom of tungsten as well as of palladium on W(211), on which diffusion is one dimensional. In the self-diffusion of a tungsten adatom, only jumps between nearest-neighbor sites contribute. For relatively weakly bound Pd atoms, however, significant contributions from jumps spanning two as well as three nearest-neighbor distances have been identified.

Tight-binding calculations of the electronic structure of small transition-metal clusters on a W(110) surface

The electronic structures of small clusters of tungsten and rhodium on a W(110) surface have been calculated within a d-electron tight-binding approximation using the recursion method. The adatom orbital energies and the total energy calculated for three tungsten adatoms on the surface were least for a triangular cluster configuration. For three rhodium adatoms, a linear-chain arrangement gave a lower total adatom orbital energy than that given by a triangular geometry, although the system energy remained higher. The calculations demonstrate the importance of non-bonding electrons for cluster structure and partially account for stabilization of the linear-chain structure for rhodium adatom clusters on a tungsten W(110) surface seen by field ion microscopy.

Field emission current fluctuations from clean tungsten field emitters in the built-up state

Field emission flicker noise spectral density functions of the (012), (113) and (111) tungsten region were measured in the built-up state of the emitter. Their dependence on temperature is analysed in terms of the spectral density and knee frequency activation energies. By comparison with FIM single tungsten adatom migration energies and build-up activation energies of the polyhedral tip and by discussion of the shape of the spectra some conclusions are drawn as to the mechanism of the flicker noise. From the given data favourable operating conditions of TF cathodes can be derived according to the requirements on bandwidth and signal-to-noise ratio.

Surface Diffusion by an Atomic Exchange Mechanism

On Ir(110), tungsten adatoms move preferentially across [$1\overline{1}0$] channels rather than along them, as observed on other channeled planes. The jump mechanism underlying this anomalous behavior has been tested in an atom probe. It is found that after cross-channel motion, an iridium adatom is left on the iridium surface rather than the tungsten originally deposited. These observations provide the first experimental evidence for atomic diffusion on a metal by exchange of the adatom with an atom from the substrate.

A study of atom zigzag chains on the surface of Tungsten

Nishigaki and Nakamura have observed zigzag chains on the central (011) face of tungsten after field evaporation at T #62; 140 K. In this paper, a study of the formation, disappearance and structure of such chains is described. Tungsten tips of small radii down to 60 Å were used. Chains of 3 to 9 spots, that are clearly visible, are found even at 90 K. Four different structure models of the zigzag chains are discussed, including the multibranch model proposed by the Japanese authors. The interpretation of our experimental results shows fairly clearly that the real zigzag chain structure is a special non-dense structure. It must be formed by a local displacement of the tungsten adatoms in the field. Without the field, a zigzag chain is transformed into a two-dimensional cluster of the nearest neighbour atom by a small increase in temperature. If the field is reintroduced, the cluster can revert to the initial zigzag structure. The zigzag structure is interpreted as being caused by forces of repulsion between the atom dipoles.

Direct observation of adatom-substitutional atom interaction on dilute metal alloy surfaces

The field ion microscope is used to study interactions of a migrating tungsten adatom with substitutional rhenium atoms in the (110) plane of a W-3%Re alloy. By observing about 300 migration periods on a single (110) surface plane of a small field ion emittier, each position of the adatom with respect to deduced Re atom locations can be identified. The interaction is found to be attractive with a strength of 90 ± 7 meV at the closest equilibrium separation, and repulsive with a strength of at least 80 meV at the second closest separation. No interaction could be observed for larger separations indicating a strength of less than 10 meV. Results of a control experiment, diffusion of single W adatoms on the (110) plane of pure W, are also presented for comparison.

A tight-binding theory of the interactions between transition metal adatoms adsorbed on a transition metal substrate

A tight-binding theory of the electronically mediated interactions between pairs of transition metal atoms adsorbed on the surface of a model transition metal is presented. The s-orbita calculation of Einstein and Schrieffer is generalised to include multi-orbital adsorbates, and the metallic d-band. Direct interactions between the adatoms can be incorporated easily in this general scheme. The parameters of the calculation have been chosen to simulate the case where the adatoms and the substrate atoms are of the same type, and detailed numerical calculations have been performed for the (100) and (110) surfaces of a bcc n etal. General considerations show that the interaction energy is an oscillatory function of both the fermienergy (which in our model determines the position of the element in the transition series) and the interatomic separation, R, although calculation shows that it decays rapidly with R for any given E F |more rapidly on the (110) than on the (100) surface |. It is found that for value of E F in the centre of the band the dimer formed when the two adatoms occupy nearest neighbour sites is stable on the (110) surface, but unstable on the (100) surface. A comparison is made with the experimental data available for tungsten adatoms on the (110) surface of tungsten.

FIM Observation of Interactions between W Atoms on W Surfaces

Kinetics of rearrangement and self-diffusion of tungsten adatoms of field-evaporated (110) and (321) planes of tungsten were studied by a field ion microscope. A zigzag chain of dimers which was left on (110) plane by field evaporation at 140 K-RT was rearranged by heating at 270 K-300 K to form stable clusters with the rhombic configuration. Evidence of saturation in W-W bindings on (110) plane is presented. Terminal W-W bindings on dimers, trimers and longer atom-rows along the channels on (321) plane exhibited the short-ranged character. However, the interaction between adatoms across the channel was suggested to be long-ranged.

Measurement of the Polarizabilities and Field Evaporation Rates of Individual Tungsten Atoms

The relative field evaporation rates of tungsten kink site atoms at (110) plane edges and the absolute field evaporation rates of individual tungsten adatoms on tungsten (110) planes have been measured covering nine orders of magnitude. From the experimental data, the effective polarizabilities of the tungsten kink site atoms and the adatoms are found to be 4.6 ± 0.6 and 6.8 ± 1.0 Å3, respectively. A theoretical consideration is given to explain why the effective polarizability of metal surface atoms depends on the atom-to-surface mirror plane distance, and therefore the atomic sites. The calculation also reveals a first power field dependent energy level shift, which is dependent upon the atom-to-surface plane distance. It is further shown that the classical image potential no longer holds at a distance smaller than 1 Å from the metal surface.

Atomic View of Surface Self-Diffusion: Tungsten on Tungsten

Surface diffusion of tungsten adatoms on several smooth, low-index planes of the tungsten lattice has for the first time been followed by direct observation of individual atoms in the field-ion microscope. Contrary to expectation, the mobility at room temperature is found to increase in the order (211) > (321) ∼ (110) > (310) ∼ (111). Migrating atoms are reflected at the boundaries of the (110), (211), and (321) planes; on the latter two, motion along atomic rows is favored over diffusion across lattice steps. From quantitative determinations of the rate of change of the mean-square displacement, diffusion coefficients are obtained as follows: (110), D=3×10−2exp(−22 000/RT)cm2/sec; (321), 1×10−3exp(−20 000/RT); (211), 2×10−7exp(−13 000/RT). Differences in diffusion on the (211) and (321), planes of very similar structure, suggest a weakening of interatomic forces at lattice edges.