Growth Of Metallic Thin Films Research Articles

Structure Formation of C60 on Insulating CaF2 Substrates: Matching Experiments with Simulations

AbstractThe epitaxial growth of metallic thin films has been studied intensively, leading to computational models that can predict diverse morphologies depending on thermodynamic and kinetic growth parameters. Much less is known about thin films of organic molecules. Here kinetic Monte Carlo simulations to study buckyball C60 molecules on dielectric CaF2(111) substrates is employed. This system undergoes a transition from mostly one‐layered growth to two‐layered growth of triangular cluster morphologies as the temperature is raised. Even though simplified, these simulations still require a large set of parametrized rates for the elementary transitions, the determination of which has been challenging. Here recent results for these parameters to measure cluster densities and second layer occupations are employed. Simulation snapshots of cluster morphologies are compared in a wide range of temperatures to test to which extent rate models can match the experimental observations. While adjustments to free diffusion parameters are necessary to reproduce the experimentally observed cluster densities and sizes, the cluster morphologies are excellently reproduced without further adjustments.

Advanced Atomic Layer Deposition: Ultrathin and Continuous Metal Thin Film Growth and Work Function Control Using the Discrete Feeding Method.

The ultrathin and continuous ruthenium (Ru) film was deposited through an improved atomic layer deposition (ALD) process with a discrete feeding method (DFM), called DF-ALD, employing a cut-in purge step during the precursor feeding. The excess precursor molecules can be physically adsorbed onto the chemisorbed precursors on the substrate during precursor feeding, which screens the reactive sites on the surface. Using DF-ALD, surface coverage of precursors was enhanced because the cut-in purge removes the physisorbed precursors securing the reactive sites beneath them; thus, nucleation density was greatly increased. Therefore, the grain size decreased, which changed the microstructure and increased oxygen impurity concentration. However, a more metallic Ru thin film was formed due to thermodynamic stability and improved physical density. Consequently, DF-ALD enables the deposition of the ultrathin (3 nm) and continuous Ru film with a low resistivity of ∼60 μΩ cm and a high effective work function of ∼4.8 eV.

Advances in E-ALD By Slrr for the Growth of Ultra Thin Films of Metal and Alloys - in Recognition to John Stickney's Contribution to Electrodeposition

The idea realization progress of electrochemical atomic layer deposition by surface limited redox replacement (E-ALD by SLRR) is among the brightest and most significant contributions of Professor John Stickney. The approach proposed by him in early nineties of last century for epitaxial growth of compound semiconductor films was extended in the beginning of 21st century with his active participation and contribution to the deposition of ultra thin coatings of metals and alloys with a variety of applications thus establishing foundation for the development of one of the most useful and versatile thin film growth methods. In the beginning of this presentation an overview of existing approaches for epitaxial metal thin film growth along with E-ALD by SLRR will first be presented. A comparison of the kinetics manipulation approaches will be made with emphasis on the ability of those methods to produce epitaxial, flat and uniform deposits. In the course of this comparison illustrations will be provided with results, illustrating the advantages and limitations of these deposition approaches in the growth of Ag and Cu thin films on Au (111) substrates. To support the discussion, on the approaches employed, results of electrochemical, in-situ scanning tunneling microscopy (STM), X-ray Photoelectron Spectroscopy characterization experiments will be presented and compared. In the second part, the applicability of SLRR and electroless SLRR in the growth of Pt and Pd on Au (111) substrate will be discussed with special emphasis on the contribution and achievements of John Stickney’s research team. Experimentally demonstrated will be the SLRR growth of Pt and Pd layers with controllable thickness taking place in “shuttling” chamber, flow cell, and “one-pot” configuration. Open circuit potentiometry during the replacement reaction will be shown as a way to control the completion of each deposition event. Anodic stripping of the entire multilayer will be used for determining the overall film thickness. Cyclic Voltammetry and STM will be employed to characterize the growth of Pt and Pd layers with up to 10 nm thickness. High-resolution STM results and modelling concepts underlying the foundation of the proposed approaches will shed light on the mechanistic aspects associated with the nature of growth by redox exchange at a monolayer level. In the third component of this presentation an overview of recent work on the design of potent catalysts with application in energy production and storage will be presented. This will include the use of SLRR approach for coating nanoparticles and continuous nanoporous metal layers with ultra thin films of metal / alloy with specific functionality. Successful outcome of this coating renders the final product catalytically active. In this part, a brief description will be provided also on the direct use of SLRR based protocols for the smart design of bimetallic alloy and/or core shell clusters with application in the most commonly used reactions in the field of fuel-cell catalysis. The discussion on the synthetic component of all relevant activities will be concluded with demonstration of results of testing of accordingly developed catalysts. Finally, a glimpse into the future of this rapidly developing field will be also provided.

(Electrodeposition Division Research Award Address) Metal Deposition Assisted by UPD Monolayers-Fundamentals and Applications

The improved understanding of nucleation and growth kinetics provided the base for design of new experimental approaches where the thin film growth was manipulated to enhance the evolution of atomically flat epitaxial overlayers. Following this progress, a new protocols for electrochemical thin film growth were developed which benefit from the Underpotentially Deposited Monolayers (UPD ML) serving as a surfactants (Surfactant Mediated Growth, SMG), mediators (Defect Mediated Growth, DMG) or sacrificial materials (Deposition via Surface Limited Redox Replacement, SLRR) in electrodeposition process of thin films. In these examples, the essential step that results in desired 2D growth mode is achieved by precise control of the electrode potential, electrodeposition sequence and/or solution chemistry which enable particular action/benefit of the UPD ML to be fully exploited in the growth process. In the first part of the talk we will briefly review fundamentals of the electrochemical thin film growth assisted by UPD ML. Our results and discussion are focused on examining the extendibility of SMG, DMG and SLRR approach for thin film growth in technologically relevant system such as Cu/Ru(hklm). The Cu/Ru(hklm) represents the overlayer/substrate system which has significant epitaxial strain (-5.5%). This fact together with room temperature growth conditions favor a 3D growth. However, as it will be shown, with convenient use of UPD ML serving as surfactant, or mediator, or sacrificial material a 2D growth of Cu on Ru can be achieved readily. In the second part of the talk we will demonstrate that UPD-like ML can be deposited on different substrates using an electroless (e-less) approach. The example of improved morphology and magnetic properties of e-lessly deposited Co overlayers on Cu assisted by e-lessly deposited Pb ML serving as surfactant are analyzed. The phenomenon of e-less UPD-like ML deposition as a prelude for e-less atomic layer epitaxial deposition (E-less ALD) will be discussed as well. We will show results demonstrating noble metal thin film growth on Cu and other substrates using e-lessly deposited Pb UPD-like ML serving as a reducing precursor in SLRR reaction with noble metal ions such as Pt, Pd, and Ru. In conclusion of the talk, (if time permitted), the results from exploratory research studying the benefit of UPD ML serving as a stress moderators in electrodeposition process of Pd thin films will be discussed. The author acknowledges support from Lam Research Corporation and NSF Chemistry division under the contract # 0955922 and #1605331.

Smallest Metallic Nanorods Using Physical Vapor Deposition

Physical vapor deposition provides a controllable means of growing two-dimensional metallic thin films and one-dimensional metallic nanorods. While theories exist for the growth of metallic thin films, their counterpart for the growth of metallic nanorods is absent. Because of this absence, the lower limit of the nanorod diameter is theoretically unknown; consequently the experimental pursuit of the smallest nanorods has no clear target. This Letter reports a closed-form theory that defines the diameter of the smallest metallic nanorods using physical vapor deposition. Further, the authors verify the theory using lattice kinetic MonteCarlo simulations and validate the theory using published experimental data. Finally, the authors carry out a series of theory-guided experiments to grow well-separated metallic nanorods of ∼10 nm in diameter, which are the smallest ever reported using physical vapor deposition.

Oxygen-assisted Ni growth on Fe(001): Observation of an “anti-surfactant” effect

The morphology and the electronic structure of ultrathin Ni films grown on the oxygen-passivated Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O surface are investigated by means of scanning tunneling microscopy and spectroscopy. We observe that oxygen efficiently floats on top of the Ni film and has an ``anti-surfactant'' effect on the Ni/Fe(001) interface, promoting the formation of two-layer-high islands since the first stages of growth, whereas the Ni growth on the oxygen-free Fe(001) surface proceeds in a layer-by-layer mode. Our experimental results show that the effectiveness of the oxygen surfactant action in the growth of metal thin films strongly depends on the choice of the substrate and overlayer combination.

Atomically resolved nucleation and initial growth of a Ag three-dimensional island on Si(001) substrate

The initial growth of a Ag three-dimensional island on an atomically resolved Si(001) substrate was investigated in situ by scanning tunneling microscopy (STM) at room temperature. It took \ensuremath{\sim}20 min for the island to grow from nucleation to a final dimension of 25 nm \ifmmode\times\else\texttimes\fi{} 35 nm \ifmmode\times\else\texttimes\fi{} 22 monolayers (ML). Uniquely, the island growth occurred under no Ag deposition. The Ag atoms required for the growth were provided from the two-dimensional Ag layer and diffusing Ag atoms on the layer that was deposited before the observation. Thanks to this unique growth mechanism, it was allowed to observe the island growth under an isotropic supply of Ag atoms without the shadowing effect of metal deposition by a scanning probe. On the other hand, the STM measurement itself affected finite effects on the growth; attractive interaction between the probe and Ag atoms promoted nucleation of the island, and tunnel current injection may have increased the effective temperature of the system. Despite such a measurement effect, some growth processes that are characteristic of typical metal thin-film growth on silicon substrates were clearly visualized, such as anisotropic and nonmonotonic growth rates that were affected by atomic surface defects, and the growth mode transition from area oriented to height oriented due to an accumulation of stress arising from the lattice mismatch.

Processing effects on microstructure in Er and ErD 2 thin-films

Erbium metal thin-films have been deposited on molybdenum-on-silicon substrates and then converted to erbium dideuteride (ErD 2). Here, we study the effects of deposition temperature (≈300 or 723 K) and deposition rate (1 or 20 nm/s) upon the initial Er metal microstructure and subsequent ErD 2 microstructure. We find that low deposition temperature and low deposition rate lead to small Er metal grain sizes, and high deposition temperature and deposition rate led to larger Er metal grain sizes, consistent with published models of metal thin-film growth. ErD 2 grain sizes are strongly influenced by the prior-metal grain size, with small metal grains leading to large ErD 2 grains. A novel sample preparation technique for electron backscatter diffraction of air-sensitive ErD 2 was developed, and allowed the quantitative measurement of ErD 2 grain size and crystallographic texture. Finer-grained ErD 2 showed a strong (1 1 1) fiber texture, whereas larger grained ErD 2 had only weak texture. We hypothesize that this inverse correlation may arise from improved hydrogen diffusion kinetics in the more defective fine-grained metal structure or due to improved nucleation in the textured large-grain Er.

Influence of Phase Transformation on Stress Evolution during Growth of Metal Thin Films on Silicon

In situ stress measurements during two-dimensional growth of low mobility metal films on amorphous Si were used to demonstrate the impact of interface reactivity and phase transformation on stress evolution. Using Mo1-xSix films as examples, the results show that the tensile stress rise, which develops after the film has become crystalline, is correlated with an increase in lateral grain size. The origin of the tensile stress is attributed to the volume change resulting from the alloy crystallization, which occurs at a concentration-dependent critical thickness.

Experimental measurements of the energetics of surface reactions

Microcalorimetric measurements of the adsorption energies of well-defined surface species are reviewed, using selected examples mainly from our own group to demonstrate the types of information that can be achieved with this technique. The adsorption energies of molecules on single crystal transition metal surfaces to produce well-characterized molecular or dissociated adsorbates allow determination of the standard enthalpies of formation of key catalytic reaction intermediates. The adsorption energies for metal atoms during metal thin-film growth allow quantitative estimates of metal-substrate bond energies, metal film/substrate adhesion energies and the energetic costs associated with lattice mismatch during thin film growth. Results for several metals on MgO(1 0 0) reveal that they bind weakly to terrace sites. Metals from the right side of the periodic table also bind weakly to step and kink sites (although more strongly than on terraces), whereas alkali and alkaline earth metals bind very strongly to these defects. At 300 K, metals tend to form 2D or 3D clusters nucleated on MgO(1 0 0) defects, via a transiently adsorbed precursor (mobile adatoms on terraces). Calorimetric measurement of the energy of metal atoms in supported 3D metal nanoparticles as a function of particle size reveals a very strong size dependence below 6 nm diameter. Metal atoms also adsorb weakly on polymer surfaces and nucleate 3D metal particles, sometimes in kinetic competition with migration to and strong reaction with the more reactive, subsurface organic functional groups. Measurements of the energies for adsorbed proteins on calcium phosphate crystals, which have been structurally characterized by NMR, reveal extremely weak binding dominated by the entropy gained from release of organized water. These experimental measurements of the energies of well-defined adsorbates serve as benchmarks against which to compare theoretical computations for accuracy, thus enabling improvement upon quantum mechanical methods. Comparisons of calorimetric adsorption energies on single crystal surfaces with state-of-the-art DFT calculations show that the latter can often be in substantial (⩾20%) error.

Real time monitoring of the growth of metallic thin films by in situ pyrometry

In situ temperature measurement is an integral part of dwosition processes and optical pyrometry is a useful technique to control the temperature of the growing film. In this work, in situ IR pyrometry has been used for real time monitoring of the early stages of the growth of metallic-type thin films selected as model system. Significant variations of the pyrometric signal were observed during MOCVD of CrC x N y films due to changes of emissivity of the film/substrate system. The pyrometric signal (or emissivity) depends predominantly on the nature, the thickness and the surface roughness of the growing film. As a result, fruitful informations as the formation of an interphase or the existence of an induction period can be obtained in real time by this technique. IR pyrometry can be used as surface diagnostic tool for a large variety of thin film materials that exhibits an emissivity sufficiently different from the substrate.

Multiscale approaches for metal thin film growth

Two recently developed multiscale approaches for simulations of metalization morphologies under ionized sputter deposition conditions are reviewed. These methods are capable of predicting the growth of thin films on the feature scale, based on calculations that are simultaneously carried out on all relevant scales and consistently employ data from atomistic simulations. In both techniques, the first step consists in molecular dynamics simulations that compute the relative probabilities of all relevant reactions taking place during physical vapor deposition. These probabilities are consequently employed in feature-scale simulators. One of the methods employs a two-dimensional cellular automaton technique to calculate the film topographies. The second approach employs a Green function method to combine the molecular dynamics results with a line of sight transport model in a two-dimensional feature, and incorporates all effects of re-emission and re-sputtering in a self-consistent manner. In the final step, a level-set method is used to describe the morphology of the growing film. Both methods provide complete growth rate models that allow the inclusion of energy and angular dependent reaction rates. Several applications of both techniques are discussed.

A rate-equation model for the growth of metallic thin films in ion beam assisted deposition

We describe a simple and fast rate-equation model for surface growth during ion beam assisted metal-on-metal deposition. In terms of the rate coefficients the model describes how island detachment and breakup, adatom diffusion and interlayer transitions of adatoms affect the growth. Results from our model can be directly compared to experimental data, also for coverages of several layers. We identify parameter values corresponding to different modes of growth and locate the phase boundary between them as a function of temperature. We also show that in ideal layer-by-layer growth island size distributions retain the regular scaling form. Our model shows that in order to obtain thin films of good quality, both efficient detachment of adatoms and sufficiently fast interlayer transitions are needed. The results validate the approximations made in an earlier, more coarse-grained model.

Modeling of metal thin film growth: Linking angstrom-scale molecular dynamics results to micron-scale film topographies

A general method for modeling ionized physical vapor deposition is presented. As an example, the method is applied to growth of an aluminum film in the presence of an ionized argon flux. Molecular dynamics techniques are used to examine the surface adsorption, reflection, and sputter reactions taking place during ionized physical vapor deposition. We predict their relative probabilities and discuss their dependence on energy and incident angle. Subsequently, we combine the information obtained from molecular dynamics with a line of sight transport model in a two-dimensional feature, incorporating all effects of reemission and resputtering. This provides a complete growth rate model that allows inclusion of energy- and angular-dependent reaction rates. Finally, a level-set approach is used to describe the morphology of the growing film. We thus arrive at a computationally highly efficient and accurate scheme to model the growth of thin films. We demonstrate the capabilities of the model predicting the major differences on Al film topographies between conventional and ionized sputter deposition techniques studying thin film growth under ionized physical vapor deposition conditions with different Ar fluxes.

Influence of a facetted surface on the epitaxial growth and properties of a magnetic thin film

We have investigated the molecular beam epitaxial growth of metallic thin films on top of Si(1 1 1) substrates misoriented 4° towards [1 1 2 ̄ ] on which we activate the step bunching mechanism using a thermal treatment. It results in a surface where atomically flat (1 1 1) terraces (200–600 Å wide) alternate with facets where up to 30 steps are bunched. The interplay between the starting surface topography and the structural properties of the metallic layers is analysed in detail. Transmission electron microscopy (TEM) observations in cross section clearly reveal the coexistence of two epitaxial relationships, which results in a self organised in-plane bicrystalline modulation of the metallic structure. We have analysed the kinetics of formation of the specific epitaxial relationship on top of the step bunches. We show that it can be unambiguously related to the number of steps bunched. Then we discuss the origin of this bicrystalline modulation through a detailed study of the Cu/Si interface formation.

Metal - Gan Contact Technology

ABSTRACTIn this talk, we summarize the experimental results obtained on metal-GaN interactions in our laboratory. These interactions include the epitaxial growth of metal thin films on chemically cleaned GaN surfaces, metal silicides for Schottky contacts and metallization schemes for ohmic contacts. We found that many fcc and hcp metals can grow epitaxially on (0001) GaN surfaces at room temperature without in-situ surface cleaning. Metal silicide contacts (such as PtSi) may be more suitable for high temperature applications than elemental contacts, due to the thermal stability of silicides. The intrinsic mechanisms for ohmic behavior for various metal contacts are not well understood at present. More consistent barrier height values measured experimentally can shed light on this issue. Due to the defective nature of the GaN layers, carrier transport across the metal/GaN interface can be due to a number of transport mechanisms, thus making accurate determination of the barrier height difficult. In spite of these difficulties, it seems possible to draw certain general conclusions on the electrical behavior of metal contacts on n-GaN.

A New Method for Fast Simulation of Adsorbate Dynamics

ABSTRACTWe present a dynamical version of the Smart Monte Carlo method to model the diffusion dynamics of a metal atom on a metal surface. This method, in conjunction with umbrella sampling, can be used to simulate the dynamics of metal thin film growth, including all relevant atomic-scale phenomenon, over long time scales, characteristic of experimental studies. To demonstrate the accuracy of this method we simulate the dynamics of Rh on Rh(111) and Cu on Cu(100). Interatomic forces are modeled with Lennard-Jones and Corrective-Effective-Medium potentials for the Rh and Cu systems, respectively. We show that this new simulation method correctly reproduces the diffusion dynamics and, with some modification, allows us to reach experimental time scales.

Diffusion behavior of single adatoms near and at steps during growth of metallic thin films on Ni surfaces

Diffusion behavior of single adatoms near and at steps during growth of metallic thin films on Ni surfaces

Diffusion behavior of single adatoms near and at steps during growth of metallic thin films on Ni surfaces

Diffusion of single adatoms approaching both descending and ascending steps on Ni(111), (110) and (100) has been investigated with the embedded atom method (EAM) and molecular statics (MS). (1) It was found that there exists a “forbidden” region near both descending and ascending steps on Ni(111). Adatoms have to overcome a slightly higher energy barrier to get into the “forbidden” region, which extends 2–3 nearest-neighboring spacings from the steps. This is consistent with FIM experiments for incorporation of Ir adatoms into ascending steps of Ir clusters. (2) Detailed calculations for incorporation of adatoms over descending steps of type B on Ni(111) have revealed that exchange diffusion, in which an adatom replaces the position of an atom in the step and exchange their roles, is energetically favored over ordinary jumps and is the dominant diffusion mechanism at the descending steps. This is consistent with recent FIM experiments of W adatom diffusion at Ir cluster edges of type B on Ir(111). (3) Exchange diffusion was also found to be favored over direct jumps at the descending steps on Ni(110) and Ni(100).

Diffusion behavior of single adatoms near and at steps during growth of metallic thin films on Ni surfaces

Diffusion behavior of single adatoms near and at steps during growth of metallic thin films on Ni surfaces