Session M32: Metal Films and Interfaces

The Path Toward Molecular Beam Epitaxy of Single Crystalline Actinide Materials

Actinide-based materials possess unique physics due to the presence of 5f electrons. However, samples with high purity and crystallinity are required for the effective examination of unique quantum phenomena and to provide accurate experimental validation of theoretical models. Molecular beam epitaxy presents an attractive avenue for the fabrication and study of monocrystalline actinide materials as it is a non-equilibrium synthesis technique which offers a high degree of control over purity, dimensionality, strain, and interfaces between monolithically grown materials. The methods for controlling and accessing the complex oxidation states of actinide elements (such as U and Th) in a thin film process are not fully understood. It is important to comprehend the possible formation mechanisms and behavior of these materials prior to introducing them to the deposition chamber. Therefore, materials with low vapor pressure and complex oxidation states should be investigated as preliminary surrogates to minimize issues around maintenance and safety. Many transition metals have intricate bonding arrangement possibilities; and when combined with nitrogen, are known (or predicted) to have crystalline symmetry similar to the actinide nitrides of interest. Additionally, understanding the growth processes of select transition metal nitrides could pave the way to integration of superconducting and magnetic materials with existing group III-nitride optoelectronics.   

Synthesis of Air-Stable, Atomically Thin Heavy Metals through Confinement Heteroepitaxy

Atomically thin heavy metals (Pb, Bi) are known to exhibit strong Rashba spin orbit coupling suggesting their viability in spintronics; however, their instability in ambient precludes device integration. We demonstrate the synthesis of centimeter-scale, two-dimensional Pb, Bi, and PbGa via confinement heteroepitaxy (CHet) which leverages the interface between epitaxial graphene and silicon carbide to assemble atomically thin, air-stable 2D metals. Through ex-situ microscopy, spectroscopy, diffraction, and transport characterization of these 2D heavy metals, we find > 90% 2D-metal coverage, unique spectroscopic features in Raman, evidence of ~500meV spin splitting in 2D-Pb, and superconductivity up to 3K in 2D-PbGa.   

Epitaxial growth of ultrathin, crystalline Au/Ag and Al/Ag metal heterostructures on silicon for plasmonic applications

The plasmonic response of ultrathin (less than 5 nm), nanostructured Ag could be powerful means of confining and manipulating light, but such films are unstable in ambient conditions without a passivating layer, which may also impact the overall electronic and optical properties. Here, we demonstrate the epitaxial growth of Au/Ag and Al/Ag heterostructures on (7x7)-Si(111) exhibiting well-defined mid-infrared plasmons after exposure to air and nanopatterning by electron beam lithography. Angle-resolved photoemission spectroscopy, X-ray photoemission spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction are used to monitor the heterostructure growth. The high quality of the metal films is further monitored by resolving the electronic band structure and quantum well states via photoemission as a function of Ag thickness on the Si substrate and subsequent coverage of the Ag by Au or Al. The results suggest that a wide variety of ultrathin, crystalline superlattices could be obtained for electronic, spintronic, or photonics applications by using Ag films on Si(111) as a platform.   

Adhesion and Wetting of Liquid/Metal Interfaces

The wetting of a liquid on a substrate is often modeled using Young’s equation.  This states that the wetting angle is determined by the surface energy of the substrate, the surface tension of the liquid, and the liquid/substrate interfacial energy, phenomena which are temperature and composition dependent. Young’s equation is based on continuum assumptions, and so open questions remain regarding the length scale at which these assumptions break down. In this work we consider the validity of Young’s equation at the nanoscale through the use of interfacial thermodynamics and molecular dynamics simulations of a liquid metal alloy on a metallic substrate. In addition, we consider the impact of compositional effects within the liquid metal alloy on wetting.

    

2D Mg-Cu Intermetallic Compounds with Nontrivial Band Topology and Dirac Nodal Lines

Synthesis of vertical heterostructures that include atomically thin layers of materials with topologically nontrivial energy bands is desirable for exploring exotic quantum states. Here, the authors report on atomic-layer-by-layer deposition of magnesium on copper(111) surface by molecular beam epitaxy, monitored in situ by low-energy electron microscopy and diffraction, and modeled by ab initio theory. It is found that a 2D MgCu₂ intermetallic compound forms during initial Mg deposition and persists till a full monolayer of Mg is formed. Deposition of additional Mg triggers a phase transition from the commensurate MgCu₂ to an incommensurate Mg₂Cu layer and enables growth of the second layer of Mg. Ab initio calculations indicate non-trivial topology of the electron bands in the interfacial Mg₂Cu layer and the existence of Dirac nodal lines near the Fermi level. The new 2D Mg₂Cu material emerges as a promising platform to study new topological states of matter.   

Growth dependent structural ordering and stability in sputtered grown β tungsten thin films for Spintronic applications

Spintronic is an emerging field for the next generation spin-based nanodevices. Heavy metals like tungsten (W) can generate nonequilibrium spins via the spin Hall effect. Recently, the β phase of W has emerged as a potential candidate for spintronic applications due to its higher spin Hall angle compared to the other elemental solids and large spin-orbit torque. However, ideally, it is a metastable phase, and its stability is found to be due to the presence of oxygen. We have shown the efficient ability of the dc magnetron sputtering technique to grow a stable β-W film at room temperature by varying a set of deposition parameters. The atomic strcuture of the films and the existence of the β phase have been recognized from the synchrotron-based X-ray diffraction studies. The stability and atomic structure of the β phase have been further confirmed from the ab-initio molecular dynamics simulations, which show the presence of oxygen is to be very important for the stability of this phase. Simulation with different oxygen concentrations suggests about 20 at.% oxygen for the β phase formation. This agrees well with the experimental findings

    

VSi2 Nanoclusters and Nanoribbons

We studied the epitaxial growth of VSi₂ on Si(111). The films were grown under UHV conditions using e-beam deposition of V on Si(111) substrates and subsequent annealing to temperatures ranging between 600-1100 °C. The films were investigated using in-situ scanning tunneling microscopy (STM), ex-situ atomic force microscopy (AFM), and ex-situ scanning electron microscopy (SEM).

At V coverages much larger than 1 monolayer (ML), we found that VSi₂ prefers to grow into 3 types of islands: hexagonal flat clusters, pyramidal hut clusters, and faceted hut clusters. All of which have low aspect ratios. At V coverages around 1 ML, however, we found that VSi₂ forms elongated nanoclusters with high aspect ratios and arrays of nanoribbons. In this presentation, we will explore: (1) the role of strain in the growth of these structures; (2) an STM methodology we developed to simultaneously map the topography, work function, and LDOS; and (3) the electronic structure of VSi₂ nanoribbons.   

Explosive Nucleation and Collective Diffusion during Low Temperature Growth of Pb/Ge(111) Islands

Lead deposited on Ge(111) at 220-283K first formed an amorphous wetting layer before undergoing explosive nucleation forming Pb(111) islands at critical coverage of 1.33±0.07ML (w.r.t substrate). LEEM real time images revealed evidence of collective diffusion as the main driving force during nucleation and growth. The islands exhibited anisotropic growth with superlinear growth rate in island size vs time dependence during the early growth stage, involving transport of millions of atoms in the wetting layer over mesoscale distances within the LEEM acquisition speed, fueled by slight compression of the wetting layer at the critical coverage time. Analysis of island growth after the nucleation event showed the number of islands eventually saturating, reducing the growth rate to linear. A Kronig-Penney model was used to predict the locations of LEEM I-V peaks to infer experimental island heights.[1] Similar behavior was previously seen for Pb/Si(111) at low temperatures, with unusually fast diffusion and island heights determined by quantum size effects.[2] To explain the explosive nucleation and critical coverage of Pb/Ge(111), we also present a first-principles DFT computation of the structure and chemical potential of Pb overlayers on Ge(111) as a function of coverage.

[1] M. S. Altman et al, Surf. Rev. Lett. 5, 1129, (1998).

[2] M. T. Hershberger et al., Phys. Rev. Lett. 1113, 236101 (2014).

    

Surface Analysis of Ru Thin Films after Device Fabrication Processing Techniques

Ruthenium is often used as an electrical contact material due to its resistance to oxidation at elevated temperatures. In addition, the most stable stoichiometry of ruthenium oxide under ambient conditions is RuO₂, which is an electrically conductive oxide. The goal of this study is to determine the stoichiometry and measured thickness of the surface oxide on Ru formed by typical semiconductor fabrication processing techniques such as reactive ion etch (RIE), plasma ashing processes, and annealing in various environments. The primary analysis techniques used for this study were angle-resolved XPS and AFM. The Ru thin films were deposited on SiO₂/Si(100) substrates. Analysis of the Ru-3d XPS spectrum of the as-grown Ru film indicated that the native oxide was in a 2+ state and was less than a nm thick. Annealing at atmospheric pressure results in the formation of RuO_2, with some higher order oxides and carbon also being detected. Performing a RIE or ashing process on the as-deposited Ru film resulted in the formation of a thinner RuO₂ film. The AFM data shows rounded clusters of material both before and after the different processing techniques.   

Wavelength Dependence of Laser-Induced Nanowelding on Silver Nanopartcles

Nanowelding of metallic nanoparticles induced by laser illumination is of particular interest because it provides convenient and controlled means for shape-conversion of nanoparticles and fabrication of nanodevices. Previously, we exploited fluorescence microscopy to directly image the real-time nanowelding kinetics of silver nanoparticles (AgNPs) when illuminated with a continuous wave blue laser and investigated the kinetics of the laser-induced nanoparticle nanowelding. In the currenty study, we measured the dependence of the shape of the nanowelded microstructures of AgNPs on the wavelengths of laser illumination. The measured wavelength-dependence was then understood by examining the surface electric field enhancement due to surface plasmon resonance. This work is expected to facilitate the development of better nanowelding strategies of metallic nanoparticles for broader applications.   

Towards A Multiscale Model Of Aluminum Corrosion: Atomistic Insights Into Metal Ion Transport At The Water/Alumina Interface

Corrosion pits can form on aluminum surfaces and reduce component lifetimes, but accurately modeling pit growth requires information on the transport rates of dissolved metal ions. Water is both a reactive antagonist and transport medium for corrosion, as it readily adsorbs to the metal and metal oxide (alumina) surface as a thin film. We investigate the properties of the water/alumina interface using molecular dynamics simulations to determine rates for aluminum ion diffusion in the corrosion process. The effects of temperature, Al ion concentration, and relative humidity on the water film’s structure and transport properties are assessed. Ion diffusion rates are highly dampened in adsorbed water films as compared to bulk liquid water due to the combined effects of a strong hydrogen bonding network that forms at the water/oxide interface and polarization at the water surface. A reductionist 1D reaction-diffusion model is used to assess the impact of diffusion rates on corrosion kinetics.

 

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Modeling Current Flow through Copper with Rough Surfaces

Interconnects are a key component of integrated circuits (IC) and play a major role in the speed and power consumption of an IC. While surface roughness is essential for ensuring adhesion to other components in an IC, it also increases electron scattering at the surface which can dramatically increase resistivity. Using a combination of density functional theory (DFT) and the non-equilibrium Green’s function technique in conjunction with DFT (NEGF-DFT), our work aims to identify key tradeoffs between surface roughness profiles and electronic performance in copper wire interconnects. Our work suggests a connection between the local potential and increases in resistance for atomic-level defects and raises the possibility that a roughness profile could be engineered so as to minimize negative effects on electronic performance. We extend this investigation to nanometer-level surface roughness profiles, and also investigate other compositions commonly used in industry, such as Cu₃Sn. This work has the potential to help realize low-resistivity interconnects, reducing power consumption and increasing speed in integrated circuits.

    

Thermal Effects in the Inverse Photoemission of Ni(110) Surface

We have studied the influence of thermal effects on the unoccupied electronic structure of Ni(110) surface using k-resolved inverse photoemission spectroscopy. The temperature dependence of the peak intensity has been measured for both the bulk and surface states. Our results can be successfully fit with the Debye-Waller model in the temperature region where the surface atoms undergo harmonic vibrations. We find the minimum value of the characteristic temperature (T_c) of ~ 600 K for the crystal-derived surface state of Ni(110). This surface state is located inside the projected bulk band gap in the ΓY direction of the surface band structure and reveals a free-electron-like energy dispersion with m*/m_e = 0.5± 0.1, where the vertex of its parabola is situated at 2.3 ± 0.1 eV above the Fermi level. The above reported minimum T_c value is found to be at the center of the projected bulk band gap. The characteristic temperature increases from this minimum as the surface state approaches the bulk band boundary. We attribute this to the surface state gaining bulk-like character near the boundary.

    

Reducing Metal-Insulator interactions in Granular Metal High-Pass Filters

Several order of magnitude (OOM) on/off ratio improvements are shown for Mo-SiN_x granular metals (GMs) compared to GMs with oxide insulators. Granular metals are nanostructured materials in which metal islands are separated by an insulator. At volume metal fractions (φ) below the percolation threshold (φ_c), GMs are insulating; electrical conduction occurs via electron tunneling and capacitive transport. These conduction mechanisms enable the creation of GM high-pass filters that are insulating below 1 kHz and conductive above 1 MHz. For high-pass filters, the DC conductivity, including tunneling, should be minimized while maintaining the capacitive transport. Au- and Ag-based GMs exhibit a desirable sharp conductivity (σ) knee at φ_c, with σ decreasing 4-6 OOM with Δφ≈0.1. However, Au and Ag GMs are not suitable for high temperature applications, and non-noble metals exhibit metal-insulator interactions that dramatically reduce Δσ at φ_c. The metal-insulator effects are minimized in high thermal stability Mo-SiN_x by sputtering in a partial N₂ environment, which reduces σ 3-4 OOM for φ<φ_c. Furthermore, post-growth annealing increases the separation distance between islands and results in >6 OOM decreases in σ at φ_c. These advancements in material quality improve high-pass filter performance.   

IPES and LEED Investigation of Ion Bombardment-Induced Damage and Thermal Recovery of the Ni(110) Surface

We have used the techniques of Inverse Photoelectron Spectroscopy (IPES) and Low Energy Electron Diffraction (LEED) to investigate ion-induced surface damage and thermal recovery on Ni(110). Ion bombardment creates surface defects which decrease the intensity of unoccupied electronic states as measured using IPES, and of diffracted electron beam spots as measured with LEED. Thermal annealing reduces these surface defects; a range of recovery temperatures demonstrates a linear dependence on the surface state intensity as a function of temperature. Isothermal annealing in the range of 350K-650K for different time periods causes the surface state intensity to reach different levels of saturation. We have modeled our results to obtain an activation energy of ~0.18eV.