Epitaxial Copper Films Research Articles

Effects of nanoscale surface roughness on the resistivity of ultrathin epitaxial copper films

The knowledge on the influence of surface roughness and the electron–phonon (el–ph) interaction on electrical transport properties of nanoscale metal films is important from both fundamental and technological points of view. Here we report a study of the temperature dependent electron transport properties of nanoscale copper films by measuring temperature dependent electrical resistivity with thickness ranging from 4 to 500 nm. We show that the residual resistivity, which is temperature independent, can be described quantitatively using both measured vertical surface root-mean-square roughness and lateral correlation length in the nanoscale, with no adjustable parameter, by a recent quasi-classical model developed by Chatterjee and Meyerovich (2010 Phys. Rev. B 81 245409–10). We also demonstrate that the temperature dependent component of the resistivity can be described using the Bloch–Grüneisen equation with a thickness dependent el–ph coupling constant and a thickness dependent Debye temperature. We show that the increase of the el–ph coupling constant with the decrease of film thickness gives rise to an enhancement of the temperature dependent component of the resistivity.

Evidence of enhanced electron-phonon coupling in ultrathin epitaxial copper films

Electron phonon (el-ph) coupling is a fundamental quantity that controls the electron transport through a conductor. We experimentally determined the el-ph coupling strength of epitaxial copper (Cu) films ranging from 5 to 1000 nm thick using both ultra-fast, optical pump-probe reflectivity and temperature-dependent resistivity measurements. An enhancement of the el-ph coupling strength was observed when the thickness of the films was reduced to below 50 nm. We suggest that this unexpected enhancement of the el-ph coupling strength is partially responsible for the observed increase of resistivity in the films below 50 nm thick.

Graphene Growth Dynamics on Epitaxial Copper Thin Films

Graphene chemical vapor deposition on copper is phenomenologically complex, yielding diverse crystal morphologies, including lobes, dendrites, stars, and hexagons, of various orientations depending on conditions. We present a comprehensive study of the evolution of these morphologies as a function of the copper surface orientation, absolute pressure, hydrogen-to-methane ratio (H2:CH4), and nucleation density. Growth was studied on ultrasmooth, epitaxial copper films inside copper enclosures to minimize copper polycrystallinity and roughness and decrease the graphene nucleation density. At low pressure and low H2:CH4, circular graphene islands initially form. After exceeding ∼1.0 μm, Mullins-Sekerka instabilities evolve into dendrites extending hundreds of micrometers in the ⟨100⟩, ⟨111⟩, and ⟨110⟩ directions on Cu(100), Cu(110), and Cu(111), respectively, indicating mass transport limited growth. Twin boundaries perturb the preferential growth direction on Cu(111) and alter graphene morphology. Increasing H2:CH4 results in compact islands that reflect the copper symmetry. At atmospheric pressure and low H2:CH4, Mullins-Sekerka instabilities develop but with multiple preferred orientations. Increasing H2:CH4 results in more hexagonal islands. Every growth regime can be tuned to yield continuous monolayers with a D:G Raman ratio <0.1. The understanding gained from this study provides a roadmap to rationally tailor the structure, morphology, and orientation of graphene crystals.

A microindentation method for estimating interfacial shear strength and its use in studying the influence of titanium transition layers on the interface strength of epitaxial copper films on sapphire

An approximate elastic model is derived that converts measurements of indentation depth as a function of the applied load in a microindentation experiment, to an estimate of the interfacial yield strength of a metal film on a ceramic substrate. The analysis is done in two stages. In the first stage the indenter lies entirely within the overgrowth. In the second, the indenter pierces through the interface and into the substrate. The load is predicted to increase as the second power of the displacement in Stage I. In Stage II the net load supported in the film is predicted to increase linearly with the displacement. The coefficients for the parabolic and linear functions are related to the effective interfacial shear strength. Experiments conducted on epitaxial films of (111) copper on (0001) sapphire, show excellent fits with the Stage I and Stage II models, yielding self consistent values for the interfacial shear strength. The technique was used to measure the influence of titanium transitional layers, varying in thickness from 0.7 nm (∼ 3 monolayers) to 110 nm. Just 0.7 nm thick titanium produced a 40% increase in the interfacial strength of the copper/sapphire interface, with thicker titanium layers producing only a marginal further improvement in the interfacial strength. Possible reasons why the elastic analysis provides a good description of the data, even through the deformation underneath the indenter is plastic, are discussed.

Structure of Cu on Ir(111): a case study in photoelectron holography and quantitative photoelectron diffraction

Angle-dependent X-ray photoelectron diffraction is used to determine the structure of epitaxial copper films on Ir(111), for coverages below six monolayers in thickness. Diffraction patterns are analyzed using the methods of photoelectron holography to provide a qualitative picture of the near-neighbor structure. Two theoretical models are used for quantitative analysis of interatomic bond-lengths. The complementarity of the two approaches is illustrated by this case study comparison of holography and quantitative photoelectron diffraction.

Photoelectron holography of Si(100)-[2×1] and other surface and thin-film structures

Direct imaging of near-neighbor atomic structure in Si(100) is demonstrated using photoelectron holography with the Si 2s core level. Photoelectron diffraction studies of the initial stages of epitaxy of copper on Si(100)-[2×1] surfaces show that room-temperature deposition does not result in ordered films. Examples of quantitative photoelectron diffraction studies of epitaxial copper films show that bond-length accuracies of 0.05 Å are possible by fitting techniques.

Epitaxial growth of thin copper layers on Cu(111) studied by high-resolution low-energy-electron-diffraction

The morphology of ultrathin epitaxial copper films (2.5 ML) on a Cu(111) substrate is investigated by high-resolution low-energy-electron-diffraction (HR-LEED). Analyzing the spot profile of the diffracted electron beam (SPA-LEED) provides detailed information about defects of the growing film, especially steps. Thus, from the broadening of the spot the density of steps and their angular distribution are evaluated for various deposition conditions. At low temperature the film grows with nearly no diffusion of the adatoms across step edges. At high temperatures the film starts to build facets whose angles with the substrate depend on the temperature during and after deposition of the film. The annealing behaviour of a film grown at low temperature is also studied with respect to step density and asperity height. Thus the morphology of copper films under various deposition and annealing conditions can be investigated quantitatively in full detail by careful measurements and evaluation of the spot profiles.

AES studies of palladium films formed on copper and mica

Thin epitaxial films of palladium were grown on epitaxial copper films and cleaved mica in ultra high vacuum. The growth modes of these films were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), transmission electron microscopy (TEM), and TEM replica techniques. Layer by layer growth of Pd on Cu and mica was observed and inelastic mean free paths of Auger electrons for energies of 60 eV (Cu MMM) and 329 eV (Pd MNN) were calculated. These values were 5.7 and 6.9 Å respectively. The thermal stability of monocrystalline and polycrystalline Pd/Cu bilayer films at 483 K was also investigated by AES and TEM. It was found that Pd agglomerates on the Cu at this temperature to form a Stranski-Krastanov growth morphology. The agglomeration is much more rapid on polycrystalline films, suggesting that high surface diffusivity paths (grain boundaries and possibly other defects) enhance the surface diffusion of Pd on Cu.

Aes studies of palladium films formed on copper and mica

Thin epitaxial films of palladium were grown on epitaxial copper films and cleaved mica in ultra high vacuum. The growth modes of these films were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), transmission electron microscopy (TEM), and TEM replica techniques. Layer by layer growth of Pd on Cu and mica was observed and inelastic mean free paths of Auger electrons for energies of 60 eV (Cu MMM) and 329 eV (Pd MNN) were calculated. These values were 5.7 and 6.9 Å respectively. The thermal stability of monocrystalline and polycrystalline Pd/Cu bilayer films at 483 K was also investigated by AES and TEM. It was found that Pd agglomerates on the Cu at this temperature to form a Stranski-Krastanov growth morphology. The agglomeration is much more rapid on polycrystalline films, suggesting that high surface diffusivity paths (grain boundaries and possibly other defects) enhance the surface diffusion of Pd on Cu.

254. Preparation and investigation of the structure of epitaxial copper films: Ya M Ksendzov et al, Neorg Mater, 8 (1), 1972, 37–39 ( in Russian)

254. Preparation and investigation of the structure of epitaxial copper films: Ya M Ksendzov et al, Neorg Mater, 8 (1), 1972, 37–39 ( in Russian)

Epitaxial Copper Films Evaporated in Ultra-High Vacuum

Recent electron microscopy and diffraction studies of vacuum-deposited films have demonstrated the importance of temperature and surface condition of substrate in determining film structure and growth. Many interesting results of thin epitaxial films of face-centered cubic metals such as gold and silver have been reported. But, relatively few studies of copper have been made. The purpose of this study is to investigate structure and epitaxy of copper thin films evaporated onto various single-crystal substrates in an ultra-high vacuum.High purity copper was evaporated onto freshly cleaved (0001)MoS2, (100)NaC1, (001)mica and (11)CaF2 surfaces heated at temperatures between ∼25° and 200°C in a vacuum of ∼10-8 Torr. Average thickness of deposited films was about 90 Å. Deposition rate was about 0.4 Å/sec. The films were studied by reflection electron diffraction and transmission electron microscopy. Oxidation of copper films during preparation for electron microscopy and diffraction observation was minimized by deposition of thin layer of amorphous carbon upon freshly formed copper films.