A major challenge for the continued downscaling of integrated circuits is the resistivity increase of Cu interconnect lines with decreasing dimensions. Alternative metals have the potential to mitigate this resistivity bottleneck by either (a) facilitating specular electron interface scattering and negligible grain boundary reflection or (b) a low bulk mean free path that renders resistivity scaling negligible. Recent research suggests that specular electron scattering at the interface between the interconnect metal and the liner layer requires a low density of states at the interface and in the liner (i.e., an insulating liner) and either a smooth epitaxial metal-liner interface or only weak van der Waals bonding as typical for 2D liner materials. The grain boundary contribution to the room-temperature resistivity becomes negligible if the grain size is large (>200 nm or ten times the linewidth for wide or narrow conductors, respectively) or if the electron reflection coefficient is small due to low-energy boundaries and electronic state matching of neighboring grains. First-principles calculations provide a list of metals (Rh, Pt, Ir, Nb, Ru, Ni, etc.) with a small product of the bulk resistivity times the bulk electron mean free path ρ_o × λ, which is an indicator for suppressed resistivity scaling. However, resistivity measurements on epitaxial layers indicate considerably larger experimental ρ_o × λ values for many metals, indicating the breakdown of the classical transport models at small (<10 nm) dimensions and suggesting that Ir is the most promising elemental metal for narrow high-conductivity interconnects, followed by Ru and Rh.

Density functional methods were used to calculate binding and diffusion energies of adatoms, molecules, and small clusters on TiN(001) and TiN(111) surfaces in order to isolate the key atomistic processes which determine texture evolution during growth of polycrystalline TiN layers. The surface energy for nonpolar TiN(001), 81 meV/Ų, was found to be lower than that of both N- and Ti-terminated TiN(111) polar surfaces, 85 and 346 meV/Ų. While ${\mathrm{N}}_2$ molecules are only weakly physisorbed, Ti adatoms form strong bonds with both TiN(001), 3.30 eV, and TiN(111), 10.09 eV. Ti adatom diffusion is fast on (001), but slow on (111) surfaces, with calculated energy barriers of 0.35 and 1.74 eV, respectively. The overall results show that growth of 111-oriented grains is favored under conditions typical for reactive sputter deposition. However, the presence of excess atomic N (due, for example, to collisionally induced dissociation of energetic ${\mathrm{N}}_2^{+}$ ions) leads to a reduced Ti diffusion length, an enhanced surface island nucleation rate, and a lower chemical potential on the (001) surface. The combination of these effects results in preferential growth of 001 grains. Thus our results provide an atomistic explanation for the previously reported transition from 111 to 001 texture observed for sputter deposition of TiN in ${\mathrm{Ar/N}}_2$ mixtures with increasing ${\mathrm{N}}_2$ partial pressure $P_{{\mathrm{N}}_2}$ and at constant $P_{{\mathrm{N}}_2}$ with increasing ${\mathrm{N}}_2^{+}/\mathrm{Ti}$ flux ratios incident at the growing film.

ScN layers, 345 nm thick, were grown on MgO(001) substrates at $\text{750\hspace{0.17em}°}\mathrm{C}$ by ultrahigh-vacuum reactive magnetron sputter deposition in pure ${\mathrm{N}}_{\mathrm{2}}$ discharges at 5 mTorr. The ${\mathrm{N}}_{\mathrm{2}}^{\mathrm{+}}$ to Sc ratio incident at the substrate and growing film was maintained constant at 14, while the ion energy $E_{{\mathrm{N}}_{\mathrm{2}}^{\mathrm{+}}}$ was varied from 13 to 50 eV. All films were stoichiometric with N/Sc ratios of $\text{1.00±0.02.}$ However, microstructural and surface morphological evolution were found to depend strongly on $E_{{\mathrm{N}}_{\mathrm{2}}^{\mathrm{+}}}.$ The nucleation and initial growth stages of ScN films deposited with $E_{{\mathrm{N}}_{\mathrm{2}}^{\mathrm{+}}}\text{=13\hspace{0.17em}}\mathrm{eV}$ are dominated by the formation of 111- and 002-oriented islands, but preferred orientation rapidly evolves toward a purely 111 texture by a film thickness of $\text{≃50\hspace{0.17em}}\mathrm{nm}$ as 002 grains grow out of existence in a kinetically limited competitive growth mode. In distinct contrast, films deposited with $E_{{\mathrm{N}}_{\mathrm{2}}^{\mathrm{+}}}\text{=20\hspace{0.17em}}\mathrm{eV}$ grow in a cube-on-cube epitaxial relationship with the substrate and exhibit no indication of 111-oriented grains, even in the earliest stages. Increasing $E_{{\mathrm{N}}_{\mathrm{2}}^{\mathrm{+}}}$ to $\text{50\hspace{0.17em}}\mathrm{eV}$ still results in epitaxial layers, but with high in-plane compressive stress and the presence of ${\mathrm{N}}_{\mathrm{2}}$ gas bubbles. All epitaxial layers contain rectangular nanopipes $\text{≃1\hspace{0.17em}}\mathrm{nm}$ wide and aligned along the growth direction. The nanopipes result from atomic shadowing near the bottom of a periodic array of surface cusps which form along orthogonal 〈100〉 directions due to kinetic roughening during growth. The hardness $H$ and elastic modulus $E$ of the epitaxial ScN(001) layer grown with $E_{{\mathrm{N}}_{\mathrm{2}}^{\mathrm{+}}}\text{=20\hspace{0.17em}}\mathrm{eV}$ are $\text{21.1±1.1}$ and $\text{356±18\hspace{0.17em}}\mathrm{GPa},$ respectively. $H$ and $E$ increase (decrease) with increasing (decreasing) $E_{{\mathrm{N}}_{\mathrm{2}}^{\mathrm{+}}}.$

Spin and valley degrees of freedom in materials without inversion symmetry promise previously unknown device functionalities, such as spin-valleytronics. Control of material symmetry with electric fields (ferroelectricity), while breaking additional symmetries, including mirror symmetry, could yield phenomena where chirality, spin, valley, and crystal potential are strongly coupled. Here we report the synthesis of a halide perovskite semiconductor that is simultaneously photoferroelectricity switchable and chiral. Spectroscopic and structural analysis, and first-principles calculations, determine the material to be a previously unknown low-dimensional hybrid perovskite (R)-(−)-1-cyclohexylethylammonium/(S)-(+)-1 cyclohexylethylammonium) PbI₃. Optical and electrical measurements characterize its semiconducting, ferroelectric, switchable pyroelectricity and switchable photoferroelectric properties. Temperature dependent structural, dielectric and transport measurements reveal a ferroelectric-paraelectric phase transition. Circular dichroism spectroscopy confirms its chirality. The development of a material with such a combination of these properties will facilitate the exploration of phenomena such as electric field and chiral enantiomer–dependent Rashba-Dresselhaus splitting and circular photogalvanic effects.

The electron scattering at surfaces, interfaces, and grain boundaries is investigated using polycrystalline and single-crystal Cu thin films and nanowires. The experimental data is described by a Fuchs–Sondheimer (FS) and Mayadas–Shatzkes (MS) model that is extended to account for the large variation in the specific resistivity of different grain boundaries as well as distinct top and bottom surfaces with different scattering specularity $p$. Textured polycrystalline Cu(111) thin films with thickness $d$ $=$ 25–50 nm are deposited on a stack of 7.5-nm Ta on SiO${}_2$/Si(001). Subsequent annealing results in small-grain (SG) thin films with an average grain size $\overline{D}$ that increases from 90 to 120 nm with increasing $d$. Corresponding large-grain (LG) thin films with $\overline{D}$ $=$ 160–220 nm are obtained by depositing 100–200-nm-thick films, followed by an in-situ anneal and a subsequent etch to match the thickness of the SG samples. Nanowires are fabricated from the SG and LG thin films using a subtractive patterning process, yielding wire widths of 75–350 nm. Single-crystal and LG layers exhibit a 18–22$\%$ and 10–15$\%$ lower resistivity than SG layers, respectively. The resistivity decrease from SG to LG Cu nanowires is 7–9$\%$. The thickness and grain size dependence of the resistivity of polycrystalline and single-crystal Cu layers is well described by an exact version of the existing FS $+$ MS model but is distinct from the commonly used approximation, which introduces an error that increases with decreasing layer thickness from 6.5$\%$ for $d$ $=$ 50 nm to 17$\%$ for $d$ $=$ 20 nm. The case of nanowires requires the FS $+$ MS model to be extended to account for variation in the grain boundary reflection coefficient $R$, which effectively increases the overall resistivity by, for example, 16$\%$ for 50 × 45 nm${}^2$ wires. The overall data from single and polycrystalline Cu layers and wires yields $R$ $=$ 0.25 ± 0.05, and $p$ $=$ 0 at Cu-air and Cu-Ta interfaces.