Conservation of dielectric constant upon amorphization in perovskite oxides

We report calculations indicating that amorphous RAO$_3$ oxides, with R and A trivalent cations, have approximately the same static dielectric constant as their perovskite crystal phase. The effect is due to the disorder-activated polar response of non-polar crystal modes at low frequency, which compensates a moderate but appreciable reduction of the ionic dynamical charges. The dielectric response was studied via density-functional perturbation theory. Amorphous samples were generated by molecular dynamics melt-and-quench simulations.Comment: 5 pages, 3 figure

First principles-based screening method for resistivity scaling of anisotropic metals

The resistivity scaling of metals is a crucial factor for further downscaling of interconnects in nanoelectronic devices that affects signal delay, heat production, and energy consumption. Here, we present a screening method for metals with highly anisotropic band structures near the Fermi level with the aim to select promising materials in terms of their electronic transport properties and their resistivity scaling at the nanoscale. For this, we consider a temperature-dependent transport tensor, based on band structures obtained from first principles. This transport tensor allows for a straightforward comparison between different anisotropic metals in nanostructures with different lattice orientations. By evaluating the temperature dependence of the tensor components, we also find strong deviations from the zero-temperature transport properties at standard operating temperature conditions around room temperature.Comment: 25 pages, 8 figure

Resistivity scaling model for metals with conduction band anisotropy

It is generally understood that the resistivity of metal thin films scales with film thickness mainly due to grain boundary and boundary surface scattering. Recently, several experiments and ab initio simulations have demonstrated the impact of crystal orientation on resistivity scaling. The crystal orientation cannot be captured by the commonly used resistivity scaling models and a qualitative understanding of its impact is currently lacking. In this work, we derive a resistivity scaling model that captures grain boundary and boundary surface scattering as well as the anisotropy of the band structure. The model is applied to Cu and Ru thin films, whose conduction bands are (quasi-)isotropic and anisotropic respectively. After calibrating the anisotropy with ab initio simulations, the resistivity scaling models are compared to experimental resistivity data and a renormalization of the fitted grain boundary reflection coefficient can be identified for textured Ru.Comment: 12 pages, 7 figure

Nature of electron trap states under inversion at In0.53Ga0.47As/Al2O3 interfaces

In and Ga impurities substitutional to Al in the oxide layer resulting from diffusion out of the substrate are identified as candidates for electron traps under inversion at In0.53Ga0.47As/Al2O3 interfaces. Through density-functional calculations, these defects are found to be thermodynamically stable in amorphous Al2O3 and to be able to capture two electrons in a dangling bond upon breaking bonds with neighboring O atoms. Through a band alignment based on hybrid functional calculations, it is inferred that the corresponding defect levels lie at similar to 1 eV above the conduction band minimum of In0.53Ga0.47As, in agreement with measured defect densities. These results support the technological importance of avoiding cation diffusion into the oxide layer. Published by AIP Publishing

Cux_xAl1−x_{1-x} films as Alternatives to Copper for Advanced Interconnect Metallization

Cu$_x$Al$_{1-x}$ thin films with $0.2 \le x \le 0.7$ have been studied as potential alternatives for the metallization of advanced interconnects. First-principles simulations were used to obtain the Cu$_x$Al$_{1-x}$ electronic structure and cohesive energy to benchmark different intermetallics and their prospects for interconnect metallization. Next, thin Cu$_x$Al$_{1-x}$ films were deposited by PVD with thicknesses in the range between 3 and 28 nm. The lowest resistivities of 9.5 $\mu\Omega$cm were obtained for 28 nm thick stochiometric CuAl and CuAl$_2$ after 400$^\circ$C post-deposition annealing. Based on the experimental results, we discuss the main challenges for the studied aluminides from an interconnect point of view, namely the control of the film stoichiometry, the phase separation observed for off-stoichiometric CuAl and CuAl$_2$, as well as the presence of a nonstoichiometric surface oxide.Comment: 24 pages, 7 figure

A demonstration of donor passivation through direct formation of V-As-i complexes in As-doped Ge1-xSnx

Positron annihilation spectroscopy in the Doppler and coincidence Doppler mode was applied on Ge1 xSnx epitaxial layers, grown by chemical vapor deposition with different total As concentrations (1019-1021 cm3), high active As concentrations (1019 cm3), and similar Sn concentrations (5.9%-6.4%). Positron traps are identified as mono-vacancy complexes. Vacancy-As complexes, V-Asi, formed during the growth were studied to deepen the understanding of the electrical passivation of the Ge1 xSnx:As epilayers. Larger monovacancy complexes, V-Asi (i 2), are formed as the As doping increases. The total As concentration shows a significant impact on the saturation of the number of As atoms (i 1/4 4) around the vacancies in the sample epilayers. The presence of V-Asi complexes decreases the dopant activation in the Ge1 xSnx:As epilayers. Furthermore, the presence of Sn failed to hinder the formation of larger V-Asi complexes and thus failed to reduce the donor-deactivation.Peer reviewe

Source/Drain Materials for Ge nMOS Devices : Phosphorus Activation in Epitaxial Si, Ge, Ge1-xSnx and SiyGe1-x-ySnx

This paper benchmarks various epitaxial growth schemes based on n-type group-IV materials as viable source/drain candidates for Ge nMOS devices. Si:P grown at low temperature on Ge, gives an active carrier concentration as high as 3.5 x 10(20) cm(-3) and a contact resistivity down to 7.5 x 10(-9) Omega.cm(2). However, Si:P growth is highly defective due to large lattice mismatch between Si and Ge. Within the material stacks assessed, one option for Ge nMOS source/drain stressors would be to stack Si:P, deposited at contact level, on top of a selectively grown n-SiyGe1-x-ySnx at source/drain level, in line with the concept of Si passivation of n-Ge surfaces to achieve low contact resistivities as reported in literature (Martens et al. 2011 Appl. Phys. Lett., 98, 013 504). The saturation in active carrier concentration with increasing P (or As)-doping is the major bottleneck in achieving low contact resistivities for as-grown Ge or SiyGe1-x-ySnx. We focus on understanding various dopant deactivation mechanisms in P-doped Ge and Ge1-xSnx alloys. First principles simulation results suggest that P deactivation in Ge and Ge1-xSnx can be explained both by P-clustering and donor-vacancy complexes. Positron annihilation spectroscopy analysis, suggests that dopant deactivation in P-doped Ge and Ge1-xSnx is primarily due to the formation of P-n-V and SnmPn-V clusters. (C) 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.Peer reviewe

Thickness dependence of the resistivity of Platinum group metal thin films

We report on the thin film resistivity of several platinum-group metals (Ru, Pd, Ir, Pt). Platinum-group thin films show comparable or lower resistivities than Cu for film thicknesses below about 5\,nm due to a weaker thickness dependence of the resistivity. Based on experimentally determined mean linear distances between grain boundaries as well as ab initio calculations of the electron mean free path, the data for Ru, Ir, and Cu were modeled within the semiclassical Mayadas--Shatzkes model [Phys. Rev. B 1, 1382 (1970)] to assess the combined contributions of surface and grain boundary scattering to the resistivity. For Ru, the modeling results indicated that surface scattering was strongly dependent on the surrounding material with nearly specular scattering at interfaces with SiO2 or air but with diffuse scattering at interfaces with TaN. The dependence of the thin film resistivity on the mean free path is also discussed within the Mayadas--Shatzkes model in consideration of the experimental findings.Comment: 28 pages, 9 figure

Al3_3Sc thin films for advanced interconnect applications

Al$_x$Sc$_{1-x}$ thin films have been studied with compositions around Al$_3$Sc ($x = 0.75$) for potential interconnect metallization applications. As-deposited 25 nm films were x-ray amorphous but crystallized at 190{\deg}C with a recrystallization observed at 440{\deg}C. After annealing at 500{\deg}C, 24 nm thick stoichiometric Al$_3$Sc showed a resistivity of 12.6 ${\mu}{\Omega}$cm, limited by a combination of grain boundary and point defect (disorder) scattering. Together with ab initio calculations that found a mean free path of the charge carriers of 7 nm for stoichiometric Al$_3$Sc, these results indicate that Al$_3$Sc bears promise for future interconnect metallization schemes. Challenges remain in minimizing the formation of secondary phases as well as in the control of the non-stoichiometric surface oxidation and interfacial reaction with the underlying dielectrics.Comment: 15 pages, 4 figure

Ovonic threshold-switching GexSey chalcogenide materials : stoichiometry, trap nature, and material relaxation from first principles

Density functional theory simulations are used to identify the structural factors that define the material properties of ovonic threshold switches (OTS). They show that the nature of mobility-gap trap states in amorphous Ge-rich Ge50Se50 is related to Ge-Ge bonds, whereas in Se-rich Ge30Se70 the Ge valence-alternating-pairs and Se lone-pairs dominate. To obtain a faithful description of the electronic structure and delocalization of states, it is required to combine hybrid exchange-correlation functionals with large unit-cell models. The extent of localization of electronic states depends on the applied external electric field. Hence, OTS materials undergo structural changes during electrical cycling of the device, with a decrease in the population of less exothermic Ge-Ge bonds in favor of more exothermic Ge-Se. This reduces the amount of charge traps, which translates into coordination changes, an increase in mobility-gap, and subsequently changes in the selector-device electrical parameters. The threshold voltage drift process can be explained by natural evolution of the nonpreferred Ge-Ge bonds (or "chains"/clusters thereof) in Ge-rich GexSe1-x. The effect of extrinsic doping is shown for Si and N, which introduce strong covalent bonds into the system, increase both mobility-gap and crystallization temperature, and decrease the leakage current