Short Electron Mean Free Path Research Articles

Surface sulfurization of liner and ruthenium metallization to reduce interface scattering for Low-Resistance interconnect

Ruthenium with a low resistivity, high melting temperature and short electron mean free path has been considered to replace copper as the next-generation interconnect metallization. However, its ascending resistivity with linewidth scaling because of the diffuse scattering of electrons at the interface with dielectrics or liners is a concern. Thus, in this study, sulfurization was applied on the surface of titanium-tantalum nitride and ruthenium films to form an ultrathin sulfide layer to reduce interface scattering. The microstructure, chemical composition, bonding configuration, and stability of the layer were characterized, and the resistivity scaling of the ruthenium films was examined. Microscopic observations suggested a crystalline TiTaSx layer formed on the titanium-tantalum nitride films while RuS2 formed on the ruthenium films, both of which changed the binding state/energy of the metals. With the surface sulfurization, the interface scattering of electrons for the ruthenium films was effectively reduced to elevate the specularity parameter from 0.2 to 0.9 and lessen the resistivity scaling for at least 40 % at a thickness of below 10 nm, without a marked loss of interfacial adhesion. However, an increase in contact resistance of molybdenum to the surface-sulfurized ruthenium films needs to be considered.

Characteristics of W thin film deposited by indirect inductively coupled plasma assisted sputtering

Tungsten (W) has a short electron mean free path (EMFP) of 19 nm and a high melting point (3673 K), and, therefore, is being actively studied as one of the next generation thin interconnector materials replacing Cu. In this study, DC magnetron sputtering of W thin film assisted by indirect inductively coupled plasma (ICP) (where, the ICP is located near the substrate) has been investigated for the deposition of 20 nm thick W thin films with a low resistivity and the results were compared with those deposited by direct ICP assisted DC magnetron (where, the ICP is located near the DC magnetron). The W thin films deposited with the indirect ICP assisted sputtering showed the continuous decrease of the W resistivity from 76.5 (0 W) to 22.2 (500 W) Ohm-cm with the increase of ICP power. In the case of W thin film deposited with direct ICP assisted sputtering, even though the resistivity was initially decreased with the ICP power, the resistivity was increased at high ICP powers due to the increased W surface roughness. The lower W resistivity at high ICP powers for the indirect ICP assisted DC sputtering was related to the change of crystal structure to BCC from A-15 and lower oxygen content in the film due to the higher ion flux to the substrate without increasing the surface roughness.

Coherent x-ray scattering in an XPEEM setup

X-ray photoemission electron microscopy, one of the most successful imaging tools at synchrotrons, is known to have limitations related to the application of external fields and to the short electron mean free path. In order to overcome such issues, we adapt an existing XPEEM instrument to simultaneously perform coherent x-ray scattering measurements in reflectivity mode, thus adding a complementary method to XPEEM. Photon-in photon-out x-ray scattering provides the sensitivity to buried interfaces as well as the possibility to work under external fields, which is challenging when using charged particles for imaging. XPEEM, in turn, greatly alleviates the difficulties associated with the reconstruction methods used in coherent diffraction imaging. The combination of the two methods is demonstrated for an artifical spin-ice lattice showing both chemical and magnetic contrast.

Thickness dependence of the superconducting properties of γ- Mo2N thin films on Si (001) grown by DC sputtering at room temperature

We study the crystalline structure and superconducting properties of γ-Mo2N thin films grown by reactive DC sputtering on AlN buffered Si (001) substrates. The films were grown at room temperature. The microstructure of the films, which was studied by X-ray diffraction and transmission electron microscopy, shows a single-phase with nanometric grains textured along the (200) direction. The films exhibit highly uniform thickness in areas larger than 20 × 20 μm2. The superconducting critical temperature Tc is suppressed from 6.6 K to ≈ 3.0 K when the thickness decreases from 40 nm to 5 nm. The residual-resistivity ratio is slightly smaller than 1 for all the films, which indicates very short electronic mean free path. The films are in the superconducting dirty limit with upper critical field Hc2 (0) ≈ 12 T for films with thickness of 40 nm, and 9 T for films with thickness of 10 nm. In addition, from the critical current densities Jc in the vortex-free state, we estimate a penetration depth λ(0) ≈ (800 ± 50) nm and a thermodynamic critical field Hc (0) = (500 ± 80 Oe).

A superconducting transistor with improved isolation between the input and outputterminals

A new design for a multi-terminal superconducting transistor-like device exploiting tunnellinginjection of quasiparticles is proposed, in which parasitic back-action of the acceptor junction (S1IS2) bias current on theinjector junction (S2FIS3) current–voltage characteristic (CVC) is dramatically reduced as compared with that forthe formerly reported quiteron (here S, I, and F denote a superconductor, aninsulator and a ferromagnetic material). Improvement of the isolation is achievedowing to the short electron mean free path and short coherence length ina thin F layer that screens the superconducting energy gap in the perturbedS2 layer preventing its manifestation in the CVC of the injector.

Nickel silicide nanowires formed in pre-patternedSiO2 trenches and their electrical transport properties

Nickel silicide (NiSi) nanowires with different linewidth (from 1000 to 32 nm) are formed in pre-patternedSiO2 trenches on asilicon substrate. SiO2 trenches are milled by focused ion beam (FIB) etching, and an electrical endpoint detectiontechnique is used to control the FIB milling depth to just reveal the silicon surface. Theformation is based on Ni thin film deposition and the subsequent annealing at550 °C. Compared with the previously reported self-aligned formation processof NiSi nanowires, the present technique can form NiSi nanowires withcontrolled length and linewidth because of the pre-patterning of theSiO2 trenches.The as-formed NiSi nanowires show metallic transport characteristics with a rather low resistivity ofabout 15 µΩ cm, which exhibits very weak dependence on linewidth above 50 nm. The lowest resistivity ofthe nanowire is observed at a linewidth of about 100 nm. The low resistivity is thought tobe due to the good crystalline structure of our as-formed NiSi nanowires. When thelinewidth of NiSi nanowires is decreased to 32 nm, the resistivity increased abruptly to22.7 µΩ cm, which can be explained by considering the short electron mean free path (about 5 nm), aswell as the obvious effect of grain boundary on electron transport properties ofNiSi.

Co-rich magnetic amorphous films and their application in magnetoelectronics

Three kinds of Co-rich magnetic amorphous films of CoFeB, CoFeNiSiB, and CoFeHfO were prepared by magnetron sputtering and applied as soft ferromagnetic (FM) electrodes in tunneling magnetoresistance (TMR) devices. Initial results exhibit a large room-temperature TMR effect of approximately 50%. The high effect can be attributed to interfacial coherence between the amorphous barrier-electrode layers and, accordingly, suggests a high local spin polarization possibly associated with strong nearest-neighbor spin correlations of the magnetic atoms. The magnetotransport behavior may be governed by details of the local spin environment in magnetic amorphous electrodes due to their short electron mean-free path $(\ensuremath{\sim}3--5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$. The annealing effect on TMR was found to be more pronounced due to the atomic cooperative structural relaxation and more thermally stable compared with the polycrystalline electrode junctions. Additionally, the use of the magnetic oxide electrode CoFeHfO has shown that the relevant FM electrode-barrier interface becomes insensitive to the oxygen, which simplifies the oxidation process used for the oxide barrier fabrication.

Phonon self-energy effects in the superconducting energy gap ofMgB2point-contact spectra

In strong-coupling superconductors with a short electron mean free path the self-energy effects in the superconducting order parameter play a major role in the phonon manifestation of the point-contact spectra at the above-gap energies. We derive asymptotic expressions for the nonlinear conductivity of tunnel, ballistic, and diffusive point contacts in the case $\mathrm{eV}\ensuremath{\gg}\ensuremath{\Delta},$ and show that these expressions not only qualitatively, but also semiquantitatively correspond to the measurements of the phonon structure in the point-contact spectra for the $\ensuremath{\pi}$ band of ${\mathrm{MgB}}_{2}$ thin films.

Surface electronic structure of the organic superconductor κ-(ET) 2Cu(NCS) 2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET) 2Cu(NCS) 2 has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.

Linear conductance of multiwalled carbon nanotubes at high temperatures

We have studied the electrical conductance of gas-desorbed multiwalled carbon nanotubes (MWNTs) at high temperatures, and found a peculiar linear temperature dependence of conductance over a wide temperature range from 100 to 800 K. We interpret this phenomenon by using a thermal activation picture of conduction channels below the gap in the vicinity of the Dirac points. The result also indicates a very short and temperature-independent electron mean free path in our MWNTs, and provides a way to determine the number of residual conduction channels in the MWNTs.

Magnetic multilayers of Fe/Au: role of the electron mean free path

The giant magnetoresistance (GMR) was measured for Fe/Au magnetic multilayers in both the CIP and CPP configurations, for a series of samples each having the same thickness for the Fe layers (10 Å) but a different thickness for the Au layers (in the range 20-50 Å). It was found that the GMR is only a few per cent in the CIP configuration, whereas the GMR ranges from 20 to 80% in the CPP configuration. We attribute this order-of-magnitude difference in the values of the GMR for the two configurations to the short electron mean free path, resulting from the relatively high resistivity of these multilayers. A short electron mean free path reduces sharply the value of the GMR in the CIP configuration, while leaving unchanged the value of the GMR in the CPP configuration.

Superconducting tunnel junction detectors: the quasiparticle diffusion in the nonlinear regime

X-ray STJ detector response is analyzed in the framework of the kinetic transport equation for the non-equilibrium quasi-particles. Numerical solutions of the resulting equation which describes the diffusion of excitations in the plane of the junction are presented and discussed. A particular attention is paid to the limits of “small” and “large” junctions, with respect to the effective diffusion length of quasi-particles. An almost linear behaviour is expected, under suitable conditions, for “large” Nb-based devices, in agreement with experiments reported in the current literature. Deviations from linearity are however possible in “small” devices or in case of short electronic mean free path.

Electronic properties of semimetallic quasicrystals

Experimental results on transport and electronic properties of selected quasicrystal systems based on good metals are presented. The effects of icosahedral symmetry and disorder on properties are examined. Semimetallic and anomalous transport behaviors as well as their strong variation with alloy composition in the highly ordered icosahedral crystals and their crystal-analog phases are reported. Comparisons are made with theories. The effects of structural order on electronic properties and the propensity toward a metal-insulator transition in ordered quasicrystals are ascribed to symmetry induced band-structure effects, despite the short electronic mean free path.

Far-infrared absorption by small particles

Detailed far-infrared (FIR) and mid-infrared (MIR) studies of aluminum small particles have been carried out. MIR absorption studies of clustered and non-clustered samples suggest that the anomalous FIR absorption does not arise from the clustering together of the individual metallic particles. The MIR absorption, which is too small to be the continuation of the anomalous FIR absorption, could be fitted to the classical theory by using an unexpectedly short electronic mean free path. Evidence is presented for the particles having a non-Drude dielectric function, suggesting that the anomalous FIR absorption is due to the fact that the small particles are not metals.

The very-low-temperature resistivity anomaly observed for potassium-rubidium alloys Effect of a short electron mean free path

Abstract The anomalous very-low-temperature resistivity data reported recently for concentrated potassium-rubidium alloys have been explained by applying the Pippard ineffectiveness condition to inelastic electron-impurity scattering, thus taking into account the fact that the electron mean free path is finite. A discussion is presented of the Pippard condition, its successes and limitations, and further experiments are suggested for clarifying the role of the Pippard condition in modifying the low-temperature resistivity of concentrated crystalline alloys.

The structure of calcium-aluminium glasses: X-ray diffraction and computer simulation studies

The authors present an investigation of the atomic structure of the metallic glass Ca60Al40 by energy-dispersive X-ray diffraction and computer simulation studies based on pseudopotential-derived interatomic forces. The characteristic features of the structure of Ca60Al40 are the large contraction of the Ca-Ca nearest-neighbour distances compared with pure Ca metal and a rather broad and asymmetric nearest-neighbour peak in the pair correlation function. The theoretical model explains the contraction of the Ca-Ca distances but not the large asymmetry of the peaks. They tentatively attribute the asymmetry to a modification of the dielectric screening function arising from the short electronic mean free path in this high-resistivity alloy. The theoretical model also points to a well defined topological short-range order and a negligible chemical short-range order.

Transport and magnetic properties of icosahedral and glassy Pd-U-Si alloys.

The glassy phase of ${\mathrm{Pd}}_{58.8}$${\mathrm{U}}_{20.6}$${\mathrm{Si}}_{20.6}$ alloys can be transformed into an icosahedral quasicrystalline phase by thermal annealing. Electrical transport properties of samples undergoing different degrees of glassy-to-icosahedral phase transformation have been measured. The icosahedral phase has an electrical resistivity of \ensuremath{\sim}220 \ensuremath{\mu}\ensuremath{\Omega} cm at room temperature and a negative temperature coefficient of resistivity. The short electronic mean free path is discussed. At low temperature, the negative magnetoresistivity is attributed to magnetic scattering effects. The low-field ac susceptibility as a function of temperature exhibits a spin-glass-type transition which is not suppressed by a dc magnetic field.

Transport properties of LaCu2Si2 and CeCu2Si2 between 1.5K and 300K

We report measurements of the electrical resistivityρ, the thermal conductivityk and the thermoelectric powerS between 1.5K and 300K on the anomalous CeCu2Si2 compound and on LaCu2Si2 as reference compound. For LaCu2Si2 the temperature dependences ofρ andS are in accord with those found in otherd band metals. For CeCu2Si2 the observed resistivity (ρ≃220 µΩ cm at 200K) leads to a very short electronic mean free path which is of the order of the Ce-Ce spacing. Correspondingly,k is almost identical with the phonon contributionkp. Below 20K, resistivity and thermoelectric power strongly suggest Fermi liquid behavior with a “degeneracy temperature” between 20K and 40K. Above 200K, bothρ andS decrease proportionally to −ln(T/1 K).

Further evidence for the [formula omitted] singularity in amorphous kondo alloys

A T − 1 2 variation of resistivity with temperature near T = 0 is found in rapidly quenched amorphous NiPdP alloys containing up to 7 at.% Mn, providing further evidence for the existence of this new singularity. It is argued that the new resistivity term arises from interference between multiple scattering of conduction electrons by atoms in magnetic and non-magnetic states and is observed as a consequence of a short electron mean free path and breakdown of conservation of electron momenta in amorphous alloys.

Theory of the proximity effect between normal metals

An analysis is made of the AC electrical conductivity of a thin metal film of relatively short electronic mean free path deposited on a pure metal substrate, assuming such good contact that a proportion of the electrons approaching the interface cross over to the other side without being scattered. An approximate but selfconsistent method is developed for combining Maxwell's equations with the solution to the Boltzmann transport equation. The results show how the effective conductivity of the film and the surface impedance of the film-substrate combination vary with the film thickness, the electric mean free path in each medium, the frequency of the applied field and the probability of electron cross over. The effect analysed is the classical analogue of the proximity effect between superconducting and normal metals.