Katayun Barmak, Sameer Ezzat, Ryan Gusle, Atharv Jog, Sit Kerdsongpanya, Asim Khanya, Erik Milosevic, William Richardson, Kadir Sentosun, Amirali Zangiabadi, Da… 展开作者
Katayun Barmak, Sameer Ezzat, Ryan Gusle, Atharv Jog, Sit Kerdsongpanya, Asim Khanya, Erik Milosevic, William Richardson, Kadir Sentosun, Amirali Zangiabadi, Daniel Gall, William E. Kaden, Eduardo R. Mucciolo, Patrick K. Schelling, Alan C. West, Kevin R. Coffey
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37 pages, 10 figures, to appear in the Journal of Vacuum Science & Technology A
The experimentally measured resistivity of Co(0001) and Ru(0001) single crystal thin films, grown on c-plane sapphire substrates, as a function of thickness is modeled using the semiclassical model of Fuchs-Sondheimer. The model fits show that the resistivity of Ru would cross below that for Co at a thickness of approximately 20 nm. For Ru films with thicknesses above 20 nm, transmission electron microscopy evidences threading and misfit dislocations, stacking faults and deformation twins. Exposure of Co films to ambient air, and the deposition of oxide layers of SiO2, MgO, Al2O3 and Cr2O3 on Ru degrade the surface specularity of the metallic layer. However, for the Ru films, annealing in a reducing ambient restores the surface specularity. Epitaxial electrochemical deposition of Co on epitaxially-deposited Ru layers is used as an example to demonstrate the feasibility of generating epitaxial interconnects for back-end of line structures. An electron transport model based on a tight-binding approach is described, with Ru interconnects used an example. The model allows conductivity to be computed for structures comprising large ensembles of atoms (10^5-10^6), scales linearly with system size and can also incorporate defects.
We perform density functional theory calculations for the determination of the structural and electronic properties of epitaxial graphene on 4H-SiC(000$\bar{1}$). Using commensurate supercells that minimize non-physical stresses we show that, in contrast with Si-face epitaxial films, the first graphene layer that forms on the C-face of SiC is purely metallic with its $π$-bands partially preserved. Typical free-standing characteristics are fully recovered with a second graphene layer. We moreover discuss on the magnetic properties of the interface and the absence of Fermi-level pinning effects …
X-ray magnetic circular dichroism (XMCD) reveal the magnetic properties of intercalated europium metal under graphene on SiC(0001). Intercalation of Eu nano-clusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu M$_{4,5}$ edges at $T = 15$ K, our samples show paramagnetic-like behavior with distinct anomaly at T $\approx$ 90 K which may be related to the N{è}el transition, T$_N$…
Transport characteristics of single crystal bismuth films on Si(111)-7 \times 7 are found to be metallic or insulating at temperature below or above TC, respectively. The transition temperature TC decreases as the film thickness increases. By combining thickness dependence of the films resistivity, we find the insulating behavior results from the states inside film, while the metallic behavior originates from the interface states. We show that quantum size effect in a Bi film, such as the semimetal-to-semiconductor transition, is only observable at a temperature higher than TC. In addition, th…
Silicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of TiSi$_2$, CoSi$_2$, NiSi and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no difference in the thermal interface conductance of epitaxial and non-epitaxial sil…
Graphene-metal contact resistance is governed by both intrinsic and extrinsic factors. Intrinsically, both the density of states bottleneck near the Dirac point and carrier reflection at the graphene-metal interface lead to a high contact resistance. Moreover, graphene exhibits insulating behavior for out-of-the-plane conduction. Extrinsically, surface contamination introduced by photoresist residue or different adsorbed species during standard lithography processing alters graphene's intrinsic properties by uncontrolled doping and increased scattering which results in high and inconsistent co…
High-quality, large-area epitaxial graphene can be grown on metal surfaces but its transport properties cannot be exploited because the electrical conduction is dominated by the substrate. Here we insulate epitaxial graphene on Ru(0001) by a step-wise intercalation of silicon and oxygen, and the eventual formation of a SiO$_2$ layer between the graphene and the metal. We follow the reaction steps by x-ray photoemission spectroscopy and demonstrate the electrical insulation using a nano-scale multipoint probe technique.
We have investigated the temperature driven first order metal-insulator (M-I) transition in thin films of NdNiO$_3$ and have compared it with the bulk behavior. The M-I transition of thin films is sensitive to epitaxial strain and a partial relaxation of epitaxial strain creates an inhomogeneous strain field in the films which broadens the M-I transition. Both the thin film and the bulk samples exhibit non equilibrium features in the transition regime which are attributed to the presence of high temperature metallic phases in their supercooled state. The degree of supercooling in the thin film…
Quantum wells made of simple polyvalent metals represent a novel family of doped 2D Mott-Hubbard insulators. As scanning tunneling microscopy experiments show, these systems exhibit an anisotropic form of metal-insulator transition. Their elementary excitations possess coherent wave-like properties along the normal axis, and show an incoherent behavior in-plane. The development of such an anisotropic coherence is most likely related to Coulomb interaction between localized and delocalized thin film electronic states - 2D Kondo screening.
Using first-principles calculations within density functional theory, we study the energetics and kinetics of carbon nucleation in the early stages of epitaxial graphene growth on three representative stepped metal surfaces: Ir(111), Ru(0001), and Cu(111). We find that on the flat surfaces of Ir(111) and Ru(0001), two carbon atoms repel each other, while they prefer to form a dimer on Cu(111). Moreover, the step edges on Ir and Ru surfaces cannot serve as effective trapping centers for single carbon adatoms, but can readily facilitate the formation of carbon dimers. These contrasting behaviors…
Ballistic electron emission microscopy (BEEM) was performed to obtain nanoscale current versus bias characteristics of non-epitaxial Au on p-type GaAs in order to accurately measure the local Schottky barrier height. Hole injection BEEM data was averaged from thousands of spectra for various metal film thicknesses and then used to determine the attenuation length of the energetic charge carriers as a function of tip bias. We report the marked increase in attenuation length at biases near the Schottky barrier, providing evidence for the existence of coherent BEEM currents in Schottky diodes. Th…
Graphene, a hexagonal sheet of $sp^2$-bonded carbon atoms, has extraordinary properties which hold immense promise for future nanoelectronic applications. Unfortunately, the popular preparation methods of micromechanical cleavage and chemical exfoliation of graphite do not easily scale up for application purposes. Epitaxial graphene provides an attractive alternative, though there are many challenges, not least of which is the absence of an understanding of the complex atomistic assembly kinetics of graphene. Here, we present a simple rate theory of epitaxial graphene growth on close-packed me…
The poor performance of copper interconnects at the nanometer scale calls for new material solutions for continued scaling of integrated circuits. We propose the use of three dimensional time-reversal-invariant topological insulators (TIs), which host backscattering-protected surface states, for this purpose. Using semiclassical methods, we demonstrate that nanoscale TI interconnects have a resistance 1-3 orders of magnitude lower than copper interconnects and graphene nanoribbons at the nanometer scale. We use the nonequilibrium Green function (NEGF) formalism to measure the change in conduct…
Graphene multilayers are grown epitaxially on single crystal silicon carbide. This system is composed of several graphene layers of which the first layer is electron doped due to the built-in electric field and the other layers are essentially undoped. Unlike graphite the charge carriers show Dirac particle properties (i.e. an anomalous Berry's phase, weak anti-localization and square root field dependence of the Landau level energies). Epitaxial graphene shows quasi-ballistic transport and long coherence lengths; properties which may persists above cryogenic temperatures. Paradoxically, in co…
The increasing resistance of Cu interconnects for decreasing dimensions is a major challenge in continued downscaling of integrated circuits beyond the 7-nm technology node as it leads to unacceptable signal delays and power consumption in computing. The resistivity of Cu increases due to electron scattering at surfaces and grain boundaries of the interconnects at the nanoscale. Topological semimetals, owing to their topologically protected surface states and suppressed electron backscattering, are promising material candidates to potentially replace current Cu interconnects as low-resistance …
Ferrimagnets, which contain the advantages of both ferromagnets (detectable moments) and antiferromagnets (ultrafast spin dynamics), have recently attracted great attention. Here we report the optimization of epitaxial growth of a tetragonal perpendicularly magnetized ferrimagnet Mn2Ga on MgO. Electrical transport, magnetic properties and the anomalous Hall effect (AHE) were systematically studied. Furthermore, we successfully integrated high-quality epitaxial ferrimagnetic Mn2Ga thin films onto ferroelectric PMN-PT single crystals with a MgO buffer layer. It was found that the AHE of such a f…
The prospect of a Dirac half metal, a material which is characterized by a bandstructure with a gap in one spin channel but a Dirac cone in the other, is of both fundamental interest and a natural candidate for use in spin-polarized current applications. However, while the possibility of such a material has been reported based on model calculations[H. Ishizuka and Y. Motome, Phys. Rev. Lett. 109, 237207 (2012)], it remains unclear what material system might realize such an exotic state. Using first-principles calculations, we show that the experimentally accessible Mn intercalated epitaxial gr…
Epitaxial growth and controllable doping of wafer-scale single-crystal transition-metal dichalcogenides (TMDCs) are two central tasks for extending Moore's law beyond silicon. However, despite considerable efforts, addressing such crucial issues simultaneously under two-dimensional (2D) confinement is yet to be realized. Here we design an ingenious epitaxial strategy to synthesize record-breaking 4-inch single-crystal Fe-doped TMDCs monolayers on industry-compatible c-plane sapphire without miscut angle. In-depth characterizations and theoretical calculations reveal that the introduction of Fe…
We report results of investigation of the current-carrying capacity of nanowires made from the quasi-1D van der Waals metal tantalum triselenide capped with quasi-2D boron nitride. The chemical vapor transport method followed by chemical and mechanical exfoliation were used to fabricate mm-long TaSe3 wires with lateral dimensions in the 20 to 70 nm range. Electrical measurements establish that TaSe3/h-BN nanowire heterostructures have a breakdown current density exceeding 10 MA/cm2 - an order-of-magnitude higher than that in copper. Some devices exhibited an intriguing step-like breakdown, whi…
A combination of theoretical modelling and experiments reveals the origin of the large perpendicular magnetic anisotropy (PMA) that appears in nanometer-thick epitaxial Co films intercalated between graphene (Gr) and a heavy metal (HM) substrate, as a function of the Co thickness. High quality epitaxial Gr/Co\n/HM(111) (HM=Pt,Ir) heterostructures are grown by intercalation below graphene, which acts as a surfactant that kinetically stabilizes the pseudomorphic growth of highly perfect Co face-centered tetragonal ($fct$) films, with a reduced number of stacking faults as the only structural def…
Establishing good electrical contacts to nanoscale devices is a major issue for modern technology and contacting 2D materials is no exception to the rule. One-dimensional edge-contacts to graphene were recently shown to outperform surface contacts but the method remains difficult to scale up. We report a resist-free and scalable method to fabricate few graphene layers with electrical contacts in a single growth step. This method derives from the discovery reported here of the growth of few graphene layers on a metallic carbide by thermal annealing of a carbide forming metallic film on SiC in h…
We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e. (001), (110) and (1-18). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, t…
Vertical and lateral heterogeneous structures of two-dimensional (2D) materials have paved the way for pioneering studies on the physics and applications of 2D materials. A hybridized hexagonal boron nitride (h-BN) and graphene lateral structure, a heterogeneous 2D structure, has been fabricated on single-crystal metals or metal foils by chemical vapor deposition (CVD). However, once fabricated on metals, the h-BN/graphene lateral structures require an additional transfer process for device applications, as reported for CVD graphene grown on metal foils. Here, we demonstrate that a single-crys…
Ultrathick MA$_2$N$_4$(M'N) Intercalated Monolayers with Sublayer-Protected Fermi Surface Conduction States: Interconnect and Metal Contact Applications
We have synthesized epitaxial Ba2IrO4 (BIO) thin-films on SrTiO3 (001) substrates by pulsed laser deposition and studied their electronic structure by dc-transport and optical spectroscopic experiments. We have observed that BIO thin-films are insulating but close to the metalinsulator transition boundary with significantly smaller transport and optical gap energies than its sister compound, Sr2IrO4. Moreover, BIO thin-films have both an enhanced electronic bandwidth and electronic-correlation energy. Our results suggest that BIO thin-films have great potential for realizing the interesting ph…
