Rare-earth Silicides Research Articles

Formation of uni-directional ultrathin metallic YSi2 nanowires on Si(110)

Ultrathin YSi2 nanowires were grown epitaxially on the Si(110) surface. High-aspect-ratio nanowire growth is induced by the strongly anisotropic lattice-match between the silicide crystal lattice and the Si(110) surface, similar to the established formation of rare-earth silicide nanowires on Si(100). In contrast to the Si(100) case, however, YSi2 nanowires on Si(110) grow in a single orientation along the [11¯0] direction and exhibit a clear preference of nucleating at step edges when these edges are aligned along the [11¯0] growth direction. This suggests a promising avenue for the fabrication of regular nanowire arrays with controlled wire separation, by varying the miscut angle of the Si wafer. The nanowires are metallic and are embedded in a reconstructed Si(110)-(23×3)R54.7°-Y semiconducting surface layer.

Oxygen-Deficient Oxide Growth by Subliming the Oxide Source Material: The Cause of Silicide Formation in Rare Earth Oxides on Silicon

The fundamental issue of oxygen stoichiometry in oxide thin film growth by subliming the source oxide is investigated by varying the additionally supplied oxygen during molecular beam epitaxy of RE2O3 (RE = Gd, La, Lu) thin films on Si(111). Supplying additional oxygen throughout the entire growth was found to prevent the formation of rare earth silicides observed in films grown without an oxygen source. Postgrowth vacuum annealing of oxygen stoichiometric films did not lead to silicide formation thereby confirming that the silicides do not form as a result of an interface instability at growth temperature in vacuum but rather due to an oxygen deficiency in the source vapor. The average oxygen deficiency of the rare-earth containing species in the source vapor was quantified by the 18O tracer technique and correlated with that of the source material, which gradually decomposed during sublimation. Therefore, any oxide growth by sublimation of the oxide source material requires additional oxygen to realize ...

Rare-earth metal gallium silicides via the gallium self-flux method. Synthesis, crystal structures, and magnetic properties of RE(Ga1−xSix)2 (RE=Y, La–Nd, Sm, Gd–Yb, Lu)

Fifteen ternary rare-earth metal gallium silicides have been synthesized using molten Ga as a molten flux. They have been structurally characterized by single-crystal and powder X-ray diffraction to form with three different structures—the early to mid-late rare-earth metals RE¼ La–Nd, Sm, Gd–Ho, Yb and Y form compounds with empirical formulae RE(GaxSi1� x)2 (0.38rxr0.63), which crystallize with the tetragonal a-ThSi2 structure type (space group I41/amd, No. 141; Pearson symbol tI12). The compounds of the late rare-earth crystallize with the orthorhombic a-GdSi2 structure type (space group Imma, No.

Metallic rare-earth silicide nanowires on silicon surfaces

The formation, atomic structure, and electronic properties of self-assembled rare-earth silicide nanowires on silicon surfaces were studied by scanning tunneling microscopy and angle-resolved photoelectron spectroscopy. Metallic dysprosium and erbium silicide nanowires were observed on both the Si(001) and Si(557) surfaces. It was found that they consist of hexagonal rare-earth disilicides for both surface orientations. On Si(001), the nanowires are characterized by a one-dimensional band structure, while the electronic dispersion is two-dimensional for the nanowires formed on Si(557). This behavior is explained by the different orientations of the hexagonal c axis of the silicide leading to different conditions for the carrier confinement. By considering this carrier confinement it is demonstrated how the one-dimensional band structure of the nanowires on Si(001) can be derived from the two-dimensional one of the silicide monolayer on Si(111).

Electronic structure and Fermi surface of iron-based superconductors R2Fe3Si5 (R = Lu;Y;Sc) from first principles

Electronic structures of three superconducting rare-earth iron silicides (Lu;Y;Sc)2Fe3Si5 and non-superconducting Lu2Ru3Si5, adopting a tetragonal crystal structure (P4/mnc), have been calculated employing the full-potential local-orbital method within the density functional theory. The investigations were focused particularly on the band structures and Fermi surfaces, existing in four bands and containing rather three-dimensional electronlike and holelike sheets. They support an idea of unconventional multi-band superconductivity in these ternaries, proposed earlier by other authors for Lu2Fe3Si5, based on heat-capacity, resistivity, electromagnetic and muon spin rotation measurements. Finally, a discussion on differences in the electronic structures between the investigated here and other common families of iron-based superconductors is carried out.

Effect of Rare Earth Additions on Grain Refinement of Plain Carbon Steels

This paper describes a set of experiments with misch metal and rare earth silicide additions in 1010 and 1030 steel to determine whether grain refinement occurs. Target rare earth (RE) contents of 0.1 and 0.2% were employed. After melting, the desired RE addition was added during tapping and then poured into green sand molds. The resulting test plates were then sectioned for tensile and metallographic testing. Yield strength increased for several of the 1010 and 1030 samples. The increase in yield strength correlated with a reduction in grain size. A dramatic increase in percent elongation was also observed in the 1010 sample with the smallest grain size. Electron microscopy found complex RE oxides. These oxides appear to act as heterogeneous nuclei. When they were coated with another slag, the grain size and mechanical properties were similar to the baseline material in both the 1010 and 1030.

MgFeGe as an isoelectronic and isostructural analog of the superconductor LiFeAs

MgFeGe with a CeFeSi-type structure was synthesized by high-pressure method, and its electrical transport, magnetic properties, and electronic structure were investigated. It is found that MgFeGe has the smallest $c$/$a$ (1.65) ratio and the shortest in-plane Fe-Fe distance (2.75 \AA{}) among CeFeSi-type and ThCr${}_{2}$Si${}_{2}$-type rare-earth iron silicides and germanides containing square lattice of iron. Further measurements demonstrate that this compound exhibits metallic conduction and Pauli paramagnetism from room temperature down to 2 K. From density functional theory calculations it is revealed that MgFeGe favors the same antiferromagnetic ground state magnetic configuration as LiFeAs. Moreover, considerable resemblance in the band dispersion, density of state, and Fermi surface was confirmed between MgFeGe and its isoelectronic superconducting counterpart LiFeAs. The results, however, suggest that the presence of the basic structural and electronic features of iron-based superconductors in MgFeGe does not necessarily lead to superconductivity, and further investigation is therefore still deserved to understand the related mechanisms.

A facile route to synthesise silica shell free silicide nanowires

We report a new synthesis route to form single-crystalline silicide nanowires (NWs) with the assistance of the reaction between SiO2 and Na2O, which has been intensively studied by glass industry for a century. Rare earth silicide NWs epitaxially grown on the clean silicon substrate surface has been well studied while the synthesis of out-of-plane grown rare earth silicide NWs has not been reported before. In this contribution, we demonstrate that by controlling the growth conditions, it is possible to obtain rare earth silicide NWs that were grown epitaxially aligned or free-standing on silicon wafers. The characterizations show that the NWs are catalyst and silica shell free. The growth mechanism was discussed and electrical properties of the obtained silicide NWs were characterized. These NWs are very attractive nanoscale building blocks for interconnects and fully silicided (FUSI) gate applications in nanoelectronics.

Electronic properties of self-assembled rare-earth silicide nanowires on Si(001)

To study the formation and the electronic properties of self-assembled dysprosium and erbium silicide nanowires on the vicinal Si(001) surface, we applied scanning tunneling microscopy and angle-resolved photoelectron spectroscopy. The formation of different types of nanowires was found, depending on the rare-earth exposure, postgrowth annealing temperature, and the rare-earth material itself. It is demonstrated that the so-called thin dysprosium silicide nanowires, which form close-packed arrays, are characterized by a nonmetallic band structure with negligible dispersion. In contrast to that, the so-called broad dysprosium silicide nanowires, which are lateral distances of several tens of nanometers apart, are metallic with one-dimensional electronlike bands crossing the Fermi energy. The effective masses of these bands amount to values ranging from 0.2 to 1.1 free-electron masses, and the overall band filling was found to be close to 1. It is shown that broad erbium silicide nanowires have a very similar electronic band structure compared to broad dysprosium silicide nanowires, while corresponding thin erbium silicide nanowires could not be found.

Modulation and Extraction of Schottky Barrier Height for Advanced Source/Drain Contact

As the device geometry dramatically shrinks, the silicice/Si contact resistance on source/drain regions is dominating the parasitic resistance and degrading the device characteristics. To lower the contact resistance one of the effective methods is to lower the Schottky barrier height (SBH) of silicide/Si contact on source/drain regions. In this paper the influence of alloy addition on Ni silicidation and the corresponding SBH, such as Ni(Y), Ni(Yb) and Ni(Al) is investigated. Besides, as one of the rare earth (RE) silicides, YbSi2-x is a promising contact material in conventional n-channel metal-oxide-semiconductor field effect transistors (nMOSFET) and n-channel Schottky barrier source/drain FETs. In this paper the oxidation suppression and pinhole-free YbSi2-x film preparation will be reported. To extract the low SBH, a new method based on admittance measurement will also be introduced.

Phase selection in the rare earth silicides

The rare earth silicides form islands with a tetragonal or a hexagonal structure that coexist when grown on the Si(100) surface. We show using medium energy ion scattering that it is possible to selectively grow one of these as a pure phase by controlling the mobility of the rare earth atoms as they are deposited. When dysprosium, holmium, and erbium are deposited onto a liquid nitrogen cooled substrate the hexagonal structural phase is formed after annealing. When erbium and holmium are deposited onto a hot substrate only the tetragonal phase results. For dysprosium silicide growth under conditions of high mobility causes approximately equal numbers of hexagonal and tetragonal islands to form. The system offers a means to obtain fine control over physical properties such as the Schottky barrier height.

The structure and growth direction of rare earth silicide nanowires on Si(100)

The growth direction and the structure of rare earth silicide nanowires grown on the Si(100) surface have been calculated from first principles. The energies of the optimum structures show that a structure related to the tetragonal bulk phase is more favorable than the hexagonal model and that growth parallel to the dimer rows is lower in energy than growth across the dimer rows.

Arsenic-Segregated Rare-Earth Silicide Junctions: Reduction of Schottky Barrier and Integration in Metallic n-MOSFETs on SOI

As an attempt to considerably reduce the equivalent contact resistivity of Schottky junctions, this letter studies the integration of rare-earth silicides, known to feature the lowest Schottky barriers (SBs) to electrons, coupled with a dopant segregation based on arsenic (As+) implantation. Both erbium (Er) and ytterbium (Yb) have been considered in the implant-before-silicide (IBS) and implant-to-silicide flavors. It is shown that the two schemes coupled with a limited thermal budget (500degC) produce an SB below the target of 0.1 eV. The implementation of IBS arsenic-segregated YbSi1.8 junctions in an n-type SB-MOSFET is demonstrated for the first time resulting in a current-drive improvement of more than one decade over the dopant-free counterpart.

Energy surfaces of rare-earth silicide films on Si(111)

We report on angle-resolved photoelectron spectroscopy results of thin dysprosium-silicide layers formed on Si(1 1 1), taken with a toroidal analyzer allowing to image the energy surfaces in k||-space. At monolayer dysprosium coverages, where hexagonal DySi2 grows with a 1×1 superstructure, electron pockets are observed at the M¯ points with highly anisotropic effective masses, and around a hole pocket at the Γ¯ point also an anisotropic dispersion is found. The band filling of these two bands amounts to one, indicating an even number of electrons assigned to the surface unit cell. Similar features are found for multilayer coverages, where hexagonal Dy3Si5 layers are formed with a 3×3R30° superstructure. Here, the influence of zone folding effects due to the 3×3R30° reconstructed layers in the bulk silicide is only weak because of the high surface sensitivity of the experiments.

Issues Associated to Rare Earth Silicide Integration in Ultra Thin FD SOI Schottky Barrier nMOSFETs

The paper focuses on specific issues associated to rare earth silicide integration on UTB-SOI substrate with a particular attention to erbium and ytterbium silicides. Due to the limited Si source, defects generation on SOI is prevented compared to bulk substrate. Reaction of RE with dielectric materials limits the temperature of silicidation. It is shown that RE S/D MOSFETs are still limited in current-drive by the Schottky barrier height.

Structural and electronic properties of rare earth silicide nanowires on Si(557)

We report on the atomic structure and electronic properties of self-organized dysprosium and erbium silicide nanowires on Si(557), studied using scanning tunneling microcopy and angle-resolved photoelectron spectroscopy. The nanowires were prepared by deposition of different rare earth amounts and subsequent annealing for silicide formation. Due to the stepped structure of the Si(557) surface, nanowires form along the step edges, showing a variety of structurally and electronically different types, depending on the preparation conditions. At submonolayer dysprosium coverages, different chainlike structures dominate, showing one-dimensional dispersion with half-metallic properties. These nanowire regions are separated by $7\ifmmode\times\else\texttimes\fi{}7$ reconstructed Si(111) facets or submonolayer dysprosium silicide patches on these facets. At monolayer coverages, in contrast, mainly nanowires with lengths exceeding $1\text{ }\ensuremath{\mu}\text{m}$ and widths of a few nanometers are found, forming on the (111) facets of the Si(557) surface. Their electronic properties are characterized by a two-dimensional band structure with strong dispersion both parallel and perpendicular to the nanowires and clear metallic behavior, and it is demonstrated that they are formed from hexagonal ${\text{DySi}}_{2}$ monolayers. In the case of thicker silicide layers, metallic nanowires form again on the Si(111) facets, but with clear structural and electronic characteristics of hexagonal ${\text{Dy}}_{3}{\text{Si}}_{5}$ multilayers. At these coverages, additional structures are found, which show an intense signal in the photoelectron spectroscopy data. Related experiments on the growth of erbium silicides indicated the formation of very similar nanowire structures.

Electronic Structure and Nesting-Driven Enhancement of the RKKY Interaction at the Magnetic Ordering Propagation Vector inGd2PdSi3andTb2PdSi3

Measurements of the low-energy electronic structure in Gd2PdSi3 and Tb2PdSi3 by means of angle-resolved photoelectron spectroscopy reveal a Fermi surface consisting of an electron barrel at the Gamma point surrounded by spindle-shaped electron pockets originating from the same band. The calculated momentum-dependent RKKY coupling strength is peaked at the 1/2GammaK wave vector, which coincides with the propagation vector of the low-temperature in-plane magnetic order observed by neutron diffraction, thereby demonstrating the decisive role of the Fermi surface geometry in explaining the complex magnetic ground state of ternary rare earth silicides.

Direct observation of remarkable crystalline-electric-field effect in quasi-two-dimensional antiferromagnet Er 5Ir 4Si 10 single crystal by low-temperature specific heat

We have intensively investigated the antiferromagnetic phase transition in the ternary rare-earth metal silicide Er 5Ir 4Si 10 single crystal by performing the high-resolution measurement of the low-temperature specific heat under zero-magnetic field and the AC magnetization. In the temperature dependence of important physical quantities associated with the antiferromagnetic phase transition, we have observed anomalies with the antiferromagnetic long-range ordering at Neel temperature T N. We have confirmed that T N is 3.5 K. In addition, we have first observed two surprising results. Firstly, a shoulder was observed in the vicinity of 2 K in addition to the sharp peak at T N corresponding to the antiferromagnetic phase transition in the high-resolution measurement of the low-temperature specific heat. Secondly, the anomaly of the AC magnetization at T N depends to the magnetic field direction. We have clearly observed the anomaly of the AC magnetization at T N when the AC magnetic field orientation is parallel to the c-axis, whereas we have observed no anomaly of the AC magnetization at T N when the AC magnetic field orientation is perpendicular to the c-axis. These results clarify that our Er 5Ir 4Si 10 single crystal is a quasi-two-dimensional antiferromagnet and has no three-dimensional magnetic structure of the Er 3+ local moments. However, we have observed a peak of the AC magnetization around 2 K when the AC magnetic field orientation is perpendicular to the c-axis. This temperature corresponds to that at which a shoulder is observed in the measurement of the low-temperature specific heat. Furthermore, we have observed no frequency dependence of the AC magnetization which is ordinarily observed in the spin glass state. This result means that there is no disorder in our Er 5Ir 4Si 10 single crystal because the crystal structure of Er 5Ir 4Si 10 has the tetragonal crystal structure in which the octagons of Er 3+ ions are stacked. In addition, both tetragons and octagons of Er 3+ local moments have no magnetic frustration. At last we can conclude that both the shoulder of the low-temperature specific heat in the vicinity of 2 K and the peak of the AC magnetization around 2 K, which is only observed when the AC magnetic field orientation is perpendicular to the c-axis, correspond to the crystalline-electric-effect in the plane which is perpendicular to the c-axis of the tetragonal crystal structure.

Ytterbium on vicinal Si(1 0 0): Growth and properties of the 2D wetting layer and the Yb silicide phase

The Yb growth on a vicinal Si(100) surface has been studied by scanning tunneling microscopy and low energy electron diffraction in the coverage range of 1–4ML. Two different methods of the Yb/Si(100) interface formation are applied, leading to a remarkable modification of structural and morphological properties of two-dimensional (2D) wetting layer and Yb silicide phase. In particular, the switchover of the 2D layer orientation, similar to the case of the Bi nanolines on Si(100) [J.H.G. Owen, K. Miki, D.R. Bowler, J. Mater. Sci. 41 (2006) 4568], is observed depending on the growth procedure. Moreover, the structure and morphology of the Yb silicide phase is found to depend critically on the growth conditions, and the ability to grow very long, unidirectional Yb silicide nanowires is demonstrated. The results obtained are discussed in the context of the previous studies of 1D nanowires and 3D islands of rare-earth silicides on Si(100).

Material and Integration Issues for Rare Earth Silicides as Gate and Diffusion Contacts in Advanced CMOS Technologies

In an integration scheme where the nFETs and pFETs of CMOS devices are silicided with different materials (Dual Silicides), rare-earth erbium (Er) and ytterbium (Yb) silicides are potential candidates for contacts to n-Si because of their lower Schottky barrier heights, as compared to more conventional nickel and cobalt silicides. [1-3] Although the lower Schottky barrier across the silicide/n-silicon interface results in reduced contact resistivity, the microstructure can exhibit defects and morphology issues [3-7] which affect device integrity and may contribute to contact resistance degradation. [8] In this study, we compared the material and integration properties of Er and Yb silicides with those of Ni (Pt-alloyed) silicide. Using four point probe, AFM, optical inspection and SEM, we compared the silicides using sheet resistance, surface morphology, defects density and ease of formation in narrow lines. We found that the silicide morphology is affected by several process parameters such as the type of metal deposition process (sputtering vs. evaporated) and the anneal formation temperature. The silicides were also tested for their ability to withstand aggressive processing after their formation. The processes tested included exposure to PECVD plasma, contact hole reactive ion etching and forming-gas annealing. The rare-earth and Ni(Pt) silicides showed similar stability upon processing. Lastly, we quantified the residual metal remaining on dielectric surfaces after silicide processing. Overall, Er silicide showed better performance than Yb silicide. By optimizing various elements of the silicidation process, higher quality silicide films can be achieved for evaluation as suitable nFet contacts.