Formation of ultrathin magnetic cobalt films on the Si(111)7 × 7 surface

The phase composition, electronic structure, and magnetic properties of ultrathin cobalt films (no thicker than 20 ?) applied on a Si(111)7 × 7 surface at room temperature are studied by high-resolution photoelectron spectroscopy using synchrotron radiation and magnetic linear dichroism. It is shown that, as the cobalt thickness increases, first interface cobalt silicide and then an island (discontinuous) film of silicon-in-cobalt solid solution form on the silicon surface. A metal cobalt film starts growing after the deposition of a ∼7-?-thick Co layer. It is found that the ferromagnetic ordering of the system, which is characterized by surface magnetization, sets in after the deposition of a ∼6-?-Co layer at the stage of Co-Si solid solution formation.     

Magnetic-dichroism study of iron silicides formed at the Fe/Si(100) interface

The interplay between the phase composition, electronic structure, and magnetic properties of the Fe/Si(100)2×1 interface has been studied at the initial stages of its formation (at Fe doses up to 8 Å). The experiments were carried out in ultra high vacuum by using high-resolution photoelectron spectroscopy with synchrotron radiation. The interface magnetic properties were examined in terms of magnetic linear dichroism in angle-resolved Fe 3p core-level photoemission. It was found that at room temperature a disordered Fe–Si solid solution is formed at the first stage of Fe deposition (≤3.4 Å). In the coverage range of 3.4–4.3 Å the solid solution transforms into Fe₃Si. However, the in-plane ferromagnetic ordering of the silicide occurs only at 6.8 Å Fe that demonstrates the thickness dependence of the magnetic properties of Fe₃Si. The subsequent sample annealing to 150°C transforms Fe₃Si to ε-FeSi, leading to the disappearance of ferromagnetic behavior.     

Reactive epitaxy of cobalt disilicide on Si(111)

A study of the mechanism governing the initial stages in silicide formation under deposition of 1–10 monolayers of cobalt on a heated Si(111) 7×7 crystal is reported. The structural data were obtained by an original method of diffraction of inelastically scattered medium-energy electrons, which maps the atomic structure of surface layers in real space. The elemental composition of the near-surface region to be analyzed was investigated by Auger electron spectroscopy. Reactive epitaxy is shown to stimulate epitaxial growth of a B-oriented CoSi₂(111) film on Si(111). In the initial stages of cobalt deposition (1–3 monolayers), the growth proceeds through island formation. The near-surface layer of a CoSi₂(111) film about 30 Å thick does not differ in elemental composition from the bulk cobalt disilicide, and the film terminates in a Si-Co-Si monolayer triad.     

Formation and magnetic properties of the silicon-cobalt interface

Formation of the Si/Co interface and its magnetic properties have been studied by high-resolution photoelectron spectroscopy with synchrotron radiation. The experiments have been performed in situ in superhigh vacuum (5 × 10^?10 Torr) with coating thicknesses up to 2 nm. It has been found that, in the initial stage of silicon deposition on the surface of polycrystalline cobalt maintained at room temperature, ultrathin layers of the Co₃Si, Co₂Si, CoSi, and CoSi₂ silicides are formed. The three last phases are nonmagnetic, and their formation gives rise to fast decay of magnetic linear dichroism in photoemission of Co 3p electrons. At deposition doses in excess of ～0.4 nm Si, a film of amorphous silicon grows on the sample surface. It has been established that the Si/Co interphase boundary is stable at temperatures up to ～250°C and that further heating of the sample brings about escape of amorphous silicon from the sample surface and initiates processes involving silicide formation.     

Intercalation synthesis of cobalt silicide under a graphene layer

The silicon intercalation under single-layer graphene formed on the surface of an epitaxial Co(0001) film was investigated. The experiments were performed under conditions of ultra-high vacuum. The thickness of silicon films was varied within the range of up to 1 nm, and the temperature of their annealing was 500°C. The characterization of the samples was carried out in situ by the methods of low-energy electron diffraction, high-energy-resolution photoelectron spectroscopy using synchrotron radiation, and magnetic linear dichroism in photoemission of Co 3p electrons. New data were obtained on the evolution of the atomic and electronic structure, as well as on the magnetic properties of the system with an increase in the amount of intercalated silicon. It was shown that the intercalation under a graphene layer is accompanied by the synthesis of surface silicide Co₂Si and a solid solution of silicon in cobalt.     

Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 × 1 surface in various sequences and during annealing of the formed structures to a temperature of 400°C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe 3p and Co 3p electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe₃Si, Co-Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co-Si solid solution, Co, Fe-Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130dgC (for the Co/Fe/Si system) and ～200°C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (Fe _x Co_{1 ? x} Si, Fe _x Co_{2 ? x} Si).     

Growth of magnetic materials and structures on Si(0 0 1) substrates using Co2Si as a template layer

We report in this paper the use of Co₂Si silicide as a template layer for the integration of magnetic materials and structures on silicon substrate. By undertaking Co deposition on silicon at a temperature of about 300 °C, we show that it is possible to obtain a smooth and epitaxial Co₂Si layer, which can act as a template layer preventing the reaction between Co and other transition metals with silicon. Two examples of over-growth of magnetic materials and structures on this template layer will be presented: growth of ferromagnetic Co layers and of magnetic tunnel junctions (Co(Fe)/AlO_x/NiFe).     

Processes of silicide formation in the Fe/Si(111)7 × 7 system

The initial stages of the formation of iron silicides in the Fe/Si(111)7 × 7 system in the course of solid-phase epitaxy are investigated using high-resolution photoelectron spectroscopy (～100 meV) with synchrotron radiation. The spectra of the Si 2p core and valence-band electrons obtained after deposition of iron coverages of up to 28 monolayers on the surface of the sample and subsequent isochronous annealings at 650°C are measured and analyzed. It is shown that the first to form under Fe deposition is an ultrathin film of the metastable silicide FeSi with a CsCl-type structure, on which a layer of the Fe-Si solid solution with segregated silicon grows. At coverages in excess of 10 monolayers, an iron film grows on the surface of the sample. Annealing of a silicon crystal coated with a Fe layer leads to the sequential formation of two stable silicide phases, namely, the ?-FeSi and β-FeSi₂ phases, in the near-surface region of the sample. It is found that the process of solid-phase synthesis of the ?-FeSi phase passes through the stage of transformation of the iron film into the Fe-Si solid solution.     

Formation and lithium doping of graphene on the surface of cobalt silicide

The intercalation of silicon under graphene on the Co(0001) surface, which is accompanied by the formation of a silicon solid solution in cobalt and by the formation of a surface crystalline Co₂Si phase, has been investigated using photoelectron spectroscopy. It has been shown that the formation of cobalt silicide leads to a substantial weakening of the hybridization of electronic states of graphene and cobalt and to the recovery of the Dirac spectrum of electronic states of graphene near the Fermi level. This has made it possible to investigate the electron doping of graphene on the cobalt silicide substrate upon deposition of lithium on its surface. It has been found that doping with lithium leads to a significant charge transfer onto graphene, and the electron concentration reaches 3.1 × 10¹⁴ cm^?2. Moreover, the specific form of the Fermi surface creates favorable conditions for the enhancement of the electron-phonon coupling. As a result, the formed system can be considered as a candidate for the creation of superconductivity in single-layer graphene.     

Pd-Si薄膜固相反应的透射电子显微镜研究

本工作利用透射电子显微术研究了Pd-Si薄膜固相反应的初始生成相及生成相Pd₂Si与(111)取向Si衬底的取向关系随Pd膜厚度、退火温度等因素的变化规律。实验结果表明:在衬底保持室温的条件下,Pd沉积到Si(111)上时也能够生成一层外延的Pd₂Si,其厚度足以在常规的选区电子衍射中产生明显的信号。在170℃退火时,Pd-Si反应即可持续到生成200nm厚的外延的Pd₂Si。在Pd膜厚度为400nm的条件下,Pd₂Si与Si(111)衬底的取向关系为[0001]_(Pd2Si)轴织构。 关键词：     

Ferromagnetic alignment of iron nanostructures on the silicon surface

The phase composition, electronic structure, and magnetic properties of nanostructures formed upon deposition of iron on the surface of the Si(556) vicinal face coated by a submonolayer silver film with a √3×√3-Ag structure have been studied using high-energy-resolution photoelectron spectroscopy and analysis of magnetic linear dichroism in photoemission of Fe 3p electrons. The effective thickness of the deposited iron layer is varied from 1 to 25 ?. It has been shown that a 1- to 2-? Fe coverage leads to the formation of a metastable iron silicide thin layer with a CsCl-type structure on the surface of the sample. A further deposition of Fe (up to ≈7 ?) brings about the formation of chains consisting of nonmagnetic islands of the Fe—Si solid solution on this layer, which are oriented along the steps of the substrate. A ferromagnetic alignment of the system along the surface of the sample appears only at coverages of approximately 10 ?, when larger (≈100 nm) iron islands start to grow on the solid solution layer.     

Characterization of ultrathin nickel layers on Si(111) using RHEED and RBS

Ultrathin nickel layers (0–40 Å) on Si(111) were characterized using reflection high energy electron diffraction (RHEED) in conjunction with high-resolution Rutherford backscattering spectrometry (RBS). RBS shows that Ni, when deposited on Si(111) at 300 K, reacts with the Si to form clusters with an average composition close to Ni₂Si, until the clusters coalesce into a continuous layer. RHEED measurements show that the microstructure of this layer also matches the Ni₂Si composition: the film consists mainly of very fine-grained Ni₂Si crystallites (∼ 15 Å grain size) which are randomly oriented. Additional Ni deposition results in the accumulation of an unreacted fine-grained Ni layer on top of the silicide film.     

FMR Investigation of the Magnetic Anisotropy in Films Synthesized by Co<Superscript>+</Superscript> Implantation into Si

Thin ferromagnetic films with the uniaxial magnetic anisotropy were synthesized by Co⁺ implantation into single-crystal silicon in the magnetic field. It was concluded that the formation of the induced magnetic anisotropy is due to the directional atomic pair ordering (Neel–Taniguchi model). The synthesized films were studied by the ferromagnetic resonance (FMR) method in the temperature range from 100 to 300 K. The FMR linewidth is almost independent of temperature, which is in agreement with the Raikher model describing the magnetic resonance of uniaxial magnetic particles. It is found that the temperature dependence of the anisotropy constant is linear. This dependence can be associated with the difference in the coefficients of thermal expansion of the Si (111) substrate and the ion-beam-synthesized cobalt silicide films.     

Study of the structural and magnetic characteristics of epitaxial Fe3Si/Si(111) films

The results of the structural and magnetic studies of the epitaxial structure prepared during the simultaneous evaporation from two iron and silicon sources on an atomically pure Si(111)7 × 7 surface at a substrate temperature of 150°C have been presented. The epitaxial structure has been identified as a single-crystal Fe₃Si silicide film with the orientation Si[111]‖Fe₃Si[111] using methods of the X-ray structural analysis, transmission electron microscopy, and reflection high-energy electron diffraction. It has been established that the epitaxial Fe₃Si film at room temperature has magnetic uniaxial anisotropy (H _a = 26 Oe) and a relatively narrow uniform ferromagnetic resonance line (ΔH = 11.57 Oe) measured at a pump frequency of 2.274 GHz.     

Magnetooptical,optical, and magnetotransport properties of Co/Cu superlattices with ultrathin cobalt layers

We investigated the field dependences of the magnetization and magnetoresistance of superlattices [Co(t _x, Å)/Cu(9.6 Å)]30 prepared by magnetron sputtering, differing in the thickness of cobalt layers (0.3 Å ≤ t _Co ≤ 15 Å). The optical and magnetooptical properties of these objects were studied by ellipsometry in the spectral region of hω= 0.09–6.2 eV and with the help of the transverse Kerr effect (hω= 0.5–6.2 eV). In the curves of an off-diagonal component of the tensor of the optical conductivity of superlattices with t _Co = 3–15 Å, a structure of oscillatory type (“loop”) was detected in the ultraviolet region, resulting from the exchange splitting of the 3d band in the energy spectrum of the face-centered cubic structure of cobalt (fcc Co). Based on magnetic experiments and measurements of the transverse Kerr effect, we found the presence of a superparamagnetic phase in Co/Cu superlattices with a thickness of the cobalt layers of 3 and 2 Å. The transition from superlattices with solid ferromagnetic layers to superparamagnetic cluster-layered nanostructures and further to the structures based on Co and Cu (t _Co = 0.3–1 Å) with a Kondo-like characteristics of the electrical resistivity at low temperatures is analyzed.     

Thickness dependent formation and properties of GdSi2/Si(100) interfaces

Epitaxial and polycrystalline orthorhombic GdSi₂ films were grown on Si(100) substrates by solid phase reaction between Si and Gd films at different thicknesses of the Gd film . The most important property of these GdSi₂/Si interfaces was defect formation. This was investigated by studying the properties of the Schottky barriers by means of current voltage and capacitance–voltage characteristics, deep level transient spectroscopy by double crystal X-ray diffractometry, and transmission electron microscopy. Epitaxial growth of the silicide layer ensured a relatively low interface defect density (about 10¹⁰ cm^-2), while the non-epitaxial growth induced defects of a much higher density (about 10¹² cm^-2). The defects generated during the silicide formation are located within a depth of about 10 nm from the GdSi₂/Si interface. PACS 68.55.-a; 73.40.-c; 81.15-zThis revised version was published in March 2005. References 10-17 were added in the online version (pdf file).     

Elaboration of self-organized magnetic nanoparticles by selective cobalt silicidation

Self-organized magnetic nanoparticles are obtained through selective silicidation of cobalt using a silicon substrate pre-structured with tri-dimensional gold islands as template. On the step bunches array of a vicinal Si(1 1 1) surface, gold deposition results in the formation of nanodroplets aligned along the step bunches. A subsequent cobalt deposition is performed onto this gold islands-covered Si surface, with two silicidation processes investigated: reactive deposition (RD) and solid phase reaction (SPR). The cobalt is converted into a non-magnetic silicide film except where the surface is locally masked by the gold islands, giving rise to cobalt nanomagnets which can be capped by a gold layer. A scanning tunneling microscopy comparative study of RD and SPR processes demonstrates that the former induces strong surface morphology changes while the latter preserves the pristine islands. Magnetic measurements performed with alternating gradient force magnetometry at room temperature are used to demonstrate the presence of ferromagnetic cobalt nanoparticles on SPR-processed samples. These nanomagnets show a clear in-plane anisotropy behavior.     

Study of phase separation in Ti-Co-N thin films on silicon substrate

73 Co₂₇ and Ti₇₃Co₂₇- N during thermal annealing has been studied by SIMS, AES and XRD methods. It has been shown that in case of Ti₇₃Co₂₇ the CoSi₂ layer was not formed and the formation of ternary silicide compounds CoTiSi and Co₃Ti₂Si took place. At the same time in case of Ti₇₃Co₂₇- N the bottom layer CoSi₂ and the upper layer based on TiN were formed. The interaction behaviour has been found to depend on nitrogen concentration in initial film. For high amount of nitrogen the diffusion of Si atoms into upper layer and Si₃N₄ phase formation were observed. The possible variants of solid-phase interaction between silicon and the alloys containing intermetallic compounds and influence of nitrogen on this process are discussed. Received: 25 September 1996 / Accepted: 11 October 1996     

Initial stages of iron silicide formation on the Si(1 0 0)2 × 1 surface

The initial stages of iron silicide growth on the Si(1 0 0)2 × 1 surface during solid-phase synthesis were investigated by photoelectron spectroscopy using synchrotron radiation. The experiments were made on iron films of 1-50 monolayer (ML) thickness in the temperature range from room temperature to 750 °С. Our results support the existence of three stages in the Fe deposition on Si(1 0 0) at room temperature, which include formation of the Fe-Si solid solution, Fe₃Si silicide and an iron film. The critical Fe dose necessary for the solid solution to be transformed to the silicide is found to be 5 ML. The solid-phase reaction was found to depend on the deposited metal dose. At 5 ML, the reaction begins at 60 °С, and the solid-phase synthesis leads to the formation of only metastable silicides (FeSi with the CsCl-type structure, γ-FeSi₂ and α-FeSi₂). A specific feature of this process is Si segregation on the silicide films. At a thickness of 15 ML and more, we observed only stable phases, namely, Fe₃Si, ε-FeSi and β-FeSi₂.     

Growth and magnetic properties of the sandwich structure Fe/magnetic silicide/Si(100) obtained from in situ optic and magneto-optic data

It is demonstrated by the Surface Magneto-Optic Kerr Effect and Differential Reflectance Spectroscopy methods that structures free of magnetically dead layers can be created by the deposition of iron at room temperature onto a prefabricated magnetic silicide layer. The magnetic silicide can be formed by the deposition of iron at 70 ^°C onto a layer of amorphous silicon prefabricated on Si(100). Both in the silicide and the iron film, magnetism onsets after the iron amount deposited reaches some critical value. The spontaneous magnetization vector in the iron film changes its direction twice during the film growth. Sufficiently thick iron films persist being ferromagnetic in air for years.