Magnetic properties of epitaxial Co nanodisk arrays packed on atomically smooth and vicinal Si substrates

Arrays of circular nanodisks have been formed from epitaxial Co films deposited on atomically smooth and vicinal Si(111) single crystals by using a focused Ga⁺ ion beam. The surface roughness has been determined by scanning tunneling microscopy. The coercive force and the processes of magnetization reversal in films and arrays of epitaxial Co nanodisks have been studied. It has been shown that the coercive force of the Co nanodisk arrays on the atomically smooth Si(111) surface is larger and that on the vicinal Si(111) surface is smaller than that in the epitaxial films. The studies of the magnetic structure of the nanodisks by magnetic-force microscopy in combination with the micromagnetic simulation have shown that the processes of magnetization reversal in the nanodisks on the atomically smooth substrates occur through the vortex-like states and, on the vicinal substrates, through the C-type state.     

Influence of the surface morphology of single-crystal Si(111) substrates on the magnetic properties of epitaxial cobalt films

The epitaxial films Co(111)/Cu(111)-R30°/Si(111) have been grown on the atomically smooth and vicinal Si(111) surfaces. The roughnesses of the substrate and the cobalt film have been determined using scanning tunneling microscopy. The dependence of the coercive force has been investigated as a function of the azimuthal angle. The dependence of the magnetic anisotropy and the coercive force on the surface roughness has been determined. It has been shown that, in the epitaxial cobalt films deposited on the atomically smooth silicon surfaces, crystalline anisotropy of the 〈110〉 type leads to the isotropy of the magnetization reversal processes. The step-induced uniaxial anisotropy has been observed upon deposition on the vicinal surfaces. The films deposited on the atomically smooth surfaces have a complex domain structure.     

Growth of Au thin film on Cu-modified Si(1 1 1) surface

Miniaturizing of electronic devices requires that conductive elements maintain advanced electrical characteristics upon reducing their geometrical sizes. For gold, which is valued for its high electrical conductivity and stability against ambient conditions, creation of extra-thin films on silicon is hampered by formation of the quite complex Au/Si interface. In the present work, by forming a Si(1 1 1)5.55 × 5.55-Cu surface reconstruction prior to Au deposition we formed Au films with smoother surface morphology and higher surface conductivity compared to Au film grown on a pristine Si(1 1 1)7 × 7 surface. Scanning tunnelling microscopy and four-point probe measurements were used to characterize the growth mode of the Au film on a Si(1 1 1)5.55 × 5.55-Cu reconstruction at room temperature.     

Scanning tunneling microscopy observation of ultrathin epitaxial CoSi<Subscript>2</Subscript>(111) films grown at a high temperature

Scanning tunneling microscopy (STM) is used to study the basic laws of growth of ultrathin epitaxial CoSi₂(111) films with Co coverages up to 4 ML formed upon sequential deposition of Co and Si atoms taken in a stoichiometric ratio onto the Co–Si(111) surface at room temperature and subsequent annealing at 600–700°C. When the coverage of Co atoms is lower than ~2.7 ML, flat CoSi₂ islands up to ~3 nm high with surface structure 2 × 2 or 1 × 1 grow. It is shown that continuous epitaxial CoSi₂ films containing 3–4 triple Si–Co–Si layers grow provided precise control of deposition. CoSi₂ films can contain inclusions of the local regions with (2 × 1)Si reconstruction. At a temperature above 700°C, a multilevel CoSi₂ film with pinholes grows because of vertical growth caused by the difference between the free energies of the CoSi₂(111) and Si(111) surfaces. According to theoretical calculations, structures of A or B type with a coordination number of 8 of Co atoms are most favorable for the CoSi₂(111)2 × 2 interface.     

STM investigation of cobalt silicide nanostructures’ growth on Si(111)-(√19 × √19) substrate

Continuing miniaturization of electronic devices necessarily requires assembly of several different objects or devices in a small space. Therefore, besides thin films growth, the possibility of fabricating wires and dots [1, 2] at the nanometre scale composed of metal silicides is of the top interest. This report is about the STM/STS investigation of cobalt silicides’ nanostructures created on Si(111)-(√19 × √19) substrates via Co evaporation and post deposition annealing. This (√19 × √19) reconstruction was induced by Ni doping. Less than 1ML of Co on surface was obtained. Surface reconstruction induced growth of agglomerates of clusters rather than an uniform layer. The post deposition annealing of a crystal sample (up to 670 K, 770 K, 870 K, 970 K, 1070 K and 1170 K) led to creation of silicides’ nanostructures. Measurements showed that coalescence of Co nanoislands begun around 970 K. Annealing above 1070 K led to alloying of a Co, Ni and Si. As a consequence the Si(111)-(7×7) reconstruction occurred at the cost of Si(111)-(√19 × √19).      

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

Structure analysis of thin iron-silicide film from φ-scan RHEED Patterson function

The atomic structure of thin iron silicide film, grown epitaxially on the Si(111) surface, has been analyzed by means of the three-dimensional RHEED Patterson function analysis. The iron-silicide-terminated surface with (2 × 2) periodicity has been prepared by a solid-phase epitaxy method. 2 ML of Fe were deposited on the Si(111)-(7 × 7) surface and annealed at 500°C. Three-dimensional Patterson function was calculated from series of φ-scanned RHEED intensity distributions converted to the k-space. The resulting model of γ-FeSi₂ structure consists of two silicide layers faulted to each other with three relaxed Si adatoms above the H₃ site.     

Growth of an Eu-Si(111) thin film structure: The stage of silicide formation

Silicide formation in thin films produced by depositing Eu atoms on the Si(111) surface is studied using LEED, Auger electron spectroscopy, contact potential difference, and isothermal thermal-desorption spectroscopy. It is shown that if Eu is deposited on a substrate at room temperature, the growing film is disordered and consists of almost pure Eu. At high temperatures (T≥500 K), the Eu-Si(111) system forms through the Stranski-Krastanow mechanism; namely, first a two-dimensional transition layer (reconstruction) with the (2×1) structure forms and then three-dimensional silicide crystallites grow on it. A specific feature of this system is a low rate of diffusion of Si atoms in the europium silicides. This feature accounts for the concentration gradient of Si atoms across the silicide film thickness and, as a consequence, the multiphase film composition.     

A leed-aes study of thin Pd films on Si(111) and (100) substrates

A system Pd (deposit)-Si (substrate) has been studied by LEED and AES. Pd₂Si formed on Si(111) became epitaxial after a short time of annealing at a temperature between 300 and 700°C, while the Pd₂Si formed on Si(100) did not, in both cases the surfaces of the Pd₂Si being covered with a very thin Si layer. A sequence of superstructures (3√3 × 3√3), (1 × 1), and (2√3 × 2√3) was observed successively in Pd/Si(111) as the annealing temperature was increased. A (√3 × √3) structure was obtained by sputtering the 3√3 surface slightly. It was found that the √3 structure corresponds to Pd²Si(0001)-(1 × 1) grown epitaxially on Si(111), and that the 3√3 structure comes from the thin Si layer accumulated over the silicide surface, while the 2√3 and 1 structures arise from a submonolayer of Pd adsorbed on Si(111). Superstructures observed on a Pd/Si(100) system are also studied.     

Growth of Cu Films on Si(111)-7×7 Surfaces at Low Temperature: A Scanning Tunnelling Microscopy Study

Morphologies of Cu(111) films on Si(111)-7×7 surfaces prepared at lowtemperature are investigated by scanning tunnelling microscopy (STM) andreflection high-energy electron diffraction (RHEED). At the initial growth stage, Cu films are flat due to the formation of silicide at the interface that decreases the mismatch between Cu films and the Si substrate. Different from the usual multilayer growth of Cu/Cu(111), on the silicide layer a layer-by-layer growth is observed. The two dimensional (2D) growth is explained by the enhanced high island density at low deposition temperature. Increasing deposition rateproduces films with different morphologies, which is the result of Ostwald ripening.     

TiSi mixing at room temperature: A high resolution ion backscattering study

Very thin Ti films (0–20 Å) have been deposited in ultra-high vacuum on atomically clean Si(111) surfaces. The films have been analyzed with Medium Energy Ion Scattering using the high depth resolution of this technique in combination with ion shadowing and blocking. The measurements show that mixing of Ti and Si already occurs at room temperature, for coverages below 2.8 × 10¹⁵ Ti atoms/cm². The composition of the uniformly mixed film is close to TiSi. For higher coverages no further mixing takes place and pure Ti is present on top of the ultrathin mixed film. The implications of these results for current silicide formation models are discussed.     

Magnetic linear dichroism in photoemission from an ultrathin iron silicide film

The effect of magnetic linear dichroism in photoemission of Fe 3p electrons was used to investigate the magnetic properties of the Si(100)2 × 1 surface on which iron films up to 10 monolayers thick were deposited at room temperature under ultrahigh vacuum. The experiments were performed with linearly polarized light (at a photon energy of 135 eV) incident at an angle of 30° to the surface. The photoelectron spectra were measured in a narrow solid angle oriented along the normal to the sample surface for two opposite magnetization directions which were parallel to the surface plane and perpendicular to the polarization vector of the light wave. An analysis of the data obtained showed that the effect has a threshold character and appears after deposition of eight Fe monolayers, when the ferromagnetic silicide Fe₃Si is formed on the surface.     

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.     

Si growth effects on the formation of Er silicide nanostructures

In this work, an ultra-high vacuum scanning tunneling microscopy has been utilized to study the effects of Si atoms to the formation and growth evolution of Er silicide nanostructures. Si evaporation is performed on the vicinal Si(0 0 1) surface as well as Er growth under different growth conditions: growth procedure, annealing temperature and duration time. The experimental results show that the Si evaporation performed at a high temperature plays a key role on the growth of Er silicide nanostructures. The deposited Si atoms become a significant source of the Si reactant and mainly affect the early growth stage of the nanostructures. It is also shown that Er atom is possibly another diffusing species during the growth of Er silicide nanostructures on the Si(0 0 1) surface.     

LEED-AES study of the AuSi(100) system

The system Au/Si(100) has been studied using LEED and AES. Au films grow as Au(111) | Si(100) having six azimuthally rotated orientations at low deposition temperatures below 50°C after the formation of intermediate gold suicide layers. Crystalline gold silicide thin layers are formed on the Au(111) film after heat treatment at 100–400°C. Two types of suicide LEED pattern observed seem to have no correlation with crystallographic data reported on quenched alloy films. Heat treatment over 450°C leads to agglomeration of the film, producing a series of Au-induced superstructures. Heat treatment of the Au film over 1000°C regenerates the clean Si surface accompanied with many etch pits.     

Formation of the Co/Si(110) interface: Phase composition and magnetic properties

The formation of the Co/Si(110)16 × 2 interface and its magnetic properties are studied by high-energy-resolution photoelectron spectroscopy using synchrotron radiation and magnetic linear dichroism in the photoemission of core electrons. It is shown that a cobalt coating less than 7 Å thick deposited on the silicon surface at room temperature results in the formation of an ultrathin (1.7 Å) interfacial cobalt silicide layer and a layer of silicon-cobalt solid solution. The ferromagnetic ordering of the interface is observed at an evaporation dose corresponding to 6–7 Å in which case a cobalt metal film begins to grow on the solid solution layer. During 300°C-annealing of the sample covered by a nanometer-thick cobalt layer, the metal film gradually disappears and four silicide phases arise: metastable ferromagnetic silicide Co₃Si and three stable nonmagnetic silicides (Co₂Si, CoSi, and CoSi₂).     

Study of ultrathin iron silicide films grown by solid phase epitaxy on the Si(001) surface

Ultrathin films of iron silicide have been grown by high-temperature annealing of 0.14-to O.5O-nm-thick Fe films deposited on the Si(001) surface at room temperature. It has been found that annealing leads to the formation of nanoislands of iron silicide on the surface, so that their type depends on the thickness of the Fe film. High-energy electron diffraction and atomic force microscopy measurements have revealed that the deposition of Fe films less than 0.32 nm thick on the Si(001) surface stimulates epitaxial growth of both three-dimensional β-FeSi₂ and two-dimensional γ-FeSi₂ islands. It has been found that, for Fe coverages of more than 0.32 nm thick, a complete transition to solide phase epitaxy is observed only for two-dimensional β-FeSi₂ islands. The effect of prolonged annealing at 850°C on the morphology of the surface of the iron silicide film has been investigated.     

Optical and Structural Phenomena at Multipulse Interference Femtosecond Laser Fabrication of Metasurfaces on a Thin Film of Amorphous Silicon

JETP Letters - The spatial dependence of the differential conductivity of ultrathin Pb films deposited on the Si(111)7 × 7 surface has been studied by low-temperature scanning tunneling...     

Effect of the crystal structure and interlayer exchange coupling on the coercive force in Co/Cu/Co films

Co/Cu/Co/Si(111) trilayer films prepared by magnetron sputtering with different thicknesses of the Cu buffer were studied. The magnitude of the coercive force was shown to oscillate with variation of the nonmagnetic-buffer thickness. A good correlation between the saturation field and the coercive force was established. It was experimentally revealed that modification of the film structure by annealing within a broad temperature range affects the indirect exchange coupling between the Co layers. A qualitative analysis of the coercive-force behavior under variation of the strength and type of the ferromagnetic-layer coupling was performed.     

Reactive epitaxy of cobalt disilicide on Si(100)

The growth of cobalt disilicide on the Si(100) surface by reactive epitaxy at T=350°C was studied within the 10–40 ML cobalt coverage range. A new method of mapping the atomic structure of the surface layer by inelastically scattered medium-energy electrons was employed. The films thus formed were shown to consist of CoSi₂(221) grains of four azimuthal orientations turned by 90° with respect to one another. This domain structure originates from substrate surface faceting by (111) planes, a process occurring during silicide formation. B-oriented CoSi₂(111) layers grow epitaxially on (111) facets.