N. Onda

Epitaxy of cubic iron silicides on Si(111)

Abstract Recently, new pseudomorphic iron silicide phases with the cubic structure have been synthesized on Si(111) by molecular beam epitaxy (MBE) at room temperature (RT). The interface between the pseudomorphic phases and Si(111) is energetically very favourable compared to stable ϵ-FeSi/Si(111) or stable β-FeSi2/Si(111) interfaces. It enables the epitaxy of various phases which are structurally similar, like Fe3Si and even pure bcc iron at room temperature. The orientation was found to be always pinned to the type B orientation of the interface. On Fe3Si a new √3 × √3 R30° surface reconstruction was observed.

Observation of misfit dislocations in epitaxial CoSi2/Si (111) layers by scanning tunneling microscopy

The surfaces of epitaxial CoSi2 layers grown on Si(111) have been examined by scanning tunneling microscopy (STM) in ultrahigh vacuum. The onset of strain relaxation above the critical thickness of about 40 A has been monitored by STM for the first time. This relaxation takes place by the formation of a honeycomb network of partial dislocations lying in the interface plane. An associated network of protruding lines has been detected in STM topographs for film thicknesses up to 104 A. The topographic cross sections perpendicular to the lines are found to have a Lorentzian shape with a height of 0.6 A and a half‐width equal to the layer thickness. Our analysis suggests that similar effects should be observable for a wider class of heteroepitaxial systems.

Investigation of the defect structure of thin single‐crystalline CoSi2 (B) films on Si(111) by transmission electron microscopy

Thin epitaxial single‐crystalline B‐type CoSi2 films (twin‐oriented) have been grown in ultrahigh vacuum by stoichiometric codeposition of Co and Si on slightly misoriented (0.1°–0.3°) Si(111) substrates. The microstructure as well as the nature of interfacial defects has been investigated in detail by transmission electron microscopy. The defect structure is found to depend closely on the initial deposition parameters, annealing temperature, and the topography of the Si substrate. It will be shown that even during the early stages of layer growth, loss of coherence is obtained and lattice strain already starts to occur with the introduction of misfit dislocations with Burgers vector b=a/2〈110〉 inclined to the interface or with Burgers vector b=a/6〈112〉 parallel to it. It is demonstrated that ultrathin CoSi2 films with thickness of about 1 nm grown on slightly misoriented substrates with parallel surface steps, exhibit quite different defect structures at annealing temperatures between 300 °C and 550 °C. ...

Surface and interface structure of epitaxial CoSi2 films on Si(111)

The surface and interface structures of molecular beam epitaxially grown CoSi2 films on Si(111) have been studied by scanning tunneling microscopy and by transmission electron microscopy, respectively. All surfaces are found to be inhomogeneous, exhibiting (2×1) and (2×2) reconstructed domains along with unreconstructed areas, depending on their stoichiometry. All of them could be imaged with atomic resolution. The surface step structure and the formation of pinholes have been examined for a wide range of growth conditions. Evidence is presented for micron‐scale surface diffusion of Si on CoSi2 at temperatures as low as 800 K. The interface step structure, studied by transmission electron microscopy, has been found to depend critically on the details of the growth procedure.

A study of epitaxially stabilized FeSi2 by surface enhanced Raman scattering

Epitaxially stabilized films with the defect CsCl structure, that were grown by molecular beam epitaxy, have been studied by surface enhanced Raman scattering using a silver overlayer. We have observed that the defect‐induced phonon density of states features in the Raman signal shift from 256 cm−1 for a coherently strained film to 263 cm−1 for a relaxed one. The lower energy observed for the former can qualitatively be explained by the expansive trigonal distortion arising from the misfit of −0.5%.

Ion channeling studies of epitaxial Fe and Co silicides on Si

High quality epitaxial Co and Fe silicides have been grown by molecular beam epitaxy on Si(111) and Si(001) substrates with film thicknesses ranging between 25 and 8400 A. We used Rutherford backscattering spectrometry channeling techniques to measure the lattice distortion as a function of film thickness. The critical thickness hc corresponding to the film thickness at which strain relieving dislocations begin to appear was determined for CoSi2 on Si(111) and Si(001) as well as for Si on CoSi2(111). For CoSi2 on Si(001), a larger critical thickness was obtained than on Si(111), where hc is ∼45 A. Epitaxial Si on CoSi2(111) was found to be under a compressive strain up to thicknesses of about 350 A depending on substrate misorientation. Strain measurements were also performed on epitaxially stabilized Co and Fe monosilicides with the CsCl structure. Channeling measurements on thick epitaxial films of bcc‐Fe, Fe3Si, FeSi, and Fe0.5Si were used to determine the crystalline quality. Excellent channeling mini...

Application of epitaxial CoSi2⧸Si⧸CoSi2 heterostructures to tunable Schottky-barrier detectors

Epitaxial CoSi2/Si/CoSi2 heterostructures on Si(111) have been grown by MBE. STM measurements during the growth process showed silicide layers below ∼ 45 A to be coherent apart from dislocations associated with the wafer misorientation, and the ∼ 1000 A thick Si spacer to be pinhole-free. Diode structures were fabricated by standard photolithography and a combination of wet chemical and plasma etching. Both the upper and the buried silicide film could be contacted separately for electrical measurements. Vertical transport measurements showed blocking behaviour confirming the Si spacer layers to be continuous. The photoresponse of the diode structures was measured as a function of bias at 77 K. A maximum shift of the cutoff energy of 0.1 eV was obtained at an electrical field of 200 kV cm-1.

Structural and electronic properties of pseudomorphic FeSi1+x films on Si(111)

Abstract Epitaxial FeSi 1+ x (0 ≤ x ≤ 1) films were grown on Si(111) by MBE at room temperature (RT). They were characterized in situ by RHEED, UPS, XPS and STM and ex situ by TEM. Kikuchi band and TEM analysis revealed that FeSi has the simple cubic CsCI structure with a lattice constant close to half the Si one. This phase which does not occur in the equilibrium Fe-Si phase diagram can be stabilized epitaxially on Si(111) through a favourable interface structure. UPS spectra display a distinct Fermi edge indicating that FeSi is metallic. Upon annealing, films thicker than ∼ 15 A exhibit a phase transformation to the stable bulk ϵ-FeSi phase, which can be either epitaxial or heavily twinned. In thinner films the Si content was observed to increase up to 1:2 without any change of symmetry.

Phase Transformations in Layered Fe-Si Structures

The kinetics of the phase transformations of the Fe-Si system and the Si/Fe and Fe/Si interfaces have been investigated by 57 Fe conversion electron Mossbauer spectroscopy (CEMS). Single crystalline Fe films grown on Si(111) by molecular beam epitaxy (MBE) and polycrystalline Fe films grown by pulsed laser ablation deposition (PLD) have been thermally treated in vacuum and the formation of the different silicide phases has been monitored as function of temperature and time by CEMS.

Interfacial dislocations detected by scanning tunneling microscopy

Abstract Thin films of CoSi 2 have been grown on Si(111) and Si(100) by molecular beam epitaxy (MBE). We present the unexpected result that scanning tunneling microscopy (STM) is sensitive to the misfit dislocations present at the CoSi 2 /Si interface in the case of overcritical, relaxed layers. Even for layer thickness above 100 A a surface contrast consisting of protruding lines which is due to the dislocations at the interface is easily visible in STM topographs. Topographic cross-sections perpendicular to these protruding lines can well be fitted to a Lorentzian with a half-width equal to the CoSi 2 layer thickness, i.e., the distance to the dislocation core. This effect is qualitatively explained by simple elasticity theory, but additional mechanisms have to be invoked in order to account for a quantitative explanation of the observed contrast. These measurements establish conventional STM as a viable technique for the imaging of interface dislocations.