J. Derrien

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO4 tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO4 tetrahedra are readily formed even at room temperature. Nevertheless, the SiO2 thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ∼400°C), silicon diffusion towards the surface is promoted and a thicker SiO2 layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.

Epitaxial growth of β-FeSi2 on silicon (111): a real-time RHEED analysis

Abstract We present a detailed analysis of the solid phase epitaxy (SPE) of β-FeSi 2 on the silicon (111) face. The initial thickness of the iron film epitaxially deposited on the (111) silicon face has been varied from 2 toA. Using a well-defined procedure of annealing (a fixed heating rate d T /d t = 1−4°C), we show that this initial iron thickness is a key parameter which controls the kinetic behaviour of Fe Si intermixing at the interface. Indeed at the same temperature after the same thermal treatment, completely different chemical and crystalline states have been observed at the surface depending essentially upon the original iron film thickness. This kinetic behaviour at the Fe Si interface is experimentally analysed through a quantitative analysis of RHEED patterns. Major facts are: (i) all the low-temperature equilibrium Fe Si phases (bcc Fe(+Si), simple cubic FeSi, β-FeSi 2 ) have been grown epitaxially on the silicon (111) face, (ii) an epitaxial FeSi 2 strained phase has been clearly identified by the RHEED technique. This result is suggested to be an intrinsic consequence of the silicon stress field applied to the distorted fluorite structure of β-FeSi 2 ; (iii) a transient amorphous phase is most probably formed at 300 T e Fe ≈ 30–50A, preceding the formation of the epitaxial β-FeSi 2 phase. The appearance of such an amorphous phase at a definite iron thickness adds new experimental insights into solid state amorphisation which is a typical process occuring at interfaces of transition-metal systems.

Kinetics of formation and properties of epitaxial CoSi2 films on Si (111)

Very thin CoSi2 films, epitaxially grown on silicon (111) surfaces, have been obtained under ultrahigh vacuum conditions by thermal reaction of Co layers deposited onto Si (111) substrates. The morphology and structural properties of the CoSi2 films depend strongly on experimental parameters such as film thickness, annealing temperature to form CoSi2, or substrate temperature during Co deposition. Usually for high‐temperature formation (≥700 °C) and large thickness (≥200 A) CoSi2 islands are observed. Below these limits the CoSi2 films display a rather smooth and homogeneous aspect. The reaction kinetics are also followed in situ monitoring the Co and Si Auger peaks versus annealing temperature. Two interfaces have been examined, namely Co–Si and Co–CoSi2. Plateaus in the Auger peak variation show the evolution of the sampled region from the Co metallic phase to successively Co2Si, CoSi, and CoSi2 phases. Finally, electrical characteristics of some nearly perfect CoSi2–Si Schottky barriers have been check...

Thin metallic silicide films epitaxially grown on Si(111) and their role in Si–metal–Si devices

Recent advances in the epitaxial growth of various d‐metal silicides on Si(111) are reviewed. The growth mechanisms, the structural, and electronic properties of epitaxial silicides are described. In situ surface techniques such as low‐energy electron diffraction, Auger electron spectroscopy, x‐ray induced photoemission spectroscopy, ultraviolet photoemission spectroscopy, extended energy‐loss fine structure, electron energy‐loss near edge spectroscopy, and ex situ techniques such as Rutherford backscattering, cross‐section transmission electron microscopy, and deep‐level transient spectroscopy have been used to study the special case of quasiperfect ultrathin films of CoSi2 on Si(111). Physical factors affecting the CoSi2 epitaxy are discussed. The role of CoSi2 film in metal base and permeable base transistors is also addressed.

Local structure determination of the CoSi(111) interface by surface electron energy-loss fine-structure technique

Extended fine structures, similar to those observed with SEXAFS technique, have been detected in electron energy-loss spectra in the reflection mode. These structures are related to the local geometry (bond length) of the investigated system. We have applied this loss technique to study the CoSi(111) interface. Our structural results suggest the formation at room temperature of an initial silicide phase followed with a nearly pure cobalt film. Upon annealing at progressively higher temperatures, sequential silicide formation (Co2Si, CoSi, CoSi2) occurs, confirming most of our recent observations by other surface techniques.

Evidence of extended fine structures in the Auger spectra: A new approach for surface structural studies

Abstract We report for the first time on extended fine structures which we have observed above core-valence-valence Auger transitions on Cu and Co samples. We interpret these oscillating structures as originated from the same interference process which produces extended fine structures observed in the x ray absorption spectra (EXAFS). Using the same EXAFS analysis procedure, we deduce from the extended fine Auger structures (hereafter called EXFAS) the radial atomic distribution F(R) of Cu and Co samples. An excellent agreement is found as compared with results from synchrotron radiation EXAFS.

Heteroepitaxy of metallic and semiconducting silicides on silicon

Abstract Recent developments in the epitaxial growth of metallic and semiconducting silicides on silicon are reviewed. The structural, electronic and electrical properties of these silicide-silicon interfaces are examined with the aid of results obtained with a large variety of in-situ and ex-situ surface techniques. The paper will focus on two topics: (i) recent progress on the epitaxial growth of thin metallic CoSi 2 films on Si(111), (ii) epitaxial growth of thin semiconducting β-FeSi 2 films on Si(111), (100) and (100) vicinal faces.

Synthesis and properties of epitaxial semiconducting silicides

Abstract The state-of-the-art in the preparation and characterization of epitaxial semiconducting silicides will be reviewed in this report. Emphasis will be put on thin FeSi 2 layers epitaxially grown on Si substrates. The mechanisms of silicide formation will be discussed through results obtained by a large variety of in-situ techniques (RHEED, Auger, photoemission, …) and ex-situ techniques (X-ray diffraction, RBS, electron microscopy). Moreover, for ultra-thin FeSi 2 films, several strained pseudomorphic and metallic phases induced by epitaxy are observed on top of Si substrates. Their transition towards the stable relaxed semiconducting β-FeSi 2 will be presented. Recent findings of the metallic α-FeSi 2 phase observed at surprisingly low temperature and its relaxation towards the β-phase will also be reported.

Silicide epilayers: recent developments and prospects for a Si-compatible technology

Semiconducting silicides epitaxially grown on silicon may be promising materials for integrated optoelectronic devices. The structure and the physical properties of FeSi2 are reviewed in the light of results obtained with a large variety of in situ an ex situ surface techniques. Dynamical transitions from strained metallic FeSi2 toward relaxed semiconducting FeSi2 and epitaxial FeSi are clearly demonstrated. New developments for silicide heteroepitaxy on silicon using gas-source molecular beam epitaxy are also discussed.

Electronic structure of Ag adsorbed on Si(111); experiment and theory

Abstract Experimental results (low energy electron loss spectroscopy) and band structure calculations relating to the early stages of Ag growth on a Si(111) surface are presented. Crystallography and thermal desorption kinetics studies of this interface, previously published, gave rise to the following conclusions. At room temperature and below 200°C, two-dimensional (2D) (111) epitaxial layers develop on top of a first ordered layer (√3 × √3), while at higher temperatures three-dimensional (3D) clusters develop over this first layer. Low energy electron loss experiments were performed at various surface coverages θ. They display different evolutions according to the growth modes. For the 2D epitaxial growth, one observes the disappearance of the peaks characteristic of a Si surface and the onset of Ag induced peaks located at 7.1 and 4.6 eV at completion of the √3 layer. These peaks narrow and shift to the bulk Ag excitation energies at 7.5 and 4 eV when a second Ag layer is deposited. In order to explain these results, we present a theoretical calculation of the electronic density of states of the interface using a tight binding approximation. This calculation accounts for the development of the Ag d band from the √3 coverage range to the (111) epitaxial Ag planes. The evolution of the spectra when θ is increased is discussed in view of these results.