Schottky barrier height lowering induced by CoSi2 nanostructure

CoSi₂ nanostructures were formed through thermal agglomeration by annealing ultrathin Co film on Si substrate at high temperatures. The characteristics of the Schottky diodes with CoSi₂ nanostructures capped by a Pt layer were measured and fitted using thermionic emission theory. All the diodes have a ideality factor less than 1.1. The results show that the Schottky barrier height of these diodes significantly decreases as the annealing temperature for CoSi₂ agglomeration increases. The barrier height lowering is correlated with the agglomeration of CoSi₂ film and the formation of CoSi₂ nano-islands. The thermal field emission may be the major reason to cause barrier lowering. Although the Schottky contact interface consists of both CoSi₂ nano-islands and Pt film whose individual contact barrier height to Si is very different, the current-voltage-temperature measurements reveal that the interface homogeneity is not degraded as expected. The study demonstrates that the CoSi₂ nanostructures can both lower the Schottky barrier height and form an ideal Schottky contact with a Pt capping layer.     

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.     

Angle-resolved photoemission study of CoSi2 nanofilms grown on Si(111) substrates

We have carried out an angle-resolved photoemission study for CoSi₂ nanofilms grown on the Si(111)-7×7 substrates. The surface of CoSi₂(111) nanofilm changes from the bulk-truncated surface to the surface with additional Si-bilayer by annealing at higher temperature above 825 K. The angle-resolved photoemission spectra of the CoSi₂ nanofilm annealed at 853 K show the spectral features originated from the surface resonance state on the CoSi₂ surface terminated by Si-bilayer. From the detailed photoemission study, we discuss the surface electronic structure in CoSi₂(111) nanofilms grown on Si(111) substrates.     

On new two-dimensional structures produced on the Si (111) and Si (100) surface upon molecular-beam epitaxy of cobalt and silicon

Highly perfect epitaxial heterostructure CoSi₂ films have been grown on the surface of Si (111) and Si (100) single crystals by the method of molecular-beam epitaxy. The optimal regimes of the film growth with different thicknesses have been determined. It has been shown that short-term annealing of epitaxial films at T = 900–950 K leads to the formation of new CoSi₂/Si(111)-2 × 2 and CoSi₂/Si(100)−2 × 4 superstructures.     

Photoelectron Si 2<Emphasis Type="Italic">p</Emphasis> spectra of ultrathin CoSi<Subscript>2</Subscript> layers formed on Si(100)2×1

Solid-phase formation of ultrathin CoSi₂ layers on Si(100)2×1 was studied using high-resolution (～140 meV) photoelectron spectroscopy with synchrotron radiation (hν=130 eV). The evolution of Si 2p spectra was recorded both under deposition of cobalt on the surface of samples maintained at room temperature and in the course of their subsequent annealing. It was shown that Co adsorption on Si(100)2×1 is accompanied by a loss of reconstruction of the original silicon surface while not bringing about the formation of a stable CoSi₂-like phase. As the amount of deposited cobalt continues to increase (up to six monolayers), a discontinuous film of the Co-Si solid solution begins to grow on the silicon surface coated by chemisorbed cobalt. The solid-phase reaction of CoSi₂ formation starts at a temperature close to 250°C and ends after the samples have been annealed to ～350°C.     

Studies of the promising material CoSi2 on GaAs substrates

The thermal stability of CoSi₂ thin films on GaAs substrates has been studied using a variety of techniques. The CoSi₂ thin films were formed by depositing Co(500 Å) and Si(1800 Å) layers on GaAs substrates by electron-beam evaporation followed by annealing processes, where the Si inter-layer was used as a diffusion/reaction barrier at the interface. The resistivity of CoSi₂ thin films formed is about 30 cm. The Schottky barrier height of CoSi₂/n-GaAs is 0.76 eV and the ideality factor is 1.14 after annealing at 750° C for 30 min. The CoSi₂/GaAs interface is determined to be thermally stable and the thin film morphologically uniform on GaAs after 900° C/30 s anneal. The CoSi₂ thin films fulfill the requirements in GaAs self-aligned gate technology.     

Low temperature ion beam mixing of Co/Si systems

Co/Si systems were ion beam mixed at 77 K using a 100 keV Ar beam. The formation of different phases as a function of irradiation dose has been studied, using Mössbauer spectroscopy (MS) and Rutherford backscattering spectroscopy (RBS). It was found that Co₂Si, CoSi and CoSi₂ are formed subsequently in parallel layers. After high dose irradiation, a phase with stoichiometry Co∶Si equal to 1∶3 was observed, suggesting CoSi₃ has been formed. However, MS gave clear evidence that this phase consists of precipitates of CoSi₂ and Si. Finally, we found that the amount of mixing scales linearly with the square root of the fluence, with a mixing rate of 1.0×10⁴Å⁴.     

CoSi2的能带结构

本文报道用自洽LMTO方法算得CoSi₂化合物的能带结构及状态密度。计算所得状态密度峰值位置与同步辐射光电子谱相应峰值位置很好地符合。计算结果表明:在CoSi₂及NiSi₂中,仍然是过渡金属原子的3d轨道与Si原子的3p轨道间的杂化成键决定它们的电子结构。Si原子并不保持像块状硅中的sp³型杂化。 关键词：     

Co-silicide layers studied by Mössbauer spectroscopy and Rutherford backscattering spectroscopy

Co-silicides were prepared with several techniques, such as annealing of evaporated Co-layers on a Si-substrate (silicide surface layers) and annealing of Co-implanted Si (buried silicide layers). By adding some⁵⁷Co to the stable⁵⁹Co, the formation of the various Co-silicides could clearly be followed as a function of annealing temperature by means of Mössbauer spectroscopy. In the case of surface silicide layers, Co₂Si, CoSi and CoSi₂ were formed subsequently. In the case of buried layers however, CoSi₂ was the only crystalline phase that could be observed. In both cases, the CoSi₂ spectra showed an anomalous side resonance. Moreover, it was found that when⁵⁷Fe was implanted (instead of⁵⁷Co), a drastic increase in the intensity of this side resonance could be detected by CEMS.     

Growth of high quality CoSi2/Si - superstructures on Si (111)

Using a newly developed solid phase epitaxy technique (SPE) it is shown that ultrathin essentially pinhole-free CoSi₂ layers can be grown epitaxially on Si (111). These form the basis of a number of short period metal/semiconductor superlattices that have been grown by combining SPE-grown CoSi₂ with MBE-grown Si. Substrate temperatures for Si-MBE have to be chosen very low (≈ 350 °C) in order to avoid a roughening of the layers.     

In-plane and out-of-plane shape transitions of heteroepitaxially self-assembled nanostructures

In this work, shapes and shape transitions of several types of self-assembled heteroepitaxial nanostructures, as observed in in situ scanning tunneling microscopy experiments during growth, are examined in the framework of several equilibrium and kinetic models. In particular, heteroepitaxial TiSi₂ and CoSi₂ islands on Si(1 1 1) are shown to behave in accordance with generalized Wulff-Kaishew theorem of equilibrium strained and supported crystal shapes. More specifically, these silicide nanocrystals exhibit out-of-plane thickening shape transition by increasing their vertical aspect ratio with growth, as long as they are strained, and inverse (flattening) transition upon relaxation by misfit dislocations. On the other hand, heteroepitaxial Ge and CoSi₂ islands on Si(0 0 1) are well-known for their in-plane anisotropic elongation. Plausible energetic and kinetic reasons for such elongation, based on the unique nucleation features of Ge-hut/Si(0 0 1) and non-planar CoSi₂-hut/Si(0 0 1) interface, are discussed.     

Two routes to polycrystalline CoSi2 thin films by co-sputtering Co and Si

Two processes for the fabrication of polycrystalline CoSi₂ thin films based on the codeposition of Co and Si by sputtering were studied and compared. The first process involved “annealing after deposition”, where Co and Si are codeposited at ambient temperature and then crystallized by annealing. This process yielded randomly oriented plate-like CoSi₂ grains with a grain size that is governed by the nanostructure of the as-deposited film. Polycrystalline CoSi₂ thin films were obtained at a process temperature of 170 °C, which was much lower than the annealing temperature of 500 °C needed for Co/Si bilayers. The second process involved “heating during deposition”, where Co and Si are codeposited on heated substrates. This process yielded CoSi₂ grains with a columnar structure, and the grain size and degree of (1 1 1) orientation are temperature dependent. The sheet resistance of the resulting films was determined by the preparation temperature regardless of the deposition process used, i.e. “annealing after deposition” or “heating during deposition”. Temperatures of 500 °C and higher were needed to achieve CoSi₂ resistivity of 40 μΩ cm or lower for both processes.     

Mössbauer and channeling measurements on buried layers of CoSi2 in Si

Single crystalline multilayered structures of Si/CoSi₂/Si were made by high dose implantation of Co into a Si wafer which was subsequently annealed. These structures were then investigated with both Mössbauer spectroscopy and channeling measurements. The experiments show that a change occurs in the structure of the CoSi₂ at a temperature between 150 K and 220 K.     

Composition and properties of nanoscale Si structures formed on the CoSi<Subscript>2</Subscript>/Si(111) surface by Ar<Superscript>+</Superscript> ion bombardment

The variations in the composition and structure of CoSi₂/Si(111) surface layers under Ar⁺ ion bombardment with subsequent annealing has been studied. It has been demonstrated that nanocluster phases enriched with Si atoms form on the CoSi₂ surface at low doses D ≤ 10¹⁵ cm^–2, and a pure Si nanofilm forms at high doses.     

Kinetics of CoSi2 from evaporated silicon

2 MeV⁴He⁺ backscattering spectrometry and CuK x-ray diffraction were used to study CoSi₂ formed by annealing at temperatures between 405° and 500 °C from CoSi with evaporated Si films. A laterally uniform layer of CoSi₂ forms, in contrast to the laterally nonuniform CoSi₂ layer that is obtained on single crystal Si substrates. The thickness of the CoSi₂ film formed is proportional to the square root of time at a fixed temperature. The activation energy of this reaction is about 2.3 eV.     

Finite Element Analysis of CoSi2 Nanocrystals on Si(001)

In this work we present a finite element analysis of pyramidal and hut-shaped CoSi₂ nanocrystals reactively deposited onto Si(001) substrates. These dots have been observed by us, as well as by other groups. Our analyses have yielded four major conclusions: (1) Elastic relaxation of CoSi₂/Si mismatch strain by three-dimensional islands drives their nucleation, rendering flat, two-dimensional, layer energetically unfavourable. (2) The effect of the nanocrystal surface and interface energies for the observed vertical aspect ratios is negligible at small nanocrystal volumes. (3) Pyramids and huts with identical vertical aspect ratios are energetically degenerate. (4) Nanocrystal growth is only energetically favourable if accompanied by an increase in vertical aspect ratio. Most of these conclusions are consistent with those found in compressively strained layers, such as Si_1?x Ge _x layers on Si.     

CoSi2/Si (111) Interface study by X-ray standing waves

Summary The CoSi₂/Si (111) interface has been studied with the X-ray standing-wave technique. The interface (a 49? thick CoSi₂ layer) has been epitaxially grown on Si (111) under ultrahigh vacuum andin situ characterized with Auger spectroscopy and low-energy electron diffraction. The perpendicular lattice mismatch between epilayer and substrate has been measured with double-crystal X-ray diffraction. The X-ray standing-wave analysis gives clear indication that the Co atoms are fivefold coordinated at the interface. This work has been partially supported by Progetto Finalizzato ?Materiali e dispositivi per l'Elettronica a Stato Solido?.     

Estimation of changes in parameters of a crystal lattice and energy bands upon variation in the size of nanocrystals and nanofilms of silicides prepared by ion implantation

Changes in the parameters of the crystal lattice and energy bands of CoSi₂ nanofilms and nanocrystals formed in the surface Si layers by ion implantation combined with annealing are studied. It is shown that the band gap E _g of CoSi₂/Si(100) nanofilms with the thickness θ ≤ 40–50 Å is higher by ～0.1 eV than for “thick” films; in the case of nanocrystals, E _g is 0.3–0.4 eV higher than for macrocrystals.     

Formations and morphological stabilities of ultrathin CoSi2 films

In this paper we investigate the formations and morphological stabilities of Co-silicide fihns using 1-8-nm thick Co layers sputter-deposited on silicon （100） substrates. These ultrathin Co-silicide films are formed via solid-state reaction of the deposited Co films with Si substrate at annealing temperatures from 450 ℃ to 850 ℃. For a Co layer with a thickness no larger than i nm, epitaxially aligned CoSi2 films readily grow on silicon （100） substrate and exhibit good morphological stabilities up to 600 ℃. For a Co layer thicker than 1 nm, polycrystalline CoSi and CoSi2 films are observed. The critical thickness below which epitaxially aligned CoSi2 film prevails is smaller than the reported critical thickness of the Ni layer for epitaxial alignment of NiSi2 on silicon （100） substrate. The larger lattice mismatch between the CoSi2 film and the silicon substrate is the root cause for the smaller critical thickness of the Co layer.     

Growth and characterization of CoSi2 films on Si (1 0 0) substrates

In the present work, a special solid phase epitaxy method has been adapted for the preparation of CoSi₂ film. This method includes an epitaxial growth of Co films on Si (1 0 0) substrate, and in situ annealing of the Co/Si films in vacuum. It has been found that at the substrate temperature of 360°C, fcc cobalt film grows epitaxially on the Si (1 0 0) surface. The crystallographic orientation relations between fcc Co film and Si substrate determined from the electron diffraction result are: (0 0 1) Co//(0 0 1) Si, [1 0 0] Co//[1 1 0]Si. Upon annealing at temperatures range from 500 to 600°C, Co film reacts with Si substrate and transforms into CoSi₂. The CoSi₂ films prepared by this way are characterized by XTEM, XPS and AFM.