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.     

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.     

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.     

Thermal oxidation of cobalt disilicide

The thermal oxidation kinetics of cobalt disilicide on Si substrates have been investigated in the temperature range of 650–1100 °C in dry oxygen and wet oxygen. A surface layer of SiO₂ grows parabolically with time. The growth rate is independent of the substrate orientation (111 or 100) and thickness of the CoSi₂ layer. We surmize that the oxidation mechanism is dominated by the diffusion of an oxidant through the growing SiO₂. Activation energies for the dry and wet oxidation are 1.49±0.05 eV and 1.05±0.05 eV, respectively. The kinetics is exactly the same as for NiSi₂ oxidation which suggest that the same mechanism controls the oxidation of these two similar suicides.     

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.     

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.     

Observation of a structural phase transition in a CoSi2 layer buried in 〈111〉 Si

Single crystalline multilayered structures of Si/CoSi₂/Si111 made by high dose implantation of Co in a Si wafer were investigated with⁵⁷Co source Mössbauer spectroscopy, channeling measurements and X-ray diffraction. The results point to a structural phase transition in the CoSi₂ buried layer between 180 and 220 K.     

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：     

Tunneling experiments on amorphous and crystalline superconducting tantalum films

The energy gap and the transition temperature of amorphous and crystalline superconducting Ta films are measured by tunneling experiments. The following values are obtained: 2(0)=0,65 meV,T _c=2,11 K for the amorphous state after quenched condensation and 2(0)=1,24 meV,T _c=4,06 K for the crystalline state after annealing at room temperature. The reduced energy gap 2(0)/kT _c=3,58 demonstrates that amorphous Ta films are weak-coupling superconductors. The crystallization of the amorphous Ta films takes place at 250 K.     

Epitaxial growth of YBa2Cu3O7−δ superconducting thin films by DC magnetron sputtering using a partially melted sintered target

In situ epitaxially grown YBa₂Cu₃O_7– (YBCO) thin films on (100)SrTiO₃ substrates with high critical current J _c=3.4 × 10⁶ A/cm² (at 77 K) and low microwave surface resistance R _s37 m (at 77 K, 50.9 GHz) are fabricated by dc magnetron sputtering using a large partially melted sintered planar target. A comparatively large homogeneous composition region can be obtained. The X-ray diffraction, X-ray double crystal diffraction, and high resolution transmission electron microscopy (HRTEM) analyses show that our deposited YBCO thin films are single crystalline containing mosaic blocks. The influence of substrate quality on the orientation behaviour of the films is also discussed. A method to improve the surface quality of the SrTiO₃ substrate, which improves the properties of the thin film, is presented.     

Electronic structure of FeSi<Subscript>2</Subscript>

The electronic structure of volume and film iron disilicides with crystal structure of the type of α leboit and fluorite was calculated using the linearized augmented plane-wave formalism. Joint and local partial densities of electronic states, x-ray emission spectra in different series of all inequivalent atoms of these phases, and photoelectron spectra for different excitation energies were obtained. γ-FeSi₂ was found to be, unlike α-FeSi₂, an unstable phase in both the volume and film realizations. X-ray L _2,3 emission spectra of silicon in the iron group disilicides NiSi₂, CoSi₂, and FeSi₂ were compared. NiSi₂, CoSi₂, and α-FeSi₂ exhibit transformation of the maximum in the near-Fermi region of the Si L _2,3 spectra as one crosses over from a bulk to a film sample. This transformation is closely connected with phase stability and may serve as a criterion of thermodynamic stability of the iron-group transition-metal disilicides.     

Pulsed laser deposition of Eu:Y2O3 thin films on (0001) α-Al2O3

This paper focuses on the preparation and characterization of crystalline thin films of rare-earth-doped sesquioxides (Y₂O₃, Lu₂O₃) grown by pulsed laser deposition on single-crystal (0001) sapphire substrates. X-ray diffraction measurements show that the films with thicknesses between 1 nm and 500 nm were highly textured along the 111 direction. Using Rutherford backscattering analysis, the correct stoichiometric composition of the films was established. The emission and excitation spectra of europium-doped films with a thickness 100 nm look similar to those of the corresponding crystalline bulk material, whereas films with a thickness 20 nm show a completely different emission behavior. PACS 68.55.Jk; 78.66.Nk; 81.15.F     

Y1Ba2Cu3O7-δ thin films from KrF laser ablation with substrate heating and in situ patterning by CO2 laser radiation

Y₁Ba₂Cu₃O_7– thin films were deposited by KrF laser ablation while replacing conventional contact heating by cw CO₂ laser irradiation of the substrate front surface. The HTSC films obtained on (100)ZrO₂ showed T _c(R=0)=90 K, T(90–10%)=0.5 K, j _c=2.5 × 10⁶ A/cm², a sharp transition in the ac susceptibility X(T), and pure c-axis orientation. Micrographs of thin films (< 0.5 m) showed a smooth morphology while thick films (>1 m) contained many crystallites sticking in the bulk material. Furthermore, in situ patterning was achieved during deposition by local laser heating of a selected substrate surface area. The resulting planar films contained amorphous, semiconducting parts only 1 mm or less apart from crystalline material showing the above HTSC quality.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Micro-Raman spectroscopy in polycrystalline CuInSe2 formation

The crystalline formation of CuInSe₂ thin films has been investigated using micro-Raman spectroscopy and AES composition analysis. It is confirmed that the Raman peaks are stongly dependent on the surface morphology and the Cu:In:Se ratio. In the films annealed at 315°C, crystalline grains larger than 2 m show Raman peaks at 174 cm^–1 and 258 cm^–1. The In content is very low and the Cu:Se ratio is about 1:1 in these grains. The low In concentration is thought to be due to the formation of In₂O₃ on the surface. On the other hand, random structures of 1–2 m grains found in films annealed at temperatures below 305°C show peaks at 174 cm^–1 and 186 cm^–1 instead of 258 cm^–1 and have a Cu:In:Se ratio of 1:1:3–4. Thus the 186 cm^–1 peak is thought to be related to a Cu, In-deficient phase when compared to stoichiometric CuInSe₂. The optimum annealing condition was found by analyzing the Raman spectra and composition of different crystalline CuInSe₂ grains. Films annealed under this condition exhibited a clear Raman peak at 174 cm^–1 and consisted of clusters of crystals less than 1 m in size.     

A high throughput approach to quantify protein adsorption on combinatorial metal/metal oxide surfaces using electron microprobe and spectroscopic ellipsometry

Although metallic biomaterials are widely used, systematic studies of protein adsorption onto such materials are generally lacking. Combinatorial binary films of Al_1−xTi_x and Al_1−xNb_x (0  x  1) and corresponding pure element films were produced on glass substrates using a unique magnetron sputtering technique. Fibrinogen and albumin adsorption amounts were measured by wavelength-dispersive spectroscopy (WDS) and spectroscopic ellipsometry (SE) equipment, both high throughput techniques with automated motion stage capabilities. X-ray diffraction revealed that the binary films have crystalline phases present near the ends of the compositional gradient with an amorphous region throughout the interior of the gradient. X-ray photoelectron spectroscopy provided the surface chemistry along the binary films and showed that Al₂O₃ preferentially formed at the surface. Protein adsorption onto these films was found to be closely correlated to the alumina surface fraction, with high alumina content at the surface leading to low amounts of adsorbed fibrinogen and albumin. Protein adsorption amounts obtained with WDS and SE were in excellent agreement for all films. This suggests that this combinatorial materials approach combined with these state-of-the-art, automated high throughput instruments provides a novel way to accurately monitor protein adsorption taking place at the surfaces of these metal/metal oxide materials.     

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.     

Optical constants of thin CoSi2 films on silicon

The optical transmission of CoSi₂ films of thickness 2.6–15 nm is measured in the wavelength range 1–20 m. The optical constants are evaluated by taking into account multiple reflections in the film and by fitting a Drude model. The plasma frequency _p=5.4–7.6 eV is equivalent to a carrier density n _eff=3×10²² cm^–3 and one carrier per unit cell. The relaxation frequency of the plasma resonance assumes high values =2 eV near the interface to silicon and decreases into the bulk film over several nanometers. Films grown off-axis from the (111) Si orientation exhibit an enhanced relaxation frequency.     

The high quality ZnO growth on c-Al2O3 substrate with Cr2O3 buffer layer using plasma-assisted molecular beam epitaxy

High quality epitaxial ZnO films were grown on c-Al₂O₃ substrates with Cr₂O₃ buffer layer by plasma-assisted molecular beam epitaxy (P-MBE). The hexagonal crystalline Cr₂O₃ layer was formed by oxidation of the Cr-metal layer deposited on the c-Al₂O₃ substrate using oxygen plasma. The epitaxial relationship was determined to be ZnO//Cr₂O₃//Cr//Al₂O₃ and ZnO//Cr₂O₃//[0 0 1]Cr//Al₂O_3. The Cr₂O₃ buffer layer was very effective in improving the surface morphology and crystal quality of the ZnO films. The photoluminescence spectrum showed the strong near band-edge emissions with the weak deep-level emission, which implies high optical quality of the ZnO films grown on the Cr₂O₃ buffer.     

Annealing study of electrodeposited CuInSe<Subscript>2</Subscript> and CuInS<Subscript>2</Subscript> thin films

In this study CuInSe₂ and CuInS₂ thin films were prepared onto ITO glass substrate using the electrodeposition technique in aqueous solution. The electrodeposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis. The annealing effects on electrodeposited precursors were investigated. The chalcopyrite structure of CuInSe₂/CuInS₂ showed an enhancement of crystallinity after subsequent selenization/sulfurization treatment in Se/S atmosphere, respectively. XRD and SEM studies revealed a dramatic improvement of the crystalline quality of CIS films after annealing treatments. Mott–Schottky measurements were used to assess the conductivity type of the films and their carrier concentration. The prepared samples underwent an etching process to remove the binary accumulated Cu_2?x(Se,S) phases shown in FESEM pictures. This etching process has shown a noticeable decrease in both, the flat band potential, V_fb (V), and the number of acceptors, N_A (cm^?3) in selenized CuInSe₂ and sulfurized CuInS₂ samples.     

Structural transformations in MoO<Subscript>x</Subscript> thin films grown by pulsed laser deposition

In this work, laser-induced crystallization in MoO_x thin films (1.8x2.1) is reported. This transformation involves a MoO_x oxidation and subsequently a crystallization process from amorphous MoO₃ to crystalline MoO₃. For comparison purposes crystallization is induced thermally, in an oven, as well. The crystallization kinetics is monitored by Raman spectroscopy; a threshold in the energy density necessary to induce the phase transformation is determined in the case of photo-crystallization. This threshold depends on the type of substrate on which the film is deposited. For the thin films deposited on glass substrates, the structural transformation is from amorphous MoO_x to the thermodynamically stable MoO₃ crystalline phase. For the thin films deposited on Si(100) the structural transformation is from amorphous MoO_x to a mixture of MoO₃ and the thermodynamically unstable MoO₃ crystalline phases. The structural transformations are also characterized by scanning electron microscopy and light-transmission experiments. PACS 81.15.Fg; 61.80.Ba; 78.30.-j