Effect of annealing on magnetic properties and silicide formation at Co/Si interface

The interaction of Co (30 nm) thin films on Si (100) substrate in UHV using solid state mixing technique has been studied. Cobalt was deposited on silicon substrate using electron beam evaporation at a vacuum of 4×10^?8 Torr having a deposition rate of about 0·1 Å/s. Reactivity at Co/Si interface is important for the understanding of silicide formation in thin film system. In the present paper, cobalt silicide films were characterized by atomic force microscopy (AFM) and secondary ion mass spectroscopy (SIMS) in terms of the surface and interface morphologies and depth profile, respectively. The roughness of the samples was found to increase up to temperature, 300°C and then decreased with further rise in temperature, which was due to the formation of crystalline CoSi₂ phase. The effect of mixing on magnetic properties such as coercivity, remanence etc at interface has been studied using magneto optic Kerr effect (MOKE) techniques at different temperatures. The value of coercivity of pristine sample and 300°C annealed sample was found to be 66 Oe and 40 Oe, respectively, while at high temperature i.e. 748°C, the hysteresis disappears which indicates the formation of CoSi₂ compound.     

Metal silicide-templated growth of quality Si films for Schottky-diodes

Quality Si films were grown on a metal silicide template and fabricated for a Schottky-diode. The thin metal was firstly deposited and reacted to the supplying Si and then formed the silicide layer, which is a template to grow quality Si film above it due to the lattice affinity to Si. Various types of metal (Co, Ni, and mixture of Co and Ni) were used as catalyst species. The morphological changes of Si grain sizes were systematically investigated. Two steps of Si supply condition were applied and revealed the formation of metal silicide phases and Si film growth.During the Si supply, Co was stable to form CoSi₂ and grew a crystalline Si (c-Si) film above it. However Ni firstly formed Ni rich silicide phases at low Si supply due to the fast Ni diffusion in Si. By increasing the Si supply, Ni diffusion has been staggered and formed NiSi₂ layer to grow a c-Si film above it. It has been also revealed that the NiSi₂ migration produced a c-Si film behind. Mixing of Co with Ni showed a stable silicide phase without a serious metal migration and improved the Si crystallinity providing an enhanced Schottky-diode performance.The investigation of silicide formation and quality Si film growth is presented. Transmission electron microscope analysis proves the volume growth of c-Si film above a metal disilicide of NiSi₂ or CoSi₂.     

Features of CoSi<Subscript>2</Subscript> phase formation by two-stage rapid thermal annealing of Ti/Co/Ti/Si(100) structures

A method of cobalt disilicide (CoSi₂) layer formation proceeding from a Ti(8 nm)/Co(10 nm)/Ti(5 nm)/Si(100) (substrate) structure prepared by magnetron sputtering is described. The initial structure was subjected to two-stage rapid thermal annealing (RTA) in nitrogen, and the samples after each stage were studied by the time-of-flight secondary-ion mass spectrometry, Auger electron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The RTA-1 stage (550°C, 45 s) resulted in the formation of a sacrificial surface layer of TiN _x O _y , which gettered residual impurities (O, C, N) from inner interfaces of the initial structure. After the chemical removal of this TiN _x O _y layer, the enrichment with cobalt at the RTA-2 stage (830°C, 25 s) led to the formation of a low-resistance CoSi₂ phase.     

Titanium Interlayer Mediated Epitaxy of CoSi2 on Si1−xGex

Titanium Interlayer Mediated Epitaxy (TIME) has been shown to promote the formation of epitaxial CoSi₂ on Si (100). Similarities between Si and Si_1−xGe_x alloys have motivated a study of whether the TIME process could be successful in forming epitaxial CoSi₂ on Si_1−xGe_x. Titanium layers of varying thickness were deposited as interlayers between a Co layer and c-Si/Si_0.8Ge_0.2 grown epitaxially onto Si (100) to investigate their role in the formation of epitaxial CoSi₂ on Si_1−xGe_x alloys. The effect of Ti interlayer thickness on the orientation of CoSi₂ to the Si_1−xGe_x substrate, and the conditions under which a polycrystalline CoSi₂ film has been formed have been studied. It was found that Ti was beneficial in promoting epitaxy to the substrate in all cases. The experimental results indicate that with a Ti interlayer thickness of ∼ 50 Å, the formation of epitaxial CoSi₂ adjacent to the substrate was achieved, and pinhole formation was minimized. It was also observed that for increased interlayer thickness, Ti reacted with Si to form a titanium silicide.     

A novel selective removal process of cobalt silicide

In this paper we study the formation of cobalt silicide on silicon and LP-CVD silicon oxide. The study was carried out on blanket and patterned wafers by the aid of Auger spectroscopy, electrochemical measurements and scanning electron microscopy (SEM). The preparation of pattern wafers follows the conventional process flow using the silicide self-aligned technology for the fabrication of Flash devices. The Auger spectra revealed the formation of the CoSi _x phase on silicon and the formation of a ternary Co–Si–O compound on silicon dioxide. The latter, that is responsible of potential electrical shortage between gate and contacts was successfully removed via a novel selective dissolution process, characterized by low cost, low temperature, and a simple formulation chemistry.     

Surface and interface morphology of CoSi2 films formed by multilayer solid-state reaction

CoSi₂ is a promising material for self-aligned silicide (salicide) applications in sub-0.25 μm complementary metal–oxide–semiconductor (CMOS) technology. In conventional salicide technology, silicides are formed by a solid-state reaction (SSR) after source/drain formation. With the continued scaling down of junction depths, surface and interface roughness of silicides is a growing concern. In this work, a comparative study has been made to investigate the morphology and thermal stability of CoSi₂ formed by SSR of different structures, i.e. Co/Si, TiN/Co/Si, Ti/Co/Si, Co/Ti/Si and Ti/Co/Ti/Si. Atomic force microscopy and other techniques were used to characterize the morphology and thermal stability. Compared with the Co/Si reaction, TiN or Ti capping reduces the roughness and improves the thermal stability. The reaction with a Ti interfacial layer shows epitaxial growth of CoSi₂ on Si (100). The morphology and thermal stability of epitaxial CoSi₂ were significantly improved. The epitaxial CoSi₂ may be useful for contact in deep submicrometer CMOS devices.     

Growth mechanism of epitaxial CoSi2 on Si and reactive deposition epitaxy of double heteroepitaxial Si/CoSi2/Si

The growth mechanism of epitaxial CoSi₂ was studied using Co/Ti/Si multilayer solid phase reaction. Results showed that phase formation was controlled by diffusion of Co through the growing CoSi_x, although at the early stage of CoSi₂ growth the diffusion of Co could be controlled by a Ti layer. A reactive deposition technique was also evaluated by using a conventional magnetron sputtering system. Results showed that an epitaxial CoSi₂ layer was formed by controlling the Co sputtering rate not to exceed the Co diffusion rate through CoSi_x. However, the surface of CoSi₂ became rough when the deposition rate was much slower than the Co diffusion rate through CoSi_x. The roughness was caused by the formation of CoSi₂ (111) facets at the interface between CoSi₂ and the Si substrate. Si/CoSi₂/Si double heteroepitaxial structures were fabricated when Si and Co were sequentially sputter-deposited on a Si (100) substrate.     

Electrical characterisation of HfYO MIM-structures deposited by ALD

By an ALD process with the solid precursors HfCl₄ and (CpCH₃)₃Y and the oxidant water Yttrium doped HfO₂ was deposited on TiN layer on highly doped silicon. The films were analysed by ellipsometry, XRR, RBS and XRD. For the electrical characterisation, capacitance and I-V measurement on MIM structure were used. By doping the HfO₂ with 6.2 at.% Yttrium and annealing the film at 500 °C in N₂ the k-value increased by 60% for a 9.5 nm thick film, the leakage current also increased. The deposited amorphous film crystallises at 450 °C into the cubic phase.     

Synthesis of new binary cobalt iron pyrophosphate CoFeP2O7

A new binary cobalt iron pyrophosphate, CoFeP₂O₇ was synthesized through solid phase reaction using cobalt carbonate, iron metal and phosphoric acid in the presence of water-methanol system with further calcinations at the temperature of 500 °C. The particle size obtained from X-ray line broadening is 36 ± 7 nm for the CoFeP₂O₇. FTIR spectrum of CoFeP₂O₇ is assigned based on the P₂O₇⁴⁻ ion in the structure. The XRD, UV-Vis near IR and FTIR results of the synthesized CoFeP₂O₇ appear to be very similar to that of M₂P₂O₇ (M = Fe and Co), which indicates the monoclinic phase with space group C2/m.     

Texture of CoSi2 films on Si(111), (110) and (001) substrates

Synchrotron radiation was used to study the texture of polycrystalline CoSi₂ films that were formed by a solid-state reaction between a 30 nm Co film and Si(111), (110) and (001) substrates. All films were strongly textured, and several texture components were identified. We discuss the simultaneous occurrence of axiotaxy (i.e. alignment of lattice planes across the interface) and several different types of epitaxy in each of the films. Comparison of the different texture components observed on the three substrate orientations suggests a strong preference for the alignment of CoSi₂{110} planes in the film with Si{110} planes in the substrate, and twinning around Si[111] directions.     

Effect of Co,Pd and Pt ultra-thin films on the Ni-silicide formation: investigating the sandwich configuration

The effect of Co, Pd and Pt ultrathin films on the kinetics of the formation of Ni-silicide by reactive diffusion is investigated. 50 nm Ni/1 nm X/ 50 nm Ni (X?=?Co, Pd, Pt) deposited on Si(100) substrates are studied using in-situ and ex-situ measurements by X-ray diffraction (XRD). The presence of Co, Pd or Pt thin films in between the Ni layers delays the formation of the metal rich phase compared to the pure Ni/Si system and thus these films act as diffusion barriers. A simultaneous silicide formation (δ-Ni₂Si and NiSi phases) different from the classic sequential formation is found during the consumption of the top Ni layer for which Ni has to diffuse through the barrier. A model for the simultaneous growth in the presence of a barrier is developed, and simulation of the kinetics measured by XRD is used to determine the permeability of the different barriers. Atom probe tomography (APT) of the Ni/Pd/Ni system shows that the Pd layer is located between the Ni top layer and δ-Ni₂Si during the silicide growth, in accordance with a silicide formation controlled by Ni diffusion through the Pd layer. The effect of the barrier on the silicide formation and properties is discussed.

     

Photocatalytic degradation of formaldehyde by nanostructured TiO2 composite films

Interest in the photocatalytic oxidation of formaldehyde from contaminated wastewater is growing rapidly. The photocatalytic activity of the nanocrystalline Fe³⁺/F^? co-doped TiO₂–SiO₂ composite film for the degradation of formaldehyde solution under visible light was discussed in this study. The films were characterised by field emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive spectroscopy, X-ray diffraction (XRD), BET surface area, UV–Vis absorption spectroscopy, and photoluminescence spectroscopy. The FE-SEM results revealed that the Fe³⁺/F^? co-doped TiO₂–SiO₂ film was composed of uniform round-like nanoparticles or aggregates with the size range of 5–10 nm. The XRD results indicated that only the anatase phase was observed in the film. Compared with a pure TiO₂ film and a singly modified TiO₂ film, the Fe³⁺/F^? co-doped TiO₂–SiO₂ composite film showed the best photocatalytic properties due to its strong visible light adsorption and diminished electrons-holes recombination.     

The effect of amorphous silicon capping on titanium during TiSi2 formation by RTA

Thin film reactions of the Ti/(1 0 0)Si structure and the amorphous-Si/Ti/(1 0 0)Si structure are performed by rapid thermal annealing (RTA) in argon at temperatures of 500–800° C. Auger depth profiling shows that the as-deposited titanium film of the Ti/(1 0 0)Si structure and the as-deposited amorphous silicon (a-Si) film of the a-Si/Ti/(1 0 0)Si structure exhibit a roughly exponential oxygen distribution decreasing from the surface when exposed to air. An electronic spectroscopy for chemical analysis (ESCA) shows that the oxygen in the a-Si film forms Si0₂ and the oxygen in the titanium film forms titanium oxide. For the Ti/(1 0 0)Si structure, the oxygen tends to be redistributed uniformly throughout the titanium film near the onset of silicide formation during RTA. As silicide formation progresses, the redistributed oxygen is snowplowed back toward the surface owing to oxygen solubility difference between Ti and TiSi₂. Consequently, the oxygen concentration in the unreacted titanium layer increases. This increased oxygen concentration retards the silicide growth even though there remains an unreacted titanium layer. The oxygen redistribution in the titanium film correlates well with the rapid increases in the sheet resistance near the onset of silicide formation. When a-Si is sputter-deposited sequentially on the titanium film without breaking the vacuum, the oxygen in a-Si is not redistributed during RTA. Thus there is no rapid increase in the sheet resistance, and the saturated sheet resistance is lower than that of Ti/(1 0 0)Si structure. The reason is that the conversion of deposited titanium film into TiSi₂ is made completely because the a-Si film on the titanium film prevents oxygen infiltration into the titanium film.     

Silicide formation by Ar+ ion bombardment of Pd/Si

Palladium films, 45 nm thick, evaporated on to Si(111) were irradiated to various doses with 78 keV Ar⁺ ions to promote silicide formation. Rutherford backscattering spectroscopy (RBS) shows that intermixing has occurred across the Pd/Si interface at room temperature. The mixing behaviour is increased with dose which coincides well with the theoretical model of cascade mixing. The absence of deep RBS tails for palladium and the small area of this for silicon spectra indicate that short-range mixing occurs. From the calculated damage profiles computed with TRIM code, the dominant diffusion species is found to be silicon atoms in the Pd/Si system. It is also found that the initial compound formed by Ar⁺ irradiation is Pd₂Si which increases with dose. At a dose of 1×10¹⁶ Ar⁺ cm^–2, a 48 nm thickness of Pd₂Si was formed by ion-beam mixing at room temperature.     

Structural analysis of an Si/CoSi2/Si heterostructure using ultrahigh resolution transmission electron microscopy

The double heteroepitaxy of the Si/CoSi₂/Si system was studied. The thickness of the CoSi₂ layer (7 nm) was chosen so as to obtain a completely strained CoSi₂ film. The structure of the material was analysed using transmission electron microscopy in the 〈111〉 and 〈011〉 directions of the specimens. The high resolution images seem to indicate that the interface model differs from that obtained for a CoSi₂-Si interface, i.e. without double epitaxy.     

Correlation of interlayer diffusion with the stoichiometric composition of RF sputtered Pt/Co/Pt sandwiched structures

Sandwiched structures comprising Pt/Co/Pt layers with varying cobalt deposition time was studied and its importance on the alloy composition, by correlating the effective interlayer diffusion with the atomic stoichiometry, has been presented. A structural phase transition from ordered L1₂ CoPt₃ –? L1₀ CoPt –? L1₂ Co₃Pt was observed with increasing Co deposition time after annealing at 700 °C. The cross-sectional SEM image of the as-deposited film clearly shows a cobalt layer sandwiched between platinum layers. Rutherford back scattering (RBS) analysis shows a multipeak signature for the as-deposited films. A detailed RBS investigation on the extent of interatomic diffusion reveals an equiatomic composition for 20 and 30 min deposition time after annealing. The equiatomic CoPt phase shows a magnetically hard behaviour with a maximum coercivity of 15000 Oe. The reported dependence of (BH)_max on coercivity concludes that by tailoring an additional parameter of deposition time, diverse combinations of structural and magnetic properties can be achieved for appropriate practical applications.     

Structural,morphological and optical properties of undoped and Co-doped ZnO thin films prepared by sol–gel process

Undoped and Co-doped ZnO thin films with different amounts of Co have been deposited onto glass substrates by sol–gel spin coating method. Zinc acetate dihydrate, cobalt acetate tetrahydrate, isopropanol and monoethanolamine (MEA) were used as a precursor, doping source, solvent and stabilizer, respectively. The molar ratio of MEA to metal ions was maintained at 1.0 and a concentration of metal ions is 0.6 mol L^?1. The Co dopant level was defined by the Co/(Co + Zn) ratio it varied from 0 to 7 % mol. The structure, morphology and optical properties of the thin films thus obtained were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDX), scanning electron microscopy (SEM), ultraviolet–visible (UV–Vis), photoluminescence (PL) and Raman. The XRD results showed that all films crystallized under hexagonal wurtzite structure and presented a preferential orientation along the c-axis with the maximum crystallite size was found is 23.5 nm for undoped film. The results of SEM indicate that the undoped ZnO thin film has smooth and uniform surface with small ZnO grains, and the doped ZnO films shows irregular fiber-like stripes and wrinkle network structure. The average transmittance of all films is about 72–97 % in the visible range and the band gap energy decreased from 3.28 to 3.02 eV with increase of Co concentration. DRX, EDX and optical transmission confirm the substitution of Co²⁺ for Zn²⁺ at the tetrahedral sites of ZnO. In addition to the vibrational modes from ZnO, the Raman spectra show prominent mode representative of Zn_yCo_3?yO₄ secondary phase at larger values of Co concentration. PL of the films showed a UV and defect related visible emissions like violet, blue and green, and indicated that cobalt doping resulted in red shifting of UV emission and the reduction in the UV and visible emissions intensity.     

SERS property of Co complex on boron nitride nanosheets/silver nanoparticles aggregates and photophysical characterisation of interactions among Co complex ion-exchanged within zirconium phosphate

ABSTRACT

The Co(II) complex of [Co(bpy)₂(NO₃)]Cl·3H₂O, bpy = 2,2'-bipyridine, has been synthesised by the solution crystallisation method. The boron nitride nanosheets/silver nanoparticles/[Co(bpy)₂(NO₃)]Cl·3H₂O nanoaggregate was prepared in order to better analyse the adsorption orientation of the cobalt complex on the surface of silver nanoparticles. The result of the surface-enhanced Raman scattering measurement indicates that the molecular plane of the cobalt complex presents a tilted orientation with respect to the surface of silver nanoparticles. The luminescence property of [Co(bpy)₂(NO₃)]⁺ two-dimensional arrangements within the layers of zirconium phosphate (ZrP) was also investigated. Steady-state luminescence spectra of the [Co(bpy)₂(NO₃)]⁺-exchanged ZrP materials show an increase in the luminescence intensity.     

Effects of aluminum doping on lanthanum oxide gate dielectric films

This work reports a novel method for improving the electrical properties of lanthanum gate oxide (La₂O₃) by using aluminum doping and rapid thermal annealing (RTA) techniques. In the bulk of the Al-doped La₂O₃ film together with 600 °C RTA, we found that the aluminum atoms were incorporated into the oxide network and the film was transformed into lanthanum aluminate complex oxide. At the interface, a thin Al₂O₃ layer was formed. This interfacial Al₂O₃ layer suppressed the out-diffusion of substrate Si, the formation of interfacial silicate layer and silicide bonds. These effects resulted in a significant reduction on the bulk and interface trap densities and hence the gate leakage current.     

Chemical structure and electrical properties of sputtered HfO2 films on Si substrates annealed by rapid thermal annealing

The chemical structure and electrical properties of HfO₂ thin film grown by rf reactive magnetron sputtering after rapid thermal annealing (RTA) were investigated. The chemical composition of HfO₂ films and interfacial chemical structure of HfO₂/Si in relation to the RTA process were examined by X-ray photoelectron spectroscopy. Hf 4f and O 1s core level spectra suggest that the as-deposited HfO₂ film is nonstoichiometric and the peaks shift towards lower binding energy after RTA. The Hf-Si bonds at the HfO₂/Si interface can be broken after RTA to form Hf-Si-O bonds. The electrical characteristics of HfO₂ films were determined by capacitance-voltage (C-V) and current density-voltage (J-V) measurements. The results showed that the density of fixed charge and leakage current density of HfO₂ film were decreased after the RTA process in N₂ atmosphere.