CoSi2 Thin Films Research Articles

Optimization of the dielectric-function modeling by B-splines in spectroscopic ellipsometry analysis: A hybrid approach

Currently, a Kramers–Kronig consistent B(asis)-spline representation of the dielectric function is an efficient and widely used method for accurate modeling of the material optical functions in ellipsometric data analysis. However, the B-spline approach to the dielectric function modeling should include an appropriate and user-independent way of a knot vector generation, i.e., the proper selection of the number and locations of knots. In this paper, we advocate for a systematic approach which combines a specific knot allocation method, based on so-called “Integral Span”, a slope-weighting factor, with a selection of optimal number of knots using the Akaike and Bayesian Information Criteria, two statistical estimators, thereby replacing an intuitive and time-consuming “trial-and-error” strategy. The proposed hybrid approach is used to optimize the B-spline models for an epitaxial cobalt disilicide (CoSi2) thin film and a crystalline silicon substrate (c-Si).

Correlated TKD/EDS - TEM - APT analysis on selected interfaces of CoSi2 thin films

Correlative analysis is a powerful way to relate crystallographic and chemical information to the properties of materials. In this work, a procedure is proposed to select and analyze interfaces of polycrystalline thin film materials through correlative transmission Kikuchi diffraction/energy dispersive X-ray spectroscopy (TKD/EDS), transmission electron microscopy (TEM) and atom probe tomography (APT). TKD provides information on the crystallographic orientation. The EDS analysis performed together with TKD in the scanning electron microscope (SEM) makes chemical information available allowing phases of similar crystal structure, but with a different composition to be distinguished. The information of TKD/EDS can be correlated to successive TEM and APT analysis on selected interfaces for structural and chemical analysis at the atomic scale. An interface of an epitaxial orientated grain of a polycrystalline CoSi2 thin film on (111)Si is selected and analyzed. The selected interface has a twin character and shows facets of different orientation and area. Site-specific segregation of Ge to junctions of the facets is evidenced. The correlation between local strain from misfit (defects) at the interface and segregation is discussed.

Effect of Mn doping on the chemical synthesis of interconnected nanoflakes-like CoS thin films for high performance supercapacitor applications

Herein, supercapacitor developed using Mn-doped CoS thin films (1–5% Mn) were prepared using the successive ionic layer adsorption and reaction (SILAR) method. The effect of the Mn-doped CoS thin films on the structural, morphological, and supercapacitor properties were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and electrochemical evaluation. Doping up to 3% Mn lead to improvements in peak intensity. Also, the morphological results indicated that doping of Mn affected the CoS nanostructures. The 3% Mn-doped CoS electrodes had an interconnected nanoflakes-like nanostructure, with a high porosity compared to the other electrodes. XPS data strongly supported the XRD results. The Mn-doped CoS electrodes showed a higher capacitance (621 F g−1) than the other electrodes, and electrochemical impedance spectroscopy indicated that the 3% Mn-doped CoS electrode was highly conductive. The characteristics of the 3% Mn-doped CoS electrode proved its applicability in supercapacitors.

In-situ multi-deposition process for cobalt-sulfide synthesis with efficient bifunctional catalytic activity

ABSTRACTUniform CoS thin films were deposited by an in-situ multi-deposition method using cobalt nitrate as cationic source and sodium sulfide as anionic source. Influence of deposition cycle numbers was studied systematically by XRD and SEM. The thin film become smoothly and compactly with increase of deposition cycles to 4, and it agglomerated seriously when up to 8 cycles. The resultant sample exhibit outstanding bifunctional electrochemical catalytic properties for oxygen evolution and oxygen reduction (OER and ORR, respectively) for 4 deposition cycles in alkaline medium. The CoS exhibit low overpotential of 0.43 V to achieve 10 mA/cm2 for its OER activity. The CoS thin film for 4 dip-cycles has exhibited good performance with the half-wave potential of 0.7 V vs. RHE.

Atomic Layer Deposition of Cobalt Silicide Thin Films Studied by in Situ Infrared Spectroscopy

Atomic layer deposition of cobalt silicide (CoSi2) thin films on H-terminated Si(111) surfaces, using the cobalt-based precursor tertiarybutylallylcobalttricarbonyl (tBu-AllylCo(CO)3) and trisilane, is investigated by in situ Fourier transform infrared spectroscopy (FTIR) and ex situ X-ray photoelectron spectroscopy (XPS) to uncover the film growth mechanisms. The strong reactivity of tBu-AllylCo(CO)3 with H-terminated silicon surfaces and inertness with silicon oxide surfaces, as previously determined by IR spectroscopy [Chem. Mater. 2012, 24, 1025], opens the door for selective deposition. Deposition of CoSi2 is observed after a brief nucleation period (∼3 cycles), during which the stabilization of the cobalt precursor takes place, as evidenced by a shift of the stretch frequency of the carbonyl groups bonded to the Co center from 2010 to 1980 cm–1. This shift is evidence for completion of the catalytic reaction and leads to a surface termination and configuration that is favorable for subsequent ligand...

Effects of NH3 flow rate on the epitaxial growth of CoSi2 thin film using a CoN x interlayer deposited by MOCVD

A uniform epitaxial CoSi2 layer could be grown on a Si(100) substrate by deposition of a CoNx interlayer followed by an in situ annealing process. The amorphous Si–N layer which is formed at the CoNx/Si interface plays a role in inhibiting excessive Co incorporation into Si layer and consequently enhancing the epitaxial growth of CoSi2 layer. However, the Si–N layer also limits the growth of thick CoSi2 layer. Therefore, the thickness of the amorphous Si–N layer is an important factor for the epitaxial growth of the CoSi2 layer with uniform structure and low sheet resistance.

Morphology reliance of cobalt sulfide thin films: A chemo-thermo-mechanical perception

We report onto the morphology dependency of CoS thin films by studying the role of mechanical agitation, thermal assistance and deposition duration in an aqueous alkaline bath (pH=9±0.1). The deposition of CoS thin films was carried out at different mechanical stirring rates, deposition temperatures and times. As-optimized CoS thin film were of polycrystalline nature and exhibited hexagonal crystal structure. Co2+ rich nature (≈85%) of optimistically grown thin film was detected. Complex multifaceted webbed network of as-grown elongated and threaded into each other CoS crystals was observed through a scanning electron microscope. Surface morphology was further studied by means of an atomic force microscopy. Existence of magnetic domains was marked in the magnetic force microscopy. As-grown CoS thin films were having transmission index of 0.5 with a band gap of ≈1.59eV.

Cobalt sulfide thin films: Chemical growth, reaction kinetics and microstructural analysis

CoS thin films were successfully deposited from an aqueous alkaline bath containing ammonia and TEA as the complexing agents. Under the pre-optimized conditions (temperature=80±0.5°C, speed of the substrate rotation=65±2rpm and deposition period=90min), ammonia and TEA quantities in the reaction bath were found to play a decisive role in the final product yield. Highly uniform, dark sea-green colored and tightly adherent deposits were obtained at our experimental conditions. As-obtained CoS thin films were polycrystalline in nature with hexagonal class of crystal system as derived from the X-ray diffraction analysis. Complex multifaceted webbed network of as-grown CoS crystals elongated and threaded into each other were observed through a scanning electron microscope. Atomic force micrographs revealed collapsing of the hillocks and filling of the valleys triggering decrease in the RMS roughness for increased TEA and NH3 quantities. Magnetic force microscopy (MFM) was employed to study surface topography in terms of magnetic mapping. MFM images highlighted the existence of the magnetic clusters imitating topography. Broad absorption edge with high absorption coefficient (α≈104cm−1) was observed for as-grown CoS thin films. Determined values of the optical bandgaps revealed influence of complexing environment on the final product.

Nitric Acid Oxidized ZrO$_2$ as the Tunneling Oxide of Cobalt Silicide Nanocrystal Memory Devices

In this study, ZrO2 formed by the nitric acid oxidation method is proposed to be the tunneling oxide for nonvolatile memory device applications. The sputtered Zr thin film was oxidized by immersing in the nitric acid solution (HNO3:H2O = 1:10) for 60 s at room temperature. The quality of the formed ZrO2 was also extracted by the capacitance-voltage and current density-voltage measurements. Then, X-ray photoelectron spectroscopy has been used to confirm that the deposited Zr can be oxidized completely after the oxidation process. Moreover, a CoSi2 thin film was deposited on the nitric acid oxidized ZrO2 as the self-assembled layer of the memory device. After the device fabrication, the electrical and material characteristics of the CoSi2 nanocrystal memory devices have also been demonstrated and discussed.

Cobalt sulphide thin films: Chemical bath deposition, growth and properties

In the present report synthesis of CoS thin films was carried out by a modified liquid phase chemical growth process. Dark green coloured CoS thin films with hexagonal wurtzite polycrystalline structure and average grain size of ≈ 15 nm were deposited. Surface morphology reveals a randomly oriented network of elongated thread like grains. The absorption coefficient of the CoS thin film is high (α ≈ 10 4–10 5 cm − 1 ) and a direct band gap of 1.13 eV has been observed. n-type conduction is found in the deposited films which can be attributed to the lack of stoichiometry.

Study of diffusion and quality control for CoSi2 formation by oxide-mediated cobalt silicidation with Ti capping

The mechanism for the enhancement of CoSi2 thin film formation and film quality control by oxide-mediated cobalt silicidation with a Ti-capping layer is discussed. It is found that Ti absorbs oxygen from the SiOx layer, which induces weak points in the SiOx layer and then enhances Co diffusion as well as CoSi2 formation. The control of the reactions between Ti and SiOx is significant because a low reaction rate cannot form a suitable thickness of CoSi2 film, whereas a high reaction rate tends to form the highly resistive CoSi phase. In addition, to maintain the SiOx layer still existing after annealing is also important because if no SiOx layer remains, unreacted Co would react with Si directly to form the highly resistive phase of CoSi, leading to the smooth interface between CoSi2 and Si and dense bulk CoSi2 thin film being destroyed.

Effect of zirconium addition on formation of CoSi2 thin films

CoSi2 thin films were prepared by annealing the amorphous Co-Si thin films. Zr was added into the amorphous thin film in order to investigate its effect on formation of CoSi2 thin film. It was found that Zr increased the crystallization nucleation barrier and accelerated the complete transformation of CoSi to CoSi2.

Effect of sputtering input powers on CoSi2 thin films prepared by magnetron sputtering

CoSi2 thin films were prepared by radio frequency magnetron sputtering using CoSi2 alloy target. Effect of sputtering input powers on characteristics of CoSi2 thin films was researched by X-ray diffraction (XRD), transparent electron microscope (TEM), energy dispersive X-ray analysis and four points probe, etc. It was shown that the deposition rate increased lineally, the selective sputtering of silicon was strengthened, the (1 1 1) texture increased, and the resistivity decreased when the input powers were increased.

Axiotaxy of CoSi2 thin films on Si(100) substrates and the effects of Ti alloying

The texture of CoSi2 films on Si(001) substrates has been examined using synchrotron based high-resolution pole figure analysis. We find that axiotaxy, a recently recognized type of texture which was observed in NiSi thin films on Si(001), is also occurring in CoSi2 layers. Typical of axiotaxy texturing, the CoSi2 pole figures show symmetrical patterns of lines. Calculations show that these features on the pole figures are created by three off-normal fiberlike texture components. These off-normal fiber orientations are a consequence of the alignment of CoSi2〈110〉 planes with Si〈110〉 planes because of very similar d spacings, which causes a periodic arrangement along Si〈100〉 directions in the plane of the interface. This one-dimensional periodicity may provide a lower interfacial energy; however, the periodic arrangement along the Si〈100〉 directions may also be indicative of growth anisotropy. The influence of Ti alloying has been investigated by using in situ x-ray diffraction measurements, as well as pole figure analysis. These results show that the nucleation of both the CoSi and CoSi2 phases is delayed to higher temperatures for the 5 at. % Ti alloyed films. The texture of the CoSi2 phase is also influenced by the addition of Ti.

Direct CoSi[sub 2] thin-film formation with homogeneous nanograin-size distribution by oxide-mediated silicidation

By annealing at 460°C for 120s followed by 600°C120s, nanocrystalline CoSi2 thin film with an average grain size of 5nm can be directly formed from a Co∕SiOx∕Si multilayer with the SiOx as a mediated layer. It is found that annealing at 460°C for enough time is crucial for generating enough diffusion channels within the SiOx layer. After these channels are created, subsequent annealing at 600°C keeps these channels open and is responsible for rapid grain growth. In other words, by using two-step annealing, nucleation and growth processes can be effectively controlled and, hence, the resulting microstructure. The homogeneous nanograin-size distribution is important for ultralarge-scale integration technology below 90nm to prevent resistance degradation induced by CoSi2 agglomeration.

Growth of epitaxial CoSi2 on 6H-SiC(0001)Si

Epitaxial growth of (111)-oriented CoSi2 has been achieved on a scratch-free 6H-SiC(0001)Si substrate. The surface was prepared using atmospheric hydrogen etching and ultrahigh vacuum Si cleaning. A high-quality CoSi2 thin film was obtained by a modified template method and co-deposition of Co and Si at 550 °C. The structure and morphology of the film is studied by means of reflection high electron energy diffraction, x-ray absorption fine structure, x-ray diffraction, and atomic force microscopy.

Structure, interface roughness, and growth mechanism of reactive deposition epitaxy of CoSi2 on Si(100) substrates

Thin CoSi2 films have been grown on Si(100) substrates using the relative deposition epitaxy method. The structure of the silicide films have been analyzed using x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry and channeling, and the interface roughness of the CoSi2/Si(100) is analyzed using specular x-ray reflectivity and cross-sectional TEM. The structure and interface roughness of CoSi2/Si(100) is found to be dependent on the substrate temperature. Highly epitaxial CoSi2 with minimum interface roughness is obtained when the film is grown at substrate temperatures around 900 K. The observed interface roughness is a parabolic function of temperature. The achievement of the best silicide at a substrate temperature around 900 K is explained on the basis of the instantaneous diffusion of Co through growing CoSi2.

Nanometer patterning of thin CoSi 2-films by application of local stress

We have developed a self-assembly method for patterning thin CoSi 2 layers on Si(100) during their formation in a solid state reaction. This technique is based on anisotropic diffusion in a local stress field during rapid thermal processing. The stress is induced by a layer structure consisting of 30 nm SiO 2 and 300 nm Si 3N 4 which is patterned with conventional optical lithography. We have investigated two different silicide formation processes. Firstly, we deposited Co on Si in a UHV MBE chamber. Rapid thermal annealing leads to the formation of polycrystalline CoSi 2. Secondly, we used a titanium oxide mediated epitaxy process. For both processes we observed nanostructures with dimensions of about 100 nm showing wave-like separation edges in the first case and good uniformity in the second case.

The influence of Ti and TiN on the thermal stability of CoSi2.

ABSTRACTIn this work, AFM and sheet resistance measurements are used to characterize the thermal stability of thin CoSi2 films. We were particularly interested in the influence of different multilayer structures on the topography and surface roughness. Four different multilayer structures (Co/Si, TiN/Co/Si, Ti/Co/Si and Co/Ti/Si) were investigated. Thermal degradation of CoSi2 formed from a standard Co/Si structure is found to have an activation energy of about 4.2 eV, independent of layer thickness (in agreement with previous results by Alberti et al.). A TiN capping layer is shown to improve the thermal stability. However, if the TiN layer is too thick (e.g. 50 nm), a new failure mode is observed: although the TiN prevents grain boundary grooving of the silicide, the thermal stress induced by the TiN causes the CoSi2 layer to crack. For a Ti capping layer, a strong increase of the thermal stability of the CoSi2 layer is observed, even if the Ti capping layer is removed by a selective etching step after the first RTP annealing. The presence of a very thin Ti-O-N containing layer on the CoSi2surface seems to strongly decrease surface diffusion and in this way reduce the tendency for grain boundary grooving.

Fractality aspects during agglomeration of solid-phase-epitaxy Co–silicide thin films

Morphology changes of thin CoSi2 films close to the agglomeration threshold have been investigated by in situ scanning tunneling microscopy. Films prior to agglomeration reveal a strong morphology anisotropy where depending on direction the roughness correlation g(r) changes from logarithmic [g(r)∼ln r] to power law [g(r)∼r2c with c≈0.78] at short lateral length scales (r⩽30 nm). Such an anisotropy reflects the complex nature of diffusion process associated with the original anisotropic substrate surface and the formation of pinhole networks. Annealing above the agglomeration threshold (>600 °C), the pinhole network becomes connected breaking the CoSi2 film into aggregates with a two-dimensional fractal dimension D≈1.66 which is close to predictions of diffusion limited aggregation within the mean field theory prediction D=(d2+1)/(d+1) for d=2, and invasion percolation models.