Amorphous Si Matrix Research Articles

Preparation of barium ferrite dot arrays by means of nano-spot crystallization

A novel process to prepare hexagonal ferrite films with dot arrays has been proposed. Two immiscible elements, Al and Si, were first deposited onto a Si (1 1 1) substrate at various temperatures. The microstructure of this kind of film depends on the concentration and deposition temperature. Scanning electron microscopy (SEM) results show that structures with Al dot arrays, in the size range of 30–50 nm, embedded into an amorphous Si matrix can be obtained by precise control of the sputtering conditions. Amorphous Ba–Fe–O films were deposited onto the dot array films at room temperature and then annealed to crystallize in part. Experimental results indicate that amorphous Ba–Fe–O films on Al dots can be crystallized by flash annealing the samples at 700 °C for 60 s. However, those on the amorphous Si keep the amorphous state. A structure with magnetic dot arrays embedded into a nonmagnetic matrix has been achieved by this process. The magnetic properties of the samples have been discussed.

Microstructure and electrical properties of Ti–Si–C–Ag nanocomposite thin films

Ti–Si–C–Ag nanocomposite coatings consisting of nanocrystalline TiC in an amorphous Si matrix with segregated Ag were deposited by dual magnetron sputtering from Ti 3SiC 2 and Ag targets. As evidenced by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, for Ag contents below 10 at.%, the Ag forms ∼ 10 nm large crystallites that are homogeneously distributed in the films. For higher Ag contents, coalescence during growth results in the formation of > ∼ 100 nm Ag islands on the film surface. The electrical resistivity of the coatings was measured in a four-point-probe setup, and ranged from 340 μΩcm (for Ti–Si–C coatings without Ag) to 40 μΩcm (for high Ag content).

Expansion and melting of Xe nanocrystals in Si

Xe agglomerates confined in a Si matrix by ion implantation were synthesized with different size depending on the implantation process and/or the thermal treatment. At low temperature Xe nanocrystals are formed, whose expansion and melting were studied in the range 15--$300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Previous high resolution x-ray diffraction spectra were corroborated with complementary techniques such as two-dimensional imaging plate patterns and transmission electron microscopy. We detected fcc Xe nanocrystals whose properties were size dependent. The experiments showed that in annealed samples epitaxial condensation of small Xe clusters, on the cavities of the Si matrix, gave in fact expanded and oriented Xe, suggesting a possible preferential growth of $\mathrm{Xe}\phantom{\rule{0.3em}{0ex}}(311)$ planes oriented orthogonally to the $\mathrm{Si}[02\text{\ensuremath{-}}2]$ direction. On the contrary, small Xe clusters in an amorphous Si matrix have a fcc lattice contracted as a consequence of surface tension. Furthermore, a solid-to-liquid phase transition size dependent was found. Expansion of fcc Xe lattice was accurately determined as a function of the temperature. Overpressurized nanocrystals and/or binary size distributions were disproved.

Formation and characterization of Cu–Si nanocomposite electrodes for rechargeable Li batteries

We have investigated the structural and electrochemical properties of Cu–Si nanocomposite electrode fabricated by co-sputtering method. Reversible capacity of an amorphous Si electrode is degraded continuously with increasing cycle number up to 40 cycles. However, a Cu–Si nanocomposite electrode, where Cu nano-dots are embedded in an amorphous Si matrix, shows an excellent reversible capacity with a stable value of ca. 400 μA h cm −2 μm −1 up to 40 cycles. The improved reversible capacity of the Cu–Si nanocomposite electrodes is attributed to the enhanced structural stability of the electrodes due to the presence of the Cu nano-dots evenly distributed throughout the Si matrix.

Residual stress in amorphous and nanocrystalline Si films prepared by PECVD with hydrogen dilution

Amorphous and nanocrystalline Si films were prepared using radio-frequency (13.56 MHz) plasma enhanced chemical vapour deposition with different SiH 4/H 2 ratios. The film residual stress was measured using curvature method as a function of SiH 4 gas flow ratio (SiH 4/(SiH 4 + H 2)). Results from high resolution scanning electron microscopy, X-ray diffraction and Raman scattering studies revealed the appearance of nanocrystallites with SiH 4 gas ratio less than 3%. With a gradual decrease of SiH 4 gas ratio, the film intrinsic stress increased significantly until SiH 4 gas ratio reached 2%. Below that critical value, the film intrinsic stress decreased sharply. Different mechanisms of stress formation and relaxation during film growth were discussed, including ion bombardment effect, hydrogen and hydrogen induced bond-reconstruction, nanocomposite effects (nanocrystal embedded in an amorphous Si matrix). Results indicated that nanocomposite effect is the dominant factor in this maximum stress condition.

Phase-separated Al–Si thin films

Phase-separated Al–Si films composed of Al nanocylinders embedded in an amorphous-Si matrix have been prepared by a sputtering method. By controlling the deposition rate, substrate temperature, and film composition, the average diameter of the Al cylinders can be varied systematically from less than 5to13nm with a cylinder density ranging from 1015 to in excess of 1016cylindersm−2. A three-dimensional simulation of phase separation in binary thin films was performed using a modified Cahn-Hilliard [J. Chem. Phys. 28, 258 (1958)] equation to understand the growth mechanism. The simulation studies indicate that the surface diffusion length and film composition are important factors which determine film morphology. Experimental and simulation studies are compared and discussed.

Li insertion/extraction characteristics of a vacuum-deposited Si–Sn two-component film

Vacuum-deposited Si–Sn two-component films were prepared by depositing Si and Sn simultaneously on a metal foil substrate. The composition of the films was estimated from the ratio of consumption of the two evaporation sources. SEM and XRD revealed the film structure consisting of two different phases, where Sn nanoparticles were dispersed homogeneously in the amorphous Si matrix. The film with higher Sn content gave unsatisfactory results, which is in contrast to the much improved results obtained with a film containing less Sn. The conductivity of the Si–Sn film was found to be higher than that of a pure Si film, which was attributed to the homogenously deposited Sn nanoparticles. This enabled us to realize a discharge capacity of 400 mAh g −1 even after 500 charge/discharge cycles at a very high rate of 20 C.

Effect of Ar + ion implantation on the nano-mechanical properties and microstructure of single crystal silicon

The effect of argon ion implantation on the nano-machanical properties of single crystal Si was examined making use of nano-indentation and nano-scratch tests. The morphologies of the scratched tracks of the unimplanted Si and that implanted at a moderate Ar + fluence were observed on a scanning electron microscope, while the changes in the microstructure of the single crystal Si by Ar + implantation were investigated on a transmittance electron microscope. It was found that the implantation of Si at a small or moderate fluence of Ar + below 1 × 10 15 ions/cm 2 had little effect on the surface roughness and a minor effect on the surface nano-hardness. At the same time, the implantation of Si with Ar + at a moderate fluence up to 1 × 10 16 ions/cm 2 led to a significant increase in the critical load. This was attributed to the desired changes in the microstructures of the single crystal Si by Ar + implantation at a proper fluence. Namely, the Si surface implanted with Ar + at a moderate fluence was composed of nano-sized polycrystalline Si uniformly distributed in amorphous Si matrix, which contributed to significantly increase the nano-scratch resistance and surface toughness of the single crystal silicon. It was suggested to implant the single crystal Si at an Ar + fluence of 1 × 10 16 ions/cm 2 so as to acquire the optimized modification effect.

Dielectric confinement and surface plasmon damping in Au: semiconductor nanocomposite thin films

Au metal clusters are embedded in amorphous Si matrix using alternating sputtering method. Although the nominal thickness of Au metal layer was adjusted as small as 1 nm, the resultant microstructure showed the Bruggeman geometry consisting of a networked Au clusters. Using the thin film data of the constituent materials Au and Si, the optical absorption in the effective medium was calculated, and the effects of composite geometry, particle size, and shape distributions were analyzed. Inhomogeneous broadening of absorption band is thought to be most significant in this BG geometric sample, but the remaining discrepancy between the calculated and the measured absorption may be ascribed to the presence of energy dissipation process due to a finite Schottky barrier built up at the metal–semiconductor junction.

Template-Assisted Growth of Nanowires Using Novel Nanoporous Films Fabricated by Inorganic Nano Phase Separation

AbstractUltrahigh pore density nanoporous films with a pore diameter of less than 10nm and a pore density exceeding 1016 pores/m2 were developed. Nano phase separation of a eutectic Al-Si system was used for the fabrication of these nanoporous films. Co-sputtered AlSi films form Al nano-cylinders, perpendicular to the substrate and parallel to each other, embedded in an amorphous Si matrix during film growth due to phase separation. Removal of the Al nano-cylinders from the co-sputtered AlSi films by chemical etching gives us ultrahigh pore density nanoporous films. The nanoporous films consist of mainly oxidized silicon. Depending on the film compositions and the film preparation conditions, such as RF power and the deposition temperature, the average pore diameter can be varied systematically from less than 5nm to 13nm with the pore density from 1015 to exceeding 1016 pores/m2. Furthermore we have demonstrated a template-assisted growth of ultrahigh-density Ni nanowire arrays with an aspect ratio of ∼100 in the nanoporous films by electrodeposition. The fabrication method for nanowire arrays using the nanoporous films is quite simple and promising for the fabrication of nanostructured devices.

Microstructure of N + ion beam induced epitaxial crystallized Si

Ion beam induced epitaxial crystallization of 300–350 nm thick amorphous Si by 1 MeV N + ions in the temperature range of 200–400 °C was investigated. The dynamic thickness recovery as a function of temperature was observed using Rutherford backscattering spectrometry-channeling. The regrowth rate showed Arrhenius behavior with an activation energy of 0.29 ± 0.03 eV. Using micro-Raman Spectroscopy the crystalline Si peak at 521 cm −1 was found to decrease, broaden and shift to the lower wave numbers as the temperature decreased from 400 to 200 °C. The bond angle deviation below 3.5° suggests the phase change from amorphous to crystalline structure during ion beam induced recovery process. The asymmetric broadening of the Raman peaks at 250 and 300 °C along with TEM micrograph indicates the coexistence of nano-crystalline structures embedded in the amorphous Si matrix.

Evidence for a compressed fluid phase of Xe clusters

We report on the formation and detection of a compressed fluid phase of Xe clusters in as- implanted Si, at room temperature. The simultaneous structural characterization of the Xe clusters and of the Si matrix was performed by X-ray diffraction at grazing incidence coupled with two-dimensional detection; in both cases, the nearest-neighbor distance and the coordination were obtained. In order to investigate the early stage of the atomic inclusion and the cluster segregation, the average compression and size of Xe fluid clusters within the amorphous Si matrix were explained within the simple Hard Sphere model.

Structure and properties of amorphous silicon-metal alloys: I. The Si1-xNix system

Structural and optical measurements have been made on a series of amorphous thinfilms of Si1-xNix with 0<x<0.75, prepared by RF sputtering.We present the results of structural studies using extended x-ray absorptionfine structure (EXAFS) and small angle x-ray scattering (SAXS) together withoptical measurements of the band gap. The EXAFS analysis gave interatomic distances, partial coordinationnumbers and Debye-Waller factors for the nearest neighbours of the two atomtypes. The Ni environment was found to be independent of composition forx<0.5. A small variation in the mean Si-Si distance was also found in thiscomposition range. The SAXS results show phase segregation, on the 30 Å scale, for this composition range. We interpret the structural data in terms ofa two-phase model, amorphous Si plus amorphous NiSi, for x<0.5 and asingle-phase model for x>0.5. The optical data show that a metal-insulator transition occurs at about10-15% Ni content. We suggest that this transition should be interpreted interms of a percolation theory involving regions of conducting amorphous NiSi inan amorphous Si matrix.

Effects of surface resonance state on the plasmon resonance absorption of Ag nanoparticles embedded in partially oxidized amorphous Si matrix

Nanocomposite films consisting of nanosized Ag particles embedded in partially oxidized amorphous Si matrices were prepared by radio frequency cosputtering deposition. Subsequent heat treatment at different temperatures ranging from 200 to 500 °C and its effects on the optical absorption of the films were investigated by spectrometry in the wavelength ranging from 200 to 1200 nm. “Surface resonance state” is introduced to discuss the broadening, blue-shift, and intensity decrease of the plasmon resonance absorption peak with decreasing Ag particle size.

Nanoscale Lead Inclusions in Silicon

TEM analyses of Si implanted with immiscible elements have shown that the implanted species characteristically segregate to form nanoscale inclusions in the amorphous Si matrix. Subsequen annealing induced recrystallization was found to lead to simultaneous removal of nearly all of the implanted species, explained in terms of stress-induced trapping of the implanted atoms at the amc phous/crystalline interface as it progresses towards the surface.The solubility of Pb in Si is negligible, and it has previously been shown-that elemental nanoscale inclusions can be formed in Si either by ion implantation at elevated temperature or by rapid solidi cation. In the present work single crystals of &lt;110&gt; Si have been implanted with 80 keV Pb+ k at 300K, 625K and 925K respectively. The implanted fluence was 8-1019 m−2 corresponding to an average concentration of about 3 at.%. Combined TEM and RBS/channeling analysis showed that implantations at 300 K and 625 K induced amorphization of the implanted layer with dense distributions of nanoscale lead inclusions embedded in the amorphous silicon matrix (fig. 1).

Microstructure and optical properties of Si–Ag nanocomposite films prepared by co-sputtering

Novel composite thin films consisting of nanosized Ag particles embedded in an amorphous Si matrix were made with Ag contents from 0 at% to 61 at% by radio frequency co-sputtering of Si and Ag. The microstructure and optical properties of the films were characterized by conventional and high resolution transmission electron microscopy and spectrometry in the wavelength range from 200 to 1500 nm. It was found that the films consist of nanosized Ag particles (2.8–6.0 nm) particle and their clusters embedded in an amorphous Si matrix. The optical absorption spectra of the films up to 40 at% Ag exhibit characteristics similar to the amorphous semiconductor Si. At higher Ag contents two absorption maxima at 350 nm and 700 nm appear. Effective medium theories were examined to predict the optical properties of the films and it was found that the predictions from the Sheng Ping theory with a modified dielectric function of bulk Ag (taking into account the mean free path limitation of Ag particle boundaries) qualitatively agree with the measured absorption spectra. The two absorption maxima are accounted for as interfacial plasma resonance absorption associated with the silver particlersilicon matrix interfaces.

Size control of Si nanocrystallites formed in amorphous Si matrix by Er-doping

Si nanocrystallites formed in amorphous Si (a-Si) show intense blue luminescence at room temperature. The samples are fabricated by crystallization of a-Si thin films, which leads to a large size dispersion of the nanocrystallites due to a random distribution of strain fields in the a-Si phase. In this work, we demonstrate a process of controlling the crystallite size by doping Er in the a-Si layers. The doped Er atom introduces additional strain in the a-Si matrix and behaves as a nucleation center during the crystallization. It is demonstrated that the average size of the Si nanocrystallites is almost independent of the crystallization time for a certain Er concentration and a certain annealing temperature. By controlling the Er density and crystallization temperature, we have fabricated a series of nanocrystalline (nc) Si samples having the average sizes from 3 to 10 nm. The smallest size achieved here is 2.7 nm. A bright blue luminescence from the Si nanocrystallites as well as a 1.54 μm emission from the doped Er 3+ ions were observed up to room temperature.

Structural Characterization and Crystallization Process of Nanostructured Silicon Thin Films Produced in Low-Pressure Silane Plasma

ABSTRACTNanostructured silicon thin films (ns-Si:H), consisting of a two-phase mixture of amorphous and ordered material, were obtained by plasma-enhanced chemical vapor deposition (PECVD) under a wide range of plasma conditions. The key to embedding Si-ordered particles in the amorphous Si matrix was the formation of silicon clusters in the gas phase (diameter &lt; 2nm) under conditions of plasma polymerization, and their incorporation into the growing films. The crystallization induced by thermal annealing in these nanostructured films can be attained faster than in conventional a-Si:H thin films, because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. In this work, we present a detailed structural characterization by using electron and X-ray diffraction patterns and Raman spectroscopy. The crystallization dynamics were studied in-situ by Raman spectroscopy.

The influence of substrate temperature, substrate material and heat treatment on the micro structure of Ag/Si nanocomposite films prepared by r.f. co-sputtering

Nanocomposite films consisting of silver and silicon were manufactured by r.f. co-sputtering deposition. The influence of substrate temperatures varying from 20 to 200 °C and different substrate materials such as cleaved NaCl crystals, C-coated Cu grids and (100) Si single-crystal wafers was investigated. Subsequent heat treatment at different temperatures ranging from 500 to 800 °C and its effect on the microstructure of the deposited films was also investigated by using a combination of differential scanning calorimetry, X-ray diffractometry, scanning electron microscopy, conventional transmission electron microscopy and high-resolution electron microscopy. The typical microstructure of the as-deposited film consisted of nanosized Ag particles embedded in an amorphous Si matrix. The crystallization temperature of the amorphous Si matrix was reduced from 550 to 490 °C with increasing Ag content in the films. The microstructure of the annealed Ag/Si film after crystallization consists of Si crystallites with sizes from several nanometres to hundreds of nanometres and Ag particles with average sizes from 12 nm (600 °C) to 28 nm (800 °C). Surface segregation of silver was also observed for the annealing temperatures of 600 °C and above. We suggest a possible explanation for the silver-induced lowering of the amorphous Si matrix crystallization temperature.

Positron/positronium annihilation in nanocrystalline silicon thin films

Positron and positronium annihilation investigations were applied to nanocrystalline silicon (nc-Si) thin films, for the first time. The nc-Si thin films with average grain diameters of 3–5 nm show intense blue luminescence at room temperature. The nanometer-sized Si crystallites formed in amorphous Si (a-Si) matrix give rise to this luminescence. Very highS-parameters up to 0.62 were observed in the as-grown a-Si thin film suggesting positronium formation in the a-Si layer. The average lifetime of the positrons in the a-Si was determined to be about 520 ps. TheS-parameters dropped significantly to 0.53 by crystallization of the thin film at 800 °C for 10 seconds, which was almost the same to the value observed in bulk Si (100) substrate. Further crystallization from 60 seconds to 1 hour showed smaller change in theS-parameters than that from the a-Si to 10 seconds. The large change in theS-parameters due to the annealing might be caused by the formation of Si nanocrystallites in a-Si matrix suggesting that positron is a sensitive probe for structural investigations of the nc-Si materials.