Amorphous Si Films Research Articles

Orientation Control of Large-Grained Si Films on Insulators by Thickness-Modulated Al-Induced Crystallization

The thickness-modulated Al-induced crystallization technique enables us to control the orientation of polycrystalline Si films on insulators. The amorphous Si film on a SiO2 substrate was crystallized at a low temperature (425 °C) through the layer exchange between Si and catalytic Al layers. Electron backscattering diffraction (EBSD) measurements revealed that the crystal orientation of the grown Si layer varied significantly depending on the Al/Si thickness: The [111]-orientation fraction reached 97% for the 50-nm-thick sample and the [100]-orientation fraction 88% for the 200-nm-thick sample. This mechanism was discussed in terms of the heterogeneous nucleation energy. The 50-nm-thick sample provided the maximum Si grain-size as large as 68-μm diameters. These large-grained Si films with controlled-orientations are promising epitaxial templates for advanced Si-based thin-film solar cells.

Layer Transfer and Simultaneous Crystallization of Amorphous Si Films with Mid-Air Structure Induced by Near-Infrared Semiconductor Diode Laser Irradiation

Layer transfer and simultaneous crystallization of amorphous Si (a-Si) films induced by near-infrared semiconductor-diode-laser (SDL) irradiation have been investigated. The a-Si films supported by columns on a starting substrate (quartz) and a counter substrate (glass and polyethylene terephthalate) were face-to-face contact, and an SDL irradiated the a-Si films. After SDL irradiation, the Si films of 6 × 6 mm2 area were completely transferred and crystallized simultaneously on the counter substrates. In-situ monitoring revealed that the layer transfer took place either in the solid phase or the liquid phase followed by phase transformation in the cooling period.

Low-temperature crystallization of amorphous silicon and amorphous germanium by soft X-ray irradiation

The low-temperature-crystallization effects of soft X-ray irradiation on the structural properties of amorphous Si and amorphous Ge films were investigated. From the differences in crystallization between Si and Ge, it was found that the effects of soft X-ray irradiation on the crystallization strongly depended on the energy band gap and energy level. The crystallization temperatures of the amorphous Si and amorphous Ge films decreased from 953K to 853K and 773K to 663K, respectively. The decrease in crystallization temperature was also related to atoms transitioning into a quasi-nucleic phase in the films. The ratio of electron excitation and migration effects to thermal effects was controlled using the storage-ring current (photon flux density). Therefore, we believe that low-temperature crystallization can be realized by controlling atomic migration through electron excitation.

Influence of rapid thermal annealing on the process of aluminum induced crystallization of amorphous Si

The effects of annealing methods on the crystallization process and microstructure of polycrystalline silicon (poly-Si) films obtained by aluminum-induced crystallization (AIC) of amorphous Si (a-Si) films were comparatively investigated. Glass/Al/a-Si structures were annealed by rapid thermal annealing (RTA) and conventional furnace at 500 °C for different times in Ar. As compared to furnace annealing, AIC of a-Si films annealed by RTA possesses a shorter period of nucleation time, a higher nucleation density and reduces the process time to form continuous poly-Si films. It is revealed that the continuous Si films obtained by both RTA and conventional furnace annealing are polycrystalline in nature, exhibiting good microstructures with Raman peaks at 518 cm−1 and full-width at half-maximums of 6.43–6.48 cm−1.

Solution-Based Metal Induced Crystallization of Amorphous Silicon Films

A viscous Nickel (Ni) solution was applied on amorphous Si films by spin coating and its effect on the crystallization of amorphous Si films was investigated with a two-step annealing process. The experimental results show that with the help of the two-step annealing, the crystallization of the film can take place at 500oC. At the same time, the crystalline fraction gets up to 79.4% after annealing at a high temperature of 520oC and the grain size of the polycrystalline Si films is approximately 200 nm.

Liquid-phase deposition of thin Si films by ballistic electro-reduction

It is shown that the nanocryatalline silicon ballistic electron emitter operates in a SiCl4 solution without using any counter electrodes and that thin amorphous Si films are efficiently deposited on the emitting surface with no contaminations and by-products. Despite the large electrochemical window of the SiCl4 solution, electrons injected with sufficiently high energies preferentially reduce Si4+ ions at the interface. Using an emitter with patterned line emission windows, a Si-wires array can be formed in parallel. This low-temperature liquid-phase deposition technique provides an alternative clean process for power-effective fabrication of advanced thin Si film structures and devices.

Encapsulation of high frequency organic Schottky diodes

Both operational lifetime and shelf-life of copper phthalocyanine (CuPc) based high frequency Schottky diodes have been studied in this work. Thin films of amorphous Si (a-Si) and polychloro-(para-xylylene) (parylene C) are deposited as a barrier layer using low temperature chemical vapor deposition to improve the lifetime of the CuPc Schottky diodes. The CuPc Schottky diodes encapsulated with the barrier layer exhibit at least 6× longer operational lifetime under both DC and AC voltage stress. Encapsulation does not significantly improve the shelf-life of the organic Schottky diodes.

Enhanced absorption in ultrathin Si by NiSi2 nanoparticles

Various forms of Si including amorphous Si (a-Si) are important photovoltaic (PV) materials. However, in order to improve the cost-to-performance aspects of Si solar cells, such as by enabling ultrathin (<500 nm) Si solar technology, new strategies are required to improve optical absorption within Si, which is a relatively poor absorber of light in the visible solar spectrum. In this study, the authors demonstrate a potential approach to enhance optical absorption in ultrathin a-Si films by embedding nickel di-silicide (NiSi2) nanoparticles (NPs). The Ni silicide NPs were engineered inside 50 nm thick a-Si by thermal annealing of co-deposited Ni and Si thin films on Si and quartz substrates. A quantitative electron energy-loss spectroscopy analysis was used to accurately determine the composition of silicide NPs. From broadband absorption optical studies, integrated optical absorption was found to increase by ~85% in the visible solar range (350–750 nm) and by ~150% in the infrared range (750–3000 nm) for the NiSi2 NP incorporated amorphous Si films. Optical modeling captured the absorption behavior of NP embedded a-Si thin films, thus suggesting an efficient route to design new photo-absorber NPs for future Si based PV devices.

Improving Crystallinity of Thin Si Film for Low-Energy-Loss Micro-/Nano-Electromechanical Systems Devices by Metal-Induced Lateral Crystallization Using Biomineralized Ni Nanoparticles

The characteristics of thin Si films were investigated in terms of crystallization for low-energy-loss micro-/nano-electromechanical systems (MEMS/NEMS) devices. Metal-induced lateral crystallization (MILC) using Ni nanoparticles accommodated within cage-shaped protein, apoferritin, was applied to an amorphous Si film to obtain a polycrystalline Si (poly-Si) film. The poly-Si film with MILC had crystallized domains of 50–60 µm, whereas the poly-Si film without MILC had grains smaller than 1 µm. Crystallized domains in the poly-Si film with MILC showed almost the same crystalline orientations, whereas those without MILC showed random crystalline orientations. Crystallization-induced tensile stress in the poly-Si film with MILC was increased to 461 MPa (without MILC: 363 MPa). The poly-Si film with MILC was applied to an electrostatically driven MEMS resonator. In the frequency responses, resonant frequency was shifted higher and the Q factor was increased by 20%.

The growth of CuSi composite nanorod arrays by oblique angle co-deposition, and their structural, electrical and optical properties

Different CuSi composite nanorods with 0–100 at.% Cu were fabricated by an oblique angle co-deposition technique. The effects of increasing Cu during deposition on the morphologies, structures and properties were investigated. During co-evaporation, the addition of Cu decreases the nanorod width and height but increases the nanorod tilting angle. The polarized optical transmission spectra reveal that all the nanorod samples show a remarkable anisotropic response to visible light with an eccentricity e ≈ 1, whereas their optical response to NIR light depends strongly on the Cu composition, and the related eccentricity increases monotonically with the increase of Cu. The obtained amorphous Si film has a resistivity of approximately 4.9 × 104 Ω cm. The incorporation of 5–75 at.% Cu increases the electrical conductance from two to eight orders of magnitude. The improved conductance and the unique optical properties of the Si-based nanocomposites could have potential applications for Li-ion battery anode and optical design.

Transmission electron microscopy study of Ni–Si nanocomposite films

Nickel induced crystallization of amorphous Si (a-Si) films is investigated using transmission electron microscopy. Metal-induced crystallization was achieved on layered films deposited onto thermally oxidized Si(3 1 1) substrates by electron beam evaporation of a-Si (400 nm) over Ni (50 nm). The multi-layer stack was subjected to post-deposition annealing at 200 and 600 °C for 1 h after the deposition. Microstructural studies reveal the formation of nanosized grains separated by dendritic channels of 5 nm width and 400 nm length. Electron diffraction on selected points within these nanostructured regions shows the presence of face centered cubic NiSi2 and diamond cubic structured Si. Z-contrast scanning transmission electron microscopy images reveal that the crystallization of Si occurs at the interface between the grains of NiSi2 and a-Si. X-ray absorption fine structure spectroscopy analysis has been carried out to understand the nature of Ni in the Ni–Si nanocomposite film. The results of the present study indicate that the metal induced crystallization is due to the diffusion of Ni into the a-Si matrix, which then reacts to form nickel silicide at temperatures of the order of 600 °C leading to crystallization of a-Si at the silicide–silicon interface.

Plasma enhanced chemical vapor deposited silicon coatings on Mg alloy for biomedical application

Amorphous Si film was prepared by plasma enhanced chemical vapor deposition (PECVD) of SiH4 on WE43 alloy for biomedical application. The microstructure and the composition of the film were investigated by glancing angle X-ray diffraction (GAXRD), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), respectively. The immersion test indicated that Si film could efficiently slow down the degradation rate of WE43 alloy in simulated body fluid (SBF) at 37±1°C. MTT assay suggested that the extract of the Si coated WE43 alloys was harmless to murine fibroblast cells (L 929). Hemolysis test and blood platelets adhesion test were conducted, and the results show that the hemolysis of WE43 alloy decreased after being coated by Si and the platelets attached on the Si film were at the inactivated stage with a round shape.

(Invited) Ballistic Electron Emission from Nanosilicon Diode and its Application to Ultra-Thin Film Deposition of Silicon and Germanium

A novel wet fabrication technique of quantum-scale Si and Ge thin films is presented based on the ballistic electron effect in solutions. A nanocrystalline silicon (nc-Si) diode, composed of a thin Au (10 nm), nc-Si layer (~1 µm), polycrystalline silicon, and n+-Si substrate, generates ballistic electrons via a multiple-tunneling cascade through a chain of nc-Si dots interconnected with tunnel oxides and ejects energetic electrons from the device surface. A remarkable function of is this nc-Si ballistic emitter is that it operates even in solutions as a supplier of highly reducing electrons. When driven in SiCl4 and GeCl4 solutions without the use of counter electrodes the reducing activity of injected electrons induces the deposition of thin amorphous Si and Ge films on the emission area without producing any by-products and contaminations. Following a brief description of the ballistic emission, the characteristic features of the observed electro-reduction deposition mode is discussed.

The composition, structure and optical properties of weakly magnetic Co-containing amorphous Si and Ge films

This study investigated the composition, structure and optical properties of amorphous SiCo and GeCo films. The samples were prepared by radio frequency sputtering. Films were deposited with Co atomic concentrations in the range of 1.7–10.3 at.%. After deposition, the films were submitted to thermal treatments up to 900 °C and investigated by energy dispersive X-ray spectrometry, X-ray diffraction, Raman scattering and optical transmission spectroscopy. Additionally, magnetic force microscopy measurements were performed at room temperature. For comparison purposes, Co-free samples were also prepared, annealed and characterized following a similar procedure. The experimental results indicated the following: (1) the Co atoms were effectively and homogeneously incorporated into the amorphous hosts; (2) the as-deposited films (either pure or containing Co) were essentially amorphous; (3) annealing the films at high temperatures induced crystallization; (4) after crystallization, non-magnetic CoSi2 (silicide) and CoGe2 (germanide) phases were identified in the Co-containing Si and Ge films, respectively; (5) the optical properties of the films were significantly affected by the insertion of Co and by the annealing temperature; and (6) the samples exhibited a reduced magnetic signal at room temperature. These experimental observations were systematically studied, which are presented and discussed in this report.

Amorphous Silicon-Coated Carbon Nanofibers Composite as Anode Material for Lithium-Ion Batteries

Silicon can be considered one of the most promising anode materials for lithium ion batteries (LIBs) as it has a low charge/discharge voltage at room temperature and exhibits the highest known theoretical lithiation capacity in combination with good safety characteristics. However, the silicon anode experiences enormous volume changes during repeated lithium insertion/extraction processes in every charge/discharge cycle. This leads to severe particle pulverization and results in a quick electrode structure failure. Moreover, the Si has to be amorphous, as crystallized Si can hardly alloy with lithium at room temperature. Amorphous Si thin films are currently manufactured by techniques such as chemical vapor deposition, that usually, are quite expensive, have complex equipment design and have slow rate of deposition. Electrodeposition of Si can offer a suitable alternative method of producing a thin film of amorphous Si by much easier and lower cost means. In this study electrodeposited amorphous silicon on carbon nanofibers (CNFs) have been investigated as the anode material for LIBs. Electrically conductive CNFs are produced by electrospinning and later heat treating of polyacrylonitrile (PAN) precursor. It is shown that the macromorphology of electrospinning-derived CNFs template provides enough space to accommodate the silicon volume expansions. By controlling electrodeposition parameters, cycle-ability of the anode material was optimized. It is envisaged that exceptional characteristics of CNFs and the electrodeposited Si will make the composite material an ideal candidate for the anode material of high-power LIBs.

Exponential depletion of neutral dangling bonds density (D0) by rare-earth doping in amorphous Si films

In this work we study the effect reduction in the density of dangling bond species D0 states in rare-earth (RE) doped a-Si films as a function concentration for different RE-specimens. The films a-Si1−xREx, RE=Y3+, Gd3+, Er3+, Lu3+) were prepared by co-sputtering and investigated by electron spin resonance (ESR) and Raman scattering experiments. According to our data the RE-doping reduces the ESR signal intensity of the D0 states with an exponential dependence on the rare-concentration. Furthermore, the reduction produced by the magnetic rare-earths Gd3+ and Er3+ is remarkably greater than that caused by Y3+ and Lu3+, which led us to suggest an exchange-like coupling between the spin of the magnetic REs3+ and the spin of silicon neutral dangling bonds.

Laser ablation and growth of Si and Ge

In this work, we investigated the laser ablation and deposition of Si and Ge at room temperature in vacuum by employing nanosecond lasers of 248nm, 355nm, 532nm and 1064nm. Time-integrated optical emission spectra were obtained for neutrals and ionized Ge and Si species in the plasma at laser fluences from 0.5 to 11J/cm2. The deposited films were characterized by using Raman spectroscopy, scanning electron microscopy and atomic force microscopy. Amorphous Si and Ge films, micron-sized crystalline droplets and nano-sized particles were deposited. The results suggested that ionized species in the plasma promote the process of subsurface implantation for both Si and Ge films while large droplets were produced from the superheated and melted layer of the target. The dependence of the properties of the materials on laser wavelength and fluence were discussed.

Impurity-free seeded crystallization of amorphous silicon by nanoindentation

We demonstrate that nanoindents formed in amorphous Si films, with dimensions as small as ∼20 nm, provide a means to seed solid phase crystallization. During post-indentation annealing at ∼600 °C, solid phase crystallization initiates from the indented sites, effectively removing the incubation time for random nucleation in the absence of seeds. The seeded crystallization is studied by optical microscopy, cross-sectional transmission electron microscopy, and electrical characterization via Hall measurements. Full crystallization can be achieved, with improved electrical characteristics attributed to the improved microstructure, using a lower thermal budget. The process is metal contaminant free and allows for selective area crystallization.

Characterization and PL Properties of Ge-Induced Crystallization of a-Si Films Deposited by Magnetron Sputtering

In this paper, we present the characterization of Ge-induced crystallization of amorphous Si (a-Si) films deposited by magnetron sputtering. The film structures of a-Si films were characterized by Raman spectroscopy, Atomic Force microscope (AFM), and field emission scanning electron microscope (FESEM). The result show that 60% of a-si film with a layer of 400 nm Ge buried is crystallized at growth temperature of 800 °C. The surface roughness and average surface grain size obtained by AFM is 2.39 nm and 60 nm for the crystallized film, respectively. The films growth at temperature of 500°C and 650 °C shows a PL spectrum band from 1.6 eV to 1.8 eV, and the PL peak shifts to lower energy as the growth temperature increased. As for the film grown at 800 °C, the PL spectrum is nearly extinguished. The crystallization of a-Si film induced by buried Ge might be a useful technology to develop high quality poly-Si film without annealing.

Graphene/Si multilayer structure anodes for advanced half and full lithium-ion cells

Graphene/Si multilayer structures were constructed through a repeated process of filtering liquid-phase exfoliated graphene film and the subsequent coating of amorphous Si film via plasma-enhanced chemical vapor deposition (PECVD) method. The multilayer heterogeneous structure films, directly fabricated on copper current collectors, were used as anodes for rechargeable lithium half-cells and full-cells without adding any polymer binder or conductive additives. The half cells based on the new anodes could easily achieve a capacity almost four times higher than the theoretical value of graphite even after 30 cycles' charging/discharging. It also demonstrated improved capacity retention compared to those of pure Si film-based anodes. Furthermore, full cells composed of the graphene/Si multilayer structure anodes and commercially available LiNi1/3Mn1/3Co1/3O2 cathodes were also assembled. Initial results showed good electrochemical performance comparable to that of commercially available rechargeable LIBs. Our prepared multilayer structures, taking advantage of the long cycle life of carbon and the high lithium-storage capacity of Si, provided a promising research platform that may eventually lead to an optimized anode structure for advanced rechargeable LIBs.