Amorphous Si Thin Films Research Articles

Phase Growth in Amorphous Si-Cu and Si-Co Systems: Combination of SNMS, XPS, XRD, and APT Techniques

Abstract. It is shown, by the combination of SNMS, (Secondary Neutral Mass Spectrometry), XRD, XPS and APT (Atom Probe Technique) that the growth of the Cu3Si crystalline layer at 408 K between the amorphous Si and nanocrystalline Cu thin films follows a linear law and the shifts of the Cu3Si/Cu and Cu3Si/a-Si interfaces approximately equally contributed to the growth of this phase. It is also illustrated that the Si atoms diffuse fast into the grain boundaries of the nanocrystalline Cu, leading to Si segregation. Both the SNMS and APT results indicate that even during the deposition of Cu on the amorphous Si an intermixed region is formed at the interface. This region easily transforms into a homogeneous Cu3Si crystalline reaction layer subsequently which further grows following apparently an interface controlled linear kinetics. Similar experiments performed in Co/a-Si system to study the formation and growth kinetics of the intermetallic phase. However, interestingly, homogenous formation of the new phase at the Co/a-Si interface was not always observed.

Mechanism Study of Crack Formation at Si Thin Film Anodes with Different Crystalline during Lithiation/Delithation

Thin film electrodes have the advantage for fundamental study owing to the ability to not use binder and conductive additive in testing them as anodes. Particularly, Si thin film electrodes have attracted the most attention due to their superior cyclic performance and high initial coulombic efficiency. It is well known that the problem is crack formation which leads to poor cycle ability during charge/discharge at Si thin films.In order to study the behavior of crack formation at Si anodes with different crystalline, amorphous Si (a-Si) thin films have been treated by laser annealing technique. When the heat exceeds the melting point of the a-Si, the Si melt and it generally form polycrystalline structures (p-Si) during re-solidification.Crystalline of a-Si is dramatically transformed to p-Si of cylindrical column type whose diameter ranges from tens of nanometers to hundred nanometers in p-Si thin film by the laser annealing as shown in Fig. 1.We prepared two adjacent Si patterns, whose size is 30 μm × 30 μm, with different crystalline. At initial stage, we found that the behavior of crack formation was very different in term of shape, size, and density of cracks.In this work, the cracking feature of Si films with different crystalline after first and 30th cycles has been investigated.Fig.1. Optical and cross-sectional TEM images of a-Si, and p-Si thin film.

An ultrabroad band omni-directional anti-reflective coating with quasi-gradient refractive index distribution based on Si–SiOxNy–SiO2 materials system

An ultrabroad band omni-directional antireflective coating was fabricated using both reactive magnetron sputtering and glancing angled electron beam evaporation methods. Gradient index amorphous Si, SiNx, and SiOxNy thin films were deposited by tuning the flow rate of the reactive gases, while the gradient index distribution of the nanoporous SiO2 stacks was obtained by rotating the angle of the substrate. A low average reflectivity of less than 2% at normal incidence in the wavelength range 280 to 3300 nm was achieved, and the average reflectivity over the angle range 15 to 89° was 3.7% for the wavelength between 300 and 1700 nm.

Thin film solar cells on honeycomb-structured substrates for photovoltaic building blocks

Honeycomb-structured solar cell is proposed for photovoltaic building block applications. Honeycomb-like substrates were prepared either by a conventional semiconductor processing or by a low cost wet-chemical method, and amorphous Si thin film solar cells were fabricated on these substrates. We have demonstrated one of the essential requirements for building block application, which is the low sensitivity of the light incidence angles on the power conversion efficiency; and we have identified the critical processing issues through the experimental study using various thin film deposition methods. This honeycomb-structured solar cell is a promising candidate for the future photovoltaic building block applications enabling the inherent high strength-to-weight ratio and higher efficiency at an oblique light incidence.

Measurement and modeling of the mechanical and electrochemical response of amorphous Si thin film electrodes during cyclic lithiation

A combination of experimental measurements and numerical simulations are used to characterize the mechanical and electrochemical response of thin film amorphous Si electrodes during cyclic lithiation. Parameters extracted from the experiment include the variation of elastic modulus and the flow stress as functions of Li concentration; the strain rate sensitivity; the diffusion coefficient for Li transport in the electrode; the free energy of mixing as a function of Li concentration in the electrode; the exchange current density for the Lithium insertion reaction; as well as reaction rates and diffusion coefficients characterizing the rate of formation of solid-electrolyte interphase layer at the electrode surface. Model predictions are compared with experimental measurements; and the implications for practical Si based electrodes are discussed.

Amorphous Si Thin Film Based Photocathodes with High Photovoltage for Efficient Hydrogen Production

An amorphous Si thin film with TiO2 encapsulation layer is demonstrated as a highly promising and stable photocathode for solar hydrogen production. With platinum as prototypical cocatalyst, a photocurrent onset potential of 0.93 V vs RHE and saturation photocurrent of 11.6 mA/cm(2) are measured. Importantly, the a-Si photocathodes exhibit impressive photocurrent of ~6.1 mA/cm(2) at a large positive bias of 0.8 V vs RHE, which is the highest for all reported photocathodes at such positive potential. Ni-Mo alloy is demonstrated as an alternative low-cost catalyst with onset potential and saturation current similar to those obtained with platinum. This low-cost photocathode with high photovoltage and current is a highly promising photocathode for solar hydrogen production.

Phase growth in an amorphous Si–Cu system, as shown by a combination of SNMS, XPS, XRD and APT techniques

It is shown, by the combination of secondary neutral mass spectrometry (SNMS), X-ray diffraction and atom probe tomography (APT), that the growth of a Cu3Si crystalline layer between amorphous Si and nanocrystalline Cu thin films at 408K follows a linear law and the shifts of the Cu3Si/Cu and Cu3Si/amorphous Si interfaces contribute approximately equally to the growth of this phase. It is also illustrated that the Si atoms diffuse rapidly into the grain boundaries of the nanocrystalline Cu, leading to Si segregation on the outer surface and to an increase in the overall Si content inside the Cu layer. Both the SNMS and APT results indicate that, even during the deposition of Cu on the amorphous Si, an intermixed region (of about 10nm thick) is formed at the interface. This readily transforms into a homogeneous Cu3Si crystalline reaction layer which grows further, apparently following an interface-controlled linear kinetics.

Nanoindentation-Induced Phase Transformation in Silicon Thin Films

Nanoindentation-induced phase transformation of amorphous, annealed amorphous and microcrystalline hydrogen-free silicon thin films were studied. Series of nanoindentation experiments were performed with a sharp Berkovich indenter at various unloading rates. The structural changes in indentation deformed regions were examined using Raman spectroscopy. Analyses of indentation curves and Raman spectra suggest that high pressure phases appear more easily in annealed amorphous Si thin films than in microcrystalline ones.

Analytical modeling and simulation of electrochemical charge/discharge behavior of Si thin film negative electrodes in Li-ion cells

Physically-based analytical models that provide insights into the diffusion and/or interface charge transfer effects in bulk (lithiating/delithiating) electrodes are needed to truly assess the performance/limitations of electrode materials for Li-ion batteries. In this context, an analytical modeling framework is constructed here to predict the electrochemical charge–discharge characteristics during lithiation and delithiation of solid amorphous Si (a-Si) thin film electrodes. The framework includes analytical expressions that satisfy Fick's second law for Li transport and the requisite flux boundary conditions of lithiation and delithiation steps. The expressions are derived here by the method of separation of variables. They enable the determination of transient Li concentration profiles in the thin film electrode as a function of state of charge/discharge. The time-dependent electrode surface concentrations (at the electrode–electrolyte interface) obtained from these profiles were used to determine the activation overpotentials and thus, the non-equilibrium cell potentials, as a function of state of charge/discharge using Butler–Volmer kinetics. The simulated charge/discharge characteristics agreed well with the experimental data of a-Si thin film electrodes obtained at different C-rates. The model offers insights into how the charge-discharge behavior is controlled by diffusion limitation within electrode and/or the activation overpotentials at the interface. The analytical framework is also shown to predict successfully the hysteretic behavior of lithiation/delithiation voltage curves.

Effects of hydrogen plasma treatment on SnO2:F substrates for amorphous Si thin film solar cells

We investigated the effects of hydrogen plasma treatment on the physical and electrical properties of fluorine-doped tin oxide (FTO) films used for amorphous silicon (a-Si) thin film solar cells. A slight increase in carrier concentration by the hydrogen doping effect was observed for the FTO film exposed to the hydrogen plasma for 5 min. For further exposure to the plasma, the chemical reduction became prominent and resulted in deterioration of the electrical and optical properties of the film. XPS analysis revealed that the chemical reduction of SnO2 to Sn metallic state occurs on the surface region. It was found that the defects formed by hydrogen plasma act as recombination centers at the interface between FTO electrode and p-layer of a-Si solar cells. This phenomenon resulted in the deterioration of the cell performance. The averaged conversion efficiency (6.82%) of the cells on pristine FTO hydrogen substrate was decreased to 5.81% for the cells on FTO treated for 5 min, which is mainly attributed to the decrease in short-circuit current density.

Pressure-sensitive plasticity of lithiated silicon in Li-ion batteries

Lithiation-induced plasticity is a key factor that enables Si electrodes to maintain long cycle life in Li-ion batteries. We study the plasticity of various lithiated silicon phases based on first-principles calculations and identify the linear dependence of the equivalent yield stress on the hydrostatic pressure. Such dependence may cause the compression-tension asymmetry in an amorphous Si thin film electrode from a lithiation to delithiation cycle, and leads to subsequent ratcheting of the electrode after cyclic lithiation. We propose a yield criterion of amorphous lithiated silicon that includes the effects of the hydrostatic stress and the lithiation reaction. We further examine the microscopic mechanism of deformation in lithiated silicon under mechanical load, which is attributed to the flow-defects mediated local bond switching and cavitation. Hydrostatic compression confines the flow defects thus effectively strengthens the amorphous structure, and vice versa.

Spectroscopic Detection of Medium Range Order in Device Grade Hydrogenated Amorphous Silicon

This article addresses low defect densities in hydrogenated amorphous silicon, a-Si1-x:Hx with approximately 10 at. % bonded H, x∼0.1. Based low defect densities at mid-gap, ∼0.5 to 1×1016 cm-3, a-Si:H thin films have been integrated into photovoltaic (PV) devices and thin film transistors (TFT's). Amorphous Si (a-Si) thin films with no detectable bonded-H have been used as precursors for polycrystalline gate electrodes in microelectronic applications. PV and TFT alloys have been deposited by glow discharge (GD), remote plasma-enhanced chemical vapor deposition (RPECVD), and reactive magnetron sputtering (RMS) with different bonded-H content determined by deposition precursors and substrate temperatures. Two conditions are required for the lowest Si dangling bond densities: (i) a monohydride, Si–H, concentration of ∼10 at. % H, and (ii) deposition, and/or a post-deposition annealing at 240 to 300 °C.

Evaluation of the microangiographic fluoroscope (MAF) using generalized system performance metrics

The performance of a newly developed, high resolution, microangiographic fluoroscope (MAF) (35 μm pixel pitch and 300 μm thick CsI phosphor) was evaluated using a generalized linear system analysis and compared with that of a standard amorphous Si thin film transistor flat panel detector (FPD) (194 μm pixel pitch and 600 μm thick CsI phosphor). The linear system metrics such as modulation transfer function (MTF), noise power spectrum, and detection quantum efficiency (DQE) are commonly used to gauge the intrinsic detector performance in the detector plane. However, these linear system metrics do not provide information about the image receptor performance in a real system since they do not include the effects of other parameters such as focal spot distribution, scatter radiation, and geometric unsharpness, which may compromise detector performance characteristics. Use of generalized linear system metrics [generalized modulation transfer function (GMTF), generalized normalized noise power spectrum (GNNPS), and generalized detection quantum efficiency (GDQE)] that include these effects gives a more meaningful, complete, and appropriate evaluation of detector performance as part of the imaging system. A uniform head equivalent phantom was used to simulate realistic clinical parameters and x-ray spectra. The detector MTFs were measured using the slanted edge method and the focal spot MTFs were measured using a pinhole assembly. The scatter MTF was simulated and the scatter fraction was measured for a head-equivalent phantom. The generalized system metrics were calculated for different combinations of three choices of focal spots and three different magnifications with two different air-gaps. The performance of the MAF was also illustrated using stent images obtained with different focal spots under similar conditions. Results for the generalized metrics provide a quantitative description of the performance of the imaging system for both detectors. This generalized analysis demonstrated that both detectors have similar imaging capabilities at lower spatial frequencies, but that the MAF has superior performance over the FPD at higher frequencies even when considering focal spot blurring and scatter. This generalized performance analysis demonstrates the significance of focal spot size, magnification, and scatter on the system performance metrics (GMTF, GNNPS, and GDQE). Although the ideal detector performance characteristics of the MAF are not fully realized due to these other system factors, it still retains an advantage in DQE at high spatial frequencies over the FPD. Similar studies based on the generalized linear system metrics can serve as an efficient tool to evaluate total system capabilities under different realistic conditions to enable optimal design for specific imaging tasks.

Rapid melt crystallization of amorphous-silicon thin films

We investigated the potential to melt crystallize amorphous Si thin films on glass with electromagnetic irradiation at 13.56 MHz, 2.45 GHz, and 110 GHz, respectively. We showed that the melt crystallization of millimeter-wave annealing could be an ideal choice for achieving high-quality semiconductor thin films with thickness in the micron range. In particular, silicon pillars with large ultra-high crystalline quality grains were experimentally realized by millimeter-wave annealing amorphous silicon on glass. Our simulation results were consistent with the experimental ones. Such understandings may enable us to prepare high-quality thin films on inexpensive substrates for solar cells and other solid-state devices.

Role of surface plasmon polaritons and photonic modes in light absorption by thin-film solar cells patterned with metallic nanogratings

Light absorption by thin-film amorphous Si solar cells patterned with metallic nanogratings is investigated theoretically. Propagative bounded modes that include the surface plasmon polariton (SPP) and the photonic mode (PM) are extracted from the total field to quantitatively evaluate their contribution to the light absorption. Our results show that after removing the propagative bounded modes from the total field, the residual field still contributes to a major part of the total light absorption. This proves, at a quantitative level, that the light absorption of the structure is largely attributed to the residual field that is composed of unbounded or evanescent modes arising from the grating scattering, and the SPP and the PM do not play a dominant role, which is out of the previous intuitive expectations.

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.

Low temperature Si-Si, SiO2-SiO2 covalent bonding structures with thin siloxane layer

We present new Si to Si, SiO2 to SiO2 bonding technologies for low temperature applications (<200°C). Direct bonding process between Si (or SiO2) substrates makes high bonding strength without contamination problems. However, high temperature over 1000°C is needed for the reliable Si to Si and SiO2 to SiO2 direct bonding processes. To reduce the bonding temperature, thin siloxane layer and low-powered oxygen plasma treatment was used in this study. We used dimethyl siloxane layer having siloxane chains (-Si-O-)n and methyl ends. Siloxane layer is able to be bonded strongly with Si-based substrates at low temperature (<200°C) when oxygen plasma is treated on it. Polymerized siloxane layer such as PDMS has much higher coefficient of thermal expansion (CTE) of 300ppm/K than Si of 2.6ppm/K. When the bonded structure is cooled or heated, the interfaces is possibly distorted and cracked by the high residual stress between siloxane layer and Si substrate. To solve these problems, we developed new fabrications of reducing the siloxane layer thickness to 3∼4nm, that is the monomer layer levels. Extremely thin thickness of siloxane layer prevented the problems of the CTE differences. The Si to Si bonding structure with siloxane layer showed strong adhesion properties in this study. The bonded body kept reliable bonding force when it was heated to high temperature (∼900°C). The feasible wafer-level bonding process was demonstrated. We investigated the siloxane layer thickness by TEM images. The bonding strength was confirmed by dicing test by 1mm and measured over 20MPa. We also expended this new development to SiO2 to SiO2 bonding structures. Low temperature bonding between non-Si substrates such as GaN was possible with thin siloxane layer when amorphous Si thin film was deposited on these substrates.

Formation of silicon nanoparticles by a pressure induced nucleation mechanism

Formation of silicon nanoparticles (SiNPs) was achieved using excimer laser crystallization of an amorphous Si (a-Si) thin film using a SiO2 capping layer (C/L) with improved thin-film transistor (TFT) performance due to the enlarged grain size of polycrystalline Si (poly-Si). After laser irradiation of an a-Si thin film covered with C/L, fluctuation in the surface morphology of the C/L was observed above the critical laser energy density (Ecr) with the formation of SiNPs. The grain size of the poly-Si layer after crystallization increased abruptly at the same time. A non-uniform pressure distribution beneath the SiO2 C/L was proposed for the initiation of nucleation, which is named pressure induced nucleation (PIN) mechanism. Following nucleation, the release of latent heat made it difficult for the remnant liquid Si to solidify and the volume increased due to the density difference between the liquid and solid Si. Consequently, the pressure on the liquid Si caused SiNPs to sprout through the SiO2 C/L as grains grew from the low temperature to high temperature point. This study offers not only a simple method to fabricate SiNPs with controllable size/density but also larger grain size with lower laser energy density, which leads to higher TFT performance.

Spectroscopic Detection of Medium Range Order in Hydrogenated Amorphous Silicon, a-Si(H): Applications in Photovolatics, Thin Film Transistors and Si-based Microelectronics

Abstract Hydrogenated amorphous silicon, a-Si1-xHx, with ~ at.10% or x ~0.1±0.02, is used in photovoltaics (PV), and thin film transistors (TFT's). Amorphous Si (a-Si) thin films, thought to be free of H, are used as precursors for polycrystalline gate electrodes in microelectronics. PV and TFT alloys has been deposited by the glow discharge method (GD), remote plasma-enhanced chemical vapor deposition (RPECVD, and reactive magnetron sputtering (RMS) with bonded-H determined by deposition precursors and substrate temperatures. Two conditions are required for low Si dangling bond densities ~0.5 to 1x1016 cm−3: (i) a bonded monohydride, Si-H, concentration of ~10 at. % H, and (ii) a deposition, and/or a post-deposition anneal at ~240 °C to 300 °C. These reduce strain by introducing medium range order (MRO) as nano-meter scale aperiodically-assembled clusters. Si L2,3 X-ray absorption spectroscopy (XAS) has been used to confirm MRO extending beyond short-range order (SRO) of continuous random networks (CRN) through observation of ligand-field split d-state features. These are associated with symmetry-adapted linear combinations (SALCs) of atomic states forming molecular orbital valence bands. XAS and photoelectron spectroscopy (PES) studies have confirmed that a-Si used for microelectronic applications also has MRO as well. This is associated with a H-transfer reaction from an Si-H bond of the cluster into a Si-H-Si bonding arrangement on near-neighbor sites. MRO has also identified by other spectroscopic techniques: spectroscopic ellipsomentry (SE), and ultra-violet or X-ray photoemission spectroscopy (UPS or XPS). The same H-atom motion induced by absorption of sun-light is responsible for the Staebler-Wronski Effect (SWE); the generation of Si-atom dangling bonds that degrade PV performance.