Showing papers on "Amorphous silicon published in 2004"

Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors

Abstract: Transparent electronic devices formed on flexible substrates are expected to meet emerging technological demands where silicon-based electronics cannot provide a solution. Examples of active flexible applications include paper displays and wearable computers1. So far, mainly flexible devices based on hydrogenated amorphous silicon (a-Si:H)2,3,4,5 and organic semiconductors2,6,7,8,9,10 have been investigated. However, the performance of these devices has been insufficient for use as transistors in practical computers and current-driven organic light-emitting diode displays. Fabricating high-performance devices is challenging, owing to a trade-off between processing temperature and device performance. Here, we propose to solve this problem by using a novel semiconducting material—namely, a transparent amorphous oxide semiconductor from the In-Ga-Zn-O system (a-IGZO)—for the active channel in transparent thin-film transistors (TTFTs). The a-IGZO is deposited on polyethylene terephthalate at room temperature and exhibits Hall effect mobilities exceeding 10 cm2 V-1 s-1, which is an order of magnitude larger than for hydrogenated amorphous silicon. TTFTs fabricated on polyethylene terephthalate sheets exhibit saturation mobilities of 6–9 cm2 V-1 s-1, and device characteristics are stable during repetitive bending of the TTFT sheet.

Structural changes in silicon anodes during lithium insertion/extraction

Abstract: The structural changes in silicon electrochemically lithiated and delithiated at room temperature were studied by X-ray powder diffraction. Crystalline silicon becomes amorphous during lithium insertion, confirming previous studies. Highly lithiated amorphous silicon suddenly crystallizes at 50 mV to form a new lithium-silicon phase, identified as This phase is the fully lithiated phase for silicon at room temperature, not as is widely believed. Delithiation of the phase results in the formation of amorphous silicon. Cycling silicon anodes above 50 mV avoids the formation of crystallized phases completely and results in better cycling performance. © 2004 The Electrochemical Society. All rights reserved.

In Situ XRD and Electrochemical Study of the Reaction of Lithium with Amorphous Silicon

Abstract: Silicon is a very promising candidate to replace graphite as the anode in Li-ion batteries because of its very high theoretical capacity. It has not yet made its way into commercial cells because of severe problems with the charge and discharge cycling of the material. It seems that amorphous silicon and amorphous silicon-containing alloys exhibit much improved cycling performance. Therefore, it is desirable to fully understand the reaction of Li with a-Si. To this end, an in situ X-ray diffraction study of the reaction of lithium with a-Si has been performed. The results confirm that a new crystalline Li 15 Si 4 phase is formed below 30 mV as Li/Li + as first reported by Obrovac and Christensen in an article published in Electrochemical and Solid-State Letters. However, the crystalline phase only forms for films of a-Si above a critical thickness of about 2 μm.

Thin‐film silicon solar cell technology

Abstract: This paper describes the use, within p–i–n- and n–i–p-type solar cells, of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) thin films (layers), both deposited at low temperatures (200°C) by plasma-assisted chemical vapour deposition (PECVD), from a mixture of silane and hydrogen. Optical and electrical properties of the i-layers are described. These properties are linked to the microstructure and hence to the i-layer deposition rate, that in turn, affects throughput in production. The importance of contact and reflection layers in achieving low electrical and optical losses is explained, particularly for the superstrate case. Especially the required properties for the transparent conductive oxide (TCO) need to be well balanced in order to provide, at the same time, for high electrical conductivity (preferably by high electron mobility), low optical absorption and surface texture (for low optical losses and pronounced light trapping). Single-junction amorphous and microcrystalline p–i–n-type solar cells, as fabricated so far, are compared in their key parameters (Jsc, FF, Voc) with the [theoretical] limiting values. Tandem and multijunction cells are introduced; the μc-Si: H/a-Si: H or [micromorph] tandem solar cell concept is explained in detail, and recent results obtained here are listed and commented. Factors governing the mass-production of thin-film silicon modules are determined both by inherent technical reasons, described in detail, and by economic considerations. The cumulative effect of these factors results in distinct efficiency reductions from values of record laboratory cells to statistical averages of production modules. Finally, applications of thin-film silicon PV modules, especially in building-integrated PV (BIPV) are shown. In this context, the energy yields of thin-film silicon modules emerge as a valuable gauge for module performance, and compare very favourably with those of other PV technologies. Copyright © 2004 John Wiley & Sons, Ltd.

Modeling thin-film PV devices

Abstract: Numerical modeling is increasingly used to obtain insight in to the details of the physical operation of thin-film solar cells. Over the years several modeling tools specific to thin-film PV devices have been developed. A number of these tools have reached a mature status and are available to the PV community. Some of the most commonly used programs are presented and the possibilities as well as the shortcomings are discussed. Also, for the different thin-film PV devices (CdTe, CIGS, and, to a lesser extent, amorphous silicon and nano-structured solar cells) an overview is given of modeling efforts and achievements. Copyright © 2004 John Wiley & Sons, Ltd.

Microcrystalline silicon.. Growth and device application

Abstract: Processes used to grow hydrogenated microcrystalline silicon (μc-Si:H) from a H 2 /SiH 4 -glow discharge plasma are explained in comparison to those for hydrogenated amorphous silicon (a-Si:H). Differences and similarities between μc-Si:H and a-Si:H-growth reactions in the plasma and on the film-growing surface are discussed, and the nucleus-formation process followed by epitaxial-like crystal growth process is illustrated as unique processes for the formulation of μc-Si:H. Determination of the effect of dangling-bond defect density on the propagation of the resulting μc-Si:H films is also discussed in parallel with the effect on a-Si:H in order to obtain a clue to improve opto-electronic properties of those materials for device applications especially for thin-film-silicon solar cells. Material issues to produce low cost and high efficiency solar cells are described, and finally recent progress in those issues is demonstrated.

Sub–damage–threshold femtosecond laser ablation from crystalline Si: surface nanostructures and phase transformation

Abstract: Surface structures and structural transformations are investigated upon femtosecond laser ablation (800 nm, 120 fs) from crystalline silicon (100) targets placed under ultra-high vacuum. After repetitive illumination with several thousand laser pulses at intensities below the single shot damage threshold, at normal incidence, the crater morphology indicates the development of periodic structures at the crater bottom, with the orientation depending on the laser beam polarization. Periods of 200 nm and 600–700 nm, respectively, are shorter than the laser wavelength and appear as a result of surface instability. The ablation dynamics monitored by time-of-flight mass spectrometry shows the emission of positive silicon ions and clusters with kinetic energies of about 7 eV. Raman spectroscopy reveals phase transformations in the irradiated spot from Si-I to the polymorphs Si-III, Si-IV, Si-XII, and amorphous silicon as well as a stable, uncommon phase of hexagonal Si-wurzite.

Structure and hydrogen content of polymorphous silicon thin films studied by spectroscopic ellipsometry and nuclear measurements

Abstract: The dielectric functions of amorphous and polymorphous silicon films prepared under various plasma conditions have been deduced from UV-visible spectroscopic ellipsometry measurements. The measured spectra have been firstsimulated by the use of the Tauc-Lorentz dispersion model and then the compositions of the films have been obtained by the use of the tetrahedron model combined with the Bruggeman effective medium approximation. This approach allows us to determine the hydrogen content, the crystalline fraction, and the void fraction of the films. This is particularly important in the case of polymorphous films in which the low crystalline fraction (below 10%) can only be detected when an accurate description of the effects of hydrogen on the dielectric function through the tetrahedron model is considered. The hydrogen content and film porosity deduced from the analysis of the spectroscopic ellipsometry measurements are in excellent agreement with the hydrogen content and film density deduced from combined elastic recoil detection analysis and Rutherford backscattering spectroscopy measurements. Moreover, despite their high hydrogen content (∼15%-20%) with respect to hydrogenated amorphous silicon films deposited at the same temperature (8%), polymorphous silicon films have a high density, which is related to their very low void fraction.

Thin-Film Silicon-Growth Process and Solar Cell Application-

Abstract: Growth processes of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) from SiH4 and H2/SiH4-glow discharge plasmas are reviewed. Differences and similarities between µc-Si:H and a-Si:H growth reactions in the plasma and on the film-growing surface are discussed, and the nucleus-formation process followed by the epitaxial-like crystal growth process is explained as being processes unique to µc-Si:H. The governing reaction of dangling-bond-defect density in the resulting a-Si:H and µc-Si:H films is also discussed in order to obtain a clue to improve the optoelectronic properties of these materials to enable device applications, particularly to thin-film silicon-based solar cells. Material issues concerning the realization of low-cost high-efficiency solar cells are described, and finally, recent progress in those issues is presented.

Surface-passivated high-resistivity silicon substrates for RFICs

Abstract: Surface passivation of high-resistivity silicon (HRS) by amorphous silicon thin-film deposition is demonstrated as a novel technique for establishing HRS as a microwave substrate. Metal-oxide-silicon (MOS) capacitor measurements are used to characterize the silicon surface properties. An increase of the quality factor (Q) of a 10-nH spiral inductor by 40% to Q=15 and a 6.5-dB lower attenuation of a coplanar waveguide (CPW) at 17 GHz indicate the beneficial effect of the surface passivation for radio frequency (RF) and microwave applications. Regarding CPW attenuation, a nonpassivated 3000-/spl Omega//spl middot/cm substrate is equivalent to a 70-/spl Omega//spl middot/cm passivated substrate. Surface-passivated HRS, having minimum losses, a high permittivity, and a high thermal conductivity, qualifies as a close-to-ideal radio frequency and microwave substrate.

Amorphous silicon exhibits a glass transition.

Abstract: Amorphous silicon is a semiconductor with a lower density than the metallic silicon liquid. It is widely believed that the amorphous-liquid transition is a first-order melting transition. In contrast to this, recent computer simulations and the experimental observation of pressure-induced amorphization of nanoporous silicon have revived the idea of an underlying liquid-liquid phase transition implying the existence of a low-density liquid and its glass transition to the amorphous solid. Here we demonstrate that during irradiation with high-energy heavy ions amorphous silicon deforms plastically in the same way as conventional glasses. This behaviour provides experimental evidence for the existence of the low-density liquid. The glass transition temperature for a timescale of 10 picoseconds is estimated to be about 1,000 K. Our results support the idea of liquid polymorphism as a general phenomenon in tetrahedral networks.

Crystallization Behavior of Amorphous Silicon Carbonitride Ceramics Derived from Organometallic Precursors

Abstract: The crystallization behavior of organometallic-precursor-derived amorphous Si-C-N ceramics was investigated under N2 atmosphere using X-ray diffractometry (XRD), transmission electron microscopy (TEM), and solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy. Amorphous Si-C-N ceramics with a C/Si atomic ratio in the range of 0.34–1.13 were prepared using polycarbosilane-polysilazane blends, single-source polysilazanes, and single-source polysilylcarbodiimides. The XRD study indicated that the crystallization temperature of Si3N4 increased consistently with the C/Si atomic ratio and reached 1500°C at C/Si atomic ratios ranging from 0.53 to 1.13. This temperature was 300°C higher than that of the carbon-free amorphous Si-N material. In contrast, the SiC crystallization temperature showed no clear relation with the C/Si atomic ratio. The TEM and NMR analyses revealed that the crystallization of amorphous Si-C-N was governed by carbon content, chemical homogeneity, and molecular structure of the amorphous Si-C-N network.

Oxidation Kinetics of an Amorphous Silicon Carbonitride Ceramic

Abstract: The oxidation kinetics of amorphous silicon carbonitride (SiCN) was measured at 1350°C in ambient air. Two types of specimens were studied: one in the form of thin disks, the other as a powder. Both specimens contained open nanoscale porosity. The disk specimens exhibited weight gain that saturated exponentially with time, analogous to the oxidation behavior of reaction-bonded Si3N4. The saturation value of the weight gain increased linearly with specimen volume, suggesting the nanoscale pore surfaces oxidized uniformly throughout the specimen. This interpretation was confirmed by high-resolution electron microscopy and secondary ion mass spectroscopy. Experiments with the powders (having a particle size much larger than the scale of the nanopores) were also consistent with measurements of the disks. However, the powder specimens, having a high surface-to-volume ratio, continued to show measurable weight gain due to oxidation of the exterior surface. The wide range of values for the surface-to-volume ratio, which included all specimens, permitted a separation of the rate of oxidation of the free surface and the oxidation of the internal surfaces of the nanopores. Surface oxidation data were used to obtain the rate constant for parabolic growth of the oxidation scale. The values for the rate constant obtained for SiCN lay at the lower end of the spectrum of oxidation rates reported in the literature for several Si3N4 and SiC materials. Convergence in the behavior of SiCN and CVD-SiC is ascribed to the purity of both materials. Conversely, it is proposed that the high rates of oxidation of sintered polycrystalline silicon carbides and nitrides, as well as the high degree of variability of these rates, might be related to the impurities introduced by the sintering aids.

Photovoltaic cell and method of fabricating the same

Abstract: An i-type amorphous silicon film and an anti-reflection film made of amorphous silicon nitride or the like are formed in this order on a main surface of an n-type single-crystalline silicon substrate. On a back surface of the n-type single-crystalline silicon substrate are provided a positive electrode and a negative electrode next to each other. The positive electrode includes an i-type amorphous silicon film, a p-type amorphous silicon film, a back electrode, and a collector electrode formed in this order on the back surface of the n-type single-crystalline silicon substrate. The negative electrode includes an i-type amorphous silicon film, an n-type amorphous silicon film, a back electrode, and a collector electrode formed in this order on the back surface of the n-type single-crystalline silicon substrate.

Fabrication of ZnO quantum dots embedded in an amorphous oxide layer

Abstract: ZnO quantum dots (QDs) have been fabricated by the growth of SiO2/ZnO films/Si substrate and subsequent rapid-thermal annealing in a N2 ambient. Transmission electron microscopy (TEM) results show that the ZnO QDs 3–7 nm in size are formed and embedded in the amorphous silicon oxide interfacial layer when annealed at 850 °C. Photoluminescence (PL) at room temperature from the 850 °C-annealed samples reveals only high-energy emission at about 3.37 eV, while PL at 10 K shows a broad spectra with a tail up to about 3.5 eV. The TEM and PL results indicate that the broad spectra are caused by the presence of the ZnO QDs and hence by the quantum confinement effect.

Tunable and switchable multiple-cavity thin film filters

Abstract: A family of thin film interference filters is described that incorporates amorphous silicon layers for wide thermo-optic tunability and the potential for multiple cavity designs. Plasma enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon (a-Si:H) onto fused silica or crystalline silicon wafer substrates produces films with high index (n=3.71), low loss at 1500 nm (k<4/spl times/10/sup -6/), and thermo-optic index coefficients approximately ten times larger than those of the dielectric materials typically used in wavelength division multiplexing (WDM) filters. Pairing amorphous silicon with low index (n=1.77, k=1/spl times/10/sup -6/) silicon nitride companion layers followed by post-process annealing leads to multilayer film stacks which may be cycled up to 400 C without delamination or failure, resulting in thermal index modulations as large as 4% and enabling a wide variety of dynamic thin film device designs. In this paper we survey the range of device applications and show experimental demonstrations for two quite different classes of functionality. One class of filters is tunable in wavelength in the conventional sense. Single and dual-cavity narrowband filters are described with temperature coefficients of center wavelength 85-172 pm//spl deg/C in the 1550 nm WDM band and tuning ranges as large as 60 nm, an order of magnitude larger thermal tunability than for conventional thin film narrowband filters. A second class of filters is fixed in wavelength but switchable in transmission, based on hybrid structures which combine tunable semiconductor films and cavities with static dielectric films and cavities in an integrated coating design. To demonstrate the principle of a switchable add/drop filter, we fabricated a five-cavity, 117 layer, 200 GHz filter by combining a single cavity of a-Si:H/a-Si/sub x/N/sub y/ films deposited by PECVD with four cavities of dielectrics Ta/sub 2/O/sub 5//SiO/sub 2/ deposited by ion assisted e-beam evaporation. By varying the temperature, this device can be switched thermo-optically between transmission and reflection states at a fixed channel 1548.3 nm with a contrast ratio 18.4 dB. Stable devices can be obtained even with large internal temperature changes in microscopic volumes provided that layer-to-layer and layer-to-substrate adhesion is robust, film stresses are well controlled through coefficients of thermal expansion matching, and devices are annealed at or above maximum operational temperatures. "Hitless" filters can be obtained by structuring thermo-optic filters in two independently heated portions. Thermo-optically actuated thin film semiconductor devices are manufacturable and testable on a wafer scale and may be packaged by methods adapted from those for conventional thin film filters.

Light-induced metastability in hydrogenated nanocrystalline silicon solar cells

Abstract: Light-induced metastability in hydrogenated nanocrystalline silicon (nc-Si:H) single-junction solar cells has been studied under different light spectra. The nc-Si:H studied contains a certain fraction of hydrogenated amorphous silicon (a-Si:H). We observe no light-induced degradation when the photon energy used is lower than the bandgap of a-Si:H, while degradation occurs when the photon energy is higher than the bandgap. We conclude that the light-induced defect generation occurs mainly in the amorphous phase. Light soaking experiments on a-Si:H∕a-SiGe:H∕nc-Si:H triple-junction solar cells show no light-induced degradation in the bottom cell, because the a-Si:H top and a a-SiGe:H middle cells absorb most of the high-energy photons.

Infrared surface plasmons in two-dimensional silver nanoparticle arrays in silicon

Abstract: We present two-dimensional arrays of silver nanoparticles embedded in amorphous silicon, fabricated by a sequential Si∕Ag∕Si electron-beam evaporation process. The particle arrays exhibit surface plasmon resonance spectra in the near-infrared (0.9eV), with tails extending below 0.5eV. The data are compared with calculations that take into account measured particle size, shape anisotropy, and separation. It is concluded that the large redshift is mainly due to the high refractive index of the matrix, with shape anisotropy and interparticle coupling contributing several tenths of an electron volt. This work enables plasmon-related applications at the telecommunication wavelength of 1.5μm(0.8eV).

Reverse Monte Carlo modeling of amorphous silicon

Abstract: An implementation of the Reverse Monte Carlo algorithm is presented for the study of amorphous tetrahedral semiconductors. By taking into account a number of constraints that describe the tetrahedral bonding geometry along with the radial distribution function, we construct a model of amorphous silicon using the reverse monte carlo technique. Starting from a completely random configuration, we generate a model of amorphous silicon containing 500 atoms closely reproducing the experimental static structure factor and bond angle distribution and in improved agreement with electronic properties. Comparison is made to existing Reverse Monte Carlo models, and the importance of suitable constraints beside experimental data is stressed.

Oxidation Behavior of a Fully Dense Polymer‐Derived Amorphous Silicon Carbonitride Ceramic

Abstract: The oxidation behavior of a polymer-derived amorphous silicon carbonitride (SiCN) ceramic was studied at temperature range of 900°–1200°C using fully dense samples, which were obtained using a novel pressure-assisted pyrolysis technique. The oxidation kinetics was investigated by measuring the thickness of oxide layers. The data were found to fit a typical parabolic kinetics. The measured oxidation rate constant and activation energy of the SiCN are close to those of CVD and single-crystal SiC. The results suggest that the oxidation mechanism of the SiCN is the same as that of SiC: oxygen diffusion through a silica layer.

Staebler-Wronski Effect in Hydrogenated Amorphous Silicon and Related Alloy Films

Abstract: Hydrogenated amorphous silicon and related alloy films have attracted much attention because of the wide application of these films in devices such as thin-film transistors and solar cells. However, the degradation of these films caused by intense illumination is a serious shortcoming. In this review, various experimental results concerning this problem and various models for the photocreation of dangling bonds which is thought to be the main origin of the degradation are introduced and discussed. Degradation in the device performance, some efforts to overcome the degradation and some metastable defects other than photocreated ones are also briefly introduced.

Near-infrared femtosecond laser-induced crystallization of amorphous silicon

Abstract: Amorphous silicon (a-Si) was crystallized by femtosecond laser annealing (FLA) using a near-infrared (λ≈800nm) ultrafast Ti:sapphire laser system. The intense ultrashort laser pulses lead to efficient nonlinear photoenergy absorption and the generation of very dense photoexcited plasma in irradiated materials, enabling nonlinear melting on transparent silicon materials. We studied the structural characteristics of recrystallized films and found that FLA assisted by spatial scanning of laser strip spot constitutes superlateral epitaxy that can crystallize a-Si films with largest grains of ∼800nm, requiring laser fluence as low as ∼45mJ∕cm2, and low laser shots. Moreover, the optimal annealing conditions are observed with a significant laser-fluence window (∼30%).

Fabrication of nanofluidic devices using glass-to-glass anodic bonding

Abstract: In this work, we present a technology for fabrication of nanochannels created in glass with which bio-analysis can be performed in combination with fluorescence microscopy. The technology is based on a glass-to-glass anodic bonding process. In the bonding process, an intermediate layer (thin insulating film) is deposited on one of the two glass wafers. The channel is then defined, with one or two photo-patterning steps, in the intermediate layer. In our approach, a 33 nm thick amorphous silicon layer deposited by low-pressure chemical vapor deposition (LPCVD) was used as an intermediate layer. The depth of the channel is defined during the etching of this layer.

Method of improving the stability of active matrix OLED displays driven by amorphous silicon thin-film transistors

Abstract: A method of improving the stability of organic light emitting diode (OLED) display devices driven by amorphous silicon thin-film transistors, in which the driving circuitry within each sub-pixel includes a driving transistor for driving organic light emitting diode (OLED), a scanning transistor and a storage capacitance. An end of the capacitance is connected to the signal resetting line, which a resetting time pulse of high potential and low potential are supplied. Since the resetting signals within the sub-pixels are synchronized, a single voltage of the resetting signal can control the positive and negative stresses for each transistor in the sub-pixels on the panel.

Improvement of pin-type amorphous silicon solar cell performance by employing double silicon-carbide p -layer structure

Abstract: We investigated a double silicon-carbide p-layer structure consisting of a undiluted p-type amorphous silicon-carbide (p-a-SiC:H) window layer and a hydrogen diluted p-a-SiC:H buffer layer to improve a pin-type amorphous silicon based solar cell. Solar cells using a lightly boron-doped (1000 ppm) buffer layer with a high conductivity, low absorption, well-ordered film structure, and slow deposition rate improves the open-circuit voltage (Voc), short-circuit current density, and fill factor by reducing recombination in the buffer layer and at p/buffer and buffer/i interfaces. It is found that a natural hydrogen treatment generated throughout the buffer layer deposition onto the p-a-SiC:H window layer is an advantage of this double p-layer structure. We achieved a considerable initial conversion efficiency of 11.2% without any back reflector.