Showing papers on "Amorphous silicon published in 2003"

Photovoltaic materials, history, status and outlook

Abstract: This paper reviews the history, the present status and possible future developments of photovoltaic (PV) materials for terrestrial applications. After a brief history and introduction of the photovoltaic effect theoretical requirements for the optimal performance of materials for pn-junction solar cells are discussed. Most important are efficiency, long-term stability and, not to be neglected, lowest possible cost. Today the market is dominated by crystalline silicon in its multicrystalline and monocrystalline form. The physical and technical limitations of this material are discussed. Although crystalline silicon is not the optimal material from a solid state physics point of view it dominates the market and will continue to do this for the next 5–10 years. Because of its importance a considerable part of this review deals with materials aspects of crystalline silicon. For reasons of cost only multicrystalline silicon and monocrystalline Czochralski (Cz) crystals are used in practical cells. Light induced instability in this Cz-material has recently been investigated and ways to eliminate this effect have been devised. For future large scale production of crystalline silicon solar cells development of a special solar grade silicon appears necessary. Ribbon growth is a possibility to avoid the costly sawing process. A very vivid R&D area is thin-film crystalline silicon (about 5–30 μm active layer thickness) which would avoid the crystal growing and sawing processes. The problems arising for this material are: assuring adequate light absorption, assuring good crystal quality and purity of the films, and finding a substrate that fulfills all requirements. Three approaches have emerged: high-temperature, low-temperature and transfer technique. Genuine thin-film materials are characterized by a direct band structure which gives them very high light absorption. Therefore, these materials have a thickness of only one micron or less. The oldest such material is amorphous silicon which is the second most important material today. It is mainly used in consumer products but is on the verge to also penetrate the power market. Other strong contenders are chalcogenides like copper indium diselenide (CIS) and cadmium telluride. The interest has expanded from CuInSe 2 , to CuGaSe 2 , CuInS 2 and their multinary alloys Cu(In,Ga)(S,Se) 2 . The two deposition techniques are either separate deposition of the components followed by annealing on one hand or coevaporation. Laboratory efficiencies for small area devices are approaching 19% and large area modules have reached 12%. Pilot production of CIS-modules has started in the US and Germany. Cadmium telluride solar cells also offer great promise. They have only slightly lower efficiency and are also at the start of production. In the future other materials and concepts can be expected to come into play. Some of these are: dye sensitized cells, organic solar cells and various concentrating systems including III/V-tandem cells. Theoretical materials that have not yet been realized are Auger generation material and intermediate metallic band material.

High-Performance Nanowire Electronics and Photonics on Glass and Plastic Substrates

Abstract: The merger of nanoscale building blocks with flexible and/or low cost substrates could enable the development of high-performance electronic and photonic devices with the potential to impact a broad spectrum of applications. Here we demonstrate that high-quality, single-crystal nanowires can be assembled onto inexpensive glass and flexible plastic substrates to create basic transistor and light-emitting diode devices. In our approach, the high-temperature synthesis of single-crystal nanowires is separated from ambient-temperature solution-based assembly to enable the fabrication of single-crystal-like devices on virtually any substrate. Silicon nanowire field-effect transistors were assembled on glass and plastic substrates and display device parameters rivaling those of single-crystal silicon and exceeding those of state-of-the-art amorphous silicon and organic transistors currently used for flexible electronics on plastic substrates. Nanowire transistor devices have been configured as low-threshold logi...

Determining the material structure of microcrystalline silicon from Raman spectra

Abstract: An easy and reliable method to extract the crystalline fractions in microcrystalline films is proposed. The method is shown to overcome, in a natural way, the inconsistencies that arise from the regular peak fitting routines. We subtract a scaled Raman spectrum that was obtained from an amorphous silicon film from the Raman spectrum of the microcrystalline silicon film. This subtraction leaves us with the Raman spectrum of the crystalline part of the microcrystalline film and the crystalline fraction can be determined. We apply this method to a series of samples covering the transition regime from amorphous to microcrystalline silicon. The crystalline fractions show good agreement with x-ray diffraction (XRD) results, in contrast to crystalline fractions obtained by the fitting of Gaussian line profiles applied to the same Raman spectra. The spectral line shape of the crystalline contribution to the Raman spectrum shows a clear asymmetry, an observation in agreement with model calculations reported previo...

Contact resistance in organic transistors that use source and drain electrodes formed by soft contact lamination

Abstract: Soft contact lamination of source/drain electrodes supported by gold-coated high-resolution rubber stamps against organic semiconductor films can yield high-performance organic transistors. This article presents a detailed study of the electrical properties of these devices, with an emphasis on the nature of the laminated contacts with the p- and n-type semiconductors pentacene and copper hexadecafluorophthalocyanine, respectively. The analysis uses models developed for characterizing amorphous silicon transistors. The results demonstrate that the parasitic resistances related to the laminated contacts and their coupling to the transistor channel are considerably lower than those associated with conventional contacts formed by evaporation of gold electrodes directly on top of the organic semiconductors. These and other attractive features of transistors built by soft contact lamination suggest that they may be important for basic and applied studies in plastic electronics and nanoelectronic systems based ...

High Mobility Thin Film Transistors with Transparent ZnO Channels.

Abstract: We have fabricated high performance ZnO thin film transistors (TFTs) using CaHfOx buffer layer between ZnO channel and amorphous silicon?nitride gate insulator. The TFT structure, dimensions, and materials set are identical to those of the commercial amorphous silicon (a-Si) TFTs in active matrix liquid crystal display, except for the channel and buffer layers replacing a-Si. The field effect mobility can be as high as 7 cm2?V-1?s-1 for devices with maximum process temperature of 300?C. The process temperature can be reduced to 150?C without much degrading the performance, showing the possibility of the use of polymer substrate.

Vacancies and voids in hydrogenated amorphous silicon

Abstract: The hydride configurations in the hydrogenated amorphous silicon (a-Si:H) network have been studied by means of infrared absorption spectroscopy. The results on the film mass density of a-Si:H deposited by means of an expanding thermal plasma reveal the presence of two distinct regions in terms of hydrogen content and microstructure: below approximately 14 at. % H a-Si:H contains predominantly divacancies decorated by hydrogen, above 14 at. % H a-Si:H contains microscopic voids. These two distinct regions provide additional information on the origin of the low and high hydride stretching modes at 1980–2010 and 2070–2100 cm−1, respectively.

An interface clusters mixture model for the structure of amorphous silicon monoxide (SiO)

Abstract: The present state of research on the structure of amorphous silicon monoxide (SiO) is reviewed. The black, coal-like modification of bulk SiO is studied by a combination of diffraction, microscopy, spectroscopy, and magnetometry methods. Partial radial distribution functions of SiO are obtained by X-ray, neutron and electron diffraction. Disproportionation of SiO into Si and SiO2 is verified. High resolution TEM gives an upper limit of less than 2 nm for the typical Si cluster size. The Si K-edge electron energy-loss near-edge structure (ELNES) data of SiO are interpreted in terms of the oxidation states Si4+ and Si0. X-ray photoelectron spectroscopy gives first details about possible stoichiometric inhomogeneities related to internal interfaces. The wipe-out effect in the 29Si MAS NMR signal of SiO is confirmed experimentally. The new estimation of the wipe-out radius is about 1.1 nm. First-time W-band, Q-band, and X-band ESR and SQUID measurements indicate an interfacial defect structure. Frequency distributions of atomic nearest-neighbours are derived. The interface clusters mixture model (ICM model) suggested here describes the SiO structure as a disproportionation in the initial state. The model implies clusters of silicon dioxide and clusters of silicon surrounded by a sub-oxide matrix that is comparable to the well-known thin Si/SiO2 interface and significant in the volume because of small cluster sizes.

High-efficiency visible photoluminescence from amorphous silicon nanoparticles embedded in silicon nitride

Abstract: Confinement of silicon nanoparticles in silicon nitride instead of an oxide matrix might materially facilitate its potential applications as a light-emitting component in optoelectronics. We report in this letter the production of high-density (up to 4.0×1012/cm2 from micrographs) silicon nanoparticles in SiNx thin films by chemical vapor deposition on cold substrates. Strong room-temperature photoluminescence was observed in the whole visible light range from the deposits that were postannealed at 500 °C for 2 min. The Si-in-SiNx films provide a significantly more effective photoluminescence than Si-in-SiOx fabricated with similar processing parameters: for blue light, the external quantum efficiency is over three times as large. The present results demonstrate that the nanostructured Si-in-SiNx system can be a very competitive candidate for the development of tunable high-efficiency light-emitting devices.

Method for forming a double-gated semiconductor device

Abstract: A method for forming a polysilicon FinFET (10) or other thin film transistor structure includes forming an insulative layer (12) over a semiconductor substrate (14). An amorphous silicon layer (32) forms over the insulative layer (12). A silicon germanium seed layer (44) forms in association with the amorphous silicon layer (32) for controlling silicon grain growth. The polysilicon layer arises from annealing the amorphous silicon layer (32). During the annealing step, silicon germanium seed layer (44), together with silicon germanium layer (34), catalyzes silicon recrystallization to promote growing larger crystalline grains, as well as fewer grain boundaries within the resulting polysilicon layer. Source (16), drain (18), and channel (20) regions are formed within the polysilicon layer. A double-gated region (24) forms in association with source (16), drain (18), and channel (20) to produce polysilicon FinFET (10).

Plasma-enhanced chemical vapor deposited silicon carbide as an implantable dielectric coating.

Abstract: Amorphous silicon carbide (a-SiC) films, deposited by plasma-enhanced chemical vapor deposition (PECVD), have been evaluated as insulating coatings for implantable microelectrodes. The a-SiC was deposited on platinum or iridium wire for measurement of electrical leakage through the coating in phosphate-buffered saline (PBS, pH 7.4). Low leakage currents of <10(-11) A were observed over a +/-5-V bias. The electronic resistivity of a-SiC was 3 x 10(13) Omega-cm. Dissolution rates of a-SiC in PBS at 37 and 90 degrees C were determined from changes in infrared absorption band intensities and compared with those of silicon nitride formed by low-pressure chemical vapor deposition (LPCVD). Dissolution rates of LPCVD silicon nitride were 2 nm/h and 0.4 nm/day at 90 and 37 degrees C, respectively, while a-SiC had a dissolution rate of 0.1 nm/h at 90 degrees C and no measurable dissolution at 37 degrees C. Biocompatibility was assessed by implanting a-SiC-coated quartz discs in the subcutaneous space of the New Zealand White rabbit. Histological evaluation showed no chronic inflammatory response and capsule thickness was comparable to silicone or uncoated quartz controls. Amorphous SiC-coated microelectrodes were implanted in the parietal cortex for periods up to 150 days and the cortical response evaluated by histological evaluation of neuronal viability at the implant site. The a-SiC was more stable in physiological saline than LPCVD Si(3)N(4) and well tolerated in the cortex.

A new a-Si:H thin-film transistor pixel circuit for active-matrix organic light-emitting diodes

Abstract: We propose a new pixel circuit using hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs), composed of three switching and one driving TFT, for active-matrix organic light-emitting diodes (AMOLEDs) with a voltage source method. The circuit simulation results based on the measured threshold voltage shift of a-Si:H TFTs by gate-bias stress indicate that this circuit compensates for the threshold voltage shifts over 10000 h of operation.

Silicon Carbide Nanosprings

Abstract: Amorphous silicon carbide nanosprings, as well as biphase (crystalline core/amorphous sheath) helical nanowires, have been synthesized by plasma enhanced chemical vapor deposition Both variants grow via the vapor−liquid−solid mechanism The formation of the amorphous silicon carbide nanosprings is explained in terms of the contact angle anisotropy model initially proposed to explain the formation of amorphous boron carbide nanosprings A modified contact angle anisotropy model, where the presence of temperature gradients within the catalyst are imposed, has been proposed to explain the formation of the biphase helical nanowires The basis for this model is that the crystalline core acts to pin the catalyst, thereby prohibiting nanospring formation The model is supported by the experimental observation of a transition to nanospring growth at the point of extinction of the crystalline core of a linear biphase nanowire at a position where the catalyst is off-center with respect to the axis of the growth di

Stacked photovoltaic device

Abstract: A stacked photovoltaic device comprises at least three p-i-n junction constituent devices superposed in layers, each having a p-type layer, an i-type layer and an n-type layer which are formed of silicon non-single crystal semiconductors. An amorphous silicon layer is used as the i-type layer of a first p-i-n junction, a microcrystalline silicon layer is used as the i-type layer of a second p-i-n junction and a microcrystalline silicon layer is used as the i-type layer of a third p-i-n junction, the first to third layers being in order from the light incident side. In this way, a stacked photovoltaic device can be provided which is practical and low-cost and yet has high reliability and high photoelectric conversion efficiency.

Nanostructured Si / TiB2 Composite Anodes for Li-Ion Batteries

Abstract: Silicon and titanium boride nanocomposites were synthesized using high-energy mechanical milling. The nanocomposites obtained after mechanical milling consist of amorphous silicon and nanocrystalline titanium boride. The nanocomposite containing 40 mol % silicon obtained after milling for 20 h exhibits a stable capacity of ∼400 mAh/g. X-ray diffraction and scanning electron microscopy analyses indicated that the nanocomposite retains its initial phase and microstructure during electrochemical cycling. Premilling of the inactive TiB 2 component appears to increase the stability of the capacity of the nanocomposite electrodes due to a probable homogeneous distribution of the induced stress during cycling.

Analysis of light scattering in amorphous Si:H solar cells by a one‐dimensional semi‐coherent optical model

Abstract: A one-dimensional semi-coherent optical model for thin-film solar cells is presented. The optical circumstances at flat interfaces are addressed and the situation at rough interfaces in the model is described for the case of direct (coherent) incident and scattered (incoherent) incident light. After the model has been experimentally verified, analysis of the light scattering process in hydrogenated amorphous silicon (a-Si:H) p–i–n solar cells is carried out. The influence of the interface root-mean-square roughness and the effect of different angular distribution functions of diffused light on quantum efficiency and short-circuit current are investigated by the optical model. Copyright © 2002 John Wiley & Sons, Ltd.

Above-threshold parameter extraction and modeling for amorphous silicon thin-film transistors

Abstract: This paper presents modeling and parameter extraction of the above-threshold characteristics of hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) in both linear and saturation regions of operation. A bias- and geometry-independent definition for field effect mobility considering the ratio of free-to-trapped carriers is introduced, which conveys the properties of the active semiconducting layer. A method for extraction of model parameters such as threshold voltage, effective mobility, band-tail slope, and contact resistance from the measurement results is presented. This not only provides insight to the device properties, which are highly fabrication-dependent, but also enables accurate and reliable TFT circuit simulation. The techniques presented here form the basis for extraction of physical parameters for other TFTs with similar gap properties, such as organic and polymer TFTs.

TEM investigation on the structure of amorphous silicon monoxide

Abstract: Commercially available powder samples of silicon monoxide have been investigated by methods of transmission electron microscopy: electron scattering, electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Pair distribution functions (PDFs) as well as EEL spectra can be shown to be a composition of the PDF and EEL spectra of elemental silicon and amorphous SiO2. The distribution of the elements silicon and oxygen calculated from ESI images proof the silicon monoxide to be inhomogeneous, i.e. it consists of amorphous silicon and amorphous SiO2. The phase separated regions measure ≈3–4 nm. One maximum in the PDF at 2.95 A does not stem from either a-Si or a-SiO2, and it is assigned to atomic configurations at the interphase boundary layer between Si and SiO2. The portion of the interphase domain in the total composite material is estimated to be between 20% and 25%.

Role of front contact work function on amorphous silicon/crystalline silicon heterojunction solar cell performance

Abstract: We investigated with numerical simulations the effects that a transparent conductive oxide front contact (TCO) can have on amorphous silicon/crystalline silicon heterojunction solar cells. We found that, due to the extremely thin emitter layer used in this kind of device, the built-in potential available cannot be merely defined by the difference between the work function of the emitter and the base, but it depends on TCO work function as well. As a consequence, because of the correlation between built-in potential and open circuit voltage, the TCO work function strongly affects the solar cell performance. Simulation results show that a higher work function leads to the best device efficiency.

Room temperature nanocrystalline silicon single-electron transistors

Abstract: Single-electron transistors operating at room temperature have been fabricated in 20-nm-thick nanocrystalline silicon thin films. These films contain crystalline silicon grains 4 – 8 nm in size, embedded in an amorphous silicon matrix. Our single-electron transistor consists of a side-gated 20 nm×20 nm point contact between source and drain electrodes. By selectively oxidizing the grain boundaries using a low-temperature oxidation and high-temperature argon annealing process, we are able to engineer tunnel barriers and increase the potential energy of these barriers. This forms a “natural” system of tunnel barriers consisting of silicon oxide tissues that encapsulate sub-10 nm size grains, which are small enough to observe room-temperature single-electron charging effects. The device characteristics are dominated by the grains at the point contact. The material growth and device fabrication process are compatible with silicon technology, raising the possibility of large-scale integrated nanoelectronic sys...

Ellipsometric study of silicon nanocrystal optical constants

Abstract: Samples of silicon nanocrystals on various substrates were prepared by cluster beam deposition of silicon nanoparticles, obtained by laser-induced pyrolysis of silane in a flow reactor. Using optical ellipsometry, the optical properties (refractive index and extinction coefficient) of the as-prepared silicon nanocrystal layers were determined in the wavelength range from 240 to 700 nm. Two dispersion models were used to describe the silicon nanocrystal optical properties: the Bruggeman effective medium approximation model and the Tauc–Lorentz model. The study showed that while a simple Bruggeman effective medium approximation model could not completely account for the silicon nanocrystal dispersion behavior, the optical response of the silicon nanocrystal layers could be satisfactorily described by a Tauc–Lorentz model. The present study also showed that, as for porous silicon, the silicon nanocrystal optical indexes significantly deviate from those of bulk crystalline and amorphous silicon. It confirms the special behavior of silicon under its nanoscale form.