Showing papers on "Amorphous silicon published in 2001"

Third generation photovoltaics: Ultra‐high conversion efficiency at low cost

Abstract: Since the early days of terrestrial photovoltaics, a common perception has been that ‘first generation’ silicon wafer-based solar cells eventually would be replaced by a ‘second generation’ of lower cost thin-film technology, probably also involving a different semiconductor. Historically, cadmium sulphide, amorphous silicon, copper indium diselenide, cadmium telluride and now thin-film polycrystalline silicon have been regarded as key thin-film candidates. Any mature solar cell technology seems likely to evolve to the stage where costs are dominated by those of the constituent materials, be they silicon wafers or glass sheet. It is argued, therefore, that photovoltaics is likely to evolve, in its most mature form, to a ‘third generation’ of high-efficiency thin-film technology. By high efficiency, what is meant is energy conversion values double or triple the 15–20% range presently targeted, closer to the thermodynamic limit of 93%. Tandem cells are the best known of such high-efficiency approaches, where efficiency can be increased merely by adding more cells of different bandgap to a cell stack, at the expense of increased complexity and spectral sensitivity. However, a range of other more ‘paralleled’ approaches offer similar efficiency to an infinite stack of tandem cells. These options are reviewed together with possible approaches for practical implementation, likely to become more feasible with the evolution of materials technology over the next two decades. Copyright © 2001 John Wiley & Sons, Ltd.

Band gap engineering of amorphous silicon quantum dots for light-emitting diodes

Abstract: Amorphous silicon quantum dots (a-Si QDs), which show a quantum confinement effect were grown in a silicon nitride film by plasma-enhanced chemical vapor deposition. Red, green, blue, and white photoluminescence were observed from the a-Si QD structures by controlling the dot size. An orange light-emitting diode (LED) was fabricated using a-Si QDs with a mean size of 2.0 nm. The turn-on voltage was less than 5 V. An external quantum efficiency of 2×10−3% was also demonstrated. These results show that a LED using a-Si QDs embedded in the silicon nitride film is superior in terms of electrical and optical properties to other Si-based LEDs.

Pressure-induced amorphization and an amorphous–amorphous transition in densified porous silicon

Abstract: Crystalline and amorphous forms of silicon are the principal materials used for solid-state electronics and photovoltaics technologies. Silicon is therefore a well-studied material, although new structures and properties are still being discovered. Compression of bulk silicon, which is tetrahedrally coordinated at atmospheric pressure, results in a transition to octahedrally coordinated metallic phases. In compressed nanocrystalline Si particles, the initial diamond structure persists to higher pressure than for bulk material, before transforming to high-density crystals. Here we report compression experiments on films of porous Si, which contains nanometre-sized domains of diamond-structured material. At pressures larger than 10 GPa we observed pressure-induced amorphization. Furthermore, we find from Raman spectroscopy measurements that the high-density amorphous form obtained by this process transforms to low-density amorphous silicon upon decompression. This amorphous-amorphous transition is remarkably similar to that reported previously for water, which suggests an underlying transition between a high-density and a low-density liquid phase in supercooled Si (refs 10, 14, 15). The Si melting temperature decreases with increasing pressure, and the crystalline semiconductor melts to a metallic liquid with average coordination approximately 5 (ref. 16).

Clean electricity from photovoltaics

Abstract: The past and present, M.D. Archer device physics of silicon solar cells, J.O. Schumacher and W. Wettling principles of cell design, J. Poortmans et al crystalline silicon solar cells, M.A. Green amorphous silicon solar cells, C.R. Wronski and D.E. Carlson cadmium telluride solar cells, D. Bonnet Cu (In,Ga)Se2 solar cells, U. Rau and H.W. Schock super-high efficiency III-V tandem and multijunction cells, M. Yamaguchi organic photovoltaic devices, J.J.M. Halls and R.H. Friend quantum well solar clls, J. Nelson thermophotovoltaic generation of electricity, T.J. Coutts concentrator cells and systems, A. Luque cells and systems for space applications, C.M. Hardingham storage of electrical energy, R.M. Dell photovoltaic modules, systems and applications, N.M. Pearsall and R. Hill the photovoltaic business - manufacturers and markets, B. McNelis the economics of photovoltaic technologies, D. Anderson the outlook for PV in the 21st century, E.H. Lysen and B. Yordi.

Crystal size and temperature measurements in nanostructured silicon using Raman spectroscopy

Abstract: In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The ...

Prediction of amorphous phase stability in the metal–silicon systems

Abstract: The stability of the amorphous phase with respect to the liquid phase in metal–silicon systems is modeled thermodynamically as a second-order phase transformation. The glass transition temperature of amorphous silicon is estimated according to the experimentally determined heat of crystallization and the Third Law of Thermodynamics. The feasibility of the model has been demonstrated using the Pd–Si, Co–Si, and Au–Si systems as examples. The predicted glass transition temperatures and heat of formation of the amorphous phase are consistent with available experimental data. The predicted amorphization stabilization at low temperatures in the Co–Si systems agrees with experimental observations.

Thermal cross-linking and pyrolytic conversion of poly(ureamethylvinyl)silazanes to silicon-based ceramics

Abstract: The aim of this work was to study the pyrolytic conversion of a novel commercial polysilazane, poly(ureamethylvinyl)silazane (PUMVS; Ceraset™, Allied Signal Composites Inc., USA), into silicon-based ceramics. The precursor was thermally cross-linked and pyrolyzed between 200 and 1700 °C under argon or nitrogen atmosphere and the products were investigated by spectroscopic techniques (FTIR and Raman spectroscopy, solid-state NMR), elemental analysis and simultaneous thermal analysis coupled with mass spectrometry. Upon heating under argon, the starting liquid precursor transformed into an infusible solid polymer at T > 250 °C with a conversion yield of >95 wt%. The cross-linking solidification occurred predominantly through hydrosilylation or addition reaction involving vinyl groups. Subsequent pyrolysis of the cross-linked products around 1000 °C in argon yielded amorphous silicon carbonitride ceramics with a composition of SiN0.82C0.86. The overall ceramic yield (with respect to the starting PUMVS) was around 70 wt%, which was found to be independent of the initial cross-linking step. Solid-state NMR (29Si and 13C) revealed that the amorphous silicon carbonitrides contain predominately CSiN3 units. There is evidence for the formation of free amorphous carbon between 700 and 800 °C. Graphitic phases were detected by X-ray diffraction in the samples heated to T > 1000 °C at high heating rates. Upon annealing at T > 1500 °C, the excess carbon reacted completely with the silicon (carbo)nitride to form SiC and nitrogen. The final ceramics contained a large amount of crystalline SiC (∼90 wt%), and were free of excess carbon or silicon. Therefore, PUMVS is an ideal precursor for the formation of high-quality SiC-based ceramics. Copyright © 2001 John Wiley & Sons, Ltd.

Fitting the Stillinger–Weber potential to amorphous silicon

Abstract: Modifications are proposed to the Stillinger–Weber (SW) potential, an empirical interaction potential for silicon. The modifications are specifically intended to improve the description of the amorphous phase and are obtained by a direct fit to the amorphous structure. The potential is adjusted to reproduce the location of the transverse optic (TO) and transverse acoustic (TA) peaks of the vibrational density of states (VDOS), properties insensitive to the details of experimental preparation. These modifications also lead to excellent agreement with structural properties. Comparison with other empirical potentials shows that amorphous silicon configurations generated with the modified potential have overall better vibrational and structural properties.

Surface passivation of p-type crystalline Si by plasma enhanced chemical vapor deposited amorphous SiC x :H films

Abstract: Excellent passivation properties of intrinsic amorphous silicon carbide (a-SiCx:H) films deposited by plasma enhanced chemical vapor deposition on single-crystalline silicon (c-Si) wafers have been obtained. The dependence of the effective surface recombination velocity, Seff, on deposition temperature, total pressure and methane (CH4) to silane (SiH4) ratio has been studied for these films using lifetime measurements made with the quasi-steady-state photoconductance technique. The dependence of the effective lifetime, τeff, on the excess carrier density, Δn, has been measured and also simulated through a physical model based on Shockley–Read–Hall statistics and an insulator/semiconductor structure with fixed charges and band bending. A Seff at the a-SiCx:H/c-Si interface lower than 30 cm s−1 was achieved with optimized deposition conditions. This passivation quality was found to be three times better than that of noncarbonated amorphous silicon (a-Si:H) films deposited under equivalent conditions.

Controlled growth of oriented amorphous silicon nanowires via a solid–liquid–solid (SLS) mechanism

Abstract: Highly oriented amorphous silicon nanowires (a-SiNWs ) were grown on Si (1 1 1). The length and diameter of oriented SiNWs are almost uniform, which are 1 μm and 25 nm , respectively. Different from the well-known vapor–liquid–solid (VLS) for conventional whisker growth, it was found that growth of the a-SiNWs was controlled by a solid–liquid–solid mechanism (SLS). This synthesis method is simple and controllable. It may be useful in large-scale synthesis of various nanowires.

Method for forming shallow trench isolation in semiconductor device

Abstract: The present invention discloses a method for forming shallow trench isolation in a semiconductor device, particularly a nonvolatile memory device. A dielectric layer, an amorphous silicon layer, and a mask layer are sequentially formed over a substrate. Isolation trenches are etched in the substrate through the layers. An oxide layer is thermally grown lining the sidewalls of the amorphous silicon layer and the trenches. Due to the lower oxidation rate of amorphous silicon, the liner oxide layer is thinner at the position lining the amorphous silicon layer than at the position lining the trench. The trenches are filled with an isolation layer to form shallow trench isolation (STI) structures. After removing the mask layer, the amorphous silicon layer can be converted into a polysilicon layer to serve as a floating gate for a nonvolatile memory device.

Imaging Characteristics of an Amorphous Silicon Flat-Panel Detector for Digital Chest

Abstract: PURPOSE To evaluate the imaging characteristics of an amorphous silicon flat-panel detector (FPD) for digital chest radiography. MATERIALS AND METHODS The 41 x 41-cm digital FPD is constructed on a single monolithic glass substrate with a structured cesium iodide scintillator layer and an amorphous silicon thin-film transistor array for image readout. Basic imaging characteristics of the FPD and associated image processing system were assessed on acquired images, including linearity, repeatability, uniformity of response, modulation transfer function (MTF), noise power spectrum, detective quantum efficiency (DQE), contrast sensitivity, and scatter content. Results with the FPD system were compared to those with a storage phosphor computed radiography (CR) system. RESULTS Images obtained with the FPD demonstrated excellent uniformity, repeatability, and linearity, as well as MTF and DQE that were superior to those with the storage phosphor CR system. The contrast and scatter content of images acquired with the FPD were equivalent to those acquired with the storage phosphor system. CONCLUSION The FPD provides radiographic images with excellent inherent physical image quality.

Raman microspectroscopy analysis of pressure-induced metallization in scratching of silicon

Abstract: A single-point diamond turning machine was used to make grooves on (111) p-type single-crystal silicon wafers at room temperature. Scratch tests have been performed with both sharp (Vickers and conical) diamond tools, and a spherical (Rockwell) diamond tool. Our results showed that material removal mechanisms differed between these tools. Pressure-induced metallization of Si allows the ductile regime mechanical micromachining of wafer surfaces. Raman microspectroscopy and electron microscopy were used to determine the machining parameters that do not introduce cracking or other types of damage. The surface of the groove, after machining, was covered by a mixture of metastable, high-pressure silicon phases and amorphous silicon. Further, these phases can be transformed into cubic silicon by annealing. The maximum depth of cut in the ductile regime has been determined for the given scratch test conditions and tools. The developed technique can be used to machine Ge, GaAs and other semiconductors. Applications drawing from this research are many. For example, channels for microfluidic devices can be engraved with a channel cross-section that is determined by the shape of the tool, which allows patterns that cannot be produced using etching. There are no limitations on the channel length or direction, and the channel width can vary from potentially a few nanometres to several micrometres.

Amorphous silicon photovoltaic devices

Abstract: A photovoltaic device comprising an amorphous silicon-containing i-layer that is more efficient at elevated operation temperatures than at lower operation temperatures.

Area sensor and display apparatus provided with an area sensor

Abstract: An area sensor of the present invention has a function of displaying an image in a sensor portion by using light-emitting elements and a reading function using photoelectric conversion devices. Therefore, an image read in the sensor portion can be displayed thereon without separately providing an electronic display on the area sensor. Furthermore, a photoelectric conversion layer of a photodiode according to the present invention is made of an amorphous silicon film and an N-type semiconductor layer and a P-type semiconductor layer are made of a polycrystalline silicon film. The amorphous silicon film is formed to be thicker than the polycrystalline silicon film. As a result, the photodiode according to the present invention can receive more light.

Evidence of light-emitting amorphous silicon clusters confined in a silicon oxide matrix

Abstract: Amorphous silicon oxide thin films were prepared by the coevaporation technique in ultrahigh vacuum. Different compositions were obtained by changing the evaporation rate of silicon. The samples were then annealed to different temperatures up to 950 °C. The composition and the structure were investigated using energy dispersive x-ray spectroscopy, infrared absorption measurements, and Raman spectroscopy. This study attests the presence of amorphous silicon clusters in a silicon oxide matrix. Optical transmission measurements were performed and interpreted in the field of the composite medium theory. The obtained results are in good agreement with the presented structural model. The photoluminescence in the red-orange domain was studied in relation with the structure. The correlation between the photoluminescence energy and intensity and the structure shows that the light emission originates from the silicon clusters embedded in the silicon oxide matrix. Moreover the dependence of the photoluminescence energy with the silicon volume fraction suggests the origin of the light emission could be due to a quantum confinement effect of carriers in the amorphous silicon clusters.

Thin film semiconductor and method for manufacturing the same, semiconductor device and method for manufacturing the same

Abstract: The present invention is related to a thin film semiconductor which can be regarded as substantially a single crystal and a semiconductor device comprising an active layer formed by the thin film semiconductor. At least a concave or convex pattern is formed intentionally on a insulating film provided in contact with the lower surface of an amorphous silicon film, whereby at least a site is formed in which a metal element for accelerating crystallization can be segregated. Therefore, a crystal nuclei is selectively formed in a portion where the concave or convex pattern is located, which carries out controlling a crystal diameter. Thus, a crystalline silicon film is obtained. A crystallinity of the crystalline silicon film is improved by the irradiation of a laser light or an intense light having an energy equivalent to that of the laser light, whereby a monodomain region in which no grain boundary substantially exit is formed.

Hydrogenated amorphous silicon deposited at very high growth rates by an expanding Ar-H2-SiH4 plasma

Abstract: The properties of hydrogenated amorphous silicon (a-Si:H) deposited at very high growth rates (6–80 nm/s) by means of a remote Ar–H2–SiH4 plasma have been investigated as a function of the H2 flow in the Ar–H2 operated plasma source. Both the structural and optoelectronic properties of the films improve with increasing H2 flow, and a-Si:H suitable for the application in solar cells has been obtained at deposition rates of 10 nm/s for high H2 flows and a substrate temperature of 400 °C. The “optimized” material has a hole drift mobility which is about a factor of 10 higher than for standard a-Si:H. The electron drift mobility, however, is slightly lower than for standard a-Si:H. Furthermore, preliminary results on solar cells with intrinsic a-Si:H deposited at 7 nm/s are presented. Relating the film properties to the SiH4 dissociation reactions reveals that optimum film quality is obtained for conditions where H from the plasma source governs SiH4 dissociation and where SiH3 contributes dominantly to film ...

Thin-Film Amorphous Silicon Position-Sensitive Detectors

Abstract: Optical position-sensitive detectors are a useful class of sensor with a wide range of applications in machine control systems, industrial alignment, and robotic vision. They have distinct advantages over most arrayed discrete optical devices in that they can produce continuous optical signals, and versions based on thin-film amorphous silicon are not restricted by crystal growth limits and so have the potential to be fabricated in large area format. Sputter-deposited hydrogenated a-Si also has features such as excellent adhesion to glass substrates, precise film thickness, and hydrogen content control, which are of some interest in device design and fabrication.

Structure and physical properties of paracrystalline atomistic models of amorphous silicon

Abstract: We have examined the structure and physical properties of paracrystalline molecular dynamics models of amorphous silicon. Simulations from these models show qualitative agreement with the results of recent mesoscale fluctuation electron microscopy experiments on amorphous silicon and germanium. Such agreement is not found in simulations from continuous random network models. The paracrystalline models consist of topologically crystalline grains which are strongly strained and a disordered matrix between them. We present extensive structural and topological characterization of the medium range order present in the paracrystalline models and examine their physical properties, such as the vibrational density of states, Raman spectra, and electron density of states. We show by direct simulation that the ratio of the transverse acoustic mode to transverse optical mode intensities ITA/ITO in the vibrational density of states and the Raman spectrum can provide a measure of medium range order. In general, we conc...

150°C Amorphous Silicon Thin-Film Transistor Technology for Polyimide Substrates

Abstract: We have developed a 150°C technology for amorphous silicon thin-film transistors (a-Si:H TFTs) on polyimide substrates deposited by plasma enhanced chemical vapor deposition. The silicon nitride gate dielectric and the a-Si:H channel material were tailored to provide the least leakage current and midgap defect density, respectively. In addition, we conducted experiments on the TFT structure and fabrication with the aim of obtaining high electron mobility. TFTs with back-channel etch and channel-passivated structures were fabricated on glass or 51 μm thick polyimide foil. The a-Si:H TFTs have an on/off current ratio of ∼10 7 and an electron mobility of ∼0.7 cm 2/V s.

Semiconductor device and fabrication method therefor

Abstract: In a crystalline silicon film fabricated by a related art method, the orientation planes of its crystal randomly exist and the orientation rate relative to a particular crystal orientation is low. A semiconductor material which contains silicon as its main component and 0.1-10 atomic % of germanium is used as a first layer, and an amorphous silicon film is used as a second layer. Laser light is irradiated to crystallize the amorphous semiconductor films, whereby a good semiconductor film is obtained. In addition, TFTs are fabricated by using such a semiconductor film.

Semiconductor material and method for forming the same and thin film transistor

Abstract: A semiconductor material and a method for forming the same, said semiconductor material having fabricated by a process comprising irradiating a laser beam or a high intensity light equivalent to a laser beam to an amorphous silicon film containing therein carbon, nitrogen, and oxygen each at a concentration of 5×1019 atoms·cm−3 or lower, preferably 1×1019 atoms·cm−3 or lower, without melting the amorphous silicon film. The present invention provides thin film semiconductors having high mobility at an excellent reproducibility, said semiconductor materials being useful for fabricating compact thin film semiconductor devices such as thin film transistors improved in device characteristics.

An investigation of laser annealed and metal-induced crystallized polycrystalline silicon thin-film transistors

Abstract: We report results on thin-film transistors (TFTs) made from a new hybrid process in which amorphous silicon (a-Si) is first converted to polycrystalline silicon (poly-Si) using Ni-metal-induced lateral crystallization (MILC), and then improved using excimer laser annealing (laser MILC or L-MILC) With only a very low shot laser process, we demonstrate that laser annealing of MILC material can improve the electron mobility from 80 to 170 cm/sup 2//Vs, and decrease the minimum leakage current by one to two orders of magnitude at a drain bias of 5 V Similar trends occur for both p- and n-type material A shift in threshold voltage upon laser annealing indicates the existence of a net positive charge in Ni-MILC material, which is neutralised upon laser exposure The MILC material in particular exhibits a very high generation state density of /spl sim/10/sup 19/ cm/sup -3/ which is reduced by an order of magnitude in L-MILC material The gate and drain field dependences of leakage current indicate that the leakage current in MILC transistors is related to this high defect level and the abruptness of the channel/drain junction This can be improved with a lightly doped drain (LDD) implant, as in other poly-Si transistors