Structure Of Amorphous Silicon Research Articles

Infrared photodetection at room temperature using photocapacitance in amorphous silicon structures

In this letter we present a device, based on amorphous silicon material, able to detect infrared light through capacitance measurement at room temperature. The device is a p-c-n stacked structure, where c indicates a compensated amorphous silicon absorber layer. Operation is based on transitions between extended states in the valence band and defects in the forbidden gap excited by the infrared radiation. We found that the measured capacitance is sensitive to radiation from 800 nm to 5 μm.

固体の機能発現単位としてのクラスター クラスレート化合物およびアモルファスシリコンの電子状態とＳｉ２０正１２面体クラスター

固体の機能発現単位としてのクラスター クラスレート化合物およびアモルファスシリコンの電子状態とＳｉ２０正１２面体クラスター

The Structure of Ion-Implanted Amorphous Silicon

AbstractUsing fluctuation microscopy, we show that ion-implanted amorphous silicon has more medium-range order than is expected from the continuous random network model. From our previous work on evaporated and sputtered amorphous silicon, we conclude that the structure is paracrystalline, i.e. it possesses crystalline-like order which decays with distance from any point. The observation might pose an explanation for the large heat of relaxation that is evolved by ion-implanted amorphous semiconductors.

Generation of amorphous silicon structures by rapid quenching: A molecular-dynamics study

Amorphous silicon ($a$-Si) networks have been generated from melted Si with various quenching rates by molecular-dynamics (MD) simulations employing the Tersoff potential. The cooling rates were set between $5\ifmmode\times\else\texttimes\fi{}{10}^{11}$ and $1\ifmmode\times\else\texttimes\fi{}{10}^{14}\mathrm{K}/\mathrm{s};$ the latter is the slowest quenching rate in MD simulations previously performed. Although the atomic configurations formed by the cooling rate of ${10}^{14}\mathrm{K}/\mathrm{s}$ could reproduce the radial distribution function of $a$-Si obtained experimentally, they contained numerous structural defects such as threefold- and fivefold-coordinated atoms. As the cooling rate decreased, the average coordination number became $\ensuremath{\approx}4$ and tetrahedral bonds predominated. The structural and dynamical properties of $a$-Si generated by a cooling rate with $\ensuremath{\sim}{10}^{12}\mathrm{K}/\mathrm{s}$ were in excellent agreement with those of $a$-Si obtained experimentally.

Low-temperature admittance measurement in thin film amorphous silicon structures

In this paper we analyze low-temperature admittance (capacitance and conductance) measurement as an effective tool for the characterization of amorphous silicon doped layers embedded in thin film structures. Temperature and frequency ranges of 20–250 K and 1–100 kHz, respectively, were used. Measurements were performed on p-i silver and p-i-n structures. Devices with different thicknesses, carbon, and boron content of the p-doped layer were examined. The evolution of the conductance of p-i-n solar cells under illumination was also investigated. Results show excellent sensitivity of the measurement to the states of the doped layer. This sensitivity is explained by using a finite difference simulation program, which proves that at low temperature the trapping process is spatially limited to the doped layers.

Sensors on low-dimensional silicon structures

The possibilities of creating sensors on the basis of silicon structures with classical and quantum-dimensional effects have been researched theoretically and experimentally. The technology of forming qnantuln-well and wire structures of porous and amorphous silicon and their modifications by multiparameter monitoring in situ has been studied. H 2S sensor samples, developed on this basis, have a high sensitivity (on the level 10-20 ppb), low operating voltage (1-1.5 V), low power consumption (not more than 10 -8-10 -6 W) and relative selectivity to hydrocarbons, CO and CO 2. The most sensitive to H 2S ate structures having p = 0.03 Ω cm, oxidized porous silicon layer thicknesses of 6-10 μcm, porosity of 0.75-0.8 and Pd layer thicknesses of 50-60 nm. The obtained results verify the principle possibilities in solving all growing contradictions between information processing and its reception by using low-dimensional structures.

Analysis of front contact heterojunction in a-Si:H one-dimensional position sensitive detectors

The influence of different transparent conducting oxides (TCO) on the transverse photoelectrical properties of one-dimensional position sensitive detectors based on p-i-n amorphous silicon structures was studied. For both SnO2 and indium tin oxide, poor quality of the p layer was revealed by secondary ion mass spectroscopy measurements. Good agreement between experimental and simulation characteristics of TCO/p-i-n structure was additionally conditioned by a strong increase in defect states at the p layer surface which can be attributed to the reduction/oxidation process at the TCO/p interface. However, the analysis showed that under reverse bias the spectral response of the p-i-n structure is not significantly affected by different TCO layers and conditions at the TCO/p heterojunction. Nevertheless, indium tin oxide is less appropriate for a front TCO layer due to the poor reverse dark current-voltage characteristic, i.e., higher leakage current component leading to lower signal to noise ratio.

Amorphous structure of grain boundaries and grain junctions in nanocrystalline silicon by molecular-dynamics simulation

Molecular-dynamics simulations using the Stillinger-Weber three-body potential are used to synthesize fully dense nanocrystalline silicon with a grain size up to 7.3 nm by crystallization from the melt. The structures of the highly-constrained grain boundaries, triple lines and point grain junctions are found to be highly disordered and similar to the structure of amorphous silicon. These results suggest that nanocrystalline silicon may be treated as a two-phase system, namely, an ordered crystalline phase in the grain interiors connected by an amorphous, intergranular glue-like phase.

Thermodynamically stable amorphous intergranular films in nanocrystalline silicon

Molecular-dynamics simulations were used to synthesize fully dense nanocrystalline silicon by crystallization from the melt. The equilibrium structures of the highly constrained grain boundaries and grain junctions in these metastable microstructures are found to be highly disordered and similar to the structure of amorphous silicon. These results demonstrate that in thermodynamic equilibrium nanocrystalline silicon may be treated as a two phase system composed of crystalline grain interiors that are connected by an amorphous intergranular phase.

Relationship between nanocrystalline and amorphous microstructures by molecular dynamics simulation

A recently developed molecular-dynamics simulation method for the growth of fully dense nanocrystalline materials by crystallization from the melt was used together with the Stillinger-Weber three-body potential to synthesize nanocrystalline silicon with a grain size up to 75Å. The structures of the highly-constrained grain boundaries (GBs), triple lines and point grain junctions were found to be highly disordered and similar to the structure of amorphous silicon. These and our earlier results for fcc metals suggest that a nanocrystalline microstructure may be viewed as a two-phase system, namely an ordered crystalline phase in the grain interiors connected by an amorphous, intergranular, glue-like phase. The analysis of the structures of bicrystalline GBs in the same materials reveals the presence of an amorphous intergranular equilibrium phase only in the high-energy but not the low-energy GBs, suggesting that only high-energy boundaries are present in nanocrystalline microstructures.

Electronic Structure of Amorphous Silicon

ABSTRACTThe electronic structure of a continuous network model of tetrahedrally bonded amorphous silicon (a-Si) and of a model hydrogenated amorphous silicon (a-Si:H) that we have built from the a-Si model are calculated in the tight binding approximation. The band edges near the gap are characterized by exponential tails of localized states induced mainly by the variations in bond angles. The spatial localization of the states is compared between a-Si and a-Si:H. Valence band offset between the amorphous and the crystalline phases is calculated.

Comparison of the Structure of Grain Boundaries in Silicon and Diamond by Molecular-Dynamics Simulations

ABSTRACTMolecular-dynamics simulations were used to synthesize nanocrystalline silicon with a grain size of up to 75 Å by crystallization of randomly misoriented crystalline seeds from the melt. The structures of the highly-constrained interfaces in the nanocrystal were found to be essentially indistinguishable from those of high-energy bicrystalline grain boundaries (GBs) and similar to the structure of amorphous silicon. Despite disorder, these GBs exhibit predominantly four-coordinated (sp3-like) atoms and therefore have very few dangling bonds. By contrast, the majority of the atoms in high-energy bicrystalline GBs in diamond are three-coordinated (sp2-like). Despite the large fraction of three-coordinated GB carbon atoms, they are rather poorly connected amongst themselves, thus likely preventing any type of graphite-like electrical conduction through the GBs.

Current gain in amorphous silicon hot electron devices

Useful current gains have been obtained from hydrogenated amorphous silicon structures containing narrow amorphous silicide base regions. The current gains have been determined from amplification of junction leakage and photo-currents with the base open-circuited. The measurements are consistent with efficient transport of hot electrons across an amorphous silicide base a few nanometres thick.

Suppression of Boron Penetration in BF2+-Implanted Poly-Si Gate

In this paper, a comprehensive study of gate engineering to suppress the penetration of boron in p-type metal-oxide-semiconductor field-effect transistor (MOSFET) with the p+-poly-Si-gate is reported. Four types of poly-Si gate structure, two types of gate dielectrics were investigated to suppress the boron penetration. Among the different gate structures, the stacked amorphous silicon structure was found to be the most effective way to retard the boron penetration. N2O oxide exhibited a better retarding of the boron diffusion as compared with the O2 oxide. It was found that a combination of stacked amorphous silicon with N2O oxide is the most effective way to suppress the boron penetration. Thermal stability, oxide integrity, and D it of this sample are superior to all the other samples.

Event-Based Relaxation of Continuous Disordered Systems.

A computational approach is presented to obtain energy-minimized structures in glassy materials. This approach, the activation-relaxation technique (ART), achieves its efficiency by focusing on significant changes in the microscopic structure (events). The application of ART is illustrated with two examples: the structure of amorphous silicon, and the structure of Ni80P20, a metallic glass.

Medium-range structure of amorphous silicon studied by the Voronoi—Delaunay method

Medium-range structure of amorphous silicon studied by the Voronoi—Delaunay method

Medium-range structure of amorphous silicon studied by the Voronoi-Delaunay method

Structural inhomogeneities and their effect on the dynamics are investigated for a molecular dynamics model of quenched amorphous silicon. The structure of the model is analysed with the help of the Voronoi—Delaunay approach, which is a convenient tool for this purpose. The silicon structure is found to be rather homogeneous. Only localized defects are found, and no medium-range regions of ‘imperfect’ structure as were observed in a Lennard-Jones glass by V. A. Luchnikov, N. N. Medvedev, Yu. I. Naberukhin and V. N. Novikov (1995, Phys. Rev. B, 51, 15 569). It is assumed that the inhomogeneity in glasses is the result of competition between the tendency of atoms to pack locally into the most favourable arrangements and the necessity to realize space filling structures. For spherical atoms a regular tetrahedron of four atoms is the densest local configuration; however, such units cannot cover the whole space. As a result, Lennard-Jones glasses have regions of structural inhomogeneity with an extension of 3–...

Theory of low temperature capacitance measurements on amorphous silicon thin film solar cells

A theory of low temperature capacitance measurements on thin film amorphous silicon structures, like p-i Schottky diodes or p-i-n solar cells, is presented. As the interface effects and the glow discharge deposition process dramatically affect the intimate structure of thin film materials, the material properties of thin films are extremely different from the material properties of thick layers. The theory takes into account the effect of the finite dimensions of the semiconductor layers in order to explain the experimental results. A small signal approximation leads to a simple analytical expression of the capacitance which can be easily used for measurement fitting. An extensive analysis of parameter sensitivity is presented together with some examples of how experimental results can be interpreted. To date, low temperature capacitance measurements appear to be the first characterization technique of thin doped layers included in a thin film structure.

The Future of Amorphous Silicon Photovoltaic Technology

AbstractAmorphous silicon modules are commercially available. They are the first truly commercial thin‐film photovoltaic (PV) devices. Well‐defined production processes over very large areas (> 1 m2) have been implemented. There are few environmental issues during manufacturing, deployment in the field or with the eventual disposal of the modules. Manufacturing safety issues are well characterized and controllable. The highest measured initial efficiency to date is 13.7% for a small triple‐stacked cell and the highest stabilized module efficiency is 10%, There is a consensus among researchers that in order to achieve a 15% stabilized efficiency, a triple‐junction amorphous silicon structure is required. Fundamental improvements in alloys are needed for higher efficiencies. This is being pursued through the National Renewable Energy Laboratory/US Department of Energy (NREL/DOE) Thin‐Film Partnership Program. Cost reductions through improved manufacturing processes are being pursued under the NREL/DOE‐sponsored research in manufacturing technology (PVMaT). Much of the work in designing a‐Si devices is a result of trying to compensate for the Staebler‐Wronski effect. Some new deposition techniques hold promise because they have produced materials with lower stabilized defect densities. However, none has yet produced a high‐efficiency device and shown it to be more stable than those from standard glow discharge‐deposited material.

Mechanical and elastic properties of amorphous hydrogenated silicon films studied by broadband surface acoustic wave spectroscopy

Mechanical and elastic properties of a-Si∶H films were measured by broadband Surface Acoustic Wave Spectroscopy (SAWS). In the frequency range achieved, the SAW dispersion curves extend to 300 MHz, which allowed the density, Young's modulus, and Poisson's ratio to be evaluated for films grown by laser CVD or plasma CVD with different hydrogen concentrations. The films deposited by either method have the best mechanical and elastic properties. at a hydrogen concentration of about 10 at. %. For this material, a density of (2300±20) kg/m3 and Young's modulus of (134±5) GPa was determined. The network structures of amorphous silicon are discussed by applying the constraint-counting model to estimate the mean coordination number.