Structures of grain boundaries in sintered silicon

The investigation of near coincidence boundaries in sintered silicon by transmission electron microscopy revealed a fine structure (steps, dislocations). For a Σ = 9 coincidence orientation it will be demonstrated that faceting of an inclined grain boundary reduces the total energy.     

Enhanced diffusion of phosphorus at grain boundaries in multicrystalline silicon

Two different methods were applied to examine the diffusion of phosphorus at grain boundaries in silicon. Five solar-cells processed on multicrystalline silicon were investigated. These cells are distinguished by different materials, diffusion-temperature, and further preparation. Enhanced diffusion at grain boundaries was observed in two of these cells with both methods. One solar cell also showed enhanced diffusion at dislocations. A correlation of the phosphorus diffusion and an enhanced recombination activity of the grain boundaries of these cells is found.     

Recombination and transport through localized states in hydrogenated amorphous and microcrystalline silicon

Electrical transport and recombination mechanisms in hydrogenated amorphous silicon, a-Si:H, are determined by localized band-tail states and deep defects. At low temperatures (T < 100 K) the photoluminescence originates from tunneling recombination between localized band-tail states and the photoconductivity arises from hopping in the band tail. This review describes the present understanding of transport and recombination mechanisms in this low-temperature regime with a focus on two aspects: (i) the kinetics of carrier recombination and the competition between geminate and non-geminate recombination, and (ii) the microscopic identification of recombination paths by magnetic resonance techniques and the proof of excitonic recombination. Inspite of its complex nanocrystalline morphology, hydrogenated microcrystalline silicon, μc-Si:H, behaves in many respects similarly to a-Si:H in that the low-temperature properties are also determined by disorder-induced localized band-tail states.     

Relation between substrate surface morphology and microcrystalline silicon solar cell performance

In the present paper, the structural and electrical performances of microcrystalline silicon (μc-Si:H) single junction solar cells co-deposited on a series of substrates having different surface morphologies varying from V-shaped to U-shaped valleys, are analyzed. Transmission electron microscopy (TEM) is used to quantify the density of cracks within the cells deposited on the various substrates. Standard 1 sun, variable illumination measurements (VIM) and Dark J(V) measurements are performed to evaluate the electrical performances of the devices. A marked increase of the reverse saturation current density (J₀) is observed for increasing crack densities. By introducing a novel equivalent circuit taking into account such cracks as non-linear shunts, the authors are able to relate the magnitude of the decrease of V_oc and FF to the increasing density of cracks.     

Effect of growth conditions on formation of grain boundaries in epitaxial silicon layers

The formation of grain boundaries of the general type, along with small-and large-angle symmetric grain boundaries with the 〈 〉 axis in the epitaxial layers grown onto bicrystal substrates by the method of thermal migration has been studied. The solvent was aluminum. It is shown that if the grain boundaries in the epitaxial layer are tilted to the crystallization front or if there is a temperature gradient tangential to this front, their orientation differs from their orientation in the substrate. The large-angle symmetric boundaries are more stable than the boundaries of the general type. The grain-boundary energy and rotation moment of large-angle symmetric boundaries are evaluated.     

Electron and hole transport in microcrystalline silicon solar cells studied by time-of-flight photocurrent spectroscopy

A photocurrent time-of-flight study of carrier transport in microcrystalline silicon pin diodes prepared over a range of crystallinities is presented. Electron and hole drift mobilities at a crystalline volume fraction >0.35 are typically 3.8 and 1.3 cm²/(V s) respectively at 300 K and a thickness to electric field ratio of 1.8 × 10⁻⁷ cm²/V. A factor of five enhancement in hole mobility over amorphous silicon persists at a crystalline volume fraction as low as 0.1. Current decays are dispersive and mobilities are thermally activated, although detailed field-dependence is still under investigation. Evidence for a sharp fall in the density of states at 0.13 eV above the valence band edge is presented. Similarities in behaviour with certain amorphous and polymorphous silicon samples are identified.     

Some new results on transport and density of state distribution in glow discharge microcrystalline silicon

The electronic properties of the material are discussed on the basis of the grain boundary trapping model proposed for polycrystalline Si. In spite of the difference in crystallite sizes it is shown that the model is applicable, leading to 10¹¹ cm^?2 filled states between crystallites. Gap state densities between 2 × 10¹⁸ and 4 × 10¹⁸cm^?3eV^?1 are deduced from field effect measurements.     

Microscopic study of the H2O vapor treatment of the silicon grain boundaries

We have proposed annealing in H₂O vapor as a new effective and low-cost method for passivating polycrystalline silicon grain boundaries and for improving the performance of poly-Si based devices. The effect of H₂O vapor treatment was experimentally found to differ from analogous anneal in nitrogen and hydrogen. Mechanism of the H₂O vapor treatment was studied by Kelvin force microscopy, used to measure the potential change at individual grain boundaries and point defects. The potential change was dependent on the grain boundary character and it correlated with the crystalline disorder and internal stress observed by microscopic Raman spectroscopy. Effect of H₂O vapor passivation was experimentally connected with the potential change at the grain boundaries before and after the treatment.     

Metastable dark and photoconductive properties of microcrystalline silicon

Metastable changes in the dark conductivity of microcrystalline silicon upon heat treatment at different temperatures obey the Meyer–Neldel rule. Dark conductivity variations are accompanied by changes in the photoconductivity or the majority-carrier mobility-lifetime product. The minority-carrier mobility-lifetime product is not affected. The observations can be related to Fermi-level induced changes of the excess carrier lifetimes.     

Dislocations and grain boundaries in semiconducting rubrene single-crystals

Assessing the fundamental limits of the charge carrier mobilities in organic semiconductors is important for the development of organic electronics. Although devices such as organic field effect transistors (OFETs), organic thin film transistors (OTFTs) and organic light emitting diodes (OLEDs) are already used in commercial applications, a complete understanding of the ultimate limitations of performance and stability in these devices is still lacking at this time. Crucial to the determination of electronic properties in organic semiconductors is the ability to grow ultra-pure, fully ordered molecular crystals for measurements of intrinsic charge transport. Likewise, sensitive tools are needed to evaluate crystalline quality. We present a high-resolution X-ray diffraction and X-ray topography analysis of single-crystals of rubrene that are of the quality being reported to show mobilities as high as amorphous silicon. We show that dislocations and grain boundaries, which may limit charge transfer, are prominent in these crystals.     

Characterization of microcrystalline silicon by Hall and photo-Hall measurements

Transport properties of microcrystalline silicon are studied by Hall and photo-Hall measurements. The temperature dependence of the mobility shows that there exist potential barriers for majority carrirs to jump over. Illumination increases the majority carrier mobility while the minority carrier mobility turns out to be very small (<0.1 cm²/Vs) compared with the majority carrier mobility (5–30 cm²/Vs).     

Effect of thermal annealing and hydrogen-plasma treatment in boron-doped microcrystalline silicon

We have investigated the effects of temperature (during film growth and post-deposition thermal annealing) and H₂-plasma treatment on the electronic and structural properties of p-type microcrystalline silicon films (p-μc-Si:H) for solar cell applications. The highest dark conductivity is obtained in the thermally annealed p-μc-Si:H prepared at low substrate temperature of 50 °C. This dark conductivity is decreased by two orders of magnitude when the film is exposed to H₂-plasma, being completely restored after thermal annealing. Namely, reversible dual-conductivity cycle is observed between thermally annealed state and H₂-plasma-treated state in p-μc-Si:H. The dual-conductivity cycle is accompanied with the reversible change in the infrared-absorption spectrum at around 1845 cm^? 1 assigned as SiHB complex in p-μc-Si:H network structure. Taking into account of the reversible structural change by H₂-plasma-exposure and thermal-annealing cycles, necessary process-procedure condition has been proposed for obtaining high photovoltaic performance in thin-film-Si solar cells with high quality p-μc-Si:H.     

Argon-plasma-induced growth of crystalline grains in microcrystalline silicon: Formation mechanism of grains

A thick (∼300 nm) microcrystalline silicon (μc-Si:H) film with a low crystalline volume fraction (∼24%) and a columnar grain size of about 100 nm was exposed to an argon plasma at a substrate temperature of 220 °C after deposition. It is shown that argon plasma treatment significantly enhances film-crystallinity throughout the μc-Si:H layer: over a factor of 2 in crystalline fraction and by a factor of 3 in columnar grain size after a 90-min argon treatment. Based on these experimental results, it is proposed that crystallization of μc-Si:H is likely mediated by the energy transferred from energetic argon atoms.     

Determination of Raman emission cross-section ratio in hydrogenated microcrystalline silicon

The determination of the crystalline volume fraction from the Raman spectra of microcrystalline silicon involves the knowledge of a material parameter called the Raman emission cross-section ratio y. This value is still debated in the literature. In the present work, the determination of y has been carried out on the basis of quantitative analysis of medium-resolution transmission electron microscopy (TEM) micrographs performed on one layer deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) close to the amorphous/microcrystalline transition. Subsequent comparison of these data with the crystallinity as evaluated from measured Raman spectra yields a surprisingly high value of y = 1.7. This result is discussed in relation to previously published values (that range from 0.1 to 0.9).     

EPR Studies of defects in silicon

Electron and neutron irradiated, plastically deformed silicon has been studied by EPR. Irradiation resulted in a strong decrease of the EPR spectra produced by the deformation and in the appearance of a new EPR center at dislocations. Thermal annealing lead to a reestablishment of the formerly present dislocation spectra. As most probable explanation is proposed a change of the dislocation strain field by irradiation induced point defect iclusters. – EPR investigations of iron in boron doped Si demonstrated the pairing of all nterstitial iron in FeB pairs during room temperature storage. Thermal treatment at temperatures higher than 100°C lead to the reappearance of Fe. Therefore the time limiting factor for the annealing of FeB pairs should be the precipitation of interstitial iron.     

Preparation and properties of microcrystalline silicon films using photochemical vapor deposition

microcrystalline silicon films have been prepared through mercury photosensitized decomposition of monosilane at low gas pressures. The dark and light conductivities of the silicon films tend to increase at reactant pressures lower than 65 Pa and become 10^?2Ω^?1· cm^?1 at 26 Pa. From the Raman scattering and x-ray diffraction, silicon films were found to consist of a mixed phase structure including both microcrystalline and amorphous regions.     

Raman scattering in low wavenumber region as a new probe to structural properties of microcrystalline silicon

The structural properties of microcrystalline silicon (μc-Si) are studied by Raman scattering. It is found that the intensity of each Raman band closely correlates with the absorption coefficient in the interband region and that the Raman band at ca. 150 cm^?1 is a sensitive probe to randomness of Si-Si bonding structure in μc-Si.     

Pressure assisted evolution of defects in silicon

The effect of enhanced hydrostatic pressure following heat treatment on the evolution of point defects in neutron‐irradiated Czochralski‐grown silicon is investigated using infrared spectroscopy. The behavior of oxygen‐related defects, particularly of the VO and the VO₂ centers, is mainly studied using samples subjected to heat treatment under hydrostatic pressure. It is observed that (1) pressure accelerates the annealing process of the VO defects and enhances the growth of the VO₂ complexes and (2) the VO₂ concentration is larger than expected from the corresponding decay of the VO defects. The faster decay of the VO defects is attributed to a pressure‐induced decrease of their migration energy. The larger VO₂ concentration is also discussed. One possible explanation is that pressure stimulates an additional mechanism for the formation of the VO₂ defects, which involves the reaction of oxygen dimers with vacancies. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transformation of microcrystalline state of hydrogenated silicon to amorphous one due to presence of more electronegative impurities

When a slight fraction (～ 2 mol %) of N₂ is added into H₂, sputtering atmosphere for microcrystalline hydrogenated Si films, without changing other fabrication parameters, the amorphous film rather than microcrystalline one forms. The stabilization mechanism of the amorphous film of Si is discussed through TEM and IR observations of this kind of transformation from microcrystal to amorphous state.