Grain Boundaries In Silicon Research Articles

Free surface relaxation of a semicoherent interface in an isotropic thin plate

A method leading to an explicit evaluation of the elastic field of a planar semicoherent interface placed in a thin isotropic plate is proposed. It is assumed that the interface contains a single family of straight, periodically distributed, misfit dislocations. The known properties of continuous distributions of dislocations and periodic elastic fields in an unbounded medium are used to solve the problem. Surface stresses are cancelled from the addition of appropriate fields applying to an infinite medium. Numerical applications illustrate, for a near Σ9{122} semicoherent grain boundary in silicon, the dependence of the deformation field on the plate thickness and on the orientation parameters of the interface.

Complex Nanotwin Substructure of an Asymmetric Σ9 Tilt Grain Boundary in a Silicon Polycrystal.

Grain boundaries in materials have substantial influences on device properties, for instance on mechanical stability or electronic minority carrier lifetime in multicrystalline silicon solar cells. This applies especially to asymmetric, less ordered or faceted interface portions. Here, we present the complex atomic interface structure of an asymmetric Σ9 tilt grain boundary in silicon, observed by high resolution scanning transmission electron microscopy (HR-STEM) and explained by atomistic modeling and computer simulation. Structural optimization of interface models for the asymmetric Σ9 and related symmetrical Σ9 and Σ3 tilt grain boundaries, by means of molecular-statics simulations with empirical silicon potentials in combination with first-principles calculations, results in a faceted asymmetric interface structure, whose grain-boundary energy is so low that it is likely to exist. The simulated local atomic structures match the observed HR-STEM images very well.

B21-O-13Metal silicide epilayers self-organized at grain boundaries in silicon

B21-O-13Metal silicide epilayers self-organized at grain boundaries in silicon

Ab initio and atomistic simulation of local structure and defect segregation on the tilt grain boundaries in silicon

This paper reports the results of atomistic and ab initio simulation of several different tilt grain boundaries in silicon. The boundary structures obtained with genetic algorithm turned out to have no coordination defects, i.e., all silicon atoms restored their tetrahedral coordination during the structure optimisation. That concerns previously known symmetric Σ5 (130), Σ3 (211), and Σ29 (520) boundaries and previously unknown asymmetric Σ9 (2¯55)/(2¯11), Σ3 (2¯55)/(211), and Σ13 (790)/(3 11 0) structures. An extensive study has been performed on defect segregation on the boundaries, including neutral vacancy and carbon, phosphorus, and boron impurities. A clear correlation has been revealed between the segregation energy of the defect and local geometry of the boundary site where the defect is segregated. The authors propose a simple purely geometric model for evaluating approximate segregation energies of the listed defects.

Impact of Phosphorous Gettering and Hydrogenation on the Surface Recombination Velocity of Grain Boundaries in p-Type Multicrystalline Silicon

We compare the recombination properties of a large number of grain boundaries in multicrystalline silicon wafers with different contamination levels and investigate their response to phosphorous gettering and hydrogenation. The recombination activity of a grain boundary is quantified in terms of the effective surface recombination velocity S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GB</sub> based on photoluminescence imaging and 2-D modeling of the emitted photoluminescence signal. Our results show that varying impurity levels along the ingot significantly impact the grain boundary behavior. Grain boundaries from the middle of the ingot become more recombination active after either gettering or hydrogenation alone, whereas grain boundaries from the top and bottom of the ingot have a more varied response. Hydrogenation, in general, is much more effective on gettered grain boundaries compared with as-grown grain boundaries. A close inspection of their injection dependence reveals that while some grain boundaries exhibit little injection dependence before gettering, others show a relatively large injection dependence, with their S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GB</sub> increasing as the injection level decreases. The former type tend not to be recombination active after both gettering and hydrogenation and are less likely to impact the final cell performance, in comparison with grain boundaries of the latter type.

Annealing effects on recombinative activity of nickel at direct silicon bonded interface

By performing capacitance transient analyses, the recombination activity at a (110)/(100) direct silicon bonded (DSB) interface contaminated with nickel diffused at different temperatures, as a model of grain boundaries in multicrystalline silicon, was studied. The trap level depth from the valence band, trap density of states, and hole capture cross section peaked at an annealing temperature of 300 °C. At temperatures ⩾400 °C, the hole capture cross section increased with temperature, but the density of states remained unchanged. Further, synchrotron-based X-ray analyses, microprobe X-ray fluorescence (μ-XRF), and X-ray absorption near edge structure (XANES) analyses were performed. The analysis results indicated that the chemical phase after the sample was annealed at 200 °C was a mixture of NiO and NiSi2.

Origin of recombination activity at small angle grain boundaries in multicrystalline silicon using multi-seed casting growth method

The effect of misorientation on the recombination activity of tilt small angle grain boundaries was studied by temperature-dependent electron beam induced current (EBIC) analyses of artificially induced grain boundaries in multicrystal grown by casting from multiple seeds. For small misorientation, there is no significant difference in the recombination of grain boundaries at the middle of a grown ingot, whereas moderate contamination on grain boundaries caused changes in the EBIC contrast, especially at room temperature. The EBIC contrast of moderately contaminated grain boundaries at room temperature has a peak at a misorientation of ∼3°, and for misorientation θ > 6°, the recombinative nature diminishes with increasing misorientation. The results indicate differences in the gettering ability of small angle grain boundaries.

Recombination activity of interfaces in multicrystalline silicon

The electrical activity of grain boundaries in multicrystalline silicon grown from metallurgical silicon by the Bridgman method is investigated by the method of electron-beam induced current. The main tendencies of atypical manifestation of the local electrical activity of Σ3{111} and Σ9{110} special boundaries are revealed. The structural features of the grain boundaries after selective etching and the impurity-distribution characteristics in multicrystalline silicon are determined by the methods of electron backscattering diffraction and electron-probe microanalysis.

Grain boundary segregation in multicrystalline silicon: correlative characterization by EBSD, EBIC, and atom probe tomography

AbstractThis study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron‐beam‐induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a non‐coherent (random) high‐angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high‐angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface–property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Accurate measurement of atomic segregation to grain boundaries or to planar faults by analytical transmission electron microscopy

AbstractA method of analytical transmission electron microscopy is described that has been successfully applied to study dopant segregation to inversion domain boundaries in zinc oxide, to quantify the thicknesses of sub‐nanometre thin epitaxial layers grown by molecular beam epitaxy of indium arsenide (InAs) on gallium arsenide (GaAs) or silicon/germanium on silicon and proved the absence of any gettering of As or Ga dopants at Sigma=3 {111} grain boundaries in silicon, with a precision of &lt;1 atom/nm2 in each case. Here, the case study of InAs/GaAs is reviewed in detail and the procedure for quantification of full hyperspectral data sets is explained. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Interstitial iron impurities at grain boundaries in silicon: A first-principles study

Interstitial iron impurities at grain boundaries in silicon: A first-principles study

A molecular dynamics study of tilt grain boundary resistance to slip and heat transfer in nanocrystalline silicon

We present a molecular dynamics study of grain boundary (GB) resistance to dislocation-mediated slip transfer and phonon-mediated heat transfer in nanocrystalline silicon bicrystal. Three most stable ⟨110⟩ tilt GBs in silicon are investigated. Under mechanical loading, the nucleation and growth of hexagonal-shaped shuffle dislocation loops are reproduced. The resistances of different GBs to slip transfer are quantified through their constitutive responses. Results show that the Σ3 coherent twin boundary (CTB) in silicon exhibits significantly higher resistance to dislocation motion than the Σ9 GB in glide symmetry and the Σ19 GB in mirror symmetry. The distinct GB strengths are explained by the atomistic details of the dislocation-GB interaction. Under thermal loading, based on a thermostat-induced heat pulse model, the resistances of the GBs to transient heat conduction in ballistic-diffusive regime are characterized. In contrast to the trend found in the dislocation-GB interaction in bicrystal models with different GBs, the resistances of the same three GBs to heat transfer are strikingly different. The strongest dislocation barrier Σ3 CTB is almost transparent to heat conduction, while the dislocation-permeable Σ9 and Σ19 GBs exhibit larger resistance to heat transfer. In addition, simulation results suggest that the GB thermal resistance not only depends on the GB energy but also on the detailed atomic structure along the GBs.

Large- and small-angle grain boundaries in multi-crystalline silicon and implications for the evolution of grain boundaries during crystal growth

A detailed study of twin-, large-angle and small-angle grain boundaries over a small volume of multi-crystalline silicon is presented on the basis of electron backscatter diffraction and synchrotron X-ray topography (SXRT) measurements. Identical areas (ca6 mm2) of two nearest neighbour wafers from a directional solidified Si column were analysed in order to gain information about the evolution of grain boundaries during the crystal growth process. Therefore, the emphasis was placed on the evolution of one particular grain and its neighbouring grains. In the case of `straight line' Σ3 grain boundaries, no change is observed if the twin plane corresponds to a {211} plane. Significant changes are found for non-straight line boundaries: while curved Σ3 grain boundaries are formed and eliminated very frequently, the Σ9 and Σ27agrain boundaries undergo only minor changes. By means of SXRT imaging, it is shown that the microstructure of the analysed grain exhibits numerous small-angle grain boundaries with angles in the range between 0.003 and 0.02°, corresponding to a theoretical dislocation density of the order of 107–108 cm−2. The experiments give evidence that the small-angle grain boundaries with the larger angles extend, while those with smaller angles vanish during growth.

The electric activity of special grain boundaries in multicrystalline silicon grown from metallurgical refined silicon

The properties of special grain boundaries in multicrystalline silicon (mc-Si) grown from metallurgical refined silicon by the Bridgman-Stockbarger method have been studied. The electric activity of grain boundaries was characterized by measuring the electron-beam-induced current. Structural features of the mc-Si samples were studied by scanning electron microscopy, electron-probe microanalysis, and atomic force microscopy techniques.

Grain boundary engineering for improved thin silicon photovoltaics.

In photovoltaic devices, the bulk disorder introduced by grain boundaries (GBs) in polycrystalline silicon is generally considered to be detrimental to the physical stability and electronic transport of the bulk material. However, at the extremum of disorder, amorphous silicon is known to have a beneficially increased band gap and enhanced optical absorption. This study is focused on understanding and utilizing the nature of the most commonly encountered Σ3 GBs, in an attempt to balance incorporation of the advantageous properties of amorphous silicon while avoiding the degraded electronic transport of a fully amorphous system. A combination of theoretical methods is employed to understand the impact of ordered Σ3 GBs on the material properties and full-device photovoltaic performance.

Investigations on residual strains and the cathodoluminescence and electron beam induced current signal of grain boundaries in silicon

Cathodoluminescence (CL) and electron beam induced current (EBIC) measurements were used to investigate the optical behavior and electrical activity of grain boundaries (GBs) in coarsely grained silicon. Electron backscatter diffraction (EBSD) was applied for a comprehensive characterization of the structural properties of the high angle and low angle GBs (HAGBs and LAGBs) in the sample. It was found that not only the EBIC but also the panchromatic (pan) CL contrast of Σ3 HAGBs strongly depends on the hkl-type of the boundary plane. At room temperature coherent Σ3 GBs exhibit no significant contrast in the CL or EBIC images, whereas at low temperatures the pan-CL contrast is strong. For incoherent Σ3 GBs, a strong pan-CL and EBIC contrast was observed in the entire temperature range. Only on a LAGB (misorientation angle 4.5°) CL investigations at low temperatures revealed a line with peak position at about (0.82 ± 0.01) eV, usually related to the dislocation associated D1 transition. Cross-correlation EBSD was applied to analyze the strain fields of Σ3 HAGBs as well as of the LAGB. All the components of the local strain tensors were quantitatively determined. The relationship between the extension of the strain field at the LAGB and the spatial D1 intensity distribution is discussed.

Morphological characteristics of grain boundaries in multicrystalline silicon

The structure of multicrystalline silicon and the distribution of the nonequlibrium charge-carrier lifetime over the surface and in the bulk of samples are investigated. Regular dependences of the electrical characteristics on the structure of grains and grain boundaries are established. Grain boundaries in multisilicon grown by the Bridgman-Stockbarger technique from a melt of metallurgical grade silicon are investigated. Metallographic and microscopic descriptions of grain boundaries can be used in fitting the crystallization conditions for growing multisilicon with the perfect structure for solar-power engineering.

Nickel distribution and recombination activity in as-grown and annealed multicrystalline silicon

To study the impact of annealing on the nickel distribution and recombination activity at Σ3n coincident site lattice grain boundaries (CSL-GBs) in multicrystalline silicon, synchrotron-based X-ray analysis and the electron beam induced current method were performed before and after annealing. For low Σ boundaries, the interfacial symmetry at GBs strongly affects the recombination activity and nickel segregation. High Σ (≥ 81) boundaries are always recombination-active even without nickel segregation. Therefore, nickel is not a dominant factor of recombination activity at GBs. The behaviors of GBs in relation to nickel segregation before and after annealing are found to be affected by other neighboring GBs, triple junctions, or intragrain strain defects.

Image analysis of polycrystalline solar cells and modelling of intergranular and transgranular cracking

An innovative image analysis technique is proposed to process real solar cell pictures, identify grains and grain boundaries in polycrystalline Silicon, and finally generate finite element meshes. Using a modified intrinsic cohesive zone model approach to avoid mesh dependency, nonlinear finite element simulations show how grain boundaries and Silicon bulk properties influence the crack pattern. Numerical results demonstrate a prevalence of transgranular over intergranular cracking for similar interface fracture properties of grains and grain boundaries, in general agreement with the experimental observation.

On the capability of deep level transient spectroscopy for characterizing multi-crystalline silicon

The suitability of the deep level transient spectroscopy (DLTS) technique in exploring locations with high and degraded carrier lifetimes containing grain-boundaries (GBs) in multicrystalline silicon (mc-Si) wafers was studied. The types and locations of GBs were determined in mc-Si samples by electron backscatter diffraction. Mesa-type Schottky diodes were prepared at (along) GBs and at reference, GB-free locations. Detected DLTS signals varied strongly along the same GB. Experiments with dislocation networks, model structures for GBs, showed that GB-related traps may be explored only using special arrangement of a GB and the diode contacts. Iron-related carrier traps were detected in locations with degraded carrier lifetimes. Densities of the traps for near-GB and for GB free locations were compared to the lifetime measurement results.