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We have developed a three dimensional Haasen-Alexander-Sumino model to investigate the distribution of dislocation density and residual strain in Si crystals and compared the calculation results with experimental data performed in mono-like and multicrystalline silicon ingots. The results show that the residual strain in a multicrystal is lower than in a mono-like crystal, whereas the dislocation density in the multicrystal is higher than that in the mono-like crystal. This phenomenon is due to the relation between dislocation density and residual strain caused by the difference of activated slip systems in a mono-like crystal and a multicrystal.

The effect of Σ3 generation on random grain boundaries (R-GBs) were investigated using multicrystalline Si (mc-Si) grown from the microcrystalline template with random orientation. There existed three cases for the contrast variation of electron-beam-induced current (EBIC) on R-GBs after Σ3 generation from them, namely decrease, increase and constant cases. No clear tendency of EBIC contrast variation was found at the initial growth stage. On the other hand, the constant case became dominant at the steady state. This result indicates that Σ3 generation does not affect the electrical activity of R-GBs.

This study concentrates on dislocation behavior during Si growth by the seed-cast method in different crystallographic orientations. Two methods were combined: (1) Si crystal growth in different seed orientations and (2) float zone Si-annealing experiment to obtain the purely thermal stress-induced dislocation density. The main focus is on the difference between the (111) and the (100) growth directions. It is found that peripheral areas are dominated by thermal stress-induced dislocation densities. Central ingot areas are dominated by other dislocation sources. By comparing the (100) and the (111) orientations, it was found that a difference in dislocation motion exists. This difference is caused by a different activation of slip systems, causing long slip lines in the (111) orientation. It is shown that numerical simulation has problems describing this long-range dislocation slip. Copyright © 2015 John Wiley & Sons, Ltd.

In an attempt to understand how and where dislocations are introduced into Si ingots by temperature gradients, bulk dislocation-free FZ crystals are exposed to temperature gradients similar to those in Bridgman Si crystal growth. This heat treatment introduces dislocations, which were analyzed using X-ray topography (XRT) and Scanning InfraRed Polariscopy (SIRP). Hereby, the orientation dependency is taken into account and ingots in (001) and (111) growth orientation are evaluated in this work. It can be found that the dislocation generation takes place at similar regions of the crystal and is independent of orientation, however, their propagation and multiplication differs. This leads to an overall different shape of the dislocation network. Especially intriguing are the long slip lines in the (111)-crystal, which cannot be found in the (001)-crystal. This suggests a different magnitude of slip propagation depending on the sample orientation. This effect should be explained by a different activation of slip systems and is discussed in the paper.

We have proposed single seed cast Si growth and developed a furnace for 50 cm square ingots. By optimizing growth parameters, improving gas condition, coating, the quality of mono Si ingot has improved. Namely, dislocation density, the concentrations of substitutional carbon and interstitial oxygen have been significantly reduced. The conversion efficiency of cast Si solar cells has become comparable with those of CZ Si wafers.

The grain evolution of multicrystalline Si was studied using the ingot grown from microcrystalline template. The grain shape evolution and width increase are not monotonic but may have 3 stages. On the other hand, the grain boundary (GB) analysis suggests that there exit 2 reactions, namely random GB annihilation at the initial stage and Σ3 generation and annihilation at the steady state.

We discuss the defect evolution in conventional cast and seed cast Si ingot growths for photovoltaic application. Three different cast Si ingots were grown in one directional solidification furnace. The two extremes are the seed cast ingots (mono-Si), where growth starts from monocrystalline silicon seeds, and the multicrystalline silicon grown from small randomly oriented grains. The conventional multicrystalline (mc-) cast Si ingots are grown without any seeds and have grain structures in between the two extremes. It was found that in mc-Si the evolution of grain boundaries take place in several steps. On the other hand, the major defects in mono-Si are dislocations and are generated by stress due to thermal gradient in the ingot. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Extended defects were controlled using polycrystalline silicon as a template for cast-growth of multicrystalline silicon. At the initial stage of growth, small randomly oriented grains with a high density of random type grain boundaries were obtained. With growth, the grain size increased and the fraction of random grain boundaries decreased. Electrical activity of defects was investigated and it was found that with growth, network of small angle grain boundaries became the most electrically active defects. This network of small angle grain boundaries were found to have a tilt angle less than 3° and were mainly found in elongating grains. The density of these highly electrically active grain boundaries increased with growth. This can be attributed to propagation and agglomeration of dislocations into small angle grain boundaries. The high density of random grain boundaries in this ingot may suppress dislocation propagation between grains, however they do not intersect elongating grains enabling dislocation propagation in elongated grains. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Applicability of the three-dimensional Alexander-Haasen (AH) model for the analysis of dislocation distributions in single-crystal silicon has been estimated. The numerical results obtained from the AH model agree well with the experimental data for both CZ-Si and FZ-Si crystals with the axis in the [001] direction but do not completely agree with the experimental data for the FZ-Si crystal with the axis in the [111] direction. The inapplicability of the AH model in a crystal with the axis in the [111] direction may arise from the neglect of dislocation propagation in this model, because the dislocation propagation in a crystal with the axis in the [111] direction is more active than that in a crystal with the axis in the [001] direction. Therefore, to increase the applicability of the AH model, it is necessary to include the effect of dislocation propagation.

The residual strain distribution in cast-grown mono-like Si ingots is analyzed. The effect of the crucible during solidification and the influence of different cooling rates is described. To clarify in which process steps residual strain accumulates, several Si ingots were grown in a laboratory scale furnace (100mm) using different cooling conditions after completion of the solidification. For the cooling, two different cooling rates were distinguished: fast cooling (12deg/min) and slow cooling (5deg/min). It was found that changes in cooling gradients greatly influence the amount of residual strain. The results show that slow cooling in any temperature range leads to strain reduction. The greatest reduction could be found when the temperature gradient was changed to slow cooling in the high temperature region.

To get the optimized condition and ideal furnace structure, we have performed seed cast growth of mono-crystalline Si by using unidirectional solidification furnace. More than 20 ingots of 10 cm diameter and 10 cm height were grown under different growth conditions. The quality of ingots was characterized by using Fourier transform infrared spectroscopy (FTIR), infrared microscopy, scanning infrared polariscope (SIRP), X-ray topography, etc. We have realized reduction of carbon, residual strain and extended defects, which may contribute the increase of solar cell efficiency.

Grain boundaries and dislocations are major crystallographic defects in multicrystalline Si materials for solar cells. Heavily dislocated grains are detrimental to the photovoltaic performance. This paper attempts to clarify the origin of inhomogeneous defect distribution in multicrystalline Si. The impacts of crystal orientation and grain boundary were investigated. The crystal orientation gives an important geometrical effect in the possibility of initiating slip in a grain when subjected to stress. The presence of grain boundary can also affect dislocation distribution depending on boundary character.

This paper concerns the precipitation mechanism of SiN x impurities in multi-crystalline silicon grown with a directional solidification system. A butterfly-shaped precipitate region was found across the middle part of ingot. These precipitates showed filament-like and fiber-like shapes of several micrometers in diameter and up to several millimeters in length. These two types of precipitates exist in different areas, forming three distribution zones in the butterfly region. The precipitate growth periodically stopped at the last stage of the butterfly formation. A non-uniform convection flow model was proposed to explain the precipitation behavior at the butterfly.

Cast-grown mono-like Si ingots for photovoltaic application are getting increasing attention due to the possibility of obtaining highly efficient solar cells at a low-cost production process. The reduction of crystallographic defects is essential to reach that potential. In this study, the residual strain and dislocation distribution in monolike ingots grown by directional solidification was experimentally determined. It was found that both strain and dislocations are mainly concentrated in the periphery of the ingots (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This study is concerned with the effect of reduced Si3N4 coating during Si growth carried out by the directional solidification cast method. Three crystals were grown and compared in terms of cracking behavior and strain distribution. It was found, that the outer areas of the grown Si ingots exhibit much higher strain than the center pieces and this corresponds with the appearance of cracks in higher density in the outer edges of the crystals. It was also found that cracks mainly propagated in vertical direction and less in horizontal direction due to the temperature gradient during growth.

a -Axis-oriented undoped n -BaSi 2 epitaxial films were grown on Si(111) substrates by molecular beam epitaxy, and the crystalline quality and grain boundaries were investigated by means of reflection high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy (TEM). The grain size of the BaSi 2 films was estimated to be approximately 0.1–0.3 μm, and straight grain boundaries (GBs) were observed in the plan-view TEM images. Dark-field TEM images under a two-beam diffraction condition showed that these GBs consist mostly of BaSi 2 {011} planes. The diffusion length of minority carriers in n -BaSi 2 was found to be approximately 10 μm by an electron-beam-induced current technique.

Multicrystalline silicon was grown from seeds with small grains of random orientation and the growth mechanism was studied with respect to grain size, shape, boundary character and orientation. The average grain size perpendicular to growth direction increased steadily initially, became constant and then increased steadily again. Grain size parallel to growth direction increased rapidly with growth due to grain elongation in the growth direction. Grain shape with respect to growth direction changed from spherical to columnar with growth. Initially non-CSL grain boundary fraction was very high but decreased with growth as the Σ3 grain boundary fraction increased. A simple model was proposed to explain the results.

We have studied the dislocation generation and propagation from the seed crystals during seed cast Si growth. The grown ingot was cut into a vertical wafer, followed by the dislocation imaging using X-ray topography and Secco etching. The dislocation behavior at the seed area was compared with the dislocation generation at the top surface due to the thermal stress during cooling. The dislocations at the seed/crystal interface have propagated on the {111} plane toward top. When the seed surface was not melted su ciently, the interface defect density became high, but no clear dislocation propagation was recognized. This suggests that the thermal shock at the seed/melt interface was not high enough to propagate dislocations to the growth direction. A certain amount of dislocations has been introduced from the top into the ingot according to the thermal stress. These observations suggest that optimizing the initial growth condition is important to dislocation control.

Grain boundaries (GBs) and crystalline quality of semiconducting BaSi 2 films grown on polycrystalline Si (pc-Si) substrates by molecular beam epitaxy were analyzed using Kelvin probe force microscopy and electron back scattered diffraction. It was found that the degree of a-axis-orientation of BaSi 2 was dependent on local crystal orientations of the pc-Si, and thus the surface electrostatic potential varied from place to place. At the GBs of Si, there was not so much difference in potential variations compared to GBs in BaSi 2 formed on single crystalline Si(111). The barrier height was approximately 20–40 meV for holes. The GBs of BaSi 2 around Σ3 GBs of Si don't work as recombination center for minority carriers.

We have grown 50 cm2 mono Si ingots by the seed cast technique. The carbon and oxygen concentrations of the ingots have been significantly reduced by improving the gas flow condition and coating. The dislocation density was also reduced by eliminating the extra dislocation generation sources. Owing to these developments, the lifetime of wafers has reached 465 µs. Finally, the efficiency of 18.7% has been achieved, which is comparable to 18.9% of the reference Czochralski (CZ) Si wafer.

Grain boundary (GB) evolution in multicrystalline silicon grown from small randomly oriented seeds was investigated by statistical analysis of GB interactions (triple junction) with respect to growth height. As grain size increased with growth, the number of GB interactions decreased. The fraction of GB annihilation interactions (which decrease the total number of GBs) is higher throughout growth and increases with growth height in comparison with that of GB generation interactions (which increase the total number of GBs). The dominant GB interaction is that involving Σ3 GBs, especially at the later stage of growth. The impact of GB interactions on grain structure evolution is also discussed.

We report the correlation between the crystallinity of ingots grown by the directional solidification technique and the residual strain and dislocation distribution. It was found that mono-like ingots have a 20–25% higher averaged residual strain than multicrystalline Si ingots grown under the same conditions. However the existence of local high-strained areas, which were frequently found in multicrystalline Si ingots, is reduced in mono-like Si ingots. In addition, a reduction in dislocation density was observed. This effect and the decrease in local high strain could be attributed to the decrease in grain boundaries in the mono-like ingots.

We have epitaxially grown undoped β-FeSi2films on Si(111) substrates via atomic-hydrogen-assisted molecular-beam epitaxy.β-FeSi2filmsgrown without atomic hydrogen exhibited p-type conduction with a hole density of over 1019 cm−3 at room temperature (RT). In contrast, those prepared with atomic hydrogen showed n-type conduction and had a residual electron density that was more than two orders of magnitude lower than the hole density of filmsgrown without atomic hydrogen (of the order of 1016 cm−3 at RT). The minority-carrier diffusion length was estimated to be approximately 16 μm using an electron-beam-induced current technique; this value is twice as large as that for β-FeSi2 prepared without atomic hydrogen. This result could be well explained in terms of the minority-carrier lifetimes measured by a microwave photoconductance decay technique. The 1/e decay time using a 904 nm laser pulse was approximately 17 μs, which is much longer than that for β-FeSi2 prepared without atomic hydrogen (3 μs). The photoresponsivity reached 13 mA/W at 1.31 μm, which is the highest value ever reported for β-FeSi2films.

The electrical properties of defects in a p-β-FeSi2/n-Si heterostructures were investigated by deep level transient spectroscopy (DLTS) and the electron-beam-induced current (EBIC) technique. DLTS revealed the presence of trap levels for holes, caused by defects in the n-Si layer near the interface during the β-FeSi2 film fabrication. The defect density became small when a 20-nm-thick β-FeSi2 template layer was grown on the n-Si prior to molecular beam epitaxy (MBE) of a 700-nm-thick β-FeSi2 layer. The diffusion length of minority carriers in the n-Si was found to be approximately 15 µm by EBIC. This is much larger than the value of approximately 3 µm for the n-Si obtained when the template layer was not inserted.