Karolin Jiptner

Grain boundary interactions in multicrystalline silicon grown from small randomly oriented seeds

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.

Advantage in solar cell efficiency of high-quality seed cast mono Si ingot

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.

Minority-carrier diffusion length, minority-carrier lifetime, and photoresponsivity of β-FeSi2 layers grown by molecular-beam epitaxy

We have epitaxially grown undoped β-FeSi2 films on Si(111) substrates via atomic-hydrogen-assisted molecular-beam epitaxy. β-FeSi2 films grown 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 films grown 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...

Effect of Crystallinity on Residual Strain Distribution in Cast-Grown Si

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.

Characterization of Residual Strain in Si Ingots Grown by the Seed-Cast Method

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.

10 cm diameter mono cast Si growth and its characterization

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.

50 cm Size Seed Cast Si Ingot Growth and its Characterization

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.

Numerical analysis of dislocation density and residual strain in multicrystalline silicon for solar cells using experimental verification

A three-dimensional (3D) Haasen-Alexander-Sumino model (HAS model) has been developed to study the dislocation density and residual strain in the crystal. We compared the difference between calculation results and experimental results. The results show that the HAS model can evaluate the dislocation density and residual strain in the crystal semiquantitatively. And the residual strain for a multicrystal is lower than that for a mono-like crystal. In the case of mono-like crystal, the dislocation generation is small and the thermal stress cannot relax easily, then residual strain is high. In the case of the multicrystal, the dislocation generation is large and there are so many grains, then the thermal stress can relax easily and residual strain is low.

Statistical Consideration of Grain Growth Mechanism of Multicrystalline Si by One-Directional Solidification Technique

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.

Orientation Dependency of Dislocation Generation in Si Growth Process

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.