Fraunhofer THM

Freiberg, Germany

Fraunhofer THM

Freiberg, Germany
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Kupka I.,Fraunhofer THM | Lehmann T.,Fraunhofer THM | Trempa M.,Fraunhofer Institute for Integrated Systems and Device Technology | Kranert C.,Fraunhofer THM | And 4 more authors.
Journal of Crystal Growth | Year: 2017

The grain structure of high-performance (HP) multicrystalline silicon (mc-Si) is characterized by a small initial grain size with randomly oriented grains and a high length fraction of random grain boundaries. However, the remaining unmelted feedstock at the ingot bottom used as seeding layer for achieving the HP mc-Si properties in the standard crystallization procedure causes yield loss. To overcome this disadvantage, the influence of wetting angle, and surface roughness of non-Si nucleation layers at the crucible bottom on the grain structure properties of mc-Si ingots with a weight of 14.5 kg was investigated and compared to classical HP mc-Si. For that purpose, SiC and SiO2 nucleation layers realized by spraying and embedding of particles with different sizes resulting in different surface morphologies and wetting angles were studied. Nucleation on rough layers of both materials with a root mean square roughness value greater than 100 µm yielded an initially fine grain structure comparable to HP mc-Si. This did not necessarily result in a random orientation distribution and high length fraction of random grain boundaries. Nucleation on SiC layers caused random grain boundary length fractions between 20 and 30% and non-uniform grain distributions. But, nucleation on SiO2 layers yielded increased random grain boundary length fractions between 50 and 70% and homogenous grain distributions, both values are similar to HP mc-Si. These differences are discussed in terms of the thermal conductivity of the different nucleation layers. © 2017 Elsevier B.V.

Karzel P.,University of Konstanz | Ackermann M.,University of Konstanz | Groner L.,University of Konstanz | Reimann C.,Fraunhofer Institute for Integrated Systems and Device Technology | And 5 more authors.
Journal of Applied Physics | Year: 2013

This investigation analyzes the dependency of minority charge carrier lifetime values at grain boundaries in multicrystalline silicon on the grain boundary type after P gettering and/or firing of SiNx:H layers deposited by plasma enhanced chemical vapor deposition. To get a broad statistics, a new method to determine the coincidence site lattice grain boundary types on large scale throughout entire 50 × 50 mm2 samples is combined with spatially resolved lifetime-calibrated photoluminescence measurements and mappings of the interstitial iron concentration. As an evaluation of the lifetime data at grain boundaries in comparison to the recombination activity of the bordering grains, lifetime contrast values are calculated. The correlation of this dependency on the grain boundary type with the impurity concentration is analyzed by the investigation of multicrystalline samples from two different ingots grown by directional solidification with different crucible material qualities. A dependency of the efficacy of all applied processes on the grain boundary type is shown based on broad statistics-higher coincidence site lattice indexes correlate with a decrease of median lifetime values after all processes. Hydrogenation of both grains and grain boundaries is found to be more effective in cleaner samples. Extended getter sinks, as a P emitter, are also beneficial to the efficacy of hydrogenation. The lifetime contrast values are dependent on the degree of contamination of the multicrystalline silicon material. In cleaner samples, they rather decrease after the processes; in standard solar-grade material, they increase after POCl3 diffusion and decrease again after subsequent hydrogenation. No correlation with the interstitial iron concentration is found. © 2013 AIP Publishing LLC.

Stockmeier L.,Fraunhofer Institute for Integrated Systems and Device Technology | Muller G.,Fraunhofer Institute for Integrated Systems and Device Technology | Seidl A.,Schott AG | Lehmann T.,Fraunhofer THM | And 2 more authors.
Journal of Crystal Growth | Year: 2014

Silicon ribbons for photovoltaic applications grown under typical industrial processing conditions by the String Ribbon and the Edge-defined Film-fed Growth (EFG) methods were quantitatively analyzed by newly developed scanning technologies with respect to the grain structure and orientation. As a result the grain structure consists typically of elongated grains with a 〈2 1 1〉 orientation nearly parallel to the growth direction and a {1 1 0} ribbon surface. These grains are mainly separated by Σ3 twin boundaries which are nearly perpendicular to the {1 1 0} ribbon surface. This result is found to be independent from the orientation of seed crystals and is in agreement with earlier studies on silicon ribbon growth. The experimental observations will be explained by a growth model which considers the surface energies of the growing grains and the need for undercooling in front of the phase boundary. © 2014 Elsevier B.V.

Stockmeier L.,Fraunhofer THM | Elsayed M.,Martin Luther University of Halle Wittenberg | Elsayed M.,Minia University | Krause-Rehberg R.,Martin Luther University of Halle Wittenberg | And 4 more authors.
Solid State Phenomena | Year: 2016

To determine the electrically inactive fraction of As or P in heavily doped as-grown Czochralski Si4-point resistivity and SIMS measurements were carried out. No clear trend for the electricalinactive fraction was found with an increasing dopant concentration, though a mean electricalinactive fraction of 11.5% for As doping could be determined.Experimental results on a dopant-vacancy complex in as-grown Si are scarce, hence temperature dependentpositron annihilation lifetime spectroscopy (PALS) was carried out on several heavily Asand P doped as-grown Si samples. The measured average positron annihilation lifetime τav isbetween 218 ps and 220 ps. No temperature dependent effect on τav could be observed. Therefore, itcan be concluded that in the studied doping range the dopant-vacancy complexes do not exist. Thereason for the inactivation of the dopant has to be found elsewhere. A possible explanation can bethe formation of dopant precipitates. © (2016) Trans Tech Publications, Switzerland.

Buchwald R.,Fraunhofer THM | Frohlich K.,Fraunhofer THM | Wurzner S.,Fraunhofer THM | Lehmann T.,Fraunhofer THM | And 2 more authors.
Energy Procedia | Year: 2013

The usage of diamond-plated wire to produce silicon wafers for the photovoltaic industry is still a new and highly investigated wafering technology. The requirements regarding the quality of the wafer surface are very high and they have to compete with the cost effectiveness and quality of wafers produced by the established loose abrasive sawing technology. Hence, the wafer topography, the fracture stress and the corresponding sub-surface damage have to be investigated and improved. This paper discusses the topographic parameters, the crack depths and the fracture stress of mono- and multi-crystalline silicon wafers that were produced on multi-wire saws using diamond-plated wire and comparable process parameters. Especially multi-crystalline silicon (mc-Si) wafers exhibit lower fracture stress values compared to mono-crystalline silicon (cz-Si) wafers. We investigated the relations between crack depth and fracture stress. In detail, we determined a 15% higher median and a 40% increased interquartile range of the crack depth of mc-Si wafers in comparison to similar produced cz-Si wafers. That correlates with lower fracture stress values of textured mc-Si wafers compared to cz-Si wafers. In the following, we studied the sub-surface damage as a function of crystal orientation in detail. It was found that the crack depths increases from the {100} plane over the {111} plane to the {101} plane. However for the {101} plane two grains were investigated, resulting in a discrepancy of 4 μm. This may be related to the unknown rotation angle between the corresponding {111} cleavage planes and the wire direction and requires further investigations. © 2013 The Authors.

Dadzis K.,Solar World Innovations GmbH | Niemietz K.,TU Bergakademie Freiberg | Patzold O.,TU Bergakademie Freiberg | Wunderwald U.,Fraunhofer THM | And 2 more authors.
Journal of Crystal Growth | Year: 2013

A new experimental setup containing a GaInSn melt with a square horizontal cross section of 10×10 cm2 and a variable melt height up to 10 cm has been developed. The melt is positioned in the center of a coil system generating a traveling magnetic field (TMF). Using a cooling system at the bottom and a heating system at the top of the melt, a vertical temperature difference up to approximately 50 K can be applied to the melt, imitating the thermal conditions during the directional solidification of multicrystalline silicon. Direct measurements of the time-dependent velocity and the temperature profiles were performed using ultrasonic Doppler velocimetry and thermocouples, respectively. Complementary three-dimensional (3D) numerical simulations of the model experiments were used to validate the numerical tools and to gain a deeper insight into the characteristics of TMF flows in square melts. The classical toroidal flow structure known from isothermal cylindrical melts is shown to obtain a large horizontal central vortex at a small height of the square melt, whereas a distinct 3D asymmetry appears at a large height. A vertical temperature gradient tends to suppress the vertical melt motion and leads to new complex horizontal flow structures. © 2013 Elsevier B.V.

Dadzis K.,Solar World Innovations GmbH | Ehrig J.,TU Bergakademie Freiberg | Niemietz K.,TU Bergakademie Freiberg | Patzold O.,TU Bergakademie Freiberg | And 3 more authors.
Journal of Crystal Growth | Year: 2011

Low-temperature model experiments and 3D, time-dependent flow simulations with relevance to the melt motion during directional solidification of multicrystalline silicon under a traveling magnetic field are presented. The influence of the inductor current, the relative inductormelt position, and the melt height on the flow pattern and velocity is studied in a square shaped GaInSn melt. Numerical simulations show a good agreement with measurements of the flow velocity by the ultrasonic Doppler velocimetry method. The toroidal flow structure already known from cylindrical melts is observed for a large parameter range. However, at small melt heights, the 3D melt geometry leads to a new flow pattern with a central horizontal vortex. The results obtained from the model experiments are transferred to silicon solidification processes using the proposed scaling laws. © 2011 Elsevier B.V. All rights reserved.

Reimann C.,Fraunhofer Institute for Integrated Systems and Device Technology | Trempa M.,Fraunhofer Institute for Integrated Systems and Device Technology | Lehmann T.,Fraunhofer THM | Rosshirt K.,Fraunhofer Institute for Integrated Systems and Device Technology | And 4 more authors.
Journal of Crystal Growth | Year: 2016

Different silicon feedstock materials, Single Crystalline Crushed (SCS), Fluidized-Bed-Reactor (FBR) and Siemens (SIE) feedstock, were used as seeding layer for growing cylindrical shaped, high performance multi-crystalline ingots with a weight of 1.2 kg. Within the investigations a systematic variation of the particle size of the seeding material in the range of <1 mm up to 15 mm was performed. Grain size, grain orientation, and grain boundary type were evaluated at different ingot heights. These results show clearly, that the microstructure size, respectively the particle size for the crushed single crystalline material, determines the resulting grain structure in the ingot near the seeding position. If the microstructure size is equal to the particle size, as it is the case for the SCS material, the particle size has a significant influence on grain size, grain orientation, and grain boundary distribution. With increasing average particle size of the SCS seed material the grain size increases, the grain orientation distribution becomes less uniform, and the random grain boundary length fraction decreases. If the microstructure size is smaller than the particle size, as it is the case for FBR and SIE feedstock materials, the particle size has no influence on the initial grain structure of the ingot. For FBR and SIE seeding material, small grains, with a homogeneous orientation distribution and a high random grain boundary length fraction are obtained. Therefore, all FBR and all SIE seeding materials, as well as the SCS with particle size <1 mm, show lowest fractions of defected areas at about the same level which were determined by etch pit analysis. © 2015 Elsevier B.V.

Friedrich J.,Fraunhofer THM | Friedrich J.,Fraunhofer Institute for Integrated Systems and Device Technology | Stockmeier L.,Fraunhofer THM | Muller G.,Fraunhofer Institute for Integrated Systems and Device Technology
Acta Physica Polonica A | Year: 2013

This study analyses the phenomenon of constitutional supercooling, which is one of the major problems in industrial growth of heavily doped (> 10 20 atoms/cm3) silicon crystals by the Czochralski technique. The systematic study is based on theoretical models and experimental data considering the effect of three important dopants (B, P, and As) in dependence of the relevant growth parameters for the Czochralski process. Based on these results, conclusions will be drawn for the stability limits of the Czochralski growth of dislocation-free heavily doped silicon crystals in dependence of the doping species and their concentration.

Wurzner S.,Fraunhofer THM | Falke A.,Fraunhofer THM | Buchwald R.,Fraunhofer THM | Moller H.J.,Fraunhofer THM
Energy Procedia | Year: 2015

The sawing of silicon wafers with diamond coated wires still requires further development for a widespread application in the photovoltaic industry. The technique has the potential for a cost reduction due to higher cutting rates and the use of water as a low-cost cooling fluid, but it is also necessary to integrate the technique into the established processing chains particularly for sawing multicrystalline silicon (mc-Si). One of the requirements is an increasing industrial demand on the wafer surface quality, such as the optical appearance, the total thickness variations (TTV), the etching behavior and the sub-surface and surface damage, which determines the mechanical wafer stability. The goal of this work is to analyze the impact of different wire velocities on the surface damage of multicrystalline silicon wafers. First, the distribution of amorphous regions was measured using Raman microscopy. The results reveal slightly higher local fractions of the amorphous phase with increasing wire velocity. This also correlates with more scattering and higher inhomogeneity in the surface roughness values. Furthermore, the microcrack depths were analyzed on polished and etched bevel cut samples of wafers using confocal laser scanning microscopy (CLSM). Additionally, the present study investigates the impact of cleaning procedures and different grain orientations on the sawing damage characteristics. © 2015 The Authors.

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