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Schönefeld, Germany

Kiessling F.-M.,Leibniz Institute for Crystal Growth | Bullesfeld F.,Schott AG | Dropka N.,Leibniz Institute for Crystal Growth | Frank-Rotsch C.,Leibniz Institute for Crystal Growth | And 2 more authors.
Journal of Crystal Growth | Year: 2012

Solar-grade boron doped silicon has been directionally solidified in a vertical gradient freeze-type furnace equipped with KRISTMAG ®- heaters to study the influence of travelling magnetic fields (TMFs) on the ingot quality. As-grown silicon ingots of 22×22×13 cm 3 in volume were cut vertically and analysed. Information was obtained on the curvature of the melt-solid interface, the grain size distribution, the content of SiC and Si 3N 4 particles and the electrical activity of defects. TMFs were used to enhance melt stirring and to control the growth interface shape. Primarily inclusion-free ingots were solidified with grains of several centimetres in size. Minority carrier lifetimes of τ=20-30 μs were measured on polished surfaces of cuts from as-grown ingots. The concentrations of carbon, oxygen and nitrogen were determined by FTIR spectroscopy to (3-4)×10 17 atoms/cm 3, (2-3)×10 17 atoms/cm 3 and (0.6-2)×10 15 atoms/cm 3, respectively. Mean etch pit densities were evaluated on vertical cuts as low as (2-3)×10 3 cm -2. © 2012 Elsevier B.V. Source


Frank-Rotsch C.,Leibniz Institute for Crystal Growth | Juda U.,Leibniz Institute for Crystal Growth | Ubbenjans B.,Institute of Electrotechnology | Rudolph P.,Crystal Technology Consulting CTC
Journal of Crystal Growth | Year: 2012

To achieve high-quality VGF-grown semiconductor single crystals of germanium a favorable nearly flat slightly convex solid/liquid interface shape is required. This has been obtained by applying a traveling magnetic field (TMF). The process was optimized by numerical simulation. For the first time a double-frequency TMF was used experimentally to control the interface shape very effectively. Due to the superposition of two independent frequencies a precise tailoring of the Lorentz force density distribution within the melt has been obtained. Besides flow pattern control and damping of convective fluctuations, the diffusion boundary layer could be reduced, that increases the morphological stability and purification effect. Additionally, to remove nearly totally the diffusion boundary layer a more effective Schlichting stream immediately at the interface was generated by ultrasonic (US) vibration. Lateral photo-voltage scanning (LPS) on longitudinal crystal cuts was used to analyze the interface curvature and micro-homogeneity. Further, the electrical properties vs. TMF and US vibration were investigated. Ga-doped crystals showed a high doping efficiency. The axial distribution of the carrier concentrations follows the regime of nearly complete melt mixing. © 2012 Elsevier B.V. Source


Dropka N.,Leibniz Institute for Crystal Growth | Frank-Rotsch C.,Leibniz Institute for Crystal Growth | Rudolph P.,Crystal Technology Consulting CTC
Journal of Crystal Growth | Year: 2013

A numerical comparison of various techniques of non-steady magnetic stirring, such as travelling (TMF), rotating (RMF), alternating (AMF) and carousel magnetic fields (CMF), during unidirectional solidification of large rectangular silicon melts of 700 kg was performed. The comparison was focused on the time required to homogenize the distribution of a point wise released tracer in the melt. All studied magnetic fields showed identical magnetic screen and interaction parameter Rω=80.3 and Nmag=7.9 x 10-6, respectively. The Lorentz forces were generated in KRISTMAG® heater magnet modules (HMM) positioned at the side or bottom of the silicon melt container. Stirring progress was studied by tracking the formation of the concentration field of the passive tracer in already fully developed melt flow. Results revealed the superiority of the CMF concept independent of the point of injection. © 2012 Elsevier B.V. All rights reserved. Source


Dropka N.,Leibniz Institute for Crystal Growth | Frank-Rotsch C.,Leibniz Institute for Crystal Growth | Rudolph P.,Crystal Technology Consulting CTC
Journal of Crystal Growth | Year: 2012

A novel electro-magnetic stirring approach using Carousel magnetic fields (CMF) generated by bottom multifunctional KRISTMAG ® heater magnets is proposed for directional solidification of large silicon ingots for photovoltaics (PV) with mass of 700 kg. 3D numerical simulations were performed for Reynolds, Rayleigh and magnetic Taylor numbers in the range from Re=7.044×10 3-2.123×10 4, Ra=2. 0083×10 9-2.0096×10 9 and Ta mag=5. 26×10 9-3.79×10 10, respectively. Stirring progress was studied by tracking the volume average velocity, vorticity and temperature in the melt using buoyancy driven flow as a benchmark. The numerical results show advantageous enhanced mixing by CMF even along the melt cross section parallel to the solidification front and axial flow intensification parallel to the growth direction, particularly in the bottom melt region without dangerous acceleration of the flow in the vicinity of the crucible side walls. © 2012 Elsevier B.V. Source

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