C. Funke

Oxygen and lattice distortions in multicrystalline silicon

Abstract Oxygen is one of the main impurities in multicrystalline silicon for photovoltaic applications. Precipitation of oxygen occurs during crystal growth and solar cell processing. It is shown that dislocations enhance the oxygen precipitation. Depending on the thermal conditions and the initial oxygen content various types of SiO 2 +precipitates and oxygen related defects are observed and investigated by fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy. The large area distribution of oxygen decorated dislocations is studied by scanning infrared microscopy (SIRM). Both inhomogeneous distributions of dislocations and oxygen precipitates occur and can lead to internal stresses. The internal stresses of multicrystalline-silicon wafers are investigated by an optical method using polarized infrared light. The results are compared with the dislocation microstructure and the oxygen distribution in wafers produced by different growth techniques.

The relationship between microstructure and dislocation density distribution in multicrystalline silicon

The investigation of the grain structure is important to understand the origin of dislocations during crystal growth of multicrystalline silicon. This paper studies the dislocation density distribution for different grain orientations that occur during crystal growth. Single grains are analyzed in detail, including their microstructure. The grain orientations are determined by means of the electron backscatter diffraction technique. The obtained information reveals grain orientations, which allow a higher number of active slip planes during crystal growth process. The number of active slip planes during solidification seems to influence the dislocation density in the final crystal.

Modeling the Tensile Strength and Crack Length of Wire-Sawn Silicon Wafers

Solar silicon wafers are mainly produced through multiwire-sawing. This sawing implies microcracks on the wafer surface, which are responsible for brittle fracture. In order to reduce the sawing-induced cracks, the wafers are damage etched after sawing. This paper develops a model for the impact of crack length manipulation on fracture stress distribution. It investigates the effect of damage-etching on the mechanical properties of solar silicon wafers. The main idea is to transform the fracture stress distribution into a crack length intensity function and to model the effect of etching in terms of crack lengths. The fracture stress distribution is determined statistically by fracture tests of wire-sawn and sawn and etched wafers. The Griffith criterion then enables the transition to crack lengths and crack length intensity functions. Two numerical parameters, called truncation parameter and scaling parameter, determine this relationship and enable a quantitative description of the effect of etching. They turn out to be dependent on etchant and geometry of load and thus tested crack population.

Growth of Silicon Carbide Filaments in Multicrystalline Silicon for Solar Cells

This work introduces two different approaches to explain the growth of silicon carbide (SiC) filaments, found in the bulk material and in grain boundaries of solar cells made from multicrystalline (mc) silicon. These filaments are responsible for ohmic shunts. The first model proposes that the SiC filaments grow at the solid-liquid interface of the mc-Si ingot, whereas the second model proposes a growth due to solid state diffusion of carbon atoms in the solid fraction of the ingot during the block-casting process. The melt interface model can explain quantitatively the observed morphologies, diameters and mean distances of SiC filaments. The modeling of the temperature- and time-dependent carbon diffusion to a grain boundary in the cooling ingot shows that solid state diffusion based on literature data is not sufficient to transport the required amount of approximately 3.4  1017 carbon atoms per cm2 to form typical SiC filaments found in grain boundaries of mc-Si for solar cells. However, possible mechanisms are discussed to explain an enhanced diffusion of carbon to the grain boundaries.

Influence of high aluminium content on the mechanical properties of directionally solidified multicrystalline silicon

The purpose of this research is the mechanical characterisation of multicrystalline silicon crystallised from silicon feedstock with a high content of aluminium for photovoltaic applications. The mechanical strength, fracture toughness and elastic modulus were measured at different positions within the multicrystalline silicon block to quantify the impact of the segregation of impurities on these mechanical properties. Aluminium segregated to the top of the block and caused extensive micro-cracking of the silicon matrix due to the thermal mismatch between silicon and the aluminium inclusions. Silicon nitride inclusions reduced the fracture toughness and caused failure by radial cracking in its surroundings due to its thermal mismatch with silicon. However, silicon carbide increased the fracture toughness and elastic modulus of silicon.

Novel combination of orientation measurements and transmission microscopy for experimental determination of grain boundary miller indices in silicon and other semiconductors

The determination of grain boundary planes in multicrystalline material has only been restricted to transmission electron microscope investigations ( Jang et al., 1992 ; Elgat et al., 1985 ) or to metallograpical investigations of the grain boundary ( Randle et al., 1993 ). The first method is expensive, and both are complex and time consuming in grain boundary preparation. This paper proposes the determination of grain boundary planes in semiconductor wafer by a combined application of Electron Back Scatter Diffraction and Infrared Transmission Microscopy. In particular, the new method is demonstrated with directional solidificated multicrystalline silicon.