Analysis of copper-rich precipitates in silicon: Chemical state, gettering, and impact on multicrystalline silicon solar cell material

Abstract

In this study, synchrotron-based x-ray absorption microspectroscopy (µ-XAS) is applied to identifying the chemical states of copper-rich clusters within a variety of silicon materials, including as-grown cast multicrystalline silicon solar cell material with high oxygen concentration and other silicon materials with varying degrees of oxygen concentration and copper contamination pathways. In all samples, copper silicide (Cu 3 Si) is the only phase of copper identified. It is noted from thermodynamic considerations that unlike certain metal species, copper tends to form a silicide and not an oxidized compound because of the strong silicon-oxygen bonding energy; consequently the likelihood of encountering an oxidized copper particle in silicon is small, in agreement with experimental data. In light of these results, the effectiveness of aluminum gettering for the removal of copper from bulk silicon is quantified via x-ray fluorescence microscopy (µ-XRF), and a segregation coefficient is determined from experimental data to be at least (1-2)×10 3 . Additionally, µ-XAS data directly demonstrates that the segregation mechanism of Cu in Al is the higher solubility of Cu in the liquid phase. In light of these results, possible limitations for the complete removal of Cu from bulk mcSi are discussed.

Transition metal co-precipitation mechanisms in silicon

“…First, the chemical states of all measured precipitates are equilibrium (silicon-rich) single-metal silicides (e.g., NiSi 2 , Cu 3 Si), in agreement with established thermodynamics [9,37]. No mixed-metal silicide systems were observed.…”

“…Many examples of metal accumulation via solid-liquid segregation exist in silicon-based systems; the simplest example occurs during silicon crystal growth from silicon-rich melt solutions, when metal impurities are rejected from the solid crystal into the melt (for a review, see [44] and references therein). More sophisticated examples include segregation of impurities to liquid M-Si eutectic layers on free surfaces (e.g., Al-Si [9,45,46], Zn-Si [47], Mn-Si [48], Au-Si [49], Pt-Si [49], Co-Si [49], Pb-Si [49], Sn-Si [49] and Ni-Si [47,[49][50][51][52]) and segregation of metals to ion-implantation-induced Al-Si eutectic droplets within bulk Si [53]. The distinguishing feature of the mechanism proposed herein would be the formation of the gettering agent; i.e., precipitation of dissolved metallic impurities into liquid metal-Si droplets.…”

“…In these cases, only the portion of the X-ray absorption spectrum close to the absorption edge was obtained, a sub-technique of l-XAS known as l-XANES (X-ray absorption near-edge microspectroscopy; see Refs. [6,9] for examples).…”

“…For multicrystalline silicon (mc-Si), material out of which over 50% of solar cells worldwide are made, metal precipitation reactions are further complicated by the inhomogeneous presence of structural defects and the possible presence of up to 15 metallic impurity species in concentrations as high as 10 12 cm À3 or greater [3][4][5]. It is also commonly accepted as well that interstitially dissolved 3d transition metals in silicon (e.g., iron or copper) can, upon supersaturation, precipitate into their solid equilibrium metal silicide phase (e.g., FeSi 2 [6] or Cu 3 Si [7][8][9]). It is also fairly well established that the presence of precipitates of one metal species may aid the precipitation of another (see [10] and references therein), e.g., by providing energetically favorable nucleation sites via lattice strain or local native point defect compensation.…”

Effects of high temperature annealing on the dislocation density and electrical properties of upgraded metallurgical grade multicrystalline silicon

“…It can be concluded that the properties of Si wafers and the resulting performance of solar cells not only depends on the total concentrations of impurities, but also on the structure, chemical state and spatial distribution of the impurities in the Si wafers. For example, Fe and Cu usually precipitate as silicides (Fe 2 Si [9] and Cu 3 Si [16]) at grain boundaries or dislocations. In addition, Buonassisi et al have also reported that a large number of nanoscale precipitates formed during slow cooling from high temperature annealing that were widely and homogeneously distributed at intragranular microdefects.…”