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190 SOLAR GRADE SILICON FEEDSTOCK
[°C]
1500
Atomic percentage nickel
50 40 30 20 10
1400
1412
1300
L
1200
1100
1000
900
967
q
994
982
1500
1412
(Si)
0.0010 0.0005 At %Ni
L
800
700
600
0
Ni
h
q′
1300
1100
994
982
900
0.003
(Si) + L
(Si) + q
(Si) + q′
0.002 0.001
Wt %Ni
70 60 50 40 30 20 10 Si
Weight percentage nickel
(Si)
Si
Figure 5.9 The Ni-Si phase diagram. The shape of the solidus line on the Si side is shown in
the inserted window. It is seen that the solid solubility of Ni in Si is low, that it increases at
temperatures above the peritectic point (at 994 ◦ C) (retrograde solubility) and reaches a maximum
at about 1300 ◦ C. Redrawn from ASM Handbook, Vol. 8, Metallography, Structures and Phase
Diagrams 8 th Edition, ASM International, Materials Park, Ni-Si Phase Diagram, p. 325
minority-carrier capture cross-section. Therefore, the tolerable impurity concentration for
acceptable lifetime values depends upon the chemical nature of the respective impurity,
and its carrier capture cross-section for electrons in p-type silicon and for holes in n-type
silicon. Both parameters can differ by orders of magnitude and consequently the acceptable
concentration for a defined impurity can be quite different in p- andn-type silicon
(Figures 5.11 and 5.12; [30]).
Copper and nickel have high diffusivities and low capture cross-sections. These
elements will rapidly enter a low solid solution level after cooling to room temperature
and may therefore be expected to have less effect on lifetime than elements with lower
diffusivities (Fe, Ti) and high capture cross-sections.
5.6.3.5 Precipitates
Except for copper, which forms Cu 3 Si particles when cooled, the other 3d metals
form MeSi 2 precipitates. Investigations of crystallographic structure, morphology and