I have multicrystalline silicon wafers (mc-Si), and I do not know their grade. Is there a simple method that can be used to classify this material? The minority carrier lifetime measurement, is it sufficient to make this classification, or I have to make further characterization such as the determination of their impurities contamination level ?
The most important difference between solar grade silicon and the metallurgical grade silicon is the purity of the material. The solar grade one is much purer than the MG. This can be directly indicated by the measuring the resistivity of the material using 4-point probe with the four points is contacted to a large crystallite
to measure the bulk resistivity of the materials.
The doping must be around 10^17/cm^3.
The other parameter is the minority carrier lifetime which may be so high that the minority carrier diffusion length in the bulk is about 200um. If it is smaller one can make tingeing process increase the minority carrier lifetime.
Best wishes
Thank you De Zekry for your answer, by the way, in my question I want to know the standard criteria used in the classification of silicon grade. I know that it depends particularly on the level of its contamination by metallic and other impurities like oxygen, but how to classify according to this level of contamination? That was the purpose of my question. For example, if I find the lifetime exceeds a given value, do I conclude that the material is SoG or I have to measure the concentration of all (or some) contaminants in the bulk. In practice, when can I say that this material is solar grade or metallurgical grade?
Dear Dr. Nabil,
Sure, you know; the grade is a purity grade. When one said solar grade silicon that is silicon with purity grade suitable as starting material for solar cells. Electronic grade silicon is also a grade of purity of silicon suitable for electronic devices and integrated circuits. Electronic grade cab be as pure as one impurity per 10^9 silicon atom. Solar grade silicon can be pure as 1ppm. The metallurgical grade Si can be as pure as 98 percent. In fact the metallurgical grade silicon was believed to be metal because of its relatively high conductivity.
The metallurgical grade silicon has to pass long and elaborate purification steps to reach the solar grade and then the electronic grade.
The different grades are now fully specified by resistivity and minimum minority lifetime as they are the staring materials for their specific applications.Therefore one has to refer to the application to get out the material properties suitable for that application.
For more information one refers to the VLSI technology
Dear Dr. Abdelhalim, thank you for your response.
I have to precise my question by giving more details; as we know, Si wafers are contaminated by several metallic impurities such as Fe, Cr, Ni, Cu, etc..., and each impurity has a specific impact on the recombination activity of charge carriers. The reason is the different capture cross-section of these impurities as well as their energy levels in the gap.
So, the effect of Ni (for exemple) on the electrical quality of material for a given concentration will not be the same with Fe in the same concentration. Fe is very harmful in p-type Si compared to Ni. In this case, I guess that one have to specify the contamination level associated to specified impurity.
When we limit the contamination level at 10^9 cm-3, is it the total concentration of all, some or just one of the contaminants ? is there a standard rule ?
Kind regards,
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