PV >Applications >Contamination
Contamination

Many chemical impurities can occur in any semiconductor material.  Generally, specifications of single crystal silicon allows only small amounts of contamination, whereas specifications of multicrysalline silicon allow higher amounts of contamination.  The commonly occuring forms of contamination afffect a parameter of the silicon called lifetime (and its counterpart called diffusion length). 

The circled elements in the periodic table below, can be detected via lifetime or diffusion length measurement.  (The circled elements in this chart are the ones mentioned in the literature, and it is quite possible that contamination due to additional elements can also be detected by lifetime and diffusion length measurements.)


Lifetime is commonly measured via a technique called microwave-PCD.  This technique relies on the fact that contamination in a semiconductor creates new allowed energy states in the bandgap.  The existance of these allowed energy states increases the probability that an excess carrier in the conduction band will be able to lose the required energy and momentum and return to the valence band.  The increased probability of returning to the valence band results in shorter recombination time (lifetime).

Some heavy metal contaminants, including Fe, Cu, and Cr, can exist in more than one molecular state in silicon.  For instance, Fe can exist in p-type silicon as either interstial iron, Fei, or paired with boron atoms, Fe-B.  The different molecular states create different energy states, with different effects on lifetime and diffusion length.  If a contaminant can exist in two different molecular states and there is a unique process that transforms all of the contaminant from one state to the other – the process uniquely affecting that contaminant – then one can identify the element causing the degradation in lifetime or diffusion length.  Further, by making comparing lifetime or diffusion length measurements before and after the transformation, one can not only identify the contaminant but also quantify it.  Semilab’s Fe measurement capability is an excellent example of this.

The WT-2000PV offers lifetime measurement and mapping for PV wafers and cells.  The WT-1000 offers measurements of lifetime at a single point on a wafer, and the WT-1000b offers measurement of lifetime of large pieces of silicon, via a hand-held probe.  The WT-2000P and WT-2000D offer lifetime measurements, via line scans and maps, of blocks and ingots.