Detecting Dopants in Compensated Silicon

Identifying dopant species and measuring dopant concentrations is important if com­pensated silicon is to be used for solar cells. Mass spectrometry (MS) methods such as secondary ion (SIMS), glow-discharge (GDMS) and inductively coupled plasma (ICP – MS) offer the advantage of indentifying individual dopant species, and are quite robust. However, they detect the total dopant concentrations, not only the electrically active component. GDMS and ICP-MS are generally only sensitive above 1016 cm-3, while the detection limit for SIMS can extend down to 1015 cm-3 in some cases. Details about these techniques are reported in Chapter 6 (Advanced analytical techniques for solar grade feedstocks).

Very low temperature (<20 K) infrared absorption (FTIR) and photoluminescence (PL) spectra can be used to identify dopants and measure their concentrations [68, 69]. How­ever, these techniques are only applicable for dopant concentrations below 1016 cm-3, which is too low for most solar-grade silicon feedstocks. Low-temperature Hall measure­ments (between 77 K and room temperature) can also be used to identify and quantify dopants, based on the different ionizing temperatures of the various dopant species, and are suitable for dopant concentrations in the range 1015 —1017 cm-3 [50]. Finally, the total acceptor concentration NA can be determined by measuring the re-pairing rate of Fe-acceptor pairs after dissociation by illumination, using room-temperature carrier­lifetime measurements [70].