Detection of CrB in silicon

The determination of the chromium concentration is very important, since chromium is one of the most abundant and also most detrimental defects in silicon. Hence it is necessary to measure the chromium density as exactly and fast as possible. The MDPmap series can be equipped with a heated chuck with which it is possible to measure the chromium concentration in wafers fully automated and with a very high resolution.

Lifetime measurements before and after chromium boron pair dissociation is a widely used method for chromium determination in silicon wafers.

In boron doped silicon with a high doping concentration, as it is used for PV applications nearly 100 % of the electrical active chromium is present as CrB pairs. With light of sufficient energy these pairs can be dissociated in Cri and B. This process is reversible and after some time all CrB pairs are associated again, which takes much longer as for FeB pairs. CrB and Cri have different recombination properties, so that the dissociation has an effect on the measured lifetime. With this effect the chromium concentration can be determined via:

For the chromium determination a calibration factor C is used, which depends on the injection and doping concentration. With the MDPmap and the heated sample stage a determination of the chromium concentration is possible for mc- and mono-Si with a high resolution.