Investigation of dopant segregation induced surface-fields using THz pulse transmission

Doping layers like the emitter or a surface field in silicon solar cells have to fulfill two important functions: They have to allow ohmic contact formation and they should also suppress Auger and surface recombination effects. The latter can be supported by an electrical field, provided by the layer itself. In the case of an emitter this is provided by a peak doping concentration beneath the surface, like a segregation layer, formed during a silicidation process. Layers, like front surface fields applied in rear emitter concepts have a significant influence on the performance of the resulting solar cell, which is very sensitive for process fluctuations. For all these reasons, it is crucial to have an appropriate tool for analyzing these layers in terms of the resulting lifetime. Current established measurement methods are not able to provide the sufficient sensitivity, to carry out further investigations. In this work a method, based on THz transmission through the silicon material is presented to analyze these layers. Corresponding measurements were carried out with two kind of samples: a sample containing a front surface field and a sample with a dopant segregation layer. Depth dependent THz measurements were carried out, by etching the surface layers back. It is shown in this work, that the segregation layer has a large influence on the resulting effective excess carrier density or lifetime, respectively of the material.