A blanket layer of silicon can be added to a silicon substrate, via a CVD process, to achieve changes in resistivity, type, or defect density (or to achieve more than one of these).  This CVD process is called epi (or expitaxial) deposition.  Part of the production process of making epi wafers is monitoring the resistivity of the epi layer.  This monitoring is done on a sample basis, or only to confirm that the epi reactor is set up properly.  When the epi layer is of opposite type to the substrate wafer, the monitoring is usually done by 4-point probe.  When the epi layer is the same type as the substrate, the monitoring is usually done by some form of CV, such as CV Schottky or Hg-probe.