Radial Resistivity Profile in High-Purity Silicon

The radial profile of ionized impurities concentration was determined in different high purity silicon wafers, used for manufacturing solid state detectors. The measurement was carried on by determining as a function of radial position the depletion voltages of surface barrier dE/dx detectors made on the samples under study and whose circular useful surfaces were concentric with the wafers. The ratio bet ween the diameter of the detectors useful surfaces and the diameter of the wafers was between 0.75 and 0.85. The point-by-point depletion voltages were measured by means of ionizing particles sent on the back side of the detectors. All the samples considered showed a maximum of ionized impurities concentration at the center of the slice and the variation between the center and the edges was of the order of 60% with respect to the maximum value. These radial non-uniformities can remarkably change the performance of detectors with respect to the uniform concentration case. Particularly in nuclear spectro metry, if possible radial variations in the depletion region depth are not taken into account, worse resolving powers and pulse height defects can result.