PL studies of oxygen precipitation related defects, stress relaxation related defects and doping striations in various silicon materials are presented. The sample spectrum includes a variety of dopant species, and the dopant concentration range covers several 1014 cm-3 to several 1019 cm-3. Lightly doped, precipitation-annealed polished wafers were intentionally contaminated with Fe, Ni and Cu. Several types of epi wafers based on heavily doped substrates have been investigated after full device processing. PL intensity in the investigated doping concentration range is controlled by three basic recombination mechanisms: radiative recombination competing with multi phonon Shockley-Read-Hall (SRH) and Auger recombination. SRH recombination is the major competing mechanism at low dopant concentration, and Auger recombination becomes important at increasing doping levels. Even though not yet fully understood, the PL technique applied in this study has generated practically useful results.