Forming extremely shallow pn junctions with very low electrical resistance is becoming a large stumbling block to the continued scaling of microelectronic device performance according to Moore's Law. Manufacturing methods employ rapid thermal annealing after ion-implantation in order to increase the activation of dopants. Empirical results have shown that shorter annealing times and the millisecond time scale by flashlamp or laser methods generally improve dopant diffusion and activation behavior compared with conventional incandescent lamp annealing at 1-2 s time scales. However, an explanation for this effect has been lacking. Via mathematical modeling, we find that increasing the heating rate permits interstitial clusters with dissociation energies lower than the maximum of 3.5-3.7 eV to survive to higher temperatures. This improved survival delays the increase in Si interstitial concentrations near the top of an annealing spike, which decreases the profile spreading. In addition, we present experimental data showing that strong illumination nonthermally influences the diffusion of dopants such as arsenic and boron. Such effects can either enhance or inhibit diffusion depending upon temperature, and depend upon changes in average charge state of both lone interstitials and interstitial clusters. Illumination by incandescent lamps. flashlamps, and lasers employ light fluxes that differ by orders of magnitude, so such nonthermally stimulated diffusion can be used as an additional tool for defect engineering by suitable choice of light source.