For the formation of ultrashallow junctions, a controlled gaseous ambient during rapid thermal annealing is indispensible. To understand the diffusion/activation mechanism, the influencing and depending variables have to be clarified precisely. Ion implantations of 1 keV boron at a fluence of Phi approximate to 1 x 10(15) cm(-2) are annealed isothermally for 10 s at 1000, 1050, and 1100 degrees C in an AST2800 epsilon rapid thermal processing system under controlled concentrations of oxygen in nitrogen ambient (0-1 ppm up to 1%). The concentration-depth profiles, measured by secondary ion mass spectroscopy, are analyzed within the framework of the kick-out model involving diffusion enhancement via supersaturation of silicon self-interstitials and the Fermi-level effect. The validity of this interpretation is supported by the simulated results which are in good agreement with experimental data. Two input parameters for the SSUPREM IV simulator yield finite values of silicon self-interstitial supersaturation as a function of temperature and oxygen concentrations, values for the boron diffusion coefficient via neutral and positively charged silicon self-interstitials, and data for transient enhanced diffusion. After rapid thermal annealing for 10 s at 1050 degrees C, the junctions vary within 800-1400 Angstrom depending on the annealing ambient.