The dopant concentration dependence of the zero phonon lines of the nitrogen bound excitons in n-type (N) 6H-SiC in high resolution low temperature photoluminescence spectra are investigated. The asymmetric line broadening of the zero phonon lines is interpreted in terms of overlapping wave functions. From the change of the ratio of the hexagonal zero phonon lines to the cubic zero phonon lines as a function of charge carrier concentration the capture cross sections of neutral nitrogen donors for excitons were determined to be (44.5 Å)2π and (33.5 Å)2π on hexagonal and cubic lattice site, respectively.