As-grown 4H-SiC epitaxial layers were investigated by deep-level transient spectroscopy to study the formation of the well-known Z1,2 defect with energy levels normally detected at about EC-0.7 eV. Chemical vapor deposition, applying various nitrogen-doping concentrations and C/Si ratios (1.2–3) in the gas phase, was used to prepare the samples. The Z1,2 defect concentration was observed to increase with the incorporated nitrogen concentration. The dependence was linear for medium C/Si ratios (1.5–2.5). The highest and lowest applied C/Si ratios (3 and 1.2) enhanced and suppressed the Z1,2 defect formation, respectively. This behavior tentatively suggests a complex of nitrogen with interstitial carbon atoms or, less probably, silicon vacancies. In particular, the correlation between the Z1,2 defect formation and the nitrogen incorporation was clearly shown in the present investigation, in contradiction to conclusions of other authors. Previously reported negative-U properties of the Z1,2 deep-level defects could be confirmed. A 1:1 relation between the concentrations of Z1 and Z2 was obtained for the present as-grown epitaxial layers. © 2002 American Institute of Physics.