Formation of the $Z1,2$ deep-level defects in 4H-SiC epitaxial layers: Evidence for nitrogen participation

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.