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 $Z_{\text{1,2}}$ defect with energy levels normally detected at about $E_C\text{−0.7\hspace{0.17em}}\mathrm{eV}.$ 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 $Z_{\text{1,2}}$ 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 $Z_{\text{1,2}}$ 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 $Z_{\text{1,2}}$ 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 $Z_{\text{1,2}}$ deep-level defects could be confirmed. A 1:1 relation between the concentrations of $Z_1$ and $Z_2$ was obtained for the present as-grown epitaxial layers.