Negative-$U$ System of Carbon Vacancy in $4H$-SiC

Using electron paramagnetic resonance (EPR), energy levels of the carbon vacancy ($V_C$) in $4H$-SiC and its negative-$U$ properties have been determined. Combining EPR and deep-level transient spectroscopy we show that the two most common defects in as-grown $4H$-SiC—the $Z_{1/2}$ lifetime-limiting defect and the $E{H}_7$ deep defect—are related to the double acceptor ($2-|0$) and single donor ($0|+$) levels of $V_C$, respectively.

Figure 1

EPR spectra in $n$-type $4H$-SiC CVD layers irradiated by 250 keV electrons with different fluences (sample $E\mathrm{\text{:}}3.1\times {10}^{18}{\mathrm{cm}}^{-2}$, $C\mathrm{\text{:}}5.7\times {10}^{18}{\mathrm{cm}}^{-2}$ and $A\mathrm{\text{:}}7.5\times {10}^{18}{\mathrm{cm}}^{-2}$) measured at 100 K (a)–(b) in darkness and (c) under illumination. The dependence of the EPR intensity of $V_C^{-}(h)$ and $V_C^{-}(k)$ on the photon energy in (d) sample $E$ and (e) sample $C$. The error in determination of the energy threshold in the spectral region is within $\pm 0.01\mathrm{eV}$.

Figure 2

Scheme of energy levels of $V_C$, determined by photo-EPR. The $Z_{1/2}$ and $E{H}_7$ levels determined by DLTS [11, 14, 15] are given for comparison. Here the optical transitions obtained by photo-EPR involve a Franck-Condon shift in the range $\sim 0.03–0.3\mathrm{eV}$. For clarity, the location of the states in the scheme does not follow the relative scale and the levels determined by photo-EPR are aligned to the corresponding levels determined by DLTS the optical transitions from the ($-|0$) and ($2-|0$) states to the conduction band involve different Franck-Condon shifts for $V_C^{-}(h)$ and $V_C^{-}(k)$. The photo-EPR data for the ($0|+$) level are from Ref. 25.

Figure 3

Comparison between experimental (open squares and circles) and simulated carrier concentration profiles: (a) at 190 and 300 K, and (b) at 700 K. In the simulations, $Z_{1/2}$ and $E{H}_{6/7}$ are considered as a double acceptor and a single donor, respectively. The influence on the carrier concentration is negligible for $E{H}_{6/7}$ at low temperatures and for $Z_{1/2}$ at high temperatures.