Carbon vacancy-related centers in $3C$-silicon carbide: Negative-$U$ properties and structural transformation

Combining electron paramagnetic resonance (EPR) spectroscopy and first-principles density functional theory calculations we have identified the carbon monovacancy center and a second carbon vacancy-related defect, the carbon vacancy–carbon antisite defect in $3C$-SiC. In close analogy to the vacancy in silicon, the carbon vacancy in $3C$-SiC with its four potentially equivalent silicon dangling bonds shows a strong Jahn-Teller effect, confirming previously predicted negative-$U$ properties. High-temperature annealing (above 700 °C) of electron or proton irradiated samples anneals out the primary silicon monovacancies and generates a new defect, which we assign to the carbon vacancy–carbon antisite complex. As predicted by theory, this defect is the result of a structural instability of the silicon vacancy in the positive charge state, which transforms at high temperatures according to $V_{\mathrm{Si}}{\mathrm{C}}_4\to V_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}}{\mathrm{C}}_3$. Both defects are deep donors and thus not suitable for achieving semi-insulating properties.

Figure 1

(a) EPR spectrum of the ${V_{\mathrm{C}}}^{+}$ center in electron-irradiated $3C$-SiC: (top) before excitation only the neutral ${\mathrm{N}}^{\circ }$ spectrum is observed; (bottom) photoexcitation generates the ${V_{\mathrm{C}}}^{+}$ spectrum and increases the intensity of ${\mathrm{N}}^{\circ }$; $T=4$ K and $\mathrm{B}\parallel \left[001\right]$ (b) Experimental (red squares) and simulated (red circles) angular variation of the ${V_{\mathrm{C}}}^{+}$ EPR spectrum and the isotropic spectrum of ${\mathrm{N}}^{\circ }$ (blue circles). (c) EPR spectrum for $\mathrm{B}\parallel \left[001\right]$ showing the central hyperfine interaction with ${}^{29}\mathrm{Si}$. (d) EPR spectra of the ${V_{\mathrm{C}}}^{+}$ center for three orientations of the magnetic field showing the resolved superhyperfine (shf) interaction (in G) with ${}^{29}\mathrm{Si}$; $T=4$ K.

Figure 2

Top: Crystal structure of the $V_{\mathrm{Si}}$ in Si, $V_{\mathrm{C}}$ in $3C$-SiC, and $V_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}}$ in $3C$-SiC; vacancies, light gray; Si atoms, blue; carbon atoms, red. Bottom: Atomic structure and calculated magnetization density for the ${V_{\mathrm{C}}}^{+}$ (middle) and $V_C{{\mathrm{C}}_{\mathrm{Si}}}^{+}$ (right) $S=1/2$ centers (“missing” atoms indicated by circles). For comparison, the isostructural monovacancy ${V_{\mathrm{Si}}}^{+}$ in silicon (24% larger lattice constant) is also shown (left). The Si nuclei giving rise to the experimentally resolved ${}^{29}\mathrm{Si}$ hyperfine (hf) splittings (cf. Tables 1 and 4) are indicated.

Figure 3

(a) EPR spectra of the $V_C{{\mathrm{C}}_{\mathrm{Si}}}^{+}$ center taken for three orientations of the magnetic field. (b) Angular variation of the $V_{\mathrm{C}}{{\mathrm{C}}_{\mathrm{Si}}}^{+}$ EPR spectra (red squares) in the [110] plane and its simulation (black circles) with the parameters: point symmetry $C_{3v}$; $g_{\parallel \left[111\right]}=2.0023,g_{\perp \left[111\right]}=2.0037$. (c) ${}^{29}\mathrm{Si}$ central hf interaction of the $V_{\mathrm{C}}{{\mathrm{C}}_{\mathrm{Si}}}^{+}$ center; resolved for $B\parallel \left[001\right]$ at $T=80$ K. (d) Formation energies (eV) for the $V_{\mathrm{C}}$-related defects in $3C$-SiC $(V_{\mathrm{C}}$ and $V_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}})$ in their possible charge states (given by the slope of the curves); EPR active $S=1/2$ centers identified in this work are indicated by areas shaded gray. The negative-$U$ effect for the metastable ${V_{\mathrm{C}}}^{+}$ is clearly visible (for details, see text and Table 2). For comparison, the data for the isolated silicon vacancy $V_{\mathrm{Si}}$ is also given, showing the bistable, strongly Fermi-level-dependent nature of the $(V_{\mathrm{C}}{C}_{\mathrm{Si}}\iff V_{\mathrm{Si}})$ center.