Electronic structure of the N donor center in 4H-SiC and 6H-SiC

Abstract

In this paper, we present high-frequency (95 GHz) pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements on the nitrogen (N) donor in $4H\ensuremath{-}\mathrm{SiC}$ (k site) and $6H\ensuremath{-}\mathrm{SiC}$ (h, ${k}_{1},$ and ${k}_{2}$ sites according to the accepted classification). From the isotropic (a) and anisotropic (b) hyperfine interaction of the unpaired electron spin of the donor with the ${}^{13}\mathrm{C}$ $(35{%}^{13}\mathrm{C}$-enriched samples were used) and ${}^{29}\mathrm{Si}$ nuclei, the distribution of the electronic wave function of the N donor is determined. It is found that this wave function is quite different in the two polytypes because the spin-density distribution over the ${}^{13}\mathrm{C}$ and ${}^{29}\mathrm{Si}$ nuclei differs between the $4H$ and $6H$ polytypes. A similar conclusion was derived from the EPR line broadening of the N donor in ${}^{13}\mathrm{C}$-enriched 4H- and 6H-SiC compared with nonenriched crystals. The main part of the spin density in 4H-SiC is located on the Si sublattice and the wave function contains a relatively large portion of Si p character. A tentative assignment is proposed for five ENDOR lines. For the three sites in 6H-SiC, the main part of the spin density is located on the C sublattice and the wave function is built mostly of s-like C atomic orbitals. Comparing the three sites in 6H-SiC, the h site wave function has the largest delocalization and is most isotropic. The ${k}_{2}$ site wave function is most localized and less isotropic. The largest part of the spin density is located far into the crystal. The difference in behavior of the wave function in $4H$ and $6H$ polytypes seems to find its origin in the difference in their conduction-band structure. Our results indicate that the conduction-band minima in $4H\ensuremath{-}\mathrm{SiC}$ (bottom of the conduction bands) are mainly Si-like, whereas in $6H\ensuremath{-}\mathrm{SiC}$ the conduction-band minima are mostly C-like.