Characterization and formation of NV centers in $3C, 4H$, and $6H$ SiC: An ab initio study

Fluorescent paramagnetic defects in solids have become attractive systems for quantum information processing in recent years. One of the leading contenders is the negatively charged nitrogen-vacancy (NV) defect in diamond with visible emission, but an alternative solution in a technologically mature host is an immediate quest for many applications in this field. It has been recently found that various polytypes of silicon carbide (SiC), that are standard semiconductors with wafer scale technology, can host a NV defect that could be an alternative qubit candidate with emission in the near infrared region. However, there is much less known about this defect than its counterpart in diamond. The inequivalent sites within a polytype and the polytype variations offer a family of NV defects. However, there is an insufficient knowledge on the magneto-optical properties of these configurations. Here we carry out density functional theory calculations, in order to characterize the numerous forms of NV defects in the most common polytypes of SiC including $3C, 4H$, and $6H$, and we also provide new experimental data in $4H$ SiC. Our calculations mediate the identification of individual NV qubits in SiC polytypes. In addition, we discuss the formation of NV defects in SiC, providing detailed ionization energies of NV defects in SiC, which reveals the critical optical excitation energies for ionizing these qubits in SiC. Our calculations unravel the challenges to produce NV defects in SiC with a desirable spin bath.

Figure 1

Single configuration ($kk$) of ${\mathrm{N}}_{\mathrm{C}}{\mathrm{V}}_{\text{Si}}$ defect in $3C$ SiC.

Figure 2

Possible configurations of ${\mathrm{N}}_{\mathrm{C}}{\mathrm{V}}_{\text{Si}}$ defect in $4H$ SiC. The $hh$ and $kk$ configurations exhibiting $C_{\text{3v}}$ symmetry are called on-axis configurations, since the axis of defects is parallel to the $c$ axis, while $kh$ and $hk$ configurations are off-axis geometries with $C_{\text{1h}}$ symmetry.

Figure 3

Possible configurations of ${\mathrm{N}}_{\mathrm{C}}{\mathrm{V}}_{\text{Si}}$ defect structure in $6H$ SiC. Three on-axis ($hh, k_1{k}_1, k_2{k}_2$) and three off-axis or basal configurations ($h{k}_1, k_1{k}_2, k_{2}h$) can be formed.

Figure 4

Scheme of ground-state electronic structure of (a) on-axis and (b) off-axis NV center configurations exhibiting $C_{\text{3v}}$ and $C_{\text{1h}}$ symmetry, respectively.

Figure 5

Formation energies of the NV defects as a function of the position of the Fermi level in (a) $3C$, (b) $6H$, and (c) $4H$ polytypes. In $4H$ SiC we plot the formation energy of ${\mathrm{V}}_{\mathrm{C}}{\mathrm{V}}_{\text{Si}}$ divacancy.

Figure 6

Isosurface of the spin density localized on the three C atoms near the C vacancy in the triplet ground state of the NV center in SiC. The supercell structure is shown in perspective view where the lattice is depicted as a wire except for the atoms in the core of the defect that are represented by balls. The corresponding atom types are labeled.

Figure 7

(a) EPR spectrum of the NV center in $4H$ SiC displaying the two axial and two basal related EPR spectra for $\mathbf{B}\parallel c$ where $\mathbf{B}$ is the applied external magnetic field and $c$ is the $c$ axis of $4H$ SiC. (b) Simulated angular variation of the basal NV centers in $4H$ SiC.

Figure 8

Formation energies of axial (a)–(c) NV centers and (d)–(f) divacancies as a function of the Fermi level in $3C$ and $4H$ polytypes. Shaded areas represent the stability of the corresponding complexes.

Figure 9

Concentration of technologically important defects in $4H$ SiC as a function of growth temperature. All the relevant charge states and configurations of each defect are considered. The concentration was calculated in thermal equilibrium assuming stoichiometric SiC. Please note the logarithmic scale on the concentration.

Figure 10

Binding energies as a function of Fermi level for axial (a) NV center and (b) divacancy configurations in $3C$ and $4H$ SiC. Dashed lines represent the conduction-band minimum of the $3C$ polytype that lies lower than that of $4H$ SiC.