The silicon vacancy $\left(V_{Si}\right)$ in $4H$-SiC at its cubic site (V2-center) has shown significant promise for quantum technologies, due to coherent spin states, the mature material system, and stable optical emission. In these SiC-based applications, doping plays a crucial role. It can be used to control the charge state of $V_{Si}$ and formation of different types of defects. Despite its importance, there has been little research on the effects of doping. In this work, we perform a study of the effects of nitrogen doping and annealing on the photoluminescence (PL), optically-detected magnetic resonance (ODMR) contrast, and dephasing times of ensembles of V2 in epilayers of $4H$-SiC. The results show an enhancement of PL that depends on the electron irradiation dose for a given electron concentration, supported by theoretical modeling of the charge state of $V_{Si}$ in the presence of nitrogen. Nitrogen substituted for carbon is shown to very efficiently donate one electron to $V_{Si}$. We also observe that the ODMR contrast can be increased from 0.5% in low doped SiC to 1.5% by nitrogen doping of ${10}^{17}$ to ${10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$ and annealing at $500-600{}^{\circ }\mathrm{C}$ for one hour, with only a 20% decrease in PL compared to unannealed. Some of the improvement in contrast is offset by a reduction in $T_2^{*}$ at these doping levels, but the estimated cw ODMR shot-noise limited sensitivity is still 1.6 times higher than that of undoped, unannealed SiC.

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• Doping effects
• Point defects
• Quantum sensing
• Silicon vacancies
• Wide band gap systems
• Density functional theory
• Optically detected magnetic resonance