  4H silicon carbide (SiC) polytype is preferred over other SiC polytypes for
high-power, high-voltage, and high-frequency applications due to its superior
electrical, thermal, and structural characteristics. In this manuscript, we
provide a comprehensive study of the spin coupling dynamic between conduction
electrons and nitrogen (N) donors in monocrystalline 4H SiC with various
concentrations of uncompensated N donors from 10^17 cm^-3 to 5x10^19 cm^-3 by
continuous wave, pulsed EPR, and microwave perturbation techniques at T=4.2-300
K. At low temperatures two triplets due to N donors in cubic (Nk) hexagonal
(Nh) positions and triplet arisen from spin-interaction between Nh and Nk were
observed in 4H SiC having Nd-Na=10^17 cm^-3. A single S-line (S=1/2) dominates
the EPR spectra in all investigated 4H SiC monocrystals at high temperatures.
It was established that this line occurs due to the exchange coupling of
localized electrons (dominate at low temperatures) and non-localized electrons
(dominate at high temperatures). The localized electrons were attributed to Nh
for Nd-Na=10^17 cm^-3 and Nk donors for Nd-Na>=5x10^18 cm^-3. We have concluded
that the conduction electrons in 4H SiC monocrystals are characterized by
gpar=2.0053(3) gper=2.0011(3) for Nd-Na<=5x10^18 cm^-3 and gpar=2.0057(3) and
gper=2.0019(3) for Nd-Na=5x10^19 cm^-3. Using the theoretical fitting of the
temperature variation of S-line EPR linewidth in 4H SiC having Nd-Na<5x10^18
cm-^3, the energy levels of 57-65 meV that correlate with the valley-orbit
splitting values for Nk donors in 4H SiC monocrystals were obtained.
