  We present the detailed study of the spin kinetics of the nitrogen (N) donor
electrons in 6H SiC wafers grown by Lely method and by sublimation "sandwich
method" (SSM) with a donor concentration of about 10^17 cm^-3 at T = 10-40 K.
The donor electrons of the N donors substituting quasi-cubic "k1" and "k2"
sites (Nk1,k2) in both types of the samples revealed the similar temperature
dependence of the spin-lattice relaxation rate (T1^-1), which was described by
the direct one-phonon and two-phonon processes induced by the acoustic phonons
proportional to T and to T^9, respectively. The character of the temperature
dependence of the T1^-1 for the donor electrons of N substituting hexagonal
("h") site (Nh) in both types of 6H SiC samples indicates that the donor
electrons relax through the fast-relaxing centers by means of the
cross-relaxation process. The observed enhancement of the phase memory
relaxation rate (Tm^-1) with the temperature increase for the Nh donors in both
types of the samples, as well as for the Nk1,k2 donors in Lely grown 6H SiC,
was explained by the growth of the free electron concentration with the
temperature increase and their exchange scattering at the N donor centers. The
observed significant shortening of the phase memory relaxation time Tm for the
Nk1,k2 donors in the SSM grown sample with the temperature lowering is caused
by hopping motion of the electrons between the occupied and unoccupied states
of the N donors at Nh and Nk1,k2 sites. The impact of the N donor pairs,
triads, distant donor pairs formed in n-type 6H SiC wafers on the spin
relaxation times was discussed.
