Relaxation of Nitrogen Donors in Silicon Carbide at High Magnetic Fields

Nitrogen centers in silicon carbide share many of the same properties as shallow donors in silicon, like phosphorus. The situation is more complicated as silicon carbide has several different crystalline polymorphs, and polymorphs like 4H-SiC and 6H-SiC have 2 and 3 distinct nitrogen sites respectively. These two polymorphs also have a hexagonal (wurtzite) crystal structure rather than a cubic crystal structure as in the case of silicon and diamond. The nitrogen substitutional sites have S=1/2 when they trap an electron at lower temperatures. We measured the phase memory time T₂ and the spin-lattice relaxation time T₁ at frequencies of 120, 240, 316, and 395 GHz. The spin-lattice relaxation time has a strong temperature dependence mostly due to Orbach-type relaxation to the energetically nearby conduction band and valley-orbit states. We find that at the lowest temperatures the direct single phonon relaxation process becomes increasingly important with increasing frequency and field. Within the magnetic field range of 4-14 Tesla, this direct spin-lattice relaxation process has a strong field dependence (~B⁴) with several orders of magnitude change in the spin-lattice relaxation over this relatively small range in field. There are large differences also in the behavior of the different sites of the nitrogen center in 4H- and 6H-SiC, and the results will be discussed in the context of the centers' wavefunctions and possible applications for quantum technology.