Thermal admittance spectroscopy has been used to study the thermal activation energy of nitrogen at the hexagonal and cubic sites in 4H-SiC as function of net doping concentration. The net doping concentration of the samples, which was determined from l/C vs. V plots, ranges from 1.5x10~(14) cm~(-3) to 4x10~(18) cm~(-3). The thermal activation energy of nitrogen was determined to be E_c-0.054 eV and E-0.101 eV for nitrogen at hexagonal and cubic sites respectively for N_D-N_A<=10~(16) cm~(-3). As the free carrier concentration increases from 10~(16) cm~(-3) to 1.0 x10~(18) cm, the thermal activation energy of nitrogen at the hexagonal site decreases from 54 meV to 24 meV.AtN_D-N_A>=1.0x10~(18) cm~(-3) hopping conduction is the only conduction mechanism and has an activation energy of 3-9 meV.
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