Electrical properties of highly nitrogen–doped synthetic single crystal diamonds grown at high pressure and temperature

Using the temperature gradient method at high pressure and temperature (TG-HPHT), we have grown 3 diamond single crystals from growth media Co–Fe–C–N and Ni–Fe–C–N with a concentration of singly substitutional nitrogen atoms (C centers) in the range (0.7–1.35) $\cdot$ 10$^{20}$cm$^{-3}$. Samples were made from 2 of them for the study of electrical properties by the Hall effect method in Van der Pau geometry. The dependences of the resistivity and the Hall coefficient on temperature are investigated, by which the temperature dependences of the concentration of free electrons and their Hall mobility are calculated. For a sample with a concentration of C-centers $\sim$10$^{20}$cm$^{-3}$, the temperature dependence of electrical conductivity was investi- gated. At $T>$ 650 K, linear sections of $\ln(\sigma)$ dependences on the inverse temperature $1/T$ are observed, on the basis of which the activation energies of conductivity 1.5–1.64 eV are determined, higher than those of previously studied samples with a lower nitrogen concentration grown by the same method. In samples with C center concentrations of 0.7$\cdot$10$^{20}$ and 1.35$\cdot$10$^{20}$cm$^{-3}$, the dependences of $\ln(n)$ on $1/T$ are linear throughout the studied temperature range. Based on them, the values of donor ionization energy 1.32, 1.53 eV, and compensation ratios equal to 25 and 45% are calculated, which are significantly higher than the values for diamonds with a lower nitrogen concentration studied before. It is assumed that the acceptors are iron atoms, complexes of iron and nitrogen atoms in the substitutional position, complexes of iron atoms with vacancies, as predicted theoretically, as well as similar impurity centers based on nickel and cobalt atoms.