Variable-range hopping conduction in epitaxial CrN(001)

Epitaxial CrN(001) layers, grown by dc magnetron sputtering on MgO(001) substrates at growth temperatures T-s = 550-850 degrees C, exhibit electronic transport that is dominated by variable-range hopping (VRH) at temperatures < 120 K. A transition from Efros-Shklovskii to Mott VRH at 30 +/- 10 K is well described by a universal scaling relation. The localization length decreases from 1.3 nm at T-s = 550 degrees C to 0.9 nm for T-s = 600-750 degrees C, but increases again to 1.9 nm for T-s = 800-850 degrees C, which is attributed to changes in the density of localized states associated with N vacancies that form due to kinetic barriers for incorporation and enhanced desorption at low and high T-s, respectively. The low-temperature transport data provide lower limits for the CrN effective electron mass of 4.9m(e), the donor ionization energy of 24 meV, and the critical vacancy concentration for the metal-insulator transition of 8.4 x 10(19) cm(-3). The room temperature conductivity is dominated by Hubbard band states near the mobility edge and decreases monotonically from 137 Omega(-1)cm(-1) for T-s = 550 degrees C to 14 Omega(-1)cm(-1) for T-s = 850 degrees C due to a decreasing structural disorder, consistent with the measured x-ray coherence length that increases from 7 to 36 nm for T-s = 550 to 850 degrees C, respectively, and a carrier density that decreases from 4 x 10(20) to 0.9 x 10(20) cm(-3), as estimated from optical reflection and Hall effect measurements. The absence of an expected discontinuity in the conductivity at similar to 280 K suggests that epitaxial constraints suppress the phase transition to a low-temperature orthorhombic antiferromagnetic phase, such that CrN remains a cubic paramagnetic insulator over the entire measured temperature range of 10-295 K. These results contradict previous experimental studies that report metallic low-temperature conduction for CrN, but support recent computational results suggesting a band gap due to strong electron correlation and a stress-induced phase transition.