Variable-range hopping conduction in epitaxial CrN(001)

increases again to 1.9 nm for Ts = 800–850 ◦ 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 Ts, respectively. The low-temperature transport data provide lower limits for the CrN effective electron mass of 4.9me, the donor ionization energy of 24 meV, and the critical vacancy concentration for the metal-insulator transition of 8.4 × 10 19 cm −3 . The room temperature conductivity is dominated by Hubbard band states near the mobility edge and decreases monotonically from 137 −1 cm −1 for Ts = 550 ◦ Ct o 14 −1 cm −1 for Ts = 850 ◦ C due to a decreasing structural disorder, consistent with the measured x-ray coherence length that increases from 7 to 36 nm for Ts = 550 to 850 ◦ C, respectively, and a carrier density that decreases from 4 × 10 20 to 0.9 × 10 20 cm −3 , as estimated from optical reflection and Hall effect measurements. The absence of an expected discontinuity in the conductivity at ∼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.