Variable-range hopping conduction in epitaxial CrN(001)

Abstract

Epitaxial CrN(001) layers, grown by dc magnetron sputtering on MgO(001) substrates at growth temperatures T s = 550-850 °C, exhibit electronic transport that is dominated by variablerange-hopping (VRH) at temperatures <120 K. A transition from Efros-Shklovskii (ES) 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 °C to 0.9 nm for T s = 600-750 °C but increases again to 1.9 nm for T s = 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 T s , respectively. The low-temperature transport data provides 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 8.4×10 19 cm-3. The room temperature conductivity is dominated by Hubbard band states near the mobility edge and decreases monotonically from 137 Ω-1 cm-1 for T s = 550 °C to 14 Ω-1 cm-1 for T s = 850 °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 °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.

Microstructure and thermoelectric properties of CrN and CrN/Cr2N thin films

“…This behavior contrasts with the Seebeck coefficient values which matches that of n-type semiconducting. A likely explanation is that the nitrogen vacancies are acting as donors by greatly increase the charge carrier density, creating non-zero quasi-metallic density of states at the Fermi level for the sample [57,58]. In this case, a rise in the temperature will probably enhance electron-electron scattering resulting in a small decrease in the conductivity values.…”

A comparative study of transport properties in polycrystalline and epitaxial chromium nitride films

“…3,4 Considerable experimental and theoretical work have demonstrated that CrN undergoes a magnetic and structural phase transition from a paramagnetic NaCl structure at room temperature to a low-temperature antiferromagnetic orthorhombic P nma phase at N eel temperature of 273-286 K. [5][6][7] A variety of electrical transport properties in CrN have been observed, such as, a semiconducting behavior with dq/dT < 0, [8][9][10][11] a metallic behavior with dq/dT > 0, [12][13][14] and continuous and discontinuous q(T) curves at 260-280 K. [8][9][10][11][12][13]15 These differences in electrical transport properties have been attributed to the sensitivity of the transport properties to N stoichiometry. 12,15 In addition, the discontinuity in q(T) curves has usually been observed in CrN powders or polycrystalline CrN films, rather than in epitaxial films, suggesting that the epitaxial constraints could affect the transition.…”

“…25,26 Here, we use a universal scaling relation considering both Mott VRH and ES hopping, which has previously been reported for epitaxial CrN(001) films by Zhang et al 11 The expression first proposed by Aharony et al 27 is…”

Epitaxial suppression of the metal-insulator transition in CrN

“…3,4 Electronic transport studies report controversial results for CrN, including (i) values for the resistivity ρ at room temperature range over more than two orders of magnitudes, from 1.7×10 -3 to 3.5×10 -1 Ωcm, 3,[5][6][7][8][9][10] even when only considering the most reliable data for single crystal CrN layers; (ii) the temperature dependence of ρ shows metallic behavior with dρ/dT > 0 in some studies, 6,7,11 but an increase in ρ with decreasing temperature in other reports, 3,5,9,12 which has been attributed to the presence of a band gap 5 or carrier localization due to crystalline defects 13 or N-vacancies; 14 (iii) some studies report a discontinuity in ρ(T) at 260-280 K, 3,6,7,14 which is associated with a magnetic and structural phase transition from a paramagnetic NaCl structure at room temperature to a low-temperature antiferromagnetic orthorhombic P nma phase 11,15 with a 0.56-0.59% higher density, 11 and a 25% lower bulk modulus, 16 while other reports show no evidence for a phase transition in the ρ(T)-curves. 5,7,9 Electronic structure calculations suggest that magnetic stress relief couples magnetic ordering with the structural phase transition, 17 and that CrN exhibits a band gap if the Hubbard Coulomb interaction term is sufficiently large.…”