Growth of single-crystal CrN on MgO(001): Effects of low-energy ion-irradiation on surface morphological evolution and physical properties

Abstract

CrN layers, 0.5 m thick, were grown on MgO͑001͒ at T s ϭ570-775°C by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition in pure N 2 discharges at 20 mTorr. Layers grown at T s р700°C are stoichiometric single crystals exhibiting cube-on-cube epitaxy: (001) CrN ͉͉(001) MgO with ͓100͔ CrN ͉͉͓100͔ MgO. At higher temperatures, N 2 desorption during deposition results in understoichiometric polycrystalline films with N fractions decreasing to 0.35, 0.28, and 0.07 with T s ϭ730, 760, and 775°C, respectively. The surface morphologies of epitaxial CrN͑001͒ layers were found to depend strongly on the incident ion-to-metal flux ratio J N 2 ϩ /J Cr which was varied between 1.7 and 14 with the ion energy maintained constant at 12 eV. The surfaces of layers grown with J N 2 ϩ /J Cr ϭ1.7 consist of self-organized square-shaped mounds, due to kinetic roughening, with edges aligned along orthogonal ͗100͘ directions. The mounds have an average peak-to-valley height ͗h͘ϭ5.1 nm and an in-plane correlation length of ͗d͘ϭ0.21 m. The combination of atomic shadowing by the mounds with low adatom mobility results in the formation of nanopipes extending along the growth direction. Increasing J N 2 ϩ /J Cr to 14 leads, due to increased adatom mobilities, to much smoother surfaces with ͗h͘ϭ2.5 nm and ͗d͘ϭ0.52 m. Correspondingly, the nanopipe density decreases from 870 to 270 m Ϫ2 to Ͻ20 m Ϫ2 as J N 2 ϩ /J Cr is increased from 1.7 to 6 to 10. The hardness of dense CrN͑001͒ is 28.5Ϯ1 GPa, but decreases to 22.5Ϯ1 GPa for layers containing significant nanopipe densities. The CrN͑001͒ elastic modulus, 405Ϯ15 GPa, room-temperature resistivity, 7.7ϫ10 Ϫ2 ⍀ cm, and relaxed lattice constant, 0.4162Ϯ0.0008 nm, are independent of J N 2 ϩ /J Cr .

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

“…In an initial study of deposition temperature, we observed that low temperature deposition at 550 • C or lower will lead to the formation of polycrystalline films with high electrical resistivity, while deposition temperatures higher than 700 • C typically results in predominant growth of Cr 2 N over cubic CrN consistent with earlier observations [49]. Thus, we limit the presentation of experiments below to the most important temper ature ranges between 600 • C and 700 • C.…”

A review of metal-ion-flux-controlled growth of metastable TiAlN by HIPIMS/DCMS co-sputtering

“…26 The incident ion energy Ei and ion-to-metal flux ratio Ji/JMe are decisive parameters controlling nanostructural evolution during low temperature (< 500 o C; Ts/Tm < 0.3 for TiN) physical vapor deposition of transition-metal (TM) nitrides by conventional reactive DCMS. 17,25,27,28,29,30,31 The dominant ion species incident at the growth surface during DCMS with N2/Ar gas mixtures optimized to obtain stoichiometric films is typically Ar + , with N2 + and N + both contributing a few percent. 32, 33 The N2 + /N + ratio increases with increasing N2/Ar fraction, while in pure N2 discharges the dominant ion species is N2 + .…”

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.…”