Epitaxial Ti1−xMgxN(001) layers were deposited on MgO(001) by reactive magnetron cosputtering from titanium and magnesium targets in 15 mTorr pure N2 at 600 °C. X-ray diffraction (XRD) indicates a solid solution rock-salt phase for the composition range x = 0–0.55, a lattice constant that increases monotonously from 4.251 Å for TiN to 4.288 Å for Ti0.45Mg0.55N, and a decreasing crystalline quality with increasing Mg content, as quantified by the XRD ω rocking curve width which increases from 0.25° to 0.80°. XRD φ-scans show that all Ti1−xMgxN layers with x ≤ 0.55 are single crystals with a cube-on-cube epitaxial relationship with the substrate: (001)TiMgN║(001)MgO and [100]TiMgN║[100]MgO. In contrast, a larger Mg concentration (x = 0.85) leads to a polycrystalline, phase-segregated, nitrogen-deficient microstructure. The room temperature electrical resistivity increases from 14 μΩ cm for x = 0 to 554 and 3197 μΩ cm for x = 0.37 and 0.49, respectively. Ti1−xMgxN layers with 0.49 ≤ x ≤ 0.55 exhibit a negative temperature coefficient of resistivity which is attributed to the decreasing electron density of states at the Fermi level and a weak carrier localization. Optical transmission and reflection measurements indicate a decreasing electron density with increasing x and absorption minima at 2.0 and 1.7 eV for Ti0.63Mg0.37N and Ti0.48Mg0.52N, respectively, suggesting an extrapolated bandgap for semiconducting Ti0.5Mg0.5N of 0.7–1.7 eV.

In situ and ex situ transport measurements on epitaxial Co(0001)/Al2O3(0001) layers with thickness d = 7-300 nm are used to quantify the resistivity ρ scaling due to electron-surface scattering. Sputter deposition at 300 °C followed by in situ annealing at 500 °C leads to singlecrystal layers with smooth surfaces (< 1 nm roughness) and an epitaxial relationship: Co[0001]║Al2O3[0001] and Co[ 0 1 10 ]║Al2O3[ 0 2 11 ]. The measured ρ vs d data is well described by the classical expression by Fuchs and Sondheimer at both 295 and 77 K, yielding a temperatureindependent product of the bulk resistivity times the mean free path ρo×λ and an effective roomtemperature λ = 19.5 ± 1.0 nm. The resistivity increases by 9-24 % upon air exposure for layers with d ≤ 21 nm, indicating a transition from partially specular (p = 0.55 ± 0.05) to completely diffuse (p = 0) surface scattering during native oxide formation. The overall results suggest that Co exhibits a resistivity scaling that is comparable to W and approximately 2× smaller than that of Cu, and that the resistance of narrow Co lines can be reduced considerably by engineering the Co-liner interface to facilitate specular electron scattering.

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Epitaxial CrN(001) layers that are exposed to an O2-containing atmosphere exhibit a conductive 2D surface oxide with a sheet conductance Gs,oxide = 5.9 × 10−5 [Ω/◻]−1. This is demonstrated using in situ transport measurements in a 90% Ar–10% O2 mixture with continuously increasing pressure from &lt;10−6 to 240 Pa, showing a conductance increase that is independent of the CrN thickness d = 10 and 300 nm but is absent for control samples that are capped with insulating AlN prior to oxygen exposure. This suggests n-type doping of semiconducting CrN through substitutional replacement of N surface atoms with O. Cooling to 77 K leads to a decrease in Gs,oxide to 3.9 × 10−5 [Ω/◻]−1, indicating that the conduction electrons are not fully delocalized. The overall results indicate a path towards 2D electron transport devices in refractory transition metal nitrides and may explain the large variation in previously reported transport properties of CrN.

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