The electronic structure of scandium nitride is determined by combining results from optical and electronic transport measurements with first-principles calculations. Hybrid functional (HSE06) calculations indicate a 0.92 eV indirect Γ-to-X band gap and direct transition energies of 2.02 and 3.75 eV at Γ-and X-points, respectively, while G o W o and GW o methods suggest 0.44-0.74 eV higher gap values. Epitaxial ScN(001) layers deposited on MgO(001) substrates by reactive sputtering exhibit degenerate n-type semiconductor properties with a temperature-independent electron density that is varied from N = 1.12-12.8×10 20 cm -3 using F impurity doping. The direct optical gap increases linearly with N from 2.18 to 2.70 eV, due to a Burstein-Moss effect. This strong dependence on N is likely the cause for the large range (2.03-3.2 eV) of previously reported gap values. However, here extrapolation to N = 0 yields 2.07±0.05 eV for the direct X-point transition of intrinsic ScN. A reflection peak at 3.80±0.02 eV is independent of N and in perfect agreement with the HSE06-predicted peak at 3.79 eV, associated with a high joint-density of states near the Γ-point. The electron mobility at 4 K is 100±30 cm 2 /Vs and decreases with temperature due to scattering at polar optical phonons with characteristic frequencies that decrease from 620 to 440±30 cm -1 with increasing N, due to free carrier screening. The transport and density-of-states electron effective mass, determined from measured intra and inter band transitions, respectively, are 0.40±0.02 m o and 0.33±0.02 m o , in good agreement with the firstprinciples predictions of m tr = 0.33±0.05 m o and m DOS = 0.43±0.05 m o . The ScN refractive index increases with increasing hν = 1.0-2.0 eV from 2.6-3.1 based on optical measurements and from 2.8-3.2 based on the calculated dielectric function. An overall comparison of experiment and simulation indicates (i) an overestimation of band gaps by GW methods but (ii) excellent agreement with a deviation of ≤0.05 eV for the hybrid functional and (iii) a value for the fundamental indirect gap of ScN of 0.92±0.05 eV.

The resistivity of 9.3-nm-thick epitaxial and polycrystalline Cu is reduced by 11%-13% when coated with 0.75 nm Ni. Sequential in situ and ex situ transport measurements show that this is due to electron surface scattering which exhibits a specularity p ¼ 0.7 for the Cu-vacuum interface that transitions to completely diffuse (p ¼ 0) when exposed to air. In contrast, Ni-coated surfaces exhibit partial specularity with p ¼ 0.3 in vacuum and p ¼ 0.15 in air, as Cu 2 O formation is suppressed, leading to a smaller surface potential perturbation and a lower density of localized surface states, yielding less diffuse electron scattering. V

Aluminum scandium nitride (Al1−xScxN) layers deposited by reactive magnetron co-sputtering on sapphire 0001 substrates at 850 °C are epitaxial single-crystals for x ≤ 0.20. Their in-plane lattice constant increases linearly (3.111 + 0.744x Å) while the out-of-plane constant remains at 4.989 ± 0.005 Å. Optical absorption indicates a band gap of 6.15–9.32x eV and a linearly increasing density of defect states within the gap. The average bond angle decreases linearly with x, suggesting a trend towards the metastable hexagonal-ScN structure. However, an anomalous decrease at x = 0.20 indicates a structural instability which ultimately leads to phase separated rock-salt ScN grains for x > 0.4.

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Optical phonons are measured to probe the origins of the reported anomalously high piezoelectric response in aluminum scandium nitride (Al1−xScxN). Epitaxial layers with 0 ≤ x ≤ 0.16 deposited on sapphire(0001) exhibit a refractive index below the band gap, which increases from 2.03 for x = 0 to 2.16 for x = 0.16, corresponding to a dielectric constant ε∞ = 4.15 + 3.2x. Raman scattering shows that zone-center E2(H) and A1(TO) phonon modes shift to lower frequencies with increasing x, following linear relationships: ω(E2(H)) = 658–233x (cm−1) and ω(A1(TO)) = 612–159x (cm−1). Similarly, zone-center E1(TO) and A1(LO) phonon mode frequencies obtained from specular polarized infrared reflectance measurements red-shift to ω(E1(TO)) = 681–209x (cm−1) and ω(A1(LO)) = 868–306x (cm−1). The measured bond angle decreases linearly from 108.2° to 106.0°, while the length of the two metal-nitrogen bonds increase by 3.2% and 2.6%, as x increases from 0 to 0.16. This is associated with a 3%–8% increase in the Born effective charge and a simultaneous 6% decrease in the covalent metal-N bond strength, as determined from the measured vibrational frequencies described with a Valence-Coulomb-Force-Field model. The overall results indicate that bonding in Al-rich Al1−xScxN qualitatively follows the trends expected from mixing wurtzite AlN with metastable hexagonal ScN. However, extrapolation suggests non-linear composition dependencies in bond angle, length, and character for x ≥ 0.2, leading to a structural instability that may be responsible for the reported steep increase in the piezoelectric response.