Optical phonon modes in Al1− x Sc x N

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.