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Title: Optical phonon modes in Al{sub 1−x}Sc{sub x}N

Optical phonons are measured to probe the origins of the reported anomalously high piezoelectric response in aluminum scandium nitride (Al{sub 1−x}Sc{sub x}N). 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{sub ∞} = 4.15 + 3.2x. Raman scattering shows that zone-center E{sub 2}(H) and A{sub 1}(TO) phonon modes shift to lower frequencies with increasing x, following linear relationships: ω(E{sub 2}(H)) = 658–233x (cm{sup −1}) and ω(A{sub 1}(TO)) = 612–159x (cm{sup −1}). Similarly, zone-center E{sub 1}(TO) and A{sub 1}(LO) phonon mode frequencies obtained from specular polarized infrared reflectance measurements red-shift to ω(E{sub 1}(TO)) = 681–209x (cm{sup −1}) and ω(A{sub 1}(LO)) = 868–306x (cm{sup −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 Al{sub 1−x}Sc{sub x}N qualitatively follows the trends expected from mixing wurtzite AlN with metastablemore » 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.« less
Authors:
;  [1] ;  [2]
  1. Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 (United States)
  2. Department of Electronic Engineering, East China Normal University, Shanghai 200241 (China)
Publication Date:
OSTI Identifier:
22271306
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 115; Journal Issue: 1; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ALUMINIUM NITRIDES; BOND ANGLE; CONCENTRATION RATIO; CUBIC LATTICES; EFFECTIVE CHARGE; EPITAXY; HEXAGONAL LATTICES; LAYERS; NONLINEAR PROBLEMS; PERMITTIVITY; PHONONS; PIEZOELECTRICITY; RAMAN EFFECT; RED SHIFT; REFRACTIVE INDEX; SAPPHIRE; SCANDIUM NITRIDES; VALENCE