Comprehensive analysis of the anisotropic dielectric response of wurtzite (0001¯)Sc x Al1− x N for 0.1 ≤  x  ≤ 0.3

We present evaluation and detailed modeling of the anisotropic dielectric response of single-phase, (0001¯) oriented wurtzite (wz) ScxAl1−xN films employing spectroscopic ellipsometry (SE) in a spectral range from 0.037 to 6.5 eV. The samples were grown by plasma-assisted molecular-beam epitaxy and cover a composition range from 0.1 ≤ x ≤ 0.3. Clear signatures of the extraordinary dielectric function are observed in the SE data, providing evidence that the anisotropy is accessible from (0001¯) oriented films in this case. We obtain both the ordinary and extraordinary dielectric functions from point-by-point and parametric model fits, respectively. In the ultraviolet spectral range, we find negative and significantly different bowing parameters for ordinary and extraordinary absorption edges, resulting in an enormous increase in the crystal field splitting. In the infrared spectral range, a linear decrease in phonon frequency is observed for transverse optical and longitudinal optical phonons of E1 (ordinary) and A1 (extraordinary) symmetry, consistent with the linear increase in crystal volume. Finally, we derive estimates for dielectric constants, Born-effective charges, and the spontaneous polarization. We find that the dielectric limits and Born-effective charges increase, while the spontaneous polarization decreases with increasing x.