Infrared dielectric functions and optical phonons of wurtzite Y_xAl_1−xN (0  ⩽  x  ⩽  0.22)

YAlN is a new member of the group-III nitride family with potential for applications in next generation piezoelectric and light emitting devices. We report the infrared dielectric functions and optical phonons of wurtzite (0001) Y_xAl_1−xN epitaxial films with 0  ⩽  x  ⩽  0.22. The films are grown by magnetron sputtering epitaxy on c-plane Al₂O₃ and their phonon properties are investigated using infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The infrared-active E ₁(TO) and LO, and the Raman active E ₂ phonons are found to exhibit one-mode behavior, which is discussed in the framework of the MREI model. The compositional dependencies of the E ₁(TO), E ₂ and LO phonon frequencies, the high-frequency limit of the dielectric constant, ${\epsilon }_{\infty }$, the static dielectric constant, ${\epsilon }_0$, and the Born effective charge Z _B are established and discussed. (paper)