Strain and composition dependence of the E1(TO) mode in hexagonal Al1−xInxN thin films

Infrared spectroscopic ellipsometry is used to study the influence of strain and composition on the transverse-optical phonon mode of $E_1$ symmetry in hexagonal ${\mathrm{Al}}_{\text{1−x}}{\mathrm{In}}_x\mathrm{N}$ films for $\text{0.12⩽x⩽0.21.}$ The 0.1–0.2-μm thick films were grown on slightly compressively strained hexagonal GaN buffer layers, or directly on [0001] sapphire by metalorganic vapor phase epitaxy. The ${\mathrm{Al}}_{\text{1−x}}{\mathrm{In}}_x\mathrm{N}$ $E_1(\mathrm{TO})$ phonon shows a one-mode behavior in contrast to recent theoretical predictions [H. Grille, C. Schnittler, and F. Bechstedt, Phys. Rev. B 61, 6091 (2000)]. Films grown on GaN reveal the influence of strain on the phonon mode frequencies due to pseudomorphic film growth. ${\mathrm{Al}}_{\text{1−x}}{\mathrm{In}}_x\mathrm{N}$ deposited directly on sapphire possesses phonon modes which indicate fully relaxed film growth.