Long-wavelength dielectric properties and infrared active optical phonon modes of molecular beam epitaxy ScxAl1-xN determined by infrared spectroscopic ellipsometry

Tuning the optical, electronic, and long-wavelength properties of group-III nitride alloys can be achieved by alloying AlN with ScN. We report here on the infrared dielectric functions determined from spectroscopic ellipsometry of (0001) wurtzite ScxAl1-xN with compositions of 0 <= x <= 0.20 grown by molecular beam epitaxy on c-plane sapphire substrates. We also report the optical phonons and their parameters determined in our analysis and compare with those in the previous literature. We find that all phonons shift to a lower wavenumber as a function of scandium incorporation, and we also see evidence of a decrease in crystal quality. Further, we report the high frequency and static dielectric constants and the Born effective charge as well as their evolution with the scandium content.