Investigation of Composition-Dependent Optical Phonon Modes in Al(x)Ga(1-x)N Epitaxial Layers Grown on Sapphire Substrates

Abstract

We reported the systematical study of optical properties of hexagonal Al(x)Ga(1-x)N epitaxial films grown on c sapphire substrate using metal organic chemical vapor deposition By performing Fourier transform infrared spectroscopy measurements, the high-frequency dielectric constants and phonon frequencies can be obtained by theoretically fitting the experimental infrared reflectance spectra using a four phase layered model The high frequency dielectric constant of Al(x)Ga(1-x)N varies between 4 98 and 4 52 for epsilon(infinity) (perpendicular to) (polarization perpendicular to the optical axis) and between 4 95 and 4 50 for epsilon(infinity//) (polarization parallel to the optical axis) respectively when the aluminum composition changes from 0 15 to 024 Furthermore, from experimental Infrared reflectance spectra of Al(x)Ga(1-x)N films, a specific absorption dip at 785 cm(-1) was observed when the aluminum composition is larger than 0 24 The dip intensity increases and the dip frequency shifts from 785 to 812 cm-1 as aluminum composition increases from 0 24 to 0 58 According to the reciprocal space map of x-ray diffraction measurements, the emergence of this dip could be resulted from the effects of strain relaxation in AlGaN epitaxial layers due to the large lattice mismatch between GaN and AlGaN epitaxial film