Temperature Dependence of the AlN E1(To) Phonon Decay, Thermal Expansion and Strain Effect in AlN/Sapphire by Infrared Reflection

Infrared reflectivity measurement is carried out for AlN films on sapphire substrates. The frequencies of the symmetry optical phonon E1(TO) in the temperature range from 77 K to 500 K are reported by fitting the experimental reflectivity with the classical multi-oscillators model. Taking the lattice thermal expansion and Klemens process of the phonon decay into account, along with the strain effect introduced by thermal mismatch between the film and the substrate, the temperature effect on the frequency of the optical phonon E1(TO) is revealed. It is shown that the shift of frequency is mainly attributed to the decay process while the strain effect induced by thermal mismatch plays a non-negligible role in the outcomes of the strength and damping parameters.