Orientation of the nanocrystallites in AlN thin film determined by FTIR spectroscopy

Aluminum Nitride (AlN) films were deposited at 450°C in nitrogen ambient at a pressure of 0.1 Pa and at a laser incident fluence of ∼3 J/cm² and pulse repetition rate of 40 Hz. Grazing Incidence X-ray Diffraction patterns evidenced the presence of nanocrystallites in the amorphous AlN matrix. In the FTIR spectra the characteristic Reststrahlen band of AlN crystal with a hexagonal lattice is observed but it is quite broadened (950-550 cm^-1). The angular dependence of the reflectance spectra in p-polarised incidence radiation demonstrates the sensitivity of the A₁LO phonon mode of the AlN nanocrystallites to their orientation toward the normal to the substrate surface. With decrease of the incidence beam angle the intensity of the A₁LO phonon mode diminishes and softening of the resonance frequency occurs.