Phonon dynamics in AlN lattice contaminated by oxygen

The phonon dynamics of wurtzite aluminum nitride contaminated by oxygen were investigated by employing the Raman back scattering, the Fourier Transform Infrared (FT-IR) reflectivity and absorption, and the Brillouin scattering techniques on unseeded polycrystalline samples of AlN built of single crystallites 1–5 mm in size. The six Raman active zone center optic modes were observed and identified. Throughout the oxygen contamination range (∼1, ∼2, and ∼6 at.%) of three samples investigated, the widths of the principal Raman modes were found to decrease with increasing the oxygen content in the single crystal. This behavior is interpreted as a change in the nature of the oxygen defect when the oxygen concentration exceeds 1 at.%. The FT-IR reflectivity spectrum exhibits two-mode behavior at low oxygen concentration, one-mode behavior tends to be dominant when the oxygen concentration increases, and only one-mode behavior can be observed at high oxygen concentration. These changes in the reststrahlen band with oxygen concentration support the hypothesis of a transition in the oxygen accommodation defect as the concentration of oxygen increases. The oxygen effects on the AlN optical parameters were investigated by calculating these optical parameters from the reflectivity data of single crystallites differing in their oxygen concentration. The FT-IR absorption measurements showed several absorption bands in the multiple-phonon region. A tentative interpretation is proposed in which these bands are considered to be due to oxygen impurity absorption and to a combination of several phonon branches at the Brillouin-zone boundaries. The absorption spectrum in the one-phonon region allowed us to obtain a reliable data on the phonon density of states function in bulk AlN. Lastly, three different configurations were used in Brillouin scattering measurements to achieve a complete determination of the elastic stiffness constants of AlN.