Infrared Absorption of AIN in the One‐Phonon Region and Finite Size Effects

The absorption of single crystals and microcrystalline samples of AlN, pressed in pellets of halides, has been measured in the region of 1000 to 400 cm−1. Three intensive absorption maxima have been found in the region between ωT and ωL. They are connected with the finite size and shape effects and their location depends on the material of the surrounding medium. Further, intensive maxima at 610 and 590 cm−1 occur in microcrystalline samples. The absorption by surface modes may be interpreted with the help of Englman and Ruppin's theory assuming that the frequencies ωT(A1) = 622 cm−1 and ωT(E1) = 660 cm−1. It is shown that it is necessary to distinguish between surface and bulk modes for the absorption measurement of microcrystalline samples and that the absorption in the region of surface modes is decisive in materials with sufficiently fine grains, i.e. in measurements with pressed pellets.