Optical characterization of AlN films: Measurement of stress

AlN films were deposited on quartz and silicon substrates by d.c. magnetron sputtering of an Al target in argon + nitrogen plasma (20-100 vol. % nitrogen). Transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies indicated the films to be polycrystalline for higher nitrogen (> 30 vol.%) and amorphous for lower nitrogen in the sputtering gas. Direct optical transition with high band gap (E-g greater than or equal to 5 eV) was observed for the polycrystalline films while for amorphous films indirect transition could be identified. The absorption band tail at photon energies less than the band gap energy (E,) in the polycrystalline films could be analysed by considering the contribution of stress (1.4-3.0 GPa) and strain ((3-7) x 10(-3)) due to band gap fluctuation in the grain boundary region. The effects of grain boundary trap state density (Q(t) approximate to(2.0-3.8) X 10(12) cm(-2)) on the optical properties of the polycrystalline films were ascertained. The microstructural information, obtained from the scanning electron microscopy, TEM and XRD studies, were correlated with the above measurements.