Structure and refractive index of thin alumina films grown by atomic layer deposition

Aluminum oxide (Al₂O₃) is a good dielectric material for optoelectronic applications. With technologies such as atomic layer deposition (ALD), homogeneous ultrathin films of Al₂O₃ can be obtained at moderate temperatures. In this work, Al₂O₃ thin films of thickness up to 310 nm were obtained by ALD at 150 and 175 °C with trimethylaluminum and H₂O as precursors. The nitrogen purging pulses were kept short (1 or 2 s) to reach the growth rate of about 0.14 nm/cycle at 150 °C and 0.15 nm/cycle at 175 °C after 800 deposition cycles. The obtained films were amorphous and showed a good homogeneity over a 2 × 2 cm² area confirmed by optical reflectance. X-ray photoelectron spectroscopy analysis on the surface of the ALD samples indicate that films deposited at 150 °C contained more carbon-related bonds, which probably arise from incomplete surface reaction or insufficient N₂ purging time. Results of spectroscopic ellipsometry (SE) reveal that the refractive indices (n) of the ALD films are systematically lower than that of crystalline α-Al₂O₃ in the wavelength range, 240–850 nm. Higher deposition temperature increased slightly the n values, but at the same time it also increased the roughness of the film surface. A good match between the film thicknesses estimated by SE and from the cross-sectional scanning electron microscopy images of the same group of samples confirms the effectiveness of the SE modeling to study the homogeneity of the alumina films deposited by ALD on silicon substrates.