Effect of nonstoichiometry upon optical properties of radio frequency sputtered Al-N thin films formed at various sputtering pressures

Colored Al-N films can be easily made through changing the content of nitrogen gas at various sputtering pressures. For films deposited at 5X10(-2) Torr, with only slight change in the proportion of nitrogen gas from 0% to 2.5%, 5%, 7.5%, and 10%, the respective visual appearance changed from silver-gray to gray, gray, green-yellow to transparent. With the further increase in the content of nitrogen gas up to 100%, the obtained films were all transparent. For films formed at 5X10(-1) Torr, with the content of nitrogen gas being 0%, 2.5%, 5%, 7.5%, 10%, and 20%, the corresponding visual appearance of the films were silver-gray, black, dark-green-yellow, green-yellow, light-yellow and transparent. When the content of nitrogen gas changed from 20% to 100%, there was almost no difference in the visual appearance. For films formed at 1 Torr, when the content of nitrogen gas was only over 2.5%, the film became clear. Measurements of transmittance, reflectance, and thickness, calculations of the absorption coefficient, refractive index, and extinction coefficient, and an x-ray photoemission spectroscopy analysis show that the coloration is mainly due to the nonstoichiometry induced by lower content of nitrogen gas at various sputtering pressures. Furthermore, the calculated extinction coefficients are found to have a much closer relationship with the coloration than other parameters, such as, refractive index and thickness. The optical band gap of the obtained stoichiometric AIN films is 5.85-5.9 eV. (C) 1996 American Vacuum Society.