MORPHOLOGY AND OPTICAL PROPERTIES OF AlN FILMS ON SAPPHIRE

Subject of Research. The paper presents the results of an experimental study on morphology and optical properties of AlN films on sapphire. Thin AlN films on sapphire were used as experimental samples. Method. To obtain thin films, an ion beam deposition setup was used, which includes the ion source of the CLAN-53M type with an ion neutralizer. The ion beam energy ranged from 600 to 900 eV. The ion beam current was 60 mA, and it was chosen based on the steady-state plasma burning in the ion source. Deposition was carried out at the residual pressure of gases in the vacuum chamber of at least 1.5×10–3 Pa; the substrates were heated using a group of halogen lamps with the total power of 2500 W; the substrate temperature was 550–850 °C. The precipitation time was one hour. The composition of the nitrogen-argon mixture was changed by increasing the volume fraction of nitrogen from 10 to 90 %. Main Results. The obtained thin films were studied by scanning electron microscopy and energy dispersive analysis. Studies have shown that thin nitrogen AlN films on sapphire obtained with a volume fraction of nitrogen in a nitrogen-argon mixture of more than 50 % have a composition close to stoichiometric one. For AlN films on sapphire, obtained with a volume fraction of nitrogen in a nitrogen-argon mixture of more than 90 %, substrate temperature of 800 °C and the beam energy of 600 eV, the transmittance in the entire optical wavelength range is at least 92 %. The direct dependence of the beam energy on the volume fraction of nitrogen in the nitrogen-argon mixture is determined: at 900 eV, as compared to 600 eV, the nitrogen content in the AlN film rises from 10 % to 30–35 %. When the beam energy is 600 eV, there is an insignificant dependence on the substrate temperature and only the direct dependence on the amount of nitrogen in the nitrogen-argon mixture remains. With partial ionization of the ion beam, the difference between the nitrogen content in the AlN film at different beam energies is in the range of 5–10 %. The increased nitrogen content in the films (more than 20 %) adversely affects the optical perfection of the films. With the partial ionization mode only at 900 eV, the temperature of 800 °C, and with the volume fraction of nitrogen in the nitrogen-argon mixture more than 50 %, the decrease in the quality of the films is observed. Under modes with a volume fraction of nitrogen in a nitrogen-argon mixture of less than 30 %, a large number of microdroplets are observed on the surface with sizes in the range of 1–6 μm. The composition of the gas mixture with the content of the volume fraction of nitrogen in the nitrogen-argon mixture of 10 % increases the concentration of microdroplets on the film surface with the increase in the proportion of large microdroplets. The most optimal mode was revealed with the beam partial ionization, the energy of 600 eV, and the volume fraction of nitrogen in the nitrogen-argon mixture more than 50%. The change in the substrate temperature has practically no effect on the nitrogen fraction in a thin film of aluminum nitride. Practical Relevance. A thin AlN film on sapphire deposited at the substrate temperature of 800 °C and volume fraction of nitrogen in the nitrogen-argon mixture equal to 90 % has a transmittance more than 92 % in the optical range of 200–1100 nm, that characterizes the obtained thin film sample as optically transparent.