Low Temperature Preparation Of AlN Films With High C-axis Oriented By RF Magnetron Reactive Sputtering

As an important kind of Ⅲ-nitride semiconductor material, wurtzite aluminum nitride (AlN) has many outstanding properties. These properties make AlN a promising material for many applications including SiC-based and Sapphire-based optoelectronic and acoustic-electronic devices, such as surface acoustic wave and bulk acoustic wave devices, field-emission display, light-emitting diodes. Here it should be noted that the highly C-axis oriented films or single crystal AlN layer are needed to be grown for above-mentioned applications. However, the highly C-axis oriented films or single crystal AlN layers were prepared by metal organic chemical vapor deposition, molecular beam epitaxy and hydride vapor phase epitaxy techniques. There are many disadvantages such as high deposition temperature, toxic exhaust emission and expensive equipment, especially, high deposition-temperature has the disadvantage of the degradation of the substrate and AlN thin films during deposition due to thermal damage, which limits the wide applications of AlN layers in the field of microelectronics. Compared with these techniques above-mentioned, RF reactive magnetron sputtering method has some advantages of simple, low temperature and low price. Hence, low-temperature deposition of AlN thin films has become increasingly important and valuable. RF reactive magnetron sputtering is regard as an important method for the fabrication of thin films at low temperature. Therefore, the research on the preparation of high C-axis orientation aluminum nitride thin film by magnetron sputtering at low temperature has very important significance. In this thesis, high C-axis orientated and epitaxial AlN thin films were prepaed successfully by RF reactive magnetron sputtering on6H-SiC substrate. The surface micropholaogy, composition, crystal structure and optical properties were studied at different parameters. Further more, a lower cost and more lattice mismatch between sapphire and AlN film was used as the substrate instead of6H-SiC substrate. The same high C-axis orientated AlN thin films were achived. Finally, the low temperature growth of the AlN films was developed on both6H-SiC and sapphire substrates at the temperature of300℃and room temperature.AlN films with the high C-axis orientation were deposited on the6H-SiC substrate. The effects of the pressure, N2concentration and RF power on the surface microphology, composition and crystal structure were studied by scaning electrical microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and two dimensions x-ray diffraction (2D-XRD), etc. The best growth conditions of the AlN films with the high C-axis orientation were achived. The Al/N atom ratio of the prepared AlN films is up to1.09:1, and the Al, N atoms comcentration is close to96%. The size of crystal grain decreases with increasing the sputtering pressure and N2concentration, and have no obvious change when the substrate temperature higher than500℃. The surface roughness decreases with the increase of sputtering pressure, N2concentration and RF power. With the increase of N2comcentration and the RF power, the Al atom concentration decreases and N atom comcentation increases, thus the Al/N atom ratio decreases. The lower sputtering pressures is in favor of the growth of C-axis oriented AlN films, the C-axis oriention became weaker when increasing the sputtering preasure, and the films are amorphous state at the preassure of1.0Pa. The oriention of the film changed from AlN (10-10) to C-axis when increasing the N2concentration from20%to60%. The deposited AlN films present highest C-axis oriented when increasing the RF power to250W. If the RF power is too high, the growth of other crystal plane can be induced.With RF magnetron reaction sputtering technique, the high C-axis oriented AlN films were successful deposited on sapphire substrate which is a lower cost compared with6H-SiC substrate. The problem of serious lattice mismatch and thermal mismatch betwween the sapphire substrate and AlN film were solved, and the influence of growth temperature on the surface morphology and structure of the film were studied. It was suggested that the window of grown temperature is narrow for depositing high C-axis oriented AlN film. Both too high and too low substrate temperature are unfavorable to the growth of high C-axis oriented AlN film. In addition, the surface roughness increases with the increase of substrate temperature, and the oxygen atom concentration decreasesd a little at a higher substrate temperature.The growth of high C-axis oriented AlN films at low temperature and room temperature by RF magnetron reaction sputtering was fistly explored. The high C-axis oriented AlN films were deposited on6H-SiC at room temperature, and the AlN film with epitaxial structure were successful deposited on6H-SiC at the substrate temperature of300℃. The deposited AlN film presents C-axis oriented structure with some weak growth of other crystal plane when growing on sapphire substrate at room temperature.The optic performance of the deposited films was measured by spectroscopic ellipsometry. The result shows that the refractive index of the deposited AlN films is closely related to the crystalline structure of the AlN film, and the refractive index (n) between1.83and2.07. The better the C-axis orientated of the deposited films, the larger the value of refractive index, vice versa. The extinction coefficient of deposited AlN films is not affected by deposition conditions. The k value (extinction coefficient) is zero when the wavelength of the light greater than300 nm, so the deposited AlN films have a high light transmittance. When the wavelength less than300nm, k value increases rapidly, the deposited AlN films present a strong absorption.