Figures of merit for niobium thin films grown on sapphire substrate

In the manufacture of optoelectronics devices, including solar cells, a very important parameter is the degree of chemical purity of die materials used during their fabrication process. There are two figures of merit that allow die comparison in quality of devices prepared by sputtering. The first figure of merit, the impurity concentration, directly depends on the sputtering pressure and die effective Leak rate of impurities into the sputtering system. The second figure of merit, the fraction of an impurity monolayer that can be formed, depends on the time a clean surface is exposed to impurities in the growth chamber until the next layer is deposited. In this work, a study to correlate die sputtering pressure with the first figure of merit for Nb thin films grown in 90 degrees oriented sapphire substrates is presented. These films were deposited by DC magnetron sputtering, using a Nb disk as the target in a high vacuum system with a base pressure of 5 x 10(-8) torr in Ar plasma. Niobium was chosen because its electrical properties allow easy measurement and comparison. The Nb films were grown at room temperature, keeping fixed all growth parameters but the plasma pressure. Morphology, elemental composition, and structure of die films were determined by scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and x-ray powder diffraction, respectively. Resistance versus temperature profiles in the 10-300 K range are presented, where a correlation between the plasma pressure and the electrical properties can be observed as all indication that the impurity concentration directly depends oil the sputtering pressure.