Figures of merit for niobium thin films grown on sapphire substrate

In the manufacture of optoelectronics devices, including solar cells, a ver y important parameter is the degree of chemical purity of die materials use d during their fabrication process. There are two figures of merit that all ow die comparison in quality of devices prepared by sputtering. The first f igure of merit, the impurity concentration, directly depends on the sputter ing pressure and die effective Leak rate of impurities into the sputtering system. The second figure of merit, the fraction of an impurity monolayer t hat can be formed, depends on the time a clean surface is exposed to impuri ties 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 meri t for Nb thin films grown in 90 degrees oriented sapphire substrates is pre sented. These films were deposited by DC magnetron sputtering, using a Nb d isk 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 a llow easy measurement and comparison. The Nb films were grown at room tempe rature, keeping fixed all growth parameters but the plasma pressure. Morpho logy, elemental composition, and structure of die films were determined by scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (E DS), and x-ray powder diffraction, respectively. Resistance versus temperat ure profiles in the 10-300 K range are presented, where a correlation betwe en the plasma pressure and the electrical properties can be observed as all indication that the impurity concentration directly depends oil the sputte ring pressure.