Grain-boundary effect in calcium thin films

Abstract

Deposition and study of calcium films were achieved under ultrahigh vacuum. The latter was obtained with the use of an ion pump and a helium cryopump. The temperature of the substrate ranged between 90 and 400 K. The electrical resistance was measured with the use of a four-probe method with a constant current. Film thicknesses were in the range between 20 and 300 nm. To obtain a stable film structure, samples were annealed to a temperature near 400 K. The electrical resistivity of the annealed films exhibited a linear and reversible variation with temperature. It was shown to be thickness dependent. For the thickest deposits, electrical resistivity reached a constant value which no longer depended on the thickness, but was a function of the film-deposition temperature. With the help of electron photomicrographs and with use of the conduction model proposed by Mayadas and Schatzkes, we calculated the fraction $p$ of electrons which are specularly reflected at the surface of the film and the parameter $\ensuremath{\alpha}$ depending on the reflection on grain boundaries. The influence of grain boundaries was calculated and the mean free path was deduced from these results.