Growth mechanism and structure of ultrathin palladium films formed by deposition on Mo(100)

The growth mechanism, structure and thermal stability of monolayer and ultrathin Pd films formed by vapor deposition on Mo(100) were studied using AES, LEED, and TPD. Pd film growth at 150 K is described well by a Frank-van der Merwe (FM) or layer-by-layer growth mechanism with a small amount of layer disorder and/or non-ideal layering. The Pd monolayer is pseudomorphic with the Mo(100) substrate lattice as shown by LEED. Pd films deposited on Mo(100) at 450 and 600 K grow by forming three-dimensional (3D) islands on top of an initially formed Pd monolayer, i.e., a Stranski-Krastanov growth mode. Alloying could also explain the AES curves at these temperatures. Thermal desorption of Pd from multilayer films begins at 1250 K with an activation energy of 100 kcal mol. This is 7 kcal mol higher than the bulk sublimation energy of palladium due to interaction with the molybdenum substrate and was observed for films as thick as 20 layers. Pd desorption is kinetically limited by decomposition of a Pd-Mo alloy and/or diffusion of Pd from the subsurface layers of Mo to the surface. Annealing studies show that the Pd monolayer is stable to 1200 K, but that agglomeration of Pd into 3D islands and possibly alloy formation occurs upon heating thicker films above 400 K.