Growth mechanism and structure of ultrathin Pd films vapor-deposited on Ta(110)

The structure and growth of Pd films formed by vapor-deposition on Ta(110) have been studied using AES and LEED over a wide range of substrate temperatures. The thermal stability of monolayer, bilayer and trilayer Pd was examined also. At 300 K, the growth of Pd on Ta(110) is best described by a Frank-van der Merwe (layer-by-layer) growth mode. Initially, the first Pd layer grows in 2D pseudomorphic islands up to a Pd coverage of θ_Pd = 0.65 (defined relative to the atomic density of the (111) surface of bulk Pd, θ_Pd = 1). Subsequent deposition of Pd atoms induces a phase structural phase transition; for 0.65 < θ_Pd< 1.0 the monolayer growth proceeds through formation of 2D islands of Pd fcc (111) structure. At 125 K, the growth mode of Pd/Ta(110) is similar, but seems to deviate slightly from the layer-by-layer mode. For a substrate temperature of 500 K the growth mechanism changes. The first layer is still pseudomorphic initially, but the phase transition to the Pd fcc(111) structure occurs at θ_Pd = 0.82. After completion of the Pd monolayer, additional Pd deposition results in the formation of 3D crystallites 2-3 layers thick on top the first Pd layer, i.e., a Stranski-Krastanov growth mode occurs at 500 K. Annealing studies show that the pseudomorphic Pd monolayer is stable to very high temperatures (up to 1350 K), but the growth of Pd crystallites occurs upon heating thicker Pd layers above 370 K.