Growth mechanism of the Pd(100)-p(2x2)-p4g-Al surface alloy

We have studied the growth mechanism of a Pd(100)-p(2 x 2)-p4g-Al surface a lloy by scanning tunneling microscopy (STM). The surface alloy has a bilaye r structure and is formed by annealing at 450-700 It (depending on the init ial aluminum coverage) after the deposition of aluminum on Pd(100) at room temperature. The ratio of the surface-alloy coverage to the initial aluminu m coverage is found to be constant (0.44) irrespective of the initial alumi num coverage from similar to 0.5 monolayers (ML) up to similar to 2 ML. The growth mechanism of the surface alloy is proposed on the basis of the STM measurements at various annealing temperatures. Upon annealing at 450 K, so me of the surface aluminum atoms migrate into the bulk and, instead, pallad ium atoms come out to the surface. These palladium atoms react with aluminu m atoms remaining on the surface to form a surface alloy. When the initial aluminum coverage is less than 1 ML, bilayer-high islands of the surface al loy with an average area of similar to 100 nm(2) are formed at 450-500 K, w hich diffuse on the terrace at 500-700 K and coalesce to form larger island s. A possible role of the percolation transition of aluminum islands in the formation of the surface alloy is discussed. (C) 2000 Elsevier Science B.V . All rights reserved.