Growth of Ni and Ni-Cr Alloy Thin Films on MgO(001): Effect of Alloy Composition on Surface Morphology

摘要

The effects of substrate treatment, temperature and composition on thesurface morphology of Ni-Cr thin films grown on MgO(001) are studied byscanning tunneling microscopy (STM) and atomic force microscopy (AFM). Acombination of acid-etching and high temperature deposition (400C) will resultin smooth films and terraces (up to 30 nm wide) suitable for the study ofprogression of chemical reactions on the surface. Two different treatments areused to prepare the MgO substrates for deposition and they introducecharacteristic differences in film surface morphology. Thin films that aregrown on the phosphoric acid-treated substrates present reduced nucleationdensity during the initial stages of film growth which results in long and wideterraces. Due to the about 16 percent lattice mismatch in the Ni(001)/MgO(001)system, film growth at 400C yields discontinuous films and a two-step growthprocess is necessary to obtain a continuous layer. Ni films are deposited at100C and subjected to a post-growth anneal at 300C for 2 hours to obtain asmoother surface. The addition of just 5 wt. percent Crchanges the growthprocesses and yields continuous films at 400C without de-wetting in contrast topure Ni films. With increasing Cr content, the films become progressivelysmoother with wider terraces. Ni5Cr alloy thin films have an rms surfaceroughness of 3.63+/-0.75 nm while Ni33Cr thin film is smoother with an rmsroughness of only 0.29 +/-0.13 nm. The changes in film growth initiated byalloying with Cr are due to changes in the interfacial chemistry whichfavorably alters the initial adsorption of the metal atoms on MgO surface andsuggests a reduction of the Ehrlich-Schwoebel barrier. The growth of smoothNi-Cr thin films with well-defined surface structure opens up a new pathway fora wide range of surface science studies related to alloy performance.