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

The effects of substrate treatment, temperature and composition on the surface morphology of Ni-Cr thin films grown on MgO(001) are studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). A combination of acid-etching and high temperature deposition (400C) will result in smooth films and terraces (up to 30 nm wide) suitable for the study of progression of chemical reactions on the surface. Two different treatments are used to prepare the MgO substrates for deposition and they introduce characteristic differences in film surface morphology. Thin films that are grown on the phosphoric acid-treated substrates present reduced nucleation density during the initial stages of film growth which results in long and wide terraces. 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 growth process is necessary to obtain a continuous layer. Ni films are deposited at 100C and subjected to a post-growth anneal at 300C for 2 hours to obtain a smoother surface. The addition of just 5 wt. percent Crchanges the growth processes and yields continuous films at 400C without de-wetting in contrast to pure Ni films. With increasing Cr content, the films become progressively smoother with wider terraces. Ni5Cr alloy thin films have an rms surface roughness of 3.63+/-0.75 nm while Ni33Cr thin film is smoother with an rms roughness of only 0.29 +/-0.13 nm. The changes in film growth initiated by alloying with Cr are due to changes in the interfacial chemistry which favorably alters the initial adsorption of the metal atoms on MgO surface and suggests a reduction of the Ehrlich-Schwoebel barrier. The growth of smooth Ni-Cr thin films with well-defined surface structure opens up a new pathway for a wide range of surface science studies related to alloy performance.