Growth of Ni and Ni-Cr alloy thin films on MgO(001): Effect of alloy composition on surface morphology

Abstract

The effects of substrate treatment, growth 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). We demonstrate that a combination of acid-etched substrates and high temperature deposition (400 °C) will result in smooth films with well-defined terraces (up to 30 nm wide) that are 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 ≈16% lattice mismatch in the Ni(001)/MgO(001) system, film growth at 400 °C yields discontinuous films and a twostep growth process is necessary to obtain a continuous layer. Ni films are deposited at 100 °C and subjected to a post-growth anneal at 300 °C for 2 hours to obtain a smoother surface. The addition of just 5 wt.% Cr drastically changes the film growth processes and yields continuous films at 400 °C 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 2 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 a well-defined surface structure opens up a new pathway for a wide range of surface science studies related to alloy performance.

From Alloy to Oxide: Capturing the Early Stages of Oxidation on Ni–Cr(100) Alloys

“…The pristine alloy surfaces consist of relatively large terraces up to several 10 nm in width, and step edges with an average apparent height of 0.17 nm ±0.01half the unit cell height of Ni ( a Ni = 0.35 nm). The modulation of surface morphology as a function of alloy composition is addressed in a previous publication . The introduction of a small amount of oxygen, 1 L at 500 °C, did not induce an observable reaction, while a cumulative exposure of 10 L (1 L + 9 L) in a second oxidation step initiated the formation of oxide islands and nanoscale clusters (Figure G).…”

“…The Ni–12 wt % Cr(100) alloy was oxidized and annealed in several sequential reaction steps as illustrated in Figure and summarized in Table . Figure A shows the surface prior to oxidation . The first oxidation step (Figure B) corresponds to reaction step E 1 as defined in Table , and leads to step edge decoration with NiO, and formation of an abundance of nanoscale clusters on the terraces.…”

“…The base pressure was 2.5 × 10 –10 mbar, and imaging was done at ambient temperature. All metal thin films were deposited on MgO(100) single crystal substrates that had undergone pretreatment outlined in ref . Ni (Alfa Aesar 99.999% purity) and Cr (American Elements, 99.95% purity) were deposited on these MgO(100) substrates by electron beam evaporation using a Mantis EV mini e-beam quad-source.…”

“…The deposition rates of Ni and Cr were measured using a quartz crystal monitor to accurately adjust the film composition. The deposition of Ni and Ni–Cr thin films followed the procedure developed previously . The Ni–8 wt % Cr(100) and Ni–12 wt % Cr(100) alloys were deposited at a substrate temperature of 400 °C.…”

“…The work presented here focuses on the comparison between Ni(100), Ni–8 wt % Cr(100), and Ni–12 wt % Cr(100) oxidation at 500 °C using Ni–Cr(100) thin films grown under ultrahigh vacuum conditions on MgO(100) . These alloy compositions and oxidation conditions favor NiO formation, and enable us to isolate the impact of Cr on the surface reactions within the pre-Cabrera–Mott regime.…”