Intermixing and subsurface alloy formation: Ir on Cu(100)

Abstract

The growth and structure of Ir films on Cu(100) up to 1.5 ML have been investigated by means of scanning tunneling microscopy (STM), low-energy ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). After Ir deposition at 200 K strong intermixing of Cu and Ir has been found, giving rise to step roughening and surface etching. With the help of ISS the amount of Ir in the first layer was determined to be less than 20% of the nominal Ir coverage. Temperature-dependent ISS and XPS measurements indicate that, upon annealing, Ir first segregates completely into the subsurface layer and finally, for temperatures above 650 K, into deeper layers. In the STM images the Ir atoms in the second layer are visible owing to a chemical contrast. Deposition of Ir at 620 K results in the formation of a long-range ordered subsurface alloy. The LEED pattern exhibit a (2×1) superstructure with maximal intensity of the half-order spots occurring at an Ir coverage of about 0.5 ML. In the STM images the surface is marked by a chain-like structure, with chains running along the 〈011〉 directions. The measured corrugation of 0.3 Å in STM appeared due to imaging the electronic differences of the embedded Ir–Cu alloy layer. The ordered alloy phase can be modeled by alternating Cu and Ir chains located in the second layer that are covered completely by one copper layer.