Strain relaxation and surface morphology of nickel oxide nanolayers J. Schoiswohl a , W. Zheng a , S. Surnev a , M.G. Ramsey a , G. Granozzi b , S. Agnoli b , F.P. Netzer a, * a Institute of Physics, Surface and Interface Physics, Karl-Franzens University Graz, A-8010 Graz, Austria b Dipartimento di Szienze Chimique and INFM Research Unit, Universita di Padua, I-35131 Padova, Italy Received 17 August 2005; accepted for publication 22 December 2005 Available online 19 January 2006 Abstract The surface morphology and the lattice constants of NiO overlayers in the thickness range of 1–20 monolayers (NiO nanolayers) on Pd(1 0 0) have been investigated by high-resolution spot profile low-energy electron diffraction (SPA-LEED) and scanning tunneling microscopy (STM). NiO islands grow epitaxially on Pd(1 0 0) on top of a c(4 · 2) Ni 3 O 4 monolayer with a compressed strained lattice, which relaxes gradually attaining the bulk lattice constant at 10–12 monolayers. The strain relaxation is accompanied by the formation of small angle mosaic defect regions at the surface, which have been characterised quantitatively by following the behaviour of the satellites to the main Bragg diffraction rods. The analysis of the diffuse scattering intensity around the (0 0) diffraction spot reveals anisotropic NiO island shapes, whose orientation depends on the growth conditions. An incommensurate superlattice in LEED and STM at intermediate NiO coverages (2–6 monolayers) is observed and its origin is discussed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Epitaxial thin films; Growth; Oxide nanolayer; Nickel oxides; Palladium (1 0 0); Scanning tunneling microscopy (STM); High-resolution LEED (HR-LEED, SPA-LEED); Morphology 1. Introduction The epitaxial growth of oxide layers by vapour phase deposition is a preferred method for the fabrication of or- dered thin films of oxides with well-defined structural and electronic properties, as required for application in diverse areas of high-level technologies, including nano-electronic devices, spintronics, nanoscale sensors or advanced hetero- geneous catalysis [1–4]. Epitaxy is the ordered growth of one crystalline layer upon a pre-existing crystalline surface, and the close lattice matching of overlayer and substrate is an important parameter to support epitaxial growth. In heteroepitaxy, i.e. the growth of one type of material on a different substrate material, perfect lattice matching is rarely encountered. The resulting lattice mismatch at the interface leads to strain in the overlayer [5] and/or defects in form of misfit dislocations to reduce this strain. Lattice strain influences the electronic structure of the material [6] and its chemical reactivity [7], whereas dislocations cause the loss of coherence of the lattice reducing the mobility of charge carriers and thus are detrimental to the electrical transport properties. In ultrathin films of a few nanometer scale dimensions (nanolayers), the disloca- tions at the interface may reach the surface of the film caus- ing surface defects and particular surface morphologies such as elastic bending in the growing film. Strained lattices are in a non-equilibrium situation and tend to relax to the stable bulk lattice configuration at a critical distance from the interface. The relaxation process in turn may be accom- panied by the creation of defect structures. In this paper we address the lattice relaxation in nickel oxide nanolayers on a Pd(1 0 0) substrate and the effects that the lattice relaxa- tion has on the surface morphology of the NiO films. The NiO(1 0 0) bulk lattice and the Pd(1 0 0) substrate have a lattice misfit of 7.8%. We have used high-resolution low 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.12.038 * Corresponding author. E-mail address: falko.netzer@uni-graz.at (F.P. Netzer). www.elsevier.com/locate/susc Surface Science 600 (2006) 1099–1106