Growth of Ni-Al alloys on Ni(111), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni3Al interfacial layer

摘要

This paper describes the second part of a study devoted to the growth of thin Ni-Al alloys after deposition of Al on Ni(111). In the previous paper [S. Le Pevedic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni3Al(111). In the present paper we show that this regime is only observed for Al deposits smaller than 8 X 10(15) Al/cm(2) and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 x 10(15) Al/cm(2). Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(110) layer (whose thickness depends on the deposited Al amount), over a Ni3Al "interfacial" layer (whose thickness-about 18 (111) planes-is independent of rhe deposited Al amount). The NiAl over-layer is composed of three variants, at 120 degrees from each other in the surface plane, in relation with the respective symmetries of NiAl(l 10) and Ni3Al(111). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-Ll(2) Ni3Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin NiAl interfacial layer, to a mixture of At and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the At film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate. (c) 2006 Elsevier B.V. All rights reserved.