ELSEVIER Surfaceand Coatings Technology83(1996) 109-114 The use of EXAFS spectroscopy to show the structural modifications in metals implanted with N+ ions A. Fernkndez a, A. Caballero a, V. Jimknez a, J.C. SBnchez a, A.R. Gonz&lez-Elipe a9*,F. Alonso b, J.I. Ofiate b a Instituto de Ciencia de Mateyiales de Sevilla (CSIC-Univ. Sevilla) and Dpto. Q. Inorgcinica, P.O. Box 1115, 41080 Sevilla, Spain b INASMET, Camino de Portuetxe 12, 20.009 San Sebastihn, Spain Abstract Tantalum and titanium wereimplantedwith different doses of N’ ions at 70 keV, and the structural modmcations of the targets wereanalysed by X-ray absorptionspectroscopy (XAS) (EXAFS and XANES). The analysis requires the use of total yield detection and the correct calibration of the extraction depth of the XAS signal by means of MO/M standards (i.e. a metal oxide layer of known thickness grown on a metal). In the implanted samples of Ta, EXAFS spectroscopy shows the formation of Ta-N bonds at approximately 2.90A and the amorphization of the structure for a doseof 3.46x 1Ol7 ionscm-‘. A reordering of the network occurs for higher doses, asrevealedby the increase in intensity of the EXAFS oscillations and the appearance of new Ta-Ta and Ta-N distances of second and higher shells. In Ti, a metal whose coordination number with neighbouring metal atomsdoes not change on implantation of nitrogen, only smalldifferences at the second and higher shells were detected by EXAFS spectroscopy. The possibility of the useof XAS to study the implantation of metals with highly energetic ions is discussed. Keyrvords: Amorphization of metals;EXAFS; Ni implantation; Ta; Ti 1. Introduction Ion bombardment of metallic compounds induces atomic rearrangements which generally lead to the for- mation of new phases. The crystalline to amorphous transition is the most typical process encountered during irradiation [ 11. Structural analysis of the amorphization and mixing processes has been studied experimentally by different techniques including Rutherford back- scattering spectrometry (RBS) and channelling [2,3], transmission electron microscopy (TEM) [ 41, nuclear reaction analysis [3], etc. Moreover, implantation of metals with Nt ions with energies in the range 50-150 keV leads to the formation of new nitride phases [ 5,6]. Improvement of the mechanical, thermal, chemical and electrical properties of the implanted metals are beneficial effects of ion implantation processes [ 7,8]. X-Ray absorption spectroscopy (XAS) is a useful method for monitoring the local environment around a given atom. It provides information on the coordination number (CN) and distances, as well as the degree of disorder, through the estimation of the magnitude of the * Corresponding author. Tel.: 34-5-4625626; fax:34-5-4611962; e-mail: agustin@cica.es. Debye-Wailer factors associated with specific bonds [9]. In this paper, we attempt to analyse by XAS the structural changes which occur when metals (Ti and Ta) are implanted with high doses of N+ ions of 70 keV. In order to be sensitive to the surface of the material, we have used a total yield detection technique which is based on the ionization of He by the emitted electrons [lo]. Under these conditions, it is necessary that the thickness of the implanted zone is similar to the extraction depth of the electrons. To check this point, we have calibrated this extraction depth with standards consisting of oxide layers of known thickness grown on a metal. We have chosen Ti and Ta metals for this preliminary study by XAS because the former changes from an h.c.p. to an f.c.c. structure on implantation (i.e. in both cases a CN of 12) and the latter changes from a b.c.c. (CN 8) to an f.c.c. (CN 12) structure at high implantation doses [ 111. XAS is very sensitive to CNs, and therefore Ta and Ti represent two extreme cases to check the sensitiv- ity of XAS in implantation experiments. 2. Experimental details Ti and Ta metals were high purity 0.2 mm thick foils provided by Goodfellow (UK).