PHYSICAL REVIE% B VOLUME 28, NUMBER 3 1 AUGUST 1983 Iron-ion — implantation effects in MgO crystals A. Perez Departement de Physique des Materiaux, Universite Claude Bernard Lyon I, F-69622 Villeurbanne, France G. Marest Institut de Physique Nucleaire, Universite Claude Bernard Lyon I, F-69622 Villeurbanne, France B. D. Sawicka Institute of Nuclear Physics, Cracow, Poland J. A. Sawicki Institute of Physics, Jagiellonian Uniuersity, Cracow, Poland T. Tyliszczak Institute of Physics, Technical Uniuersity, Cracow, Poland (Received 17 February 1983) Magnesium oxide crystals implanted with ' Fe+ ions at doses ranging from 10' to 10' ions/cm (ion energy 70, 100, and 150 keV) were studied with the conversion-electron Mossbauer-spectroscopy technique. Supplementary data were obtained using the techniques of Rutherford backscattering and channeling of a particles, optical absorption, electron microscopy, and electrical conductivity. It was found that implantation introduces iron in MgO in three charge states: Fe +, Fe +, and metal- lic precipitates (Fe ) with the dominant role of Fe + at low doses, Fe + at medium doses, and metal- lic iron clusters at the highest doses. The phase created in a medium range of doses can be com- pared with the magnesio-wustite solid solution. The isochronal thermal annealings in air at tempera- tures between 300 and 700'C gradually cause the oxidation and the nucleation of highly dispersed spinel-like clusters and then, at about 800 — 900'C, the growth of magnesioferrite particles. In con- trast, the heat treatment in vacuum converts all iron into well-diluted Fe + in MgO phase. The na- ture of point defects and their role in annealing processes are discussed on the basis of the optical ab- sorption data. A good correspondence between the results of Mossbauer and channeling data is indi- cated. The effect of the insulator-conductor transition occurring under iron-ion implantation in MgO and observed by electrical conductivity measurements is explained in terms of the atomistic properties of implanted crystals under study. I. INTRODUCTION Heavy-ion bombardment of ionic crystals causes very complex structural changes and chemical effects, the knowledge of which is still limited until now, ' The solid phase which is created as a result of high-density cascades can incorporate implanted impurities in various charge states, which in turn can associate with various lattice de- fects, thus creating a multitude of metastable impurity- defect structures. Thermal annealing usually removes a great deal of lattice damage and leads via nucleation and diffusion to the growth of different precipitates of various equilibrium systems, which can be therefore predicted (and are sometimes well known) on the basis of classical phase diagrams. Therefore, when describing the nature of the as-implanted phases, the systematic investigations of the annealing behavior of implanted crystals are very help- ful. A characterization of metastable phases created by a high-dose (& 10' ions/cm ) implantation of metallic ions into a refractory oxide MgO has been a subject of particu- lar interest for several years. The results obtained until now show that the nature of precipitates in MgQ depends strongly on the kind of ions used: (i) Alkali-metal ions (I. i, Na, K, Rb) tended to form small metallic precipitates, (ii) indium and gold were used to form binary alloys with magnesium, and (iii) iron was found to oxidize and at elevated temperatures to form spinel ferrite particles. ' Various experimental methods employed thus far in the investigation of iron implantation in MgO included conversion-electron Mossbauer spectroscopy (CEMS), op- tical absorption (OA), Rutherford backscattering (RBS) and channeling techniques, transmission electron micros- copy (TEM), and electrical conductivity measurements. In order to characterize the investigated structures and processes, we performed systematic CEMS measurements which concentrated on two problems: (i) the dose depen- dence of Mossbauer spectra (from 10' to 10' ions/cm ) and (ii) the isochronal thermal annealing behavior (up to 1000'C); these studies are the subject of this paper. Sys- tematic complementary measurements by means of OA, RBS, and channeling methods are also reported. The study of iron-ion implantation in MgO presents for 28 1227 Q~1983 The American Physical Society