Synthesis of NiSi 2 by 6 MeV Ni implantation into silicon
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I. INTRODUCTION During the last years, studies on buried compound layer formation by high-dose ion implantation into silicon have mainly been concentrated on the production and characterization of buried SiO2 and Si3N4 in silicon. Meanwhile, there is a growing interest in applications of high-dose metal ion implantation for the synthesis of buried metal silicide layers.1"12 Due to their importance as Ohmic or Schottky contacts and interconnections in very large scale integration (VLSI) technology, metal silicides formed by solidstate reactions of deposited metal films on silicon have been studied to a large extent. Among the silicon-rich silicides that do not require overly high implantation doses, NiSi2 as well as CoSi2 are of special interest. They grow in a cubic (CaF 2 ) crystal structure with lattice constants close to that of silicon (0.4% and 1.2% mismatch, respectively). Therefore, these disilicides can be grown epitaxially on silicon. Using molecular beam epitaxy (MBE) techniques, even heteroepitaxial layer systems such as Si/MSi 2 /Si can be produced,13'14 enabling the realization of novel devices.l5 White et al.1 were the first to show that buried single-crystal epitaxial CoSi2 layers can be grown by highdose implantation and annealing. Similar results have been reported for the implantation of Cr, Ni, Y, and Co + Ni. 2 In order to understand the processes occurring during high-dose metal implantation in more detail, we irradiated Si wafers with 6 MeV Ni ions up to stoichiometric Ni concentrations. The use of high-energy ions is advantageous for such a study since problems due to surface sputtering, as reported for studies on low-energy implantation synthesis,1'3"6 are avoided. Moreover, the comparatively deeper and broader implantation profiles 1238
J. Mater. Res. 3 (6), Nov/Dec 1988
http://journals.cambridge.org
facilitate detailed investigations of the depth dependence of structural changes. In this paper, the results on optical and electrical investigations as well as transmission electron microscopy (TEM) are presented.
II. EXPERIMENTAL High-purity (111)-oriented silicon crystals (n-type material) of 300/im thickness were implanted from a 7° off-normal direction with 6 MeV 5 8 Ni 2 + ions in a dose range from 2 X 1013 to 1.3 X 1018 Ni/cm 2 . Homogeneous irradiations of sample areas of about 0.5 cm2 were achieved by electrostatically scanning a beam of 2 mm in diameter. Current densities were 0.3 //A/cm 2 in the dose interval 2X10 13 to 2X10 16 Ni/cm 2 and 3.5-12 //A/cm 2 for doses of 7X 1015 Ni/cm 2 and above; for some implantations the current densities were varied in order to assess possible dose rate effects. Implantation temperatures, measured at the substrate holder, were kept constant at values between 125 and 450 K to an accuracy of 2 K, using a resistivity heater against a liquid-nitrogen heat sink. In order to obtain good thermal contact, samples were pressed against the wafer holder with vacuum grease (Apiezon H) as a contact medium. All implantations were carried out at the Dynamitron
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