Ion Beam Synthesis of Buried Single Crystal Erbium Silicide
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ION BEAM SYNTHESIS OF BURIED SINGLE CRYSTAL ERBIUM SILICIDE A. GOLANSKI,* R. FEENSTRA,** M. D. GALLOWAY,** J. L. PARK,** S. J. PENNYCOOK,** H. E. HARMON,** AND C. W. WHITE** *Centre National d'Etudes des Telecommunications, B.P.98, 38240 Meylan, France **Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA
ABSTRACT High doses (10 16 -10 1 7 /cm 2 ) of 170 keV Er' were implanted into single-crystal (111)Si at implantation temperatures between 350'C and 520'C. Annealing at 800'C in vacuum following the implant, the growth and coalescence of ErSi2 precipitates leads to a buried single crystalline ErSi2 layer. This has been studied using Rutherford backscattering/channeling, X-ray diffraction, cross-sectional TEM and resistance versus temperature measurements. Samples implanted at 520*C using an Er dose of 7 x 1016 /cm 2 and thermally annealed were subsequently uscd as seeds for the mesocpitaxial growth of the buried layer during a second implantation and annealing process. Growth occurs meso-epitaxially along both interfaces through beam induced, defect mediated mobility of Er atoms. The crystalline quality of the ErSi2 layer strongly depends on the temperature during the second implantation. INTRODUCTION One of the interesting features of the rare earth silicides is that they form the lowest known Schottky barrier heights on n-type Si [1,2] and appear to be potentially attractive as low contact resistance materials for high integration density device structures. Until now, most investigations have involved interaction of deposited rare-earth thin films with single crystal Si. Silicides of Tb, Ho, Er, Tin, Yb, Lu, and Y have been shown to grow by solid phase epitaxy [3,4) while silicides of Tin, Yb, Lu, Gd, and Dy can be formed epitaxially in liquid phase reaction using electron beam melting. Single crystal ErSi2 layers have also been formed using ultrahigh vacuum co-deposition of Er and Si followed by an appropriate thermal annealing [5,61. While there are convincing demonstrations of the utility of ion beam synthesis in the formation of transition metal silicides [7 and ref. therein], only a few results concerning ion beam synthesis of rare earth silicides have been reported so far [81, possibly reflecting the potential difficulties related to the high theoretical value of the sputtering coefficient which may act as a dose-limiting factor for rare earth ions. It has been recently shlown, however, that ion implantation at elevated temperatures followed by an appropriate thermal annealing may be used to synthesize single crystalline erbium di-silicide [9]. It has also been shown [91 that at implantation temperature Ti > 280'C, the morphology of Er implanted Si is strongly influenced by two ion beam induced phenomena. Firstly, the defect mediated diffusion of erbium occurring at temperatures at least 400'C below the minimum temperature required for the thermally activated Er diffusion to become observable. Secondly, the ion beam induced formation of the polycrystalline ErSi2 phase occurring at Ti > 280 0 C, at lea
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