Reactive Growth of Tantalum Silicides in Ta-Si Diffusion Couples Chiara Milanese, ² Vincenzo Buscaglia, ‡ Filippo Maglia, ² and Umberto Anselmi-Tamburini* ,² Department of Physical Chemistry and C.S.T.E./CNR, UniVersity of PaVia, V. le Taramelli 16, I-27100 PaVia, Italy, and Institute of Physical Chemistry of Materials, National Research Council, Via De Marini 6, I-16149 GenoVa, Italy ReceiVed: December 31, 2001 The reactive growth of tantalum silicides was studied by means of Ta-Si diffusion couples annealed at 1250-1350 °C for 2-36 h. Two layers, corresponding to Ta 5 Si 3 and TaSi 2 , were observed in the diffusion zone. The TaSi 2 layer was about 1 order of magnitude thicker than the Ta 5 Si 3 layer. Formation of the compounds Ta 3 Si and Ta 2 Si was not detected, probably because their growth rate is very small. The concept of rate constant of the second kind was used to describe the growth kinetics of the silicides. For Ta 5 Si 3 , different values of the rate constant of the second kind can be obtained depending on the boundary conditions adopted at the Ta/Ta 5 Si 3 interface. The rate constant of the second kind was related to diffusion properties and thermodynamic stability of the given phase. Average values of the interdiffusion coefficient were calculated for the Ta 5 Si 3 and TaSi 2 compounds. The corresponding activation energy is ∼450 kJ mol -1 for Ta 5 Si 3 and ∼560 kJ mol -1 for TaSi 2 . 1. Introduction The transition-metal silicides have attracted much attention mainly for their application as thin films in integrated circuit technology because of their high electrical conductivity and their ability to form good contacts to silicon. 1,2 In particular, MoSi 2 , TaSi 2 , TiSi 2 , and WSi 2 are the compounds used for Schottky barriers and ohmic contacts, gate and interconnectors, and epitaxial conductors in heterostructures. At present, CoSi 2 and NiSi are also very promising for microelectronics. The semi- conducting (band gap 0.87-0.89 eV) silicide FeSi 2 has potential application for electroluminescent devices (high-intensity light- emitting diodes, LASERs). 3 Silicides are also known for their superconductive properties, in particular V 3 Si has a T c of 17 K. Transition-metal silicides show also remarkable mechanical strength and oxidation resistance up to high temperatures, and their use is currently under consideration for structural applica- tions and for coating of alloys sensitive to oxidation. 4-9 However, only MoSi 2 has been extensively investigated for its high-temperature properties. Many other silicides of the systems Cr-Si, Nb-Si, Ta-Si, Ti-Si, and V-Si might be potential candidates for high-temperature applications. In particular, the compounds of the Ta-Si system are among the most refractory silicides (melting points in the range 2200-2500 °C). Much attention has been spent on the study of the reactivity of transition-metal silicides, although these studies have been almost totally focused on thin films in view of their application in microelectronics. Many studies were devoted to the growth kinetics, to the identification of the dominant diffusion species, and to the determination of the activation energy for growth. However, despite the huge amount of literature available, the basic aspects of diffusion in silicides are still scarcely known. 10 In particular, most of the investigations on thin films were carried out at relatively low temperature, at which diffusion is dominated by processes along grain boundaries, surfaces, and extended defects. In addition, the interpretation of interdiffusion in thin films may be complicated by the importance of phase- boundary reactions and nucleation phenomena at the early stage of interaction, by the absence of phases or the formation of nonequilibrium compounds, and by the presence of impurities. 11 As a result, there is a lack of diffusion data above ∼1000 °C, at which the diffusion process is dominated by lattice diffusion. Only in recent years, reliable diffusion data have been collected for a few systems: Co-Si, 12,13 Fe-Si, 14,15 Mo-Si, 16-18 and Ti-Si. 19,20 This has contributed to a better understanding of the reactive growth of silicides at high temperature. No reliable diffusion data exist for Cr-Si, Nb-Si, Ta-Si, V-Si, and Zr- Si. For this reason, we undertook a systematic investigation on the diffusional growth of silicides of the aforementioned systems. In a first paper, 21 we have discussed the growth of vanadium silicides (V 3 Si, V 6 Si 5 ,V 5 Si 3 , VSi 2 ) in V-Si diffusion couples at 1150-1390 °C and the related diffusion properties. The present study deals with the reactive growth of tantalum silicides in Ta-Si diffusion couples. 2. Generalities 2.1. Growth Kinetics of Stoichiometric Binary Silicides in Metal-Silicon Diffusion Couples and Relation with Diffusion Coefficients. Let us consider a generic M-Si diffusion couple in which n binary silicides with narrow homogeneity range grow simultaneously as parallel bands. For the sake of simplicity, the stoichiometry of each silicide is normalized to 1 mol of silicon, M νi Si, and the layer sequence is, accordingly, M/M ν1 Si/M ν2 Si/.../M νn Si/Si with ν 1 > ν 2 ... > ν n . For the pure components M and Si, it is ν 0 ) ∞ and ν n+1 ) 0, respectively. If local thermodynamic equilibrium is estab- lished at the phase boundaries and interface processes are all faster than solid-state diffusion, the isothermal growth kinetics of each intermediate silicide (i ) 1, ..., n) will be described by * To whom correspondence should be addressed. Dipartimento di Chimica fisica, Universita ` di Pavia, V. le Taramelli 16, 27100 Pavia, Italy. Tel: +39-382-507208. Fax: +39-382-507575. E-mail: tau@chifis.unipv.it. ² University of Pavia. ‡ National Research Council. ∆x i 2 ) 2k i I t (1) 5859 J. Phys. Chem. B 2002, 106, 5859-5863 10.1021/jp014719k CCC: $22.00 © 2002 American Chemical Society Published on Web 05/16/2002