Reactions between palladium and gallium arsenide: Bulk versus thin-film studies
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I. INTRODUCTION Two major goals in the metallization of compound semiconductors such as gallium arsenide are rectification (gate metallization) and low resistance (Ohmic contacts). The resulting contacts from this metallization must be chemically stable in addition to exhibiting the appropriate electrical properties. The chemical stability of the contacts as well as the electrical properties are governed by the thermodynamics and kinetics of phase formation between the metallizing elements and the compound semiconductors. Most of the studies so far have focused on interfacial reactions between thin metal films and gallium arsenide.1"2' These studies include the identification of the phases formed and to some extent the characterization of resulting morphologies when the metal/gallium arsenide contacts were exposed to specific environments. Since relevant phase equilibria, thermodynamic, and kinetic data are not available for the ternary Ga-As-M systems, a rationalization of these results (some of which show apparent contradictions) is difficult if not impossible. The recent studies of Pugh and Williams,22 Tsai and Williams,23'24 and Beyers, Kim, and Sinclair25 have pointed out the importance of phase diagrams in rationalizing interfacial reaction studies. More importantly, phase diagrams provide a framework for selecting metals that would be the most promising candidates for metallization of a particular compound semiconductor. Beyers,26 Rudy,27"29 Chang and Naujack,30 and Brewer and Wengert31 among others have properly related the thermo-
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J. Mater. Res. 3 (1), Jan/Feb 1988
http://journals.cambridge.org
dynamic properties of the competing phases in a ternary to its phase diagrams. In other words, if phase diagrams are not known, they may be calculated if all the thermodynamic data are available. Frequently neither a complete phase diagram nor thermodynamic data are available. Nevertheless, thermodynamic values may be estimated either from similar compounds where data are available or from the semiempirical model of Miedema.32'33 The phase diagrams calculated in this manner may not be correct, but they do provide an excellent guide in carrying out critical experiments and in selecting candidate materials for metallization of a particular compound semiconductor. It is evident from the above discussion that phase diagrams are essential not only for the understanding but also for the successful metallization of compound semiconductors. Nevertheless, phase-diagram information is not sufficient. There is a need for kinetic data such as the diffusion path of a metal/compound semiconductor contact. According to the theory of diffusion in ternary systems, the diffusion path, the sequence of phases in equilibrium across a diffusion couple, is unique at a constant temperature and pressure, i.e., there is only one diffusion path. 34 This path may be obtained from bulk-diffusion couple experiments. A knowledge of the bulk-diffusion path should allow us to predict the sequence of phase formation in thin metal film/compound
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