Thermodynamically Stable Metal / III-V Compound-Semiconductor Interfaces
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During the past decade, there have been an enormous number of studies of the chemistry at metal/III-V compound-semiconductor interfaces. Several workers have attempted to systematize this work by correlating chemical properties with the electrical properties of the metal/semiconductor contact; for example, the Schottky barrier height. The trend has been to look at increasingly thinner metal 'films', since the Schottky barrier height appears to be established for submonolayer coverages of metal. Because highly sophisticated instrumentation is required to study submonolayer amounts of material, the investigation of metal contacts has become a very exciting high-technology endeavor. In attempting to understand the microstructure of such systems, however, the study of macroscopic chemical properties has been almost completely ignored. This situation is probably caused by the fact that most researchers and funding agencies have little interest in the lowtechnology techniques used for determining phase equilibria, since not much scientific glamour or glory is associated with them. This paper takes an alternative approach to understanding metal/III-V ocopound-semiconductor interfaces. The philosophy is to make certain that we understand the bulk chemistry of these contacts first. This knowledge is directly relevant to such topics as the morphology of the interfacial region and the compounds that may be found there; these in turn determine the ability to pattern extremely small features into the metallization and the reliability of a device structure. If a chemical reaction occurs at the contact interface, driven either by an anneal during the forming process or by Ohmic heating during device operatiori, then the gecmetry and electrical properties may change drastically in an uncontrollable fashion. Furthermore, establishing the bulk chemical properties of these systems will determine which materials may react with one another and what product species can be formed. With this information as a foundation, one may then examine thin film behavior to determine the relative importance of other macroscopic effects, such as bulk reaction kinetics and diffusion, as well as microscopic phenomena, such as defects and interfacial stresses and strains. Without a basic thermodynamic
Mat. Res. Soc. Symp. Proc. Vol. 54.-1986
Materials Research Society
336
understanding of the bulk systems in contact with one another, any model of the interfacial chemistry is just speculation. In fact, the bulk thexrmdynamics of metal/III-V systems has not been examined very thoroughly in the past. Very little information exists for such systems regarding which phases are stable in contact with one another and which will undergo chemical reactions to form solid products. Indeed, perhaps the most coamon misconception is to ignore the importance of entropy in determining the reactivity of a particular metal/III-V system, even though many experiments have shown that gaseous elemental group V species are evolved at relatively low temperatures when these
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