Entropy-driven loss of gas phase group V species from gold/III-V compound semiconductor systems
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Temperature-dependent chemical interactions between Au and nine III-V compound semiconductors (III = Al,Ga,In and V = P,As,Sb) have been calculated using bulk thermodynamic properties. Enthalpic considerations alone are insufficient to predict metal/ compound semiconductor reactivities. The entropy of vaporization of the group V elements is shown to be an extremely important driving force for chemical reactions involving the III-V's, since it enables several endothermic reactions to occur spontaneously under certain temperature and pressure conditions. Plots of either Gibbs' free energies of reaction or equilibrium vapor pressure of the group V element versus temperature are used to predict critical reaction temperatures for each of the systems studied. These plots agree extremely well with previous experimental observations of thin film reactions of Au on GaAs.
I. INTRODUCTION Chemical reactions that occur at the interface between a metal film and a compound semiconductor have been the subject of active academic and technical interest for many years. Brillson1 has proposed that the relative chemical reactivity of the constituents at a metal/compound semiconductor interface controls the Schottky barrier height of the system. When reactions proceed, they consume a portion of the semiconductor material and may induce defects or even etch deep pits in the substrate.2 Thus it is extremely important to have a basic understanding of the chemistry of metal/semiconductor systems in order to be able to control the electronic properties and reliability of devices. In the absence of sufficient information about reactivity in thin films and at interfaces, many investigators have used bulk thermodynamic properties of metals and semiconductors to model and predict thin film reactivities.1'3-4 Deviations between calculations and observed behavior were often attributed to interfacial effects.34 These investigators calculated enthalpies of reaction (Ai/fl ) for various metal/semiconductor combinations to determine the feasibility of interaction. The true measure of reaction spontaneity, however, is the change in the Gibbs free energy of the system, (AGR), which takes into account the entropy change of the reaction (&SR ) of a system as well as AHR .5 Based on the assumption that metal/semiconductor reactions involve only solid-phase products, the contribution of entropy was considered insignificant.3 However, loss of gasphase species of the group V element from Hl-V compound semiconductors has been observed during reactions of thin metal films with substrates.6 The formation of gas-phase products requires the inclusion of the entropy change, which is positive and very large for a vaJ. Mater. Res. 1 (2), Mar/Apr 1986 http://journals.cambridge.org
porization process, in reaction feasibility calculations. The determination of AGR as a function of temperature indicates the temperatures at which a reaction may occur spontaneously. However, thermodynamics cannot predict the rate at which it does take place; hence a reaction that is the
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