Initial stages of the Pd-GaAs reaction: Formation and decomposition of ternary phases

Abstract

Investigations of the metallurgical reactions occuring at metal-compound semiconductor interfaces are essential for the understanding of ohmic contact and Schottky barrier formation mechanisms. The resulting film and interface morphologies are also important for the performance of discrete devices and integrated circuits. In this study the reactions occurring during thermal annealing of the Pd-GaAs system are investigated. Reported here are the initial stages of the Pd-GaAs reactions as studied by high resolution transmission electron microscopy, electron diffraction and energy-dispersive analysis of X-rays. The first Pd-GaAs reaction product, observed in the low temperature range (less than about 315°C), is the hexagonal ternary phase Pd 5(GaAs) 2 (phase I with a 0 = 0.673 nm and c 0 = 0.338 nm). The second stage of the reaction leads to the formation of a second hexagonal ternary phase, Pd 4GaAs (phase II with a 0 = 0.92 nm and c 0 = 0.370 nm), and the binary phases Pd 2Ga and Pd 2As. The concurrent application of energy-dispersive X-ray analysis and high resolution cross-sectional transmission electron microscopy leads to the unambiguous identification of the ternary Pd 5(GaAs) 2 and Pd 4GaAs phases. X-ray diffraction data alone could erroneously lead to the identification of Pd 5(GaAs) 2 as PdGa and of Pd 4GaAs as PdAs 2. In view of this source of ambiguity in previous work, the sequence of reactions in the Pd-GaAs system is re-evaluated and the evolution of the film and interface morphologies is discussed.