OBSERVATION OF GOLD SILICON ALLOY FORMATION IN THIN-FILMS BY HIGH-RESOLUTION ELECTRON-MICROSCOPY

Thin films (< 30 nm) containing mixtures of gold and silicon were co-deposited on (100) surfaces of NaCl from the pure elements co-evaporated at equivalent molar rates. The resulting samples, believed to be homogenous distributions of the elements, were used to observed alloy formation during electron beam heating in an electron microscope. The formation of the alloy was shown to accompany the displacement and subsequent crystallization of excess silicon. The crystallographic data derived from the microscopic study were found to agree within experimental error with those reported from studies of bulk samples prepared by the rapid quenching of gold/silicon melts and assigned the composition Au2Si. Samples heated under intense electron beam irradiation resulted in the decomposition of the alloy and the formation of crystalline gold and silicon. Thicker samples (approximately 100 nm) were prepared by evaporation on (100) NaCl and subsequently heated at temperatures up to 250-degrees-C. These specimens by the "thin-film" X-ray diffraction technique confirmed the formation of Au2Si as well as its subsequent decomposition at temperatures above 100-degrees-C.