A study of the Si-Au-Ag interface by surface techniques

Auger spectroscopy, electron energy loss spectroscopy, and ion depth profiling techniques, under ultrahigh vacuum conditions, have been applied to the study of the formation of the Si-Au-Ag interface. The results show that the presence of gold atoms on the (111) face of a silicon substrate modifies drastically the structure of the silver layer subsequently deposited. At room temperature, from the Si substrate, one finds successively (i) a diffuse zone composed of an Si-Au-Ag alloy, the Ag concentration of which increases progressively (both gold-silver intermixing and silicon atoms diffusion from the bulk contribute to its formation), (ii) a pure silver layer, and (iii) a surface layer where silicon atoms have segregated and formed an Ag-Si alloy. The influence of the predeposited gold layer thickness has been investigated. Similar interface structures are obtained with gold coverage as low as 1.5 monolayers. These peculiar properties must be contrasted with the Si-Ag abrupt interface case where no silicon diffusion takes place. Results can be explained within a qualitative model involving interstitial diffusion of gold. The annealing of a room-temperature-grown interface shows two distinct regimes: for temperatures ≲ 400 °C, both silicon diffusion and gold-silver intermixing are enhanced. Beyond 400 °C, agglomeration of the metal overlayers takes place.