Bonding in transition-metal silicides studied via soft-x-ray-emission spectroscopy

Transitional metal (TM) silicides have been studied extensively due to their potential value as conducting overlayers in devices, and for the insight they provide into interactions at metal-semiconductor interfaces. A fundamental understanding of the electronic structure of the bulk electronic structure is helpful in understanding bonding in both metal-silicon and silicide-silicon systems. Soft-x-ray-emission spectroscopy has been used by present author to study a number of silicide systems. This unique experimental method provides a local, symmetry resolved partial density of states (LPDOS) of a compound. This spectroscopy helps establish a complete picture of the electronic structure of materials by providing information which is complementary to that provided by other spectroscopic methods such as photoemission and x-ray absorption spectroscopy. The s+d LPDOS of Si and d LPDOS of transition metals were measured for the pure phase mono- and disilicides of Fe, Co, and Ni and for samples prepared by ion implantation of Co and Fe in Si. For the disilicides, the spectra are in excellent agreement with available band structure calculations, and confirm the significant role that s electrons play in chemical bonding and structural stability. The understanding of the monosilicides is less satisfactory because good band structure calculations are not available. Consequently, only tentative and qualitative conclusions may be drawn about these materials. Comparisons of the spectra of ion-implanted samples with those of the pure phase materials gives useful insights into the silicide formation processes in these samples.