Study of boron effects on the reaction of Co and $Si1−xGex$ at various temperatures

The effects of boron on Co and $Si1−xGex$ interfacial reaction were studied. Undoped and in situ boron-doped strained $Si0.91Ge0.09$ and $Si0.86Ge0.14$ layers prepared at 550 °C by an ultrahigh vacuum chemical vapor deposition system were subjected to Co silicidation at various rapid thermal annealing (RTA) temperatures ranging from 500 to 1000 °C. The resulting films were characterized by a sheet resistance measurement, Auger electron spectroscopy, x-ray diffractometry (XRD), high resolution x-ray diffractometry, secondary ion mass spectroscopy, scanning electron microscopy, and transmission electron microscopy. Seen from XRD spectroscopy, a $Co(Si1−yGey)$ cubic structure was formed with RTAs ranging from 500 to 700 °C. For boron-doped samples, the CoGe fraction in $Co(Si1−yGey)$ was less than that in undoped samples, indicating that boron atoms retarded the incorporation of Ge into the $Co(Si1−yGey)$ ternary phase. It also led to a large Ge pileup at the interface between the Co-rich and silicidation regions. On the other hand, from the high resolution x-ray spectra, the presence of boron led to less relaxation of the strained $Si1−xGex$ lattice. It is the first time that small boron atoms inhibiting the relaxation of the $Si1−xGex$ layer during silicidation was observed. Furthermore, from the sheet resistance measurement, the formation of $CoSi2$ was found to be slightly retarded in boron-doped samples, due probably to the decrease of Co or Si diffusivities as a result of boron accumulation at the Co/SiGe reaction interface. At temperatures above 800 °C, $CoSi2$ formed and Ge segregated to the silicide boundaries and the upper reaction region was discovered. These phenomena caused by B dopants are explained in detail.