Investigation of the microstructure of hetero-epitaxial Si/CoSi2/Si (001) and (111) structures by TEM

Abstract

Hetero-epitaxial Si/CoSi2/Si structures have been formed by high dose implantation of Co+ ions into (001) and (111) Si substrates and subsequent annealing of the substrates. Such structures are of interest due to their application as metal base base and permeable base transistors. We have studied the microstructure of both as-implanted and annealed structures by transmission electron microscopy (TEM), including high-resolution electron microscopy (HREM). HREM was performed using a Philips 300 kV electron microscope with a point resolution of approximately 0.19 nm. CoSi2 layers have been formed by implantation of 170 keV Co+ ions at a temperature of 450°C and to doses of 1× 1017 and 2× 1017 Co+ / cm2. The wafers were annealed for 30 minutes in a N2/H2 ambient at a temperature of 1000°C. In the as implanted structures, the Co is present in the form of epitaxial CoSi2 precipitates. Precipitates occur both in an aligned (A-type) and twinned (B-type) orientation. Annealing of the implanted structures results in the formation of a buried CoSi2 layer of aligned orientation. A striking observation is that the CoSi2 layer has an aligned orientation with respect to the Si matrix, whereas CoSi2 grown on top of (111) Si has a twinned orientation. The mechanism behind this phenomenon is not fully understood. We think that geometrical aspects play a crucial role. Therefore we have studied in detail the geometry of the coordination of coherent CoSi2 precipitates.