A comparison of the reaction of titanium with amorphous and monocrystalline silicon

Abstract

This paper describes investigations into the reactions occurring between Ti and monocrystalline Si (x-Si) or sputter-deposited amorphous Si (a-Si). Samples were structured so as to have the same Ti layer in contact with both crystalline and amorphous Si, and were thus ideally suited to compare the reactions in the same sample. Reactions were mainly investigated with cross-sectional transmission electron microscopy, but also with chemical characterization techniques such as Rutherford backscattering and Auger electron spectroscopy. We demonstrated that the reaction between Ti and x-Si or a-Si proceeded very similarly at low temperatures (≤450 °C). In both cases an amorphous silicide layer was observed to grow. Reaction rates were found to be nearly equal, even if some impurities were present on the x-Si surface prior to Ti deposition. One important difference was noted between the reactions, however. The reaction with a-Si was associated with Kirkendall void formation, while these voids were absent in the reaction with x-Si. We argued that the absence of voids in the case of the reaction with x-Si is due to a higher vacancy mobility in x-Si than in a-Si. At higher temperatures (500 °C), marked differences were observed in the reaction of Ti with crystalline or amorphous Si. These differences could be explained satisfactorily from thermodynamical considerations involving the heat of crystallization of a-Si. We postulated the existence of two kinetically competing pathways to go from the low-temperature configuration, where an amorphous silicide grows, to the stationary situation where diffusion-controlled growth of crystalline silicides occurs. The first one is crystallization of the amorphous silicide. The second one is the formation of a compositionally different crystalline silicide at the amorphous-silicide/x-Si interface. This postulate of kinetically competing pathways was used successfully to explain observations of amorphous and crystalline silicide growth in the Ni and Co/Si systems.