Effect of Co, Pt, and Au additions on the stability and epitaxy of NiSi2 films on (111)Si

We studied the effect of the addition of cobalt, platinum, or gold on the cell parameter of NiSi2 deposited epitaxially on (111)Si. Namely, the formation and the microstructure of NiSi2 films containing one of these elements are compared to those of the pure disilicide. The solubility of Co, Pt, and Au in NiSi2 ranges from a total substitution to nickel in the case of cobalt to a very weak quantity (less than 1%) in the case of platinum. An intermediate behavior was observed for gold which can occupy more than 10% of the metal sites. This important solubility has been confirmed by analysis of bulk Ni(Au) disilicides and is understood as a consequence of the metastability of gold silicides. Cobalt, Pt, and Au additions change the temperature of formation of NiSi2: from 800 degrees C for reaction with pure nickel, this temperature is lowered to 650 degrees C by Co and Au additions while it is increased to 950 degrees C for Pt. These modifications are consistent with the nucleation controlled formation of NiSi2 and the effect of added elements on both the interfacial energy and the stability of the disilicide. Measurements of the cell parameters (perpendicular and parallel) by x-ray diffraction revealed the relaxation state and the cell evolution of NiSi2 films. Platinum addition does not modify the cell in agreement with a very weak incorporation of platinum in NiSi2. The rigid interface model is well adapted to the growth of NiSi2, Ni(Au)Si-2, and Ni(Co)Si-2 films. According to this model and in spite of a large rhombohedral deformation, the system which better accommodates silicon is Ni(Co)Si-2: in this case, there is practically no misfit between the film and the substrate. We also showed that misfit dislocations are generated principally during the NiSi2 formation and that their density evolves very little afterwards. (C) 1998 American Institute of Physics. [S0021-8979(98)01717-4].