Influence of the addition of Co and Ni on the formation of epitaxial semiconducting β-FeSi₂: Comparison of different evaporation methods

β-FeSi₂ films containing 4-25 at. % of Co or Ni have been grown on (111) Si using three different evaporation methods: deposition of the metal film at room temperature followed by solid state reaction (solid phase epitaxy), deposition of the metal on a heated wafer [reactive deposition epitaxy: (RDE)] codeposition of metal and Si in the stoichiometric proportions [molecular beam epitaxy: (MBE)], The films have been analysed with Rutherford backscattering spectrometry, x-ray diffraction, secondary ion mass spectroscopy, and scanning electron microscopy. We show that β-FeSi₂ can be obtained with the three methods, however the quality, the epitaxial character and the morphology of the films differ. By far, the best results are obtained by MBE. In that case the codeposition of a Fe(Ni) alloy and Si at 550 °C leads to a large, epitaxial, mirror like β-Fe(Ni)Si₂ layer with a homogeneous Ni concentration. Detailed analysis of the Fe(Ni) and Fe(Co)/Si reaction and comparison with the Fe/Si one shows that: (i) Ni and Co do not modify the temperature of formation and the stability of the different Fe suicides providing the solubility limits in the different phases are not exceeded (e.g., 6 and 12 at. % in β-FeSi₂), (ii) the first stages of the reaction are characterized by the formation of a nonhomogeneous layer of FeSi. We did not observe the formation of a Si solid solution in Fe or of the ordered Fe₃Si phase, (iii) the transformation from FeSi to β-FeSi₂ induces a degradation of the β- FeSi₂ layer morphology. This explains why the processes (RDE and MBE) bypassing the formation of FeSi give better epitaxial layers.