Epitaxial deposition mechanism of barium fluoride on single crystal silicon substrate using molecular beam epitaxy

The deposition of films made of fluoride compounds has been a goal for researchers because of their application as insulators in integrated microelectronics and as substrates. Films of barium fluoride (BaF$\sb2)$ have been grown directly on both (111), and (100) oriented silicon (Si) crystals. BaF$\sb2$ films of excellent quality grow oriented along the (111) direction regardless of the substrate orientation. The Si crystal has a lattice constant of 5.45 A in contrast with BaF$\sb2$ with a lattice constant of 6.20 A. It is difficult to explain how BaF$\sb2$ can be grown on Si using a simple atomic stacking mechanism. Strains at the interface due to differences in lattice constants should not allow the hetero-epitaxy system to exist. This work deals with the first attempt to explain how BaF$\sb2$ can be grown on Si by investigating the chemistry at the interface. X-ray photoelectron spectroscopy was used to analyze the interface between BaF$\sb2$ and Si. Results show that fluorine is not present at the interface but rather a barium-silicon compound is formed at the interface. X-ray diffraction measurements confirm the formation of a new compound at the interface. Further evidence of the chemical reaction at the interface is that by heating the BaF$\sb2$ films on Si, all of the film can be converted to the barium-silicon compound. This new Ba-Si film was used as a substrate for deposition of PbTe with excellent results. PbTe like BaF$\sb2$ has a lattice constant considerable larger that silicon (6.49 A). The PbTe films grew (100) oriented and had very little strain according to X-ray diffraction results. The results reported here suggest that a new procedure for making silicide films can be developed. Also the results can be used to explain the chemistry in other fluorides on Si.