Epitaxial layers of nickel fluoride on Si(111): Growth and stabilization of the orthorhombic phase

The growth and crystal structure of NiF$_2$ layers on CaF$_2$/Si(111) heteroepitaxial substrates have been investigated. It has been shown that molecular beam epitaxy at temperatures of 350–450$^\circ$C provides a stable epitaxial growth of the metastable orthorhombic NiF$_2$ phase (structural type CaCl$_2$) with a nickel fluoride layer thickness up to 1 $\mu$m in the metastable phase. According to X-ray diffraction, the unit cell parameters in layers of orthorhombic nickel fluoride are $a$ = 4.5680(1) $\mathring{\mathrm{A}}$, $b$ = 4.7566(3) $\mathring{\mathrm{A}}$, and $c$ = 3.0505(2) $\mathring{\mathrm{A}}$, which are very close to the known values for this phase. It has been established that the condition (100)$_{\mathrm{NiF}_2}$ || (111)$_{\mathrm{CaF}_2}$ holds over a wide range of growth parameters, which agrees with the results of the qualitative crystallographic analysis of the elements of similarity of the structures under consideration. The formation of a domain texture, the character of which depends on the growth temperature and nickel fluoride layer thickness, has been observed in the heterojunction plane.