Epitaxial growth of alkaline earth fluorides on the (0 0 1) surface of lithium fluoride. III. The system — epitaxial growth of LiBaF3

Abstract

Abstract The development of overlayers during the interaction of molecular beams of BaF 2 with (0 0 1) surfaces of LiF is studied for crystal temperatures T = 573–673 K and for impinging BaF 2 fluxes j on = 1 × 10 12 −1.5 × 10 13 cm −2 s −1 . As long as the LiF supply from the substrate is not hindered formation and epitaxial growth of LiBaF 3 dominates strongly, and growth of BaF 2 in the orientation BaF 2 (0 0 1)[1 0 0]|LiF(0 0 1)[1 1 0] with a misfit m = + 8.8% occurs only as a transient and altogether negligible phenomenon. Grwoth of LiBaF 3 proceeds in the orientation with parallel axes, i.e. with LiBaF 3 (0 0 1)[1 0 0]|LiF(0 0 1)[1 0 0], with a misfit m = − 0.82%. As growth mode, island growth with nucleation, growth and coalescence of three-dimensional islands is found. The formation of closed layers can be accelerated by decreasing the crystal temperature or increasing the impinging molecular beam flux. With a BaF 2 flux only, layer thicknesses, because of the necessary supply of LiF from the substrate, are limited, while with congruent LiF and BaF 2 fluxes, layers of virtually unlimited thicknesses can be grown. By evaporation of LiF onto already grown LiBaF 3 layers, epitaxial growth of LiF in an orientation corresponding to that of the underlying LiF crystal is found. Closed LiF layers are achieved for T = 573 K and j on (LiF) = 1 × 10 12 cm −2 s −1 at a thickness of ≈ 30 nm. This would, in principle, enable the fabrication of superlattices LiF/LiBaF 3 /LiF etc. with layer thicknesses suitable for optical applications.