Initial Stage of CaF₂/Si(111) Epitaxy Investigated by Lateral/Atomic Force Microscopy

Abstract

The initial stages of the high temperature (~ 700°C) MBE growth of CaF 2 on well-oriented Si(111) substrates have been studied by atomic force microscopy (AFM) and lateral force microscopy (LFM) in the atmosphere. At these temperatures CaF 2 molecules react with the silicon surface and form a CaF 1–Si interface layer. The CaF 1 layer covers the silicon surface completely before CaF 2 islands start to form, and it is stable in the atmospheric environment for some days. The subsequent growth on this interface layer results in multilayer CaF 2 islands (about 5 TL in height). The friction force experienced by the Si tip was found to be larger on the CaF 1 interface layer than that on the CaF 2 layer and allows a direct distinction between the CaF 1–Si interface layer and CaF 2. Two different types of islands are observed. One has a regular shape with a flat top surface; the other has an irregular shape and exhibits a 0.3–0.6-nm-high corrugation. We suppose that the formation of disturbed islands can be attributed to the dissociation of the CaF 1 interface at the growth temperature of 700°C. These results suggest that high growth temperatures (> 600°C) do not always result in the best CaF 2 film.

Formation routes and structural details of the CaF1 layer on Si(111) from high-resolution noncontact atomic force microscopy data

“…Thus, the following analysis assumes the existence of an interface layer with CaF 1 stoichiometry. With this result, the island types identified here are different from the two island types on a multilayer sample observed by Wang et al [61] and, additionally, we can exclude high-temperature interface reconstructions due to an F:Ca ratio falling significantly below 1 [37]. Furthermore, as all experiments were performed under ultra-high-vacuum conditions, we are confident that we do not investigate a contaminated surface after air exposure [22,30].…”

“…A small number of studies have been performed using atomic force microscopy, most of them in contact mode [34,[60][61][62]. Recently, Klust et al presented materialdependent [63] as well as atomically resolved [64] data using in vacuo frequency-modulated NC-AFM, identifying the CaF 1 and CaF 2 areas and revealing the atomic lattice with similar contrast as has been observed before on (111) surfaces of CaF 2 crystals [65,66].…”

“…For the case of CaF 2 on Si(111), Nakayama et al [55] identified before a total of four excess silicon atoms in the first layer during the interface formation or as a result of filling the cornerholes, and discussed their presence within the resulting interface. However, and also as an important difference to the processes revealed for Ca or for Na restructuring silicon surfaces [40,85], the known desorption of SiF x species during the etching step [22,61] offers a route to remove excess silicon and, consequently, cannot be ignored in the species quantification.…”

“…Under the assumption that only F 2 or SiF x (with x 1) species desorb from the surface during CaF 1 interface growth 125418-7 [22,61], it is instructive to quantify the involved species. The CaF x film forms a (1 × 1) superstructure, thus 49 excess fluorine atoms are present within the Si (7 × 7) unit cell for a closed CaF 1 film.…”

Temperature-dependent growth mechanisms of CaF2 on Si(111)

“…Meanwhile, some CaF 2 molecules nucleate on CaF terraces. In our previous work, 26,27 we observed that some parts of the CaF interface layer were disturbed by thermal decomposition at 700°C. 9 and 23, and a nonuniform CaF 2 film will be obtained, as depicted in Fig.…”