Initial growth stages of CaF2 on Si(111) investigated by scanning tunneling microscopy

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].…”

“…The atomic resolution NC-AFM data acquired on type-II islands reveal a (1 × 1) lattice structure as observed for type-I islands; we can therefore exclude that this island type is already formed by one of the high-temperature reconstructions [36]. Consequently, the imaged rows are different from row structures due to reconstructions that have been observed with STM on samples prepared at higher temperatures before [37,52,57,59,78]. In fact, we can induce high-temperature reconstructions by annealing the sample above 600 • C (data not shown).…”

“…Third, upon further increasing the substrate temperature T Si , the F:Ca ratio decreases due to further dissociation of CaF 1 and the desorption of fluorine. With the decreasing F:Ca ratio, a series of surface reconstructions has been observed for the intermediate F-depleted film above 650 • C [36,37], finally evolving into an apparent 3 × 1 reconstruction where fluorine is absent [38] (see T 4 in Fig. 1).…”

“…Nakayama et al [55,56] investigated defect structures within the CaF 1 interface layer grown on Si(111) and suggested that the depressions imaged in STM are formed by clusters of excess Si atoms. Further studies focused on the morphology of submonolayer and multilayer films, where different growth modes and morphologies from varying the substrate temperature during deposition or measurement could directly be imaged [34,37,[57][58][59].…”

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

“…For such RT devices, only a few Si-based epitaxial heterostructures have been found. [10][11][12][13][14][15][16][17] Recently, Olmstead 9 has given a full overview on the temperature-dependent CaF 2 / Si interface structures and surface morphologies, pointing out that the CaF 2 / Si interface shows three different interface stoichiometries at different growth temperatures: (i) at temperatures above ϳ600°C, the CaF 2 molecules dissociate at the Si surface into CaF and F. The CaF generates a reacted Si-Ca-F interface layer resulting in a B-stacking epitaxy, while the excess F atoms leave the surface as F 2 or SiF x . [4][5][6][7][8] The lattice mismatch between Si and CaF 2 is only 0.6% at room temperature and the crystal structures are compatible.…”