Formation routes and structural details of the ${\mathrm{CaF}}_1$ layer on Si(111) from high-resolution noncontact atomic force microscopy data

We investigate the ${\mathrm{CaF}}_1/\mathrm{Si}\left(111\right)$ interface using a combination of high-resolution scanning tunneling and noncontact atomic force microscopy operated at cryogenic temperature as well as x-ray photoelectron spectroscopy. Submonolayer ${\mathrm{CaF}}_1$ films grown at substrate temperatures between 550 and 600 ${}^{\circ }\mathrm{C}$ on $\mathrm{Si}\left(111\right)$ surfaces reveal the existence of two island types that are distinguished by their edge topology, nucleation position, measured height, and inner defect structure. Our data suggest a growth model where the two island types are the result of two reaction pathways during ${\mathrm{CaF}}_1$ interface formation. A key difference between these two pathways is identified to arise from the excess species during the growth process, which can be either fluorine or silicon. Structural details as a result of this difference are identified by means of high-resolution noncontact atomic force microscopy and add insights into the growth mode of this heteroepitaxial insulator-on-semiconductor system.

Figure 1

Models proposed in literature for the interface structures after (sub-)monolayer deposition of ${\mathrm{CaF}}_2$ on the $\mathrm{Si}\left(111\right)-(7\times 7)$ surface. The different models are arranged by the substrate temperature $T_{\text{Si}}$ used during film growth or in a second annealing step after deposition. Due to the complex dependence on other preparation parameters, structures including a mix of the presented phases are not uncommon. Directions referring to the Si (${\mathrm{CaF}}_2$) lattice are named ${〈hkl〉}_{\text{Si}}$ (${〈hkl〉}_{{\text{CaF}}_2}$).

Figure 2

Geometric structures of (a) ${\mathrm{CaF}}_1$ and (b) ${\mathrm{CaF}}_2/{\mathrm{CaF}}_1$ on $\mathrm{Si}\left(111\right)$ surfaces. Following Ref. [22], the Ca atoms are depicted at the $T_4$ site on the $\mathrm{Si}\left(111\right)-(1\times 1)$ surface. Equivalent directions of ${\mathrm{CaF}}_2$ are included in panel (b).

Figure 3

Two island types identified at submonolayer coverages of ${\mathrm{CaF}}_1$ on $\mathrm{Si}\left(111\right)$ marked by “type-I” and “type-II.” Islands are surrounded by bare $(7\times 7)$ regions as indicated. Imaging parameters: (a) $U_{\text{bias}}=-3.0\mathrm{V}, 220\mathrm{pA}$ and (b,c) $U_{\text{bias}}=-3.0\mathrm{V}, 50\mathrm{pA}$. Line profiles extracted at positions marked in panels (b) and (c).

Figure 4

(a), (c) STM and (b), (d) NC-AFM images of type-I ${\mathrm{CaF}}_1$ island edges showing (a), (c) the island outline and (b), (d) details of island edges. (e) Model of a ${\mathrm{CaF}}_1$ island with edges having normal vectors along ${〈\overline{1}\overline{1}2〉}_{{\text{CaF}}_2}$ directions. The three equivalent surface directions in the $\left(111\right)$ plane are drawn to the left. Imaging parameters: (a) STM in constant-current feedback, $U_{\text{bias}}=-3\mathrm{V},I_{\text{set-point}}=50\mathrm{pA}$. (b) NC-AFM in constant-$\mathrm{\Delta }f$ feedback at $U_{\text{bias}}=0\mathrm{V},\mathrm{\Delta }{f}_{\text{set-point}}=-17.6\mathrm{Hz}$. (c) STM in constant-current feedback, $-3\mathrm{V}, 10\mathrm{pA}$. (d) Constant-height NC-AFM imaging at $0\mathrm{V}$.

Figure 5

(a) STM and (b), (c) constant-height NC-AFM images of type-I ${\mathrm{CaF}}_1$ islands with defects. The positions of the detailed scans in panels (b) and (c) are marked in panels (a) and (b), respectively. STM imaging parameters: $U_{\text{bias}}=-3\mathrm{V}, 50\mathrm{pA}$. NC-AFM imaging at $0\mathrm{V}$.

Figure 6

(a) STM and (b), (c) constant-height NC-AFM images of type-II islands with rowlike defect structures. The positions of the detailed scans in panels (b) and (c) are marked in panels (a) and (b), respectively. STM imaging parameters: $U_{\text{bias}}=-3\mathrm{V}, 50\mathrm{pA}$. NC-AFM imaging at $0\mathrm{V}$.

Figure 7

High-resolution XPS scans of the (a) Ca $2p$ and (b) F $1s$ peak regions for the sample shown in Fig. 3 and acquired at one position. The ${\mathrm{CaF}}_1$ interface layer was prepared using substrate temperatures between 550 and ${600}^{\circ }\mathrm{C}$ during deposition.

Figure 8

Structures of the (a) dimer-adatom-stacking fault (DAS) model of the $\mathrm{Si}\left(111\right)-(7\times 7)$ reconstruction [84] and (b) reacted ${\mathrm{CaF}}_1/\mathrm{Si}\left(111\right)-(1\times 1)$ interface.