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

Abstract

We investigate the CaF 1 /Si(111) 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 CaF 1 films grown at substrate temperatures between 550 and 600 • C on Si (111) 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 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.

PTCDA adsorption on CaF2 thin films

“…A detailed statistical analysis is shown in Figure 3a, where the orientation along the double-lobe shape (i.e., along the long molecular axis, see also white dashed line in Figure 3c) was measured with respect to a CaF 2 direction from more than 1000 molecules in a total of 10 images. The CaF 2 directions were determined from filled-state imaging of the (7 × 7) reconstruction [27]. Due to the type-B epitaxy of CaF 2 on Si(111) at the chosen growth parameters, the CaF 2 direc- tion is identical to the Si direction that was determined from STM images.…”

“…These positions are marked by solid white circles and arrows in Figure 3f, while stationary molecules are marked by white dashed circles in both panels. Defects are imaged in STM as black depressions although they are of atomic size [27]. In contrast, the CaF 2 surfaces are mostly defect-free.…”

“…After PTCDA deposition, individual double-lobe features are apparent in STM at negative sample bias on the CaF 1 /Si(111) areas (see also Fig. 1 ), in addition to the dark spots that were identified before as single atomic-size defects within the CaF 1 interface layer [ 27 ]. Each of these double-lobe features is tentatively assigned to a single PTCDA molecule.…”

“…3 ) was measured with respect to a CaF 2 direction from more than 1000 molecules in a total of 10 images. The CaF 2 directions were determined from filled-state imaging of the (7 × 7) reconstruction [ 27 ]. Due to the type-B epitaxy of CaF 2 on Si(111) at the chosen growth parameters, the CaF 2 direction is identical to the Si direction that was determined from STM images.…”

“…CaF 2 material (cleanliness 99.9%, Alfa Aesar, Kandel, Germany) was deposited from an e-beam evaporator (type EFM3T, Focus GmbH, Huenstetten, Germany) on silicon samples held at about 600 °C by direct-current heating. Further details on the sample preparation and CaF 2 /Si(111) surface properties can be found in [ 27 – 29 ]. PTCDA molecules were deposited from custom-built Knudsen cells heated to 290–300 °C.…”