Initial growth stages of CaF 2 on Si(111) investigated by scanning tunneling microscopy

Abstract

The initial stages of calcium fluoride (CaF 2) growth on Si(111)-(7×7) have been studied using ultrahigh-vacuum (UHV) scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) from the submonolayer range up to two monolayers (ML: 1 ML CaF 2=7.84×10 14 cm −2). The STM images directly indicate that the initial growth mode changes from an island formation at around 640°C to the growth of a wetting row-like region above about 750°C. At a substrate temperature of around 640°C, the islands of characteristic shapes, with steps arranged in the [11̄0] directions of the substrate, are formed initially both at steps and on the flats of Si terraces. The island is a CaF layer that has a (1×1) periodicity. At a higher temperature of around 750°C, the region that has a well-ordered row structure along the [11̄0] directions are observed only at Si step edges. The LEED pattern indicates that the row-like region has a (3×1) periodicity, and XPS measurements show that the row-like region still has a Ca:F stoichiometry of 1:1. Based on the coverage of the deposited CaF 2 molecules, the row-like region is a reconstructed layer induced by CaF adsorption. Furthermore, in situ STM measurements of the initial growth stages of CaF 2 have been performed at 700°C and 800°C. The results clearly show that the first row-like region and the second layer grown on the first region have different growth modes.