Investigation of the surface properties of CaF2 layers on (1 1 1) Si as a function of growth temperature

Abstract

This work reports on the study of surface properties of CaF2 films (30 and 10 nm thick) grown on (1 1 1) Si by molecular beam epitaxy at substrate temperatures from 400 to 700 °C. Reflection high-energy electron diffraction (RHEED) analysis indicated that CaF2 films with smooth surfaces were obtained in temperature ranges 500–550 °C and 620–700 °C, while at temperatures from 400 to 500 °C and in the vicinity of 600 °C the films showed grains randomly oriented on top of the surface. Atomic force microscopy (AFM) investigation corroborated with the RHEED results and confirmed the presence of grains on the film surface, with an evident transition near 600 °C. The dependence of grain density on the growth temperature followed the expectation from the RHEED analysis. The arithmetical average roughness of the CaF2 surface obtained from the AFM images remained below 1 nm for the best quality films. The x-ray reflectivity curves of all samples exhibited well-defined interference fringes, whose oscillation damping behaviour agreed with the RHEED and AFM results. The CaF2 layer thickness and roughness were accurately determined by a best-fit procedure applied to the x-ray reflectivity data. By combining all results, the temperature range between 525 and 550 °C was found to be the most suitable to grow CaF2 layers on (1 1 1) Si. For growth temperatures above 650 °C, pinholes and cracks started to reduce the CaF2 surface quality.