Effects of Electron Beam Exposure Conditions on the Surface Modification of CaF₂(111) for Heteroepitaxy of GaAs/CaF₂ Structure

Abstract

Optimal conditions of electron beam exposure for the surface modification of CaF 2 (111) by simultaneous exposure of an electron beam and an As 4 molecular beam were studied for the purpose of obtaining good electrical properties of heteroepitaxial GaAs films grown on modified CaF 2 . The amount of adsorbed As atoms on the CaF 2 surface and the crystallinity of grown GaAs film evaluated from electron mobility and cross-sectional transmission electron microscopy were examined by changing the electron energy ( V E ) and dose ( D E ). It was found that with adsorption of 1 atomic monolayer of As on the modified CaF 2 surface, the highest mobility of electrons and low defect density in the GaAs films were obtained when the value of V E × D E was a constant of about 300 mJ/cm 2 in the range of 40 eV≤ V E ≤300 eV. These results indicate that the degree of surface modification is governed by the total energy of incident electrons per unit area rather than V E or D E individually. In addition, a deviation from this relationship was observed at V E of 20 eV and 3,000 eV. Possible models for this constant V E × D E rule are proposed, in which F vacancies or secondary electrons generated in the bulk CaF 2 propagate to the surface of CaF 2 to provide occupation sites of As atoms.