Effects of electron beam exposure conditions on the surface modification of CaF2 (111) for heteroepitaxy of GaAs/CaF2 structure

Optimal conditions of electron beam exposure for the surface modification of CaF2(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 CaF2. The amount of adsorbed As atoms on the CaF2 surface and the crystallinity of grown GaAs aim 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 CaF2 surface, the highest mobility of electrons and low defect density in the GaAs films were obtained when the value of V-E x D-E was a constant of about 300 mJ/cm(2) in the range of 40 eV less than or equal to V-E less than or equal to 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 x D-E rule are proposed, in which F vacancies or secondary electrons generated in the bulk CaF2 propagate to the surface of CaF2 to provide occupation sites of As atoms.