SYNTHESIS OF EPITAXIAL SNXGE1-X ALLOY-FILMS BY ION-ASSISTED MOLECULAR-BEAM EPITAXY

The group IV metastable SnxGe1-x alloy system is an interesting semico nductor material with potential applications in the fabrication of Si- based heterojunctions and long wavelength infrared optoelectronic devi ces. Band structure calculations have suggested that the SnxGe1-x allo ys may have direct energy gaps continuously tunable from 0.55 to 0 eV for compositions x from 0.2 to 0.6 with very small electron effective masses and hence high electron mobilities. However, syntheses of SnxGe 1-x alloy films in the direct gap composition range by conventional ep itaxial or polycrystalline thin film growth techniques have not been s uccessful due to the severe surface segregation of Sn during the film growth. In this work, we report the synthesis of epitaxial SnxGe1-x/Ge /Si(001) with compositions up to x=0.34 by ion-assisted molecular beam epitaxy with 30-100 eV Ar+ ions produced by an electron cyclotron res onance ionization source with ion to atom flux ratios of the order of unity in the substrate temperature range of 120 to 200 degrees C. High flux low energy ion beam irradiation greatly inhibits Sn segregation without interrupting epitaxy. In situ reflection high energy electron diffraction as well as X-ray rocking curve indicated epitaxial SnxGe1- x alloy films, and Rutherford backscattering spectra confirmed the Snx Ge1-x alloy compositions and indicated an absence of Sn segregation.