FABRICATION AND PROPERTIES OF EPITAXIALLY STABILIZED GE/ALPHA-SN HETEROSTRUCTURES ON GE(001)

We have investigated the influence of the growth parameters during molecular beam epitaxy on the realizibility of diamond crystal structure Ge/alpha-Sn alloys and superlattices on Ge(001) substrates. The segregation behaviour of Sn during Ge overgrowth has been studied. We find that for growth temperatures higher than 300-degrees-C the incorporation rates are less than 0.005 ML-1. The low-energy electron diffraction data of a series of Ge0.9Sn0.1 films deposited at substrate temperatures in the range of 185 to 275-degrees-C indicate a transition to amorphous growth for thicknesses beyond 20 angstrom. Single-crystal Ge(n)Sn(m) superlattices with alpha-Sn layer thicknesses m of 1 and 2 monolayers and periodicities n + m between 10 and 22 monolayers have been fabricated by an unconventional molecular beam epitaxy technique which involves large substrate temperature modulations during growth. Structural characterization of the samples by means of transmission electron microscopy, Raman spectroscopy and X-ray diffraction exhibits distinct superlattice effects. The downward shift of the fundamental energy gap of the superlattices with increasing Sn content, as extracted from absorption measurents with a Fourier transform spectrometer, is in excellent agreement with theoretical values obtained from pseudopotential band structure calculations. The films were found to be stable against phase transition up to temperatures of 430-465-degrees-C, depending on the average Sn content.