DIFFERENTIAL OPTICAL-ABSORPTION SPECTROSCOPY AND X-RAY CHARACTERIZATION OF SYMMETRICALLY STRAINED GE-SI SUPERLATTICES

We have used a differential spectroscopy technique to measure and characterize the optical absorbance spectrum of a 2500 angstrom symmetrically strained Ge-Si superlattice. Using X-ray diffraction we found that the average periodicity of the superlattice in the growth direction is 11 atomic layers, to measure the strain and disorder of the superlattice, and to show that the superlattice is pseudomorphic with its Ge0.75Si0.25 buffer substrate with a precision of 2 x 10(-3). At room temperature, the absorption coefficient increases linearly with photon energy above the band edge, exceeding 10(4) cm-1 at 1.55 mum. Because the band-edge electronic structure is fundamentally different in nature from either that of Si or Ge, we argue that conventional models for absorption versus photon energy in bulk semiconductors may not apply to this kind of Ge-Si superlattice.