X-ray absorption studies of strain in epitaxial (Si-Ge) atomic layer superlattice and alloy films

The Si 1s (K-shell) X-ray absorption spectra of a series of strained Si(x)Ge(100-x) alloy thin films and several {(Si)(m)(Ge)(n)}(p) atomic layer superlattices (ALS) grown epitaxially on Si(100) and Ge(100) substrates have been investigated using plane polarized synchrotron radiation. Polarization dependent components of the signal are attributed to anisotropic states associated with strain-induced tetragonal distortions. The sense of the polarization is shown to be identical for all compositions (x = 25 to 92) of SiGe alloys grown on Si(100) substrates. The opposite polarization dependence is found to occur for all Si(x)Ge(100-x) alloys (x = 12 W 50) grown on Ge(100) substrates. The polarization dependence and shape of the near edge spectral features of alloy and ALS samples which have similar (average) chemical composition are remarkably similar. A preliminary comparison of the alloy results with literature band structure calculations is made.