SI 1S X-RAY-ABSORPTION SPECTRA OF EPITAXIAL SI-GE ATOMIC LAYER SUPERLATTICE AND ALLOY-FILMS

The Si ls (K-shell) x-ray absorption spectra of several [(Si)m(Ge)n]p atomic layer superlattices (ALS) and a range of SixGe1-x alloy thin fi lms grown epitaxially on both Si(100) and Ge(100) have been investigat ed using plane-polarized synchrotron radiation. The near-edge spectral features of ALS and alloy samples with similar (average) chemical com position are remarkably similar. The spectra of both the strained ALS and alloy samples contain features at the Si ls threshold which exhibi t a small but characteristic polarization dependence. The polarization dependence is reduced or absent in strain-relaxed materials such as a nnealed ALS, annealed alloys, or thick alloy films. The polarization-d ependent components of the signal are attributed to anisotropic states associated with strain-induced tetragonal distortions. The sense of t he polarization is inverted between samples grown on Si and those grow n on Ge, consistent with the expected inversion in the spatial orienta tion of the strain field. An explanation is proposed for the dependenc e of the magnitudes of the Si Is polarization effect on the compositio n of alloys and ALS.