POLARIZATION DEPENDENCE OF THE SIK-EDGE X-RAY-ABSORPTION SPECTRA OF SI-GE ATOMIC LAYER SUPERLATTICES

The polarization dependence of the SiK-edge X-ray absorption spectra of several [(Si)(m)(Ge)(n)](p) atomic layer superlattice (ALS) materials grown on both Si(100) and Ge(100) have been investigated using plane polarized synchrotron radiation. These spectra exhibit sharp, polarization dependent, Si 1s --> conduction band (CB) resonance features which are absent in the spectrum of amorphous Si (a-Si). Subtraction of the spectrum of a-Si from that of the crystalline ALS materials is used to isolate the conduction band structure. A constrained curve fit analysis of up to eight data files simultaneously has been used to quantitatively analyze the signal. The CB structure is composed of number of polarization independent components and several polarization dependent components. In [(Si)(2)(Ge)(6)](40)/Ge(100) the lowest energy transition at 1839.1 eV is polarized along the surface normal (the growth direction) while a doublet structure centred at 1841 eV is polarized in the surface plane (perpendicular to the growth direction). A similar spectral pattern is found in [(Si)(6)(Ge)(2)](48)/Si(100) but the polarization effect is weaker and the sense of the polarization effect is reversed. The polarization dependent signal is attributed to anisotropic states associated with strain-induced tetragonal distortions in the strained-ALS materials.