Polarization dependence of the Si K-edge X-ray absorption spectra of Si-Ge atomic layer superlattices

The polarization dependence of the Si K-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 ls → 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 a 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.