Formation of β-FeSi_<2-x>Ge_x by Ge-Segregation-Controlled Solid-Phase Growth of [a-Si/a-FeSiGe]_n Multilayered Structure

The solid-phase growth of [a-Si/a-Fe<SUB>0.4</SUB>Si<SUB>0.5</SUB>Ge<SUB>0.1</SUB>]<I><SUB>n</SUB></I> (<I>n</I>=1, 2, 4; total thickness: 500 nm) multilayered structures has been investigated. After annealing at 700°C, [a-SiGe/polycrystalline β-FeSi<SUB>2−<I>x</I></SUB>Ge<I><SUB>x</SUB></I>]<I><SUB>n</SUB></I> (<I>x</I>=0.5, 0.4, 0.2 for <I>n</I>=1, 2, 4, respectively) multilayered structures were formed. From the analysis of X-ray diffraction spectra, it was found that the lattice constants of β-FeSi<SUB>1.5</SUB>Ge<SUB>0.5</SUB> changed from those of relaxed β-FeSi<SUB>2</SUB> by 0.4–0.5%. The change decreased with increasing <I>n</I>, which was due to the segregation of Ge atoms from the a-Fe<SUB>0.4</SUB>Si<SUB>0.5</SUB>Ge<SUB>0.1</SUB> layers to the a-Si layers becoming larger with increasing <I>n</I>. After annealing at 800°C, Ge atoms were completely swept out from the β-FeSi<SUB>2−<I>x</I></SUB>Ge<I><SUB>x</SUB></I> lattice. In addition, the agglomeration of β-FeSi<SUB>2</SUB> occurred, and nanocrystals of relaxed β-FeSi<SUB>2</SUB> and c-Si<SUB>0.7</SUB>Ge<SUB>0.3</SUB> were formed. This technique for the formation of β-FeSi<SUB>2−<I>x</I></SUB>Ge<I><SUB>x</SUB></I> is expected to be useful for energy gap modulation for advanced optoelectrical devices.