EXAFS sheds light on short-range ordering in G e 1 − x S n x heteroepitaxial layers grown by MBE and CVD

The local arrangement of atoms in an alloy impacts its electronic band structure and, consequently, its fundamental physical properties. This work investigates the short-range order of tin atoms in binary <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi mathvariant="normal">G</a:mi> <a:msub> <a:mi mathvariant="normal">e</a:mi> <a:mrow> <a:mn>1</a:mn> <a:mo>−</a:mo> <a:mi>x</a:mi> </a:mrow> </a:msub> <a:mi mathvariant="normal">S</a:mi> <a:msub> <a:mi mathvariant="normal">n</a:mi> <a:mi>x</a:mi> </a:msub> </a:mrow> </a:math> semiconductors epitaxially grown on Ge-buffered Si (001) substrates. Samples featuring Sn content up to 13 at.% were analyzed by x-ray absorption fine structure spectroscopy at the Sn-K edge. The study reveals how the growth method and its process parameters, like temperature, or the epitaxial built-up strain affect the local atomic ordering of tin within the alloys. While the deposition technique seems to have a marginal effect on the Sn short-range ordering, the growth temperature and Sn content systematically influence the bonding angles Sn. Ge-Sn next neighbor coordination shells, whereas the interatomic distances remain largely unaffected. In the first coordination shell Sn-Ge are strongly favored over Sn-Sn, while the opposite happens in the next coordination shell, where Sn-Sn are systematically favored. These insights into the relationship of growth condition and layer properties enable to identify the nature of the large variety of electronic and optical properties measured in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:mi mathvariant="normal">G</f:mi> <f:msub> <f:mi mathvariant="normal">e</f:mi> <f:mrow> <f:mn>1</f:mn> <f:mo>−</f:mo> <f:mi>x</f:mi> </f:mrow> </f:msub> <f:mi mathvariant="normal">S</f:mi> <f:msub> <f:mi mathvariant="normal">n</f:mi> <f:mi>x</f:mi> </f:msub> </f:mrow> </f:math> layers.