Diffraction-anomalous-fine-structure spectroscopy applied to the study of III-V strained semiconductors

Abstract

The effect of built-in strain on III-V epitaxial semiconductors has been investigated by extended diffraction anomalous fine structure (EDAFS) at the Ga and As K edges. A general formalism is presented for analyzing the diffraction-anomalous-fine-structure (DAFS) oscillations, valid for any type of crystallographic structure. The EDAFS spatial selectivity provides a unique tool for studying systems that are out of the reach of other x-ray techniques. We study two different systems grown on a GaAs(001) substrate: a strained layer superlattice of ${(\mathrm{GaP})}_{2}{(\mathrm{InP})}_{3}$ and three single epilayers of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ $(x=0.20--0.23)$ partially relaxed, with a different amount of residual strain. The bond distance Ga-P in the SLS is stretched by about 0.04 \AA{} in agreement with the predictions of the elastic theory. The Ga-As and Ga-P bond lengths in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ remain very close to their respective bulk values, independent of the residual strain. The ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ epilayers have also been measured by switching the light polarization vector from the [110] to the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ crystallographic direction. An effect is observed on the EDAFS at the Ga K edge for the most strained sample, suggesting an ordering of the P atoms in the [001] growth direction. We also point out the interest of the DAFS spectra analysis for obtaining further information about the average crystallographic structure.