STOICHIOMETRY AND DISCOMMENSURATION ON INXGA1-XAS/GAAS(001) RECONSTRUCTED SURFACES : A QUANTITATIVE X-RAY DIFFUSE-SCATTERING STUDY

A quantitative estimate of the In/Ga surface concentration ratio in ultrathin (In, Ga)As strained layers, grown by molecular-beam epitaxy on a GaAs(001) substrate, is obtained using grazing incidence x-ray diffraction and diffuse-scattering measurements. The commensurate $2\ifmmode\times\else\texttimes\fi{}3$ reconstruction is interpreted as due to cation ordering in the surface unit cell, locking the surface composition at the value ${\mathrm{In}}_{2/3}{\mathrm{Ga}}_{1/3}\mathrm{As}.$ Incommensurate $2\ifmmode\times\else\texttimes\fi{}n$ reconstructions with $nl3$ $(ng3)$ are described in terms of indium-depleted (-enriched) surface layers characterized by a statistical distribution of faults in the ideal $2\ifmmode\times\else\texttimes\fi{}3$ atomic arrangement. Within a defined temperature range 450--490 \ifmmode^\circ\else\textdegree\fi{}C, a unique correspondence between the incommensurability parameter n and the indium surface fraction is established on the basis of a formulation of the diffuse scattering distribution.