X-ray scattering from epitaxial GaSb/InAs thin films below and above the criticalthickness

Abstract

The structure of layers of GaSb grown on InAs substrates has been investigatedby means of high-resolution x-ray diffraction. The samples were grown in Oxfordusing the metal–organic vapour phase epitaxy facility to produce high-qualitysingle-crystal layers with thicknesses between 60 and 3000 Å. The x-rayscattering experiments were performed with a Philips MRD diffractometer inOxford and with the XMaS facility at the ESRF. The results show thatthe scattering for layers with a thickness below Tc′ ∼ 1250 Åis different from the scattering for those with larger thicknesses. The scatteringfrom the thinner layers shows that the in-plane lattice constant of the GaSb isvery close to that of the InAs substrate and that the strain does not vary throughthe film, while the measured diffuse scattering is in good agreement withcalculations of the scattering from isolated 60̂ dislocations. The thicker layersshow no diffuse scattering but single-Gaussian Bragg peaks and the scattering isthat expected from a mosaic layer with a large concentration of 60̂ dislocations.Analysis of the peak parameters shows that the average in-plane latticeconstant is intermediate between those of bulk GaSb and bulk InAs andthat there is a changing strain through the film. The critical thicknessTcfor GaSb on InAs is calculated as about 204 Å. We argue that between 204 and1250 Å there are only a few dislocations, but thicker films are relaxed byspontaneous creation of a dislocation network. The results demonstrate thepower of high-resolution x-ray scattering for studying non-destructively thestructures of thin films containing dislocations, and show that there is amarked change in the scattering for layers above the critical thicknessesTc andTc′.