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

The structure of layers of GaSb grown on InAs. substrates has been investigated by means of high-resolution x-ray diffraction. The samples were grown in Oxford using the metal-organic vapour phase epitaxy facility to produce high-quality single-crystal layers with thicknesses between 60 and 3000 Angstrom. The x-ray scattering experiments were performed with a Philips MRD diffractometer, in Oxford and with the XMaS facility at the ESRE The results show that the scattering for layers with a thickness below T-c' similar to 1250 Angstrom is different from the scattering for those with larger thicknesses. The scattering from the thinner. layers shows that the in-plane lattice constant of the GaSb is very close to that of the InAs substrate and that the strain does not vary through the film, while the measured diffuse scattering is in good agreement with calculations of the scattering from isolated 60degrees dislocations., The thicker layers show no diffuse scattering but single-Gaussian Bragg peaks and the scattering is that expected from a mosaic layer with a large. concentration of 60degrees dislocations. Analysis of the peak parameters shows that the average in-plane lattice constant is intermediate between those of bulk GaSb and bulk InAs and that there is a changing strain through the film. The critical thickness T-c for GaSb on InAs is calculated as about 204 Angstrom. We argue that between 204 and 1250 Angstrom there are only a few dislocations, but thicket films are relaxed by spontaneous creation of a dislocation network. The results demonstrate the power of high-resolution X-ray scattering for studying non-destructively the structures of thin films containing dislocations, and show that there is a marked change in the scattering for layers above the critical thicknesses T-c and T-c'.