The structure of rare earth thin films: holmium and gadolinium on yttrium

Single-crystal holmium and gadolinium layers have been grown on yttrium substrates using the molecular beam epitaxy technique and their structures investigated using high resolution x-ray scattering. The experiments were performed using a Philips MRD diffractometer in Oxford, and with the XMaS facility at the ESRF. Holmium layers with a thickness below T'c = 115 Angst. give scattering that is characteristic of a pseudomorphic film structure with the same in-plane lattice parameter as the yttrium substrate to within 0.05%. For layers thicker than T'c, there is a sharp reduction in misfit strain due to the creation of edge dislocations. The transverse lineshape of the holmium peaks exhibits a two-component lineshape for thicknesses above T'c but below about 500 Angst. Above 500 Angst. the lineshape of the transverse scans becomes Gaussian and is characteristic of a mosaic crystal. The gadolinium layers show no sharp change of strain for layers as thick as 2920 Angst. and the transverse peak shape remained similar for all films. This is characteristic of pseudomorphic film growth and a failure to nucleate dislocations