Optimizing scans on asymmetric reflections

Abstract

When high-resolution x-ray analysis is performed on thin layers, asymmetric reflections provide information on the relaxation state. High-quality strained layers, superlattices or multi-quantum wells give rise at every reciprocal lattice point to long rods along a direction normal to the surface. Neither the so-called 'Omega scans' also named 'rocking curves' nor the ω/2θ scans are fully suitable to scan along these rods as they follow different directions in the reciprocal space. We theoretically and experimentally show that scan quality may be much improved by choosing an appropriate coupling coefficient between the detector and sample axis, so as to follow the rod direction. This coupling coefficient depends on both θ and the angle between the normal to the surface and the scattering vector. We give examples which emphasize that large scans can be performed on asymmetric reflections, even using the triple axis analyser. Much time can thus be saved on reciprocal space mapping, as 'empty parts' of the maps have no longer to be explored. Finally, we discuss the practical use of the non-integer coupling coefficients found and we present the computer programs which have been written to calculate the data.