INVESTIGATIONS OF SEMICONDUCTOR SUPERLATTICES BY DEPTH-SENSITIVE X-RAY-METHODS

Focusing on the structure determination of a GaInAs/InP superlattice (SL), the potential of grazing incidence diffraction (GID) to resolve structure parameters on a microscopic scale is compared to x-ray reflectivity and conventional x-ray diffraction (XRD) measurements. Usually, information on the density profile perpendicular to the surface is obtained by x-ray reflectivity and on lattice mismatch by XRD. Since the penetration depth of x rays is much larger than the total thickness of the SL these methods measure parameters averaged over the whole structure. Furthermore, the depth sensitivity of both methods is small in the case of extremly high thickness ratios of the sublayers within the SL period. These disadvantages can be overcome using GID geometry for which the information depth can be reduced by keeping the angle of incidence alpha(i) and exit alpha(f) of the x-ray beam with respect to the surface in the range close to the critical angle of total external reflection alpha(C). This enables a depth-selective structure determination. As an example measurements and computer simulations corresponding to the three different methods of a lattice-matched (GaIn)As/InP SL are presented. The periodic length of the present SL and the thickness of the top layer was determined by all three methods to monolayer accuracy.