Interference structures in double-crystal X-ray rocking curves from very thin multiple epitaxial layers

Interference fringes have been observed in X-ray double-crystal rocking curves from a two layer epitaxial structure consisting of 920 AA Ga x In 1-x As 1-y P y grown on InP and capped by 0.38 mu m of InP. It is shown that the fringes can be used to measure the InP cap thickness to a precision of about 100 AA. The effects on the fringe spacing of variations in layer thickness and lattice mismatch are investigated by simulation. The fringe spacing is independent of the lattice parameter difference between cap and substrate. Simulations reveal that the fringes are modulated with a periodicity equal to the Bragg case Pendellosung period for a layer of thickness equal to that of the confined quaternary layer. This modulation is shown to be independent of cap thickness and layer mismatch. The fringe spacing and the modulation period thus provide an easy and non-destructive method for the determination of layer thicknesses to a precision of 100 AA for two-layer hetero-epitaxial structures.