Compositional depth profiling of polycrystalline thin films by grazing-incidence X-ray diffraction

This paper uses an innovative peak profile matching method to determine compositional depth profiles of Cu(In,Ga)(S,Se)₂ thin films and other suitable solid-solution compounds. A layer absorption model allows the generation of grazing-incidence diffraction patterns on the basis of the known diffractometer instrument function, compositional depth profiles and the corresponding mass absorption characteristics. In contrast to other methods, the modelling approach exploits the information stored in the shape evolution of the peak profile as a function of incidence angle. A small number of diffraction patterns of the same peak are sufficient to track depth-dependent shifts of the principal lattice parameters. The method is adapted to the specific properties of the chalcopyrite Cu(In,Ga)(S,Se)₂ system. In fact, the multinary chalcopyrite compounds grown by physical vapour deposition have some remarkable properties which allow the successful application of the method. Issues like surface roughness, refraction at low incidence angles and their effects upon relative intensities can be successfully circumvented by the modelling approach. Ambiguities in the modelling results are very small and can be minimized by comparison with complementary depth profiles using secondary ion mass spectrometry or cross-sectional energy-dispersive X-ray analysis.