Section 2.3.2.3. Grazing-incidence diffraction

In conventional focusing geometry, the specimen and detector are coupled in θ–2θ relation at all 2θ's to avoid defocusing and profile broadening. In Seemann–Bohlin geometry, changing the specimen position necessitates realigning the diffractometer and very small incidence angles are inaccessible. In parallel-beam geometry, the specimen and detector positions can be uncoupled without loss of resolution. This freedom makes possible the use of different geometries for new applications. The specimen can be set at any angle from grazing incidence to slightly less than 2θ, and the detector scanned. Because the incident and exit angles are unequal, the relative intensities may differ by small amounts from those of the θ–2θ scan due to specimen absorption. The reflections occur from differently oriented crystallites whose planes are inclined (rather than parallel) to the specimen surface so that particle statistics becomes an important factor. The method is thus similar to Seemann–Bohlin but without focusing.

The method can be used for depth-profiling analysis of polycrystalline thin films using grazing-incidence diffraction (GID) (Lim, Parrish, Ortiz, Bellotto & Hart, 1987). If the angle of incidence is less than the critical angle of total reflection , diffraction occurs only from the top 35 to 60 Å of the film. Comparison of the GID pattern with a conventional θ–2θ pattern in which the penetration is much greater gives structural information for phase identification as a function of film depth. The intrinsic profile shapes are the same in the two patterns and broadening may indicate smaller particle sizes. However, if the film is epitaxic or highly oriented, it may not be possible to obtain a GID pattern.

For , the penetration depth is (Vineyard, 1982) and, for , where μ is the linear absorption coefficient. The thinnest top layer of the film that can be sampled is determined by the film density, which may be less than the bulk value. As approaches , the penetration depth increases rapidly and fine control becomes more difficult. Fig. 2.3.2.7 shows this relation and the advantage of using longer wavelengths for a wider range of penetration control. For example, for a film with μ = 200 cm⁻¹, λ = 1.75 Å, and , only the top 45 Å contribute, and increasing to 0.35° increases the depth to 130 Å. The patterns have much lower intensity than a θ–2θ scan because of the smaller diffracting volume.

Figure 2.3.2.7| top | pdf | Penetration depth t' as a function of grazing-incidence angle α for γ-Fe2O3 thin film. The critical angle of total reflection αc is shown by the vertical arrows for different wavelengths.

Fig. 2.3.2.8 shows patterns of a 5000 Å polycrystalline film of iron oxide deposited on a glass substrate and recorded with (a) θ–2θ scanning and (b) 0.25° GID. The film has preferred orientation as shown by the numbers above the peaks in (a), which are the relative intensities of a random powder sample. The relative intensities are different because in (a) they come from planes oriented parallel to the surface and in (b) the planes are inclined. The glass scattering that is prominent in (a) is absent in (b) because the beam does not penetrate to the substrate.

Figure 2.3.2.8| top | pdf | Synchrotron diffraction patterns of annealed 5000 Å iron oxide film, λ = 1.75 Å, (a) θ–2θ scan; relative intensities of random powder sample shown above each reflection. (b) Grazing incidence pattern of same film with α = 0.25° showing only reflections from top 60 Å of film, superstructure peak S.S. and α-Fe2O3 peaks not seen in (a). Absolute intensity is an order of magnitude lower than (a).