Among various experimental possibilities, structural analysis of compound semi­conductors by diffraction techniques using X-rays or neutrons has become a technique of choice. The reasons are various, and here we place the focus on the use of neutrons as a radiation source.

The highest efficiency thin-film devices consist of a Cu-poor Cu(In, Ga)Se2 absorber layer. The interplay of structural with electronic and optical properties is therefore interesting to study for this quaternary compound and the corresponding ternaries (CuGaSe2 and CuInSe2). In the case of CuGaSe2 or Cu(In, Ga)Se2 the cations Cu+ and Ga3+ have an identical number of electrons (28). This is a problem for the differentiation of these two cations by conventional X-ray diffraction, where the diffraction is at the electron shell of the atoms. Since atomic scattering form factors /for X-rays are proportional to the atomic number Z, the positions of the unit cell atoms of similar atomic number and the fractional occupation of the Wyckoff sites are difficult to distinguish. Hence, a differentiation of these cations in the atomic structure by conventional X-ray diffraction is impossible. In the case of neutron diffraction, the scattering is at the nucleus and the neutron-scattering lengths of copper and gallium are different (bCu = 7.718(4) fm, bGa = 7.288(2) fm [18]). Moreover, in the case of X-rays, destructive interference effects lead to a decrease of the scattering amplitude with angle. In contrast to diffraction at the electron shell, the atomic nuclei cross-sections are very small and the interference effects are also very small. Therefore, neutron-scattering amplitudes do not decrease rapidly with angle, resulting in the advantage of high intensities of Bragg-reflections observed even at high Q-values. This is especially important for the determination of atomic positions and atomic site occupations. For the analysis of point defects in compound semi­conductors and position parameters, for instance of the anion, neutron powder dif­fraction is advantageous.