This application note discusses how to determine the composition of epitaxial thin films by X-ray diffraction and will focus on the analysis of AlxGa1-xAs thin films. Since the discovery of X-ray Crystallography in the 1920s, it has been possible to indirectly measure the sizes of atoms. Researchers discovered that atomic radius varies from element to element. This means that when an atom of one element is substituted for a different element in an epitaxial (single-crystal) thin film, there will be a change in lattice parameter. Figure 1 shows this effect for a series of epitaxial AlxGa1-xAs thin films on GaAs substrates. As the aluminum content in the thin film decreases, the lattice parameter of the layer peak on the left due to the film moves toward the location of the GaAs substrate peak on the right. In principle, this change in lattice parameter can be used to determine the composition of an epitaxial thin film as long as the replacement atoms are substitutional. That is, the replacement atoms must replace other atoms in the actual crystal structure. Interstitial atoms do not significantly change a crystal’s lattice parameters.
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