Accurate charge referencing in XPS of insulating specimens is a delicate issue. This difficulty is illustrated in the case of Al-Si-N composite thin films deposited by reactive magnetron sputtering with variable composition from pure aluminum nitride to pure silicon nitride. The samples were mounted with Au-coated metallic clamps. Argon sputter cleaning was required to remove a surface native oxide before analysis. For charge referencing implanted argon atoms from the sputter gas and a small amount of gold re-deposited from the metallic clamps onto the specimen surface during sputter cleaning were evaluated. For the argon atoms, a surprisingly large chemical shift (∼1 eV) and a significant peak broadening (0.6 eV) of the Ar 2p 3/2 photoelectron line were found with varying the Si content of the films. This could be related to chemical and structural changes of the Al-Si-N films. Hence implanted argon could not be used for charge referencing of Al-Si-N samples. In contrast to the implanted argon, the Au 4f 7/2 line width of the gold re-deposited onto the sample surface did not depend on the Si content of Al-Si-N films. A constant energy shift (∼1.2 eV) of the Au 4f 7/2 line as compared with bulk gold was, however, found, which was related to the size of gold particles formed on the insulating films. Therefore gold could be reliably used to study chemical shifts of sample-relevant species in Al-Si-N films, but the absolute binding energies of Al 2p, Si 2p and N 1s photoelectrons could not be determined.

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Experiments reveal that incorporation of substitutional Si in wurtzite AlN up to 6 at. % results in a lattice contraction in the [0001] direction. The contraction is linear and, for higher silicon contents, the lattice parameters remain constant. We investigate the geometric and electronic properties of Al–Si–N compounds with Si content varying from 0 to 9 at. % by means of ab initio simulations based on density functional theory. We demonstrate that charged defects are necessary to support the experimental evidence of a shrinking cell parameter: an ideal Al–Si–N wurtzite structure with delocalized charges would undergo lattice expansion due to Coulomb repulsion upon Si incorporation. Charged defects that act as acceptors and compensate the excess charge coming from Si overcompensate the lattice expansion and therefore promote a lattice contraction.

The chemical state evolution of the Al-SiN thin films at various Si contents is investigated by x-ray photoelectron spectroscopy ͑XPS͒. The detailed evolution of the Al 2p,S i2 p, and N 1s photoelectrons line positions and widths are used to identify different chemical environments as the Si content is changed. The results are compared to x-ray diffraction ͑XRD͒ data that indicate the formation of a two-phase Al 1−x Si x N / SiN y composite when the solubility limit of 6 at. % of Si in AlN is exceeded. In contrast to XRD data, no particular effect is observed in the XPS data at the solubility limit of Si. Instead, two compositional regions can be identified that are separated by a distinct change in the evolution of core level binding energy differences and chemical shifts at about 10-15 at. % of Si. This silicon concentration is identified as the onset of the formation of a SiN y intergranular phase that is a few monolayers thick, having a chemical bonding similar to that in bulk silicon nitride. The observed changes in the XPS data coincide well with the structural changes in the material at different silicon contents. The unambiguous identification of phases, especially of minority phases from XPS data, is, however, not possible.