Preparation and Core-Electron Spectroscopy of Y- or Sc- Doped AlN Films

Thin films of doped aluminum nitride, (Al1-xDox)N, have evoked significant interest during the last decade as a lead-free, biocompatible, environmentally friendly piezoelectric/ferroelectric material. Chemical stability, large thermal conductivity, large elastic modulus and compatibility with Si-based microfabrication methods make (Al1-xDox)N thin films a most promising candidate as active materials for piezoelectric MEMS and ferroelectric devices. However, depositing fully oriented (001) films without DoN (Do=Sc, Y) segregation remains challenging, despite almost two decades of intensive development. This is because both ScN and YN are completely immiscible with AlN in bulk form. This is the driving force for their segregation and loss of orientation during deposition. Here, we describe the preparation of >2 µm thick wurtzite <002> textured, Sc, x=0.25, 0.30 or Y, x=0.25) sputtered thin films as well as the investigation of the local environments of Sc and Y using X-ray absorption (XAS) and photoelectron (XPS) spectroscopies. We present evidence from the X-ray absorption fine structure (XAFS) spectra that, when x=0.25, both and ions are able to substitute for , thereby acquiring four tetrahedrally coordinated nitrogen ligands, i.e., coordination number (CN) of 4. On this basis, the crystal radius of the dopant species in the wurtzite lattice, not available heretofore, could be calculated. By modelling the scandium local environment, extended XAFS (EXAFS) analysis suggests that, when x increases from 0.25 to 0.30, CN for a fraction of the Sc ions increases from 4 to 6, signaling octahedral coordination. This change occurs at a dopant concentration significantly lower than the reported maximum concentration of Sc (42 at% Sc) in wurtzite (Al, Sc)N. XPS provides support for our observation that the local environment of Sc in (Al, Sc)N may include more than one type of coordination.