Crystallographic properties and electronic structure of V-doped AlN films that absorb near ultraviolet-visible-infrared light

Abstract

For highly efficient photoconversion devices, 3d-transition-metal-doped AlN is a candidate intermediate-band material. Here, we synthesized and investigated V-doped AlN (AlVN; V ≤ 11%) films. The optical absorption spectra of the films showed characteristic features including a peak in the infrared region and shoulders in the visible light region. These features remained essentially unchanged for the various V concentrations. X-ray diffraction (XRD), transmission electron microscopy (TEM), and V K-edge X-ray absorption fine structure (XAFS) measurements were carried out to clarify the crystallographic origin of the optical absorption features. The XRD profiles revealed that all films had a c-axis-oriented wurtzite structure. The TEM analyses supported the XRD results. The V K-edge X-ray absorption near-edge structure indicated that the V atoms in the AlN lattice were surrounded by N atoms with non-centrosymmetric conditions and had an oxidation state close to 3+. Extended XAFS (EXAFS) analyses implied that the V atoms had C3v symmetry. The results of ab initio lattice relaxation calculations for a model wurtzite structure of an Al35V1N36 supercell were consistent with the EXAFS data. Electronic structure calculations using this model showed that additional energy bands, mainly consisting of V d states, were formed in the band gap of AlN, and the Fermi level was between the additional bands. Hence, in the optical absorption spectra, the peak was explained by d-d transitions partially allowed thorough hybridization with the p component, and the shoulders were attributed to transitions from the valence band to the new bands in the band gap of AlN.