Band structure and photoconductivity of blue-green light absorbing AlTiN films

We investigated the band structure and photoconductivity of heavily Ti-doped (up to 11.0% of Ti) AlN films (AlTiN) with an optical absorption starting at around 2.2 eV. X-ray diffraction (XRD) and transmission electron microscopy results revealed that the films consisted of a c-axis oriented wurtzite structure. Ti K-edge extended X-ray absorption fine structure and X-ray absorption near edge structure (XANES) results suggested that most of the Ti atoms occupied Al sites having a 3+/4+ mixed valence. Ab initio calculations were conducted using lattice constants estimated from XRD data for supercells with Ti atoms occupying Al sites. It was revealed that the Ti-driven states were formed just below the bottom of the conduction band (CB) of AlN, and almost merged with it at high Ti concentrations. The calculated band structure was experimentally verified using X-ray photoemission spectroscopy of the valence band (VB) region and N K-edge XANES. At the excitation energy near the absorption edge, photoconduction was observed for films with 8.3% and 11.0% Ti, meaning that the Ti-driven states started forming a CB at these Ti concentrations. Photoelectron yield spectroscopy results implied that the potential energy of the VB was around 6.0 eV with respect to the vacuum level. Together with the absorption edge energy, it is concluded that AlTiN is one of the promising candidate materials for solar energy conversion.