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Wurtzite AlN films with high Fe concentrations were deposited on Si(100) substrates using radio-frequency sputtering, and the growth process was investigated. X-Ray diffraction (XRD) analysis with parallel incident X-rays showed that a thick film (∼2.5 μm) had a non-polar a-axis orientation, similar to the films deposited on SiO2 glass substrates. Through powder XRD analysis, it was found that non-wurtzite fine crystals and/or amorphous materials formed during the initial stage of deposition, followed by wurtzite nucleation with various orientations of the axes, such as a, m, and r orientations. The XRD results also implied that the a-axis-oriented regions drastically increased with increasing film thickness. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy analyses suggested that one of the candidates for the non-wurtzite fine crystalline material was γ-Al2O3. STEM analysis also demonstrated that a-axis-oriented single-crystal grains grew from randomly oriented small wurtzite grains that increased in diameter with increasing deposition time. Al K-edge X-ray absorption near-edge structure results suggested that the electronic structure in the polar c-axis direction of wurtzite lying in the film plane developed with deposition time, thereby further supporting the findings of the XRD and STEM analyses.

Wurtzite AlN film is a promising material for deep ultraviolet light-emitting diodes. However, some properties that attribute to its crystal orientation, i.e., c-axis orientation, are obstacles in realizing high efficiency devices. Constructing devices with non-c-axis oriented films is a solution to this problem; however, achieving it with conventional growth techniques is difficult. Recently, we succeeded in growing a-axis oriented wurtzite heavily Fe-doped AlN (AlFeN) films via sputtering. In this article, we report the electronic structures of AlFeN films investigated using soft X-ray spectroscopies. As-grown films were found to have conduction and valence band structures for a film with c-axis in film planes. Simultaneously, it was found that large gap states were formed via N-p and Fe-d hybridization. To remove the gap states, the films were annealed, thereby resulting in a drastic decrease of the gap states while maintaining a-axis orientation. We offer heavy Fe-doping and post annealing as a new technique to obtain non-polar AlN films.

Band structure calculations for radiofrequency-sputtered AlN-films doped with various 3d-transition-metals (TM: V, Cr, and Mn) were conducted to investigate the origin of the characteristic optical absorption structures. Experimentally evaluated crystal structures and lattice constants of the synthesized films were adopted for supercells. The model calculations showed that additional energy bands mainly consisting of 3d e and t states of TMs are formed in the band gap of AlN (6.2 eV), and that their potentials depend on the TM species. It was also shown that the Fermi levels of Cr- and Mn-doped AlN lie within the spin-up t band, while the Fermi level of V-doped AlN lies between the spin-up e and t bands. These findings imply that the materials have TM species-dependent, multiple absorption paths with lower energy than the band gap energy of AlN, resulting in optical absorption in the near-ultraviolet, visible, and infrared regions.

We report herein on a high degree of wurtzite [11-20] orientation in AlN films heavily doped with Fe (AlFeN). The AlFeN films were deposited by RF sputtering on SiO2 glass and sapphire (0001) substrates. X-ray diffraction measurements revealed the Fe concentration dependence of the preferred orientation of these films. At a high Fe concentration (10–26.1%), the films had a [11-20] orientation, whereas films with less than 5% of Fe had a [0001] orientation. This tendency was evident on sapphire (0001) substrate as well as on the SiO2 glass substrate. The highest degree of [11-20] orientation was achieved for 19.4% of Fe. Transmission electron microscopy observations of the [11-20]-oriented AlFeN films (Fe: 19.4%) showed that randomly oriented nuclei form near the interface with the substrate, and subsequently, columnar grains selectively grow with a [11-20] orientation. These results suggest that AlFeN (11-20) films may be potentially applicable as a buffer layer to grow non-polar AlN on various substrates.

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 tha...

The valence band (VB) structures of wurtzite AlCrN (Cr concentration: 0-17.1%), which show optical absorption in the ultraviolet-visible-infrared light region, were investigated via photoelectron yield spectroscopy (PYS), x-ray/ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio density of states (DOS) calculations. An obvious photoelectron emission threshold was observed ~5.3 eV from the vacuum level for AlCrN, whereas no emission was observed for AlN in the PYS spectra. Comparisons of XPS and UPS VB spectra and the calculated DOS imply that Cr 3d states are formed both at the top of the VB and in the AlN gap. These data suggest that Cr doping could be a viable option to produce new materials with relevant energy band structures for solar photoelectric conversion.