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 Fed 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. Wurtzite III-nitride semiconductors centred on GaN are proven materials for light emitting diodes (LEDs) because they have direct band gaps covering the spectral region of ultraviolet (UV), visible, and infrared lights via band gap engineering for the past quarter-century 1-4. AlN is one of the III-nitrides and it is a promising material for deep UV LEDs 5,6 by virtue of its wide band gap of 6.1 eV 7 along with its high thermal conductivity and chemical stability 8. However, AlN and AlGaN with a high AlN mole ratio confront difficult issues to realize high efficiency devices owing to some electronic properties in the form of thin films, which are traced back to its crystal axis orientations, i.e. c-axis orientation. In the c-axis direction of a wurtzite structure, electronic polarization can exist owing to a large deference of electronegativities between Al and N, which align along the c-axis, as shown in Fig. 1a. Electronic polarization can build internal electric fields in vertical LED structures, which cause low recombination efficiencies due to the quantum confined Stark effect 9,10. Additionally, the split regimes in the top of valence band (VB) can be another obstacle. Optical dipole transitions between the top of the VB and the bottom of the conduction band (CB) are forbidden for the light with electric fields in the c-plane 11 , which result in low extraction efficiency. Considering the above mechanisms of obstacles for the realization of high efficiency deep UV LEDs, laying c-axes in film planes would be a simple solution. Significant efforts have been made to grow films with m-and a-axes orientations, which are known as non-polar films 12,13. However, with conventional techniques based on epitaxial growth regimes, there are difficulties in choosing an appropriate substrate 14-17 ; moreover, wurtzite films are subjected to the formation c-axis orientation. Very recently, we succeeded in obtaining a-axis oriented films via heavily Fe-doping AlN by radio-frequency sput...
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.
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.
AlTiN is one of the promising candidate materials for solar energy conversion.
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