First-principles Calculation of L<SUB>3</SUB> X-ray Absorption Near Edge Structures (XANES) and Electron Energy Loss Near Edge Structures (ELNES) of GaN and InN Polymorphs

Abstract

First principles calculations of L 3 XANES/ELNES of GaN and InN with both wurtzite and zinc-blende structures have been made using OLCAO (orthogonalized linear combinations of atomic orbitals) method. Supercells with more than 100 atoms were employed. A core-hole was rigorously included in the calculation, and the photo absorption cross section (PACS) between the initial and final states was computed. Quantitative reproduction of experimental spectrum that is available in literature can be found when the PACS was computed. Although spectral shapes of two phases look similar, characteristic differences are predicted to appear at the first peak of the L 3 XANES/ELNES. The first peak is notably broader in the zinc-blende phases. The origin of the broadness is analyzed using partial density of unoccupied states (PDOS) and Mulliken charge. We then conclude that the broadness can be related to greater covalency of the zinc-blende phase as compared to the wurtzite phase.

First-principles calculation of spectral features, chemical shift and absolute threshold of ELNES and XANES using a plane wave pseudopotential method

“…For simplicity, isolated Ga 3+ ions located at the center of an ideal octahedron and that of a tetrahedron are assumed. The p-orbitals, which are responsible for the K edge, cannot be hybridized with dorbitals of the octahedral Ga 3+ , whereas p-d hybridization is allowed for the tetrahedral Ga 3+ [36][37][38]. In the β-Ga 2 O 3 crystals, those orbitals are more broadened because both sites are distorted from the ideal symmetries and are bonded with oxygens.…”

Overlap population diagram for ELNES and XANES: peak assignment and interpretation

“…The OP diagram was applied to the ELNES and XANES from AlN, GaN, InN, and ZnO. These compounds have wurtzite structure under ambient conditions and have similar unoccupied band structure, which is composed of s-, p-, and d-type bands from the conduction band minimum [42,43]. All calculations were performed with 108-atom supercells.…”

“…Atoms in MgO, rock salt structure, form octahedral coordination with O h symmetry, whereas atoms in the wurtzite structure have tetragonal coordination. When an isolated atom in these crystals is assumed, p orbitals cannot be hybridized with d orbitals in the O h symmetry, whereas the p-d hybridization is allowed in the wurtzite structure [42,43,46]. In the crystals, cations and anions are bonded with each other and these orbitals are broadened.…”