The electronic structure and interband transitions of a-Al,O, have been studied using temperature-dependent vacuum ultraviolet spectroscopy from 5 to 43 eV, at temperatures ranging from 293 up to 2167 K, which is approaching the melting point of 2327 K. The energy range of the spectra spans the full range of interband electronic transitions. Kramers-Kronig analysis has been employed to recover the phase information, and the interband transition strength (.Jcv) of the valence to conduction band transitions. Critical p i n t (CP) anaLysis of J c v , a modeling technique based on band structure topology, is then applied to the study of temperature-induced changes in the interband electronic structure. This approach offers new insights into the nature of the electronic structure of a-Al,O,-by allowing us to decompose the interband transitions into subsets associated with states of 0 2p nonbonding and AI=0 bonding character, and by allowing us to apply partial optical sum rules to these subsets to determine changes in their electron occupancy as a function of temperature. Up to 1700 K the temperature dependence of the electronic structure is finear, corresponding to the linear behavior of the thermal lattice expansion and the vibrational Debye-Waller factors. Below 1700 K the absorption edge shifts at -1.1 meV/K while the exciton and band gap, decomposed through CP modeling, shift at 0.93 and 0.85 meV/K with an exciton binding energy of 0.13 eV. The electron occupancy of the 0 2p nonbonding CP set decreases and the occupancy of the Al=O bonding CP set increases. Above 1700 K the temperature dependence of the electronic structure is nonlinear, reflecting the interaction of electrons with phonons in the anharmonic regime, and related to the nonlinearity observed in the vibrational Debye-Waller factors. Also above 1700 K, the 0 2p nonbonding and AI=0 bonding CP sets merge and this is discussed in the context of temperature-induced changes in the interatomic bonding.

We report the results of a vacuum ultraviolet (VUV) study of single crystal and polycrystalline AlN over the range 4–40 eV and compare these with theoretical optical properties calculated from first principles using an orthogonalized linear combination of atomic orbitals in the local density approximation. The electronic structure of AlN has a two-dimensional (2D) character indicated by logarithmic divergences at 8.7 and 14 eV. These mark the centers of two sets of 2D critical points which are associated with N 2p→Al 3s transitions and Al=N→Al 3p transitions, respectively. A third feature is centered at 33 eV and associated with N 2s→Al 3d transitions.

Optical and electron-energy-loss spectroscopies are well established methods of probing the electronic structure of materials. Comparison of experimental spectroscopic results with theory is complicated by the fact that the experiments extract information about the interband transition strength of electrons, whereas theoretical calculations provide information about individual valence and conduction bands. Based on the observation that prominent features in the optical response arise from critical points in the joint density of states, critical point modelling was developed to gain an understanding of these spectral features in terms of specific critical points in the band structure. These models were usually applied to derivative spectra and restricted to the consideration of isolated critical points. The authors present a new approach to critical point modelling of the undifferentiated spectra and interpret the model in terms of balanced sets of critical points which describe the interband transition strength arising from individual pairings of valence and conduction bands. This approach is then applied to achieve a direct, quantitative comparison of theoretical and experimental data on aluminium nitride.

Precise and accurate knowledge of the optical properties of aluminum nitride (AlN) in the ultraviolet (UV) and visible (VIS) regions is important because of the increasing application of AlN in optical and electro-optical devices, including compact disks, phase shift lithography masks, and AlN/GaN multilayer devices. The interband optical properties in the vacuum ultraviolet (VUV) region of 6-44 eV have been investigated previously because they convey detailed information on the electronic structure and interatomic bonding of the material. In this work, we have combined spectroscopic ellipsometry with UV/VIS and VUV spectroscopy to directly determine the optical constants of AlN in this range, thereby reducing the uncertainty in the preparation of the low-energy data extrapolation essential for Kramers-Kronig analysis of VUV reflectance. We report the complex optical properties of AlN, over the range of 1.5-42 eV, showing improved agreement with theory when contrasted with earlier results.