AlN/GaN and InAlN/GaN DBRs

This chapter presents the high resolution monochromated STEM-EELS characterization of two distributed Bragg reflector (DBR) multilayer heterostructures, composed of a periodic staking of III-nitride layers. These heterostructures were grown by the group of E. Calleja at the Instituto de Sistemas Optoelectrónicos y Microtecnología (ISOM), from Universidad Politécnica de Madrid. One of these DBR is composed of an alternate staking of AlN and GaN layers, and the other one, of InAlN lattice matched to GaN. EELS at sub-nanometric spatial resolution and <200 meV energy resolution was used to assess the electronic properties of the structures. The EELS signal was treated using ZLP subtraction and deconvolution methods, and non-linear fitting tools complemented with theoretical modeling of the electron scattering distribution. In this sense, the log-ratio formula was used to calculate the relative thickness, related to the electron inelastic mean free path. Moreover, fitting of the bulk plasmon peak was performed using Lorentzian and Drude free-electron models. As we have seen, in group-III nitride alloys, the energy position of this peak can be related to the chemical composition variation through Vegard's law. Also, within the context of the Drude plasmon model, information regarding the structural properties of the material can be obtained from the lifetime of the oscillation. This structural and chemical characterization of the layers was complemented with experimental and simulated high angle annular dark field (HAADF) images. Finally, information related to the dielectric response of the materials was extracted using Kramers-Kronig analysis. Our results significantly improve the understanding of previous macroscopic characterizations of the electro-optical properties of these structures. © 2019 Elsevier Inc.