Probing the local dielectric optical properties of group III-Nitrides by spatially resolved EELS on the nanometer scale

Usually, information on the dielectric-optical properties of semiconductors is obtained from synchrotron spectroscopic ellipsometry to cover the desired spectral range between 2 and 25 eV; but such measurements lack spatial resolution. In this work the local electronic structure of (In, Ga, Al) N heterostructures has been investigated by electron energy loss spectroscopy. Using subnanometer electron probes the spatial resolution of the measurements depends on the physical localization of the scattering process itself, typically in the range of nanometers. The low-loss region of the energy spectra reveals information on plasmon excitations and transitions across the band gap and the characteristic shape of the joint density of states. From these results the local dielectric/optical properties can be deduced via Kramers-Kronig transformation. The results obtained are in excellent agreement with theoretical studies and ellipsometry measurements. (C) 1999 Elsevier Science S.A. All rights reserved.