Nanoscale EELS analysis of dielectric function and bandgap properties in GaN and related materials

The low loss region of an EEL spectrum (<50 eV) contains information about excitations of outer shell electrons and thus the electronic structure of a specimen which determines its optical properties. In this work, dedicated electron energy loss spectroscopy (EELS) methods for the experimental acqui sition and analysis of spectra are described, which give improved informati on about the electronic structure near the bandgap region at a spatial reso lution in the range of nanometers. For this:purpose, we made use of a cold field emission scanning transmission electron microscope (STEM) equipped wi th a dedicated EELS system. This device provides a subnanometer electron pr obe and offers an energy resolution of 0.35 eV. Application of suitable dec onvolution routines for removal of the zero loss peak extracts information on the bandgap region while the Kramers-Kronig transformation deduces the d ielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap ma terials for the case of III-nitride compounds, which are currently the most promising material for applications on optoelectronic devices working in t he blue and ultraviolet spectral range. The obtained results are in excelle nt agreement with experimental measurements by synchrotron ellipsometry and theoretical studies. The potential of the superior spatial resolution of E ELS in a STEM is demonstrated by the analysis of dielectric properties of i ndividual layers of heterostructures and individual defects within wurtzite GaN. (C) 2000 Elsevier Science Ltd. All rights reserved.