Electronic structure analysis of (In, Ga, Al) N heterostructures on the nanometre scale using EELS.
In this work the local electronic structure of MOVPE-grown (In, Ga, Al) N heterostructures has been investigated by electron energy loss spectroscopy (EELS). The cold field-emission scanning transmission electron microscope (VG HB501) used was equipped with a dedicated parallel EELS system which provides high dispersions at an energy resolution of 0.35 eV with the use of subnanometre electron probes the spatial resolution of the measurements depends on the physical localization of the scattering process itself and thus is in the order of nanometres. The low-loss region of the energy spectra gives information on plasmon excitations and transitions across the bandgap. The main problem on looking at the bandgap region of EELS spectra is to separate the bandgap signal from the fading tail of the zero-loss peak. High energy resolution and application of suitable deconvolution routines for removal of the zero-loss peak extract improved information from this energy region. Thus the EEL spectra of different group III nitrides reveal the onset of the bandgap itself and the characteristic shape of the joint density of states. From these results the local optical properties can be deduced via a Kramers-Kronig transformation. The data obtained show detailed structure on the energy scale and are in excellent agreement with optical ellipsometric results. In comparison with these techniques EELS methods yield a superior spatial resolution of better than 10 nm which enables detailed investigation of the effect of local defects and boundaries on the optical properties.