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 h eterostructures 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 pro vides high dispersions at an energy resolution of 0.35 eV with the use of s ubnanometre electron probes the spatial resolution of the measurements depe nds 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 exci tations and transitions across the bandgap. The main problem on looking at the bandgap region of EELS spectra is to separate the bandgap signal from t he fading tail of the zero-loss peak, High energy resolution and applicatio n of suitable deconvolution routines for removal of the zero-loss peak extr act improved information from this energy region. Thus the EEL spectra of different group III nitrides reveal the onset of th e bandgap itself and the characteristic shape of the joint density of state s. From these results the local optical properties can be deduced via a Kra mers-Kronig transformation. The data obtained show detailed structure on the energy scale and are in ex cellent agreement with optical ellipsometric results. In comparison with th ese techniques EELS methods yield a superior spatial resolution of better t han 10 nm which enables detailed investigation of the effect of local defec ts and boundaries on the optical properties.