The development of new optoelectronic devices using group-III-nitride-heterostructures requires characterization techniques which give spatially resolved information about the local physical and chemical material properties. The goal of this work was the improvement of analytical STEM-techniques and its application on nano-characterization of III-N-heterostructures. The optimization of electron energy-loss spectroscopy provides a measurement techniques which yields spatially resolved information about bandgap energy and dielectric function on a nanometer scale. A combination of STEM-techniques like EELS, convergent electron diffcration (CBED) and Z-contrast imaging was used to characterize and improve structural properties, interfaces and chemical composition of InGaN-layers. The influence of growth parameters on the enhancement of interface sharpness was demonstrated. Additionally nanoscale fluctuations of the In-content could be proved by Z-contrast imaging and quantified by EELS. It was shown how v-defects in InGaN/GaN-superlattices can be eliminated by modulating the growth temperature during epitaxial growth. The improved EELS-techniques were used for measuring optical properties within III-N-heterostructures. The significance of the results could be demonstrated by a comparison to synchrotron measurements and bandstructure calculations. Due to the high spatial resolution optical properties of local separations and defects could be measured and correlated to chemical and structural properties.