Quantum well structures for nitride-based LEDs are mainly grown in c-direction, whereby the quantum confined Stark-effect (QCSE) reduces the overlap of the electron and hole wave function and with it the internal quantum efficiency. The reason for that is the difference in polarization of the quantum well and barrier materials. In order to balance these polarizations, quaternary (AlGaIn)N layers for the later use as barrier material have been grown. In addition to the bandgap energy, another parameter, the polarization, can be controlled in this way. As a standard characterization method X-ray diffraction measurements have been performed. Due to the impossibility of fully determining the composition using this method, total reflection X-ray-fluorescence (TXRF) has been used to get the missing information.