Cross-sectional scanning capacitance microscopy measurements of unintentionally doped model structures for InGaN quantum wells, GaN barriers and AlInN electron-blocking layers showed n -type conductivity for the In-containing layers. Secondary ion mass spectrometry indicated that oxygen impurities are the likely source of the electron density in the model layers. The n -type conductivity as well as the oxygen impurity level increases to ∼1018 cm-3 for AlInN lattice-matched to GaN. These results suggest that the background electron concentration due to oxygen impurities in indium-containing layers needs to be considered in the design and theoretical modelling of device structures. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)