Cathodoluminescence (CL) imaging and temperature-dependent cathodoluminescence spectroscopy (CLS) have been used to probe the spatial distribution and energies of electronic defects near GaN/Al2O3 interfaces grown by hydride vapor phase epitaxy (HVPE). Cross sectional secondary electron microscopy imaging, CLS, and CL imaging show systematic variations in defect emissions with a wide range of HVPE GaN/sapphire electronic properties. These data, along with electrochemical capacitance–voltage profiling and secondary ion mass spectrometry provide a consistent picture of near-interface doping by O out-diffusion from Al2O3 into GaN over hundreds of nanometers. Low-temperature CL spectra exhibit a new donor level at 3.447 meV near the interface for such samples, characteristic of O impurities spatially localized to the nanoscale interface. CLS emissions indicate the formation of amorphous Al–N–O complexes at 3.8 eV extending into the Al2O3 near the GaN/sapphire interface. CLS and CL images also reveal emissions due to excitons bound to stacking faults and cubic phase GaN. The temperature dependence of the various optical transitions in the 10–300 K range provides additional information to identify the near interface defects and impurity doping. © 2002 American Institute of Physics.