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We present spectrally resolved luminescence data of wide-bandgap group-III nitride layers grown by molecular beam epitaxy on sapphire substrates. The luminescence is locally excited by electrons provided by the tip of a scanning tunneling microscope (STM) operated at room temperature. Nominally undoped, the group-III nitride layers naturally show n-type character, thus guaranteeing sufficient electrical conductivity for the STM studies. Luminescence spectra recorded with the STM typically reveal the near-bandgap signal peaking at ≈ 3.4 eV. By incorporating Mg with a concentration of 10 20 cm −3 , the background n-type character can be overcompensated, giving rise to a pronounced radiative transition into the acceptor level located at ≈ 3.2 eV. The absolute peak intensities, however, markedly depend on the site of excitation at the sample surface. This effect allows to distinguish unequivocally between high-quality and defect-rich film regions, also offering the opportunity to correlate nearsurface optical properties with morphological features. By adding 10% Al, the resulting crystal alloy exhibits a wider energy-band gap which is reflected in a blue shift of the respective STM-excited luminescence signal.Direct bandgap group-III nitrides, covering a spectral range from 1.9 eV (InN) or 3.4 eV (GaN) up to 6.2 eV (AlN), have great potential in the realm of optoelectronics. The possibility of preparing ternary crystal alloys makes these materials most attractive for bandgap engineering. Great efforts have been devoted to the development of light-emitting devices, culminating in the successful demonstration of a laser diode [1]. Further applications of this family of materials include the absorption of ultraviolet radiation using appropriate sensor devices [2].Notwithstanding the remarkable efficiency of blue lightemitting devices based on GaN, the epilayers usually exhibit a high density of different types of dislocations, in- * On leave to: volving low-angle grain boundaries associated with crystalline columns [3]. Several techniques have been utilized so far in characterizing group-III nitride layers, including Xray diffraction, photoluminescence, Hall measurements, and cathodoluminescence. However, there is still a need to examine the initial stage of nucleation and its impact on subsequent layer growth. Furthermore, there is great interest in gaining a deeper insight into the detailed material composition and the related optical properties, particularly at the nanometer scale. A first approach has been to study the surface morphology of thin GaN films grown by molecular beam epitaxy (MBE) on sapphire substrates using atomic force microscopy (AFM). These measurements clearly reveal the presence of individual columns or crystallites having sizes in the µm range [4,5]. Scanning tunneling microscope (STM) and transmission electron microscope (TEM) investigations have been carried out to examine the structural properties of GaN layers grown by low-pressure chemical vapor deposition (LP-CVD) techniques [6]. The local ...