We have used scanning capacitance microscopy (SCM) and atomic force microscopy (AFM) to characterize structural and electronic properties of InxGa1-xN/GaN quantum-well structures at the nanoscale. Macroscopic capacitance-voltage measurements combined with numerical simulations indicate that either electron or hole accumulation in the quantum-well layer can be induced by application of forward or reverse bias, respectively. Under reverse bias conditions (hole accumulation), features corresponding to monolayer fluctuations in InxGa1-xN quantum-well thickness are clearly evident. Under forward bias conditions (electron accumulation), samples exhibiting high luminescence efficiencies are found to contain regions of increased carrier accumulation within the quantum well, which on the basis of spatially resolved spectroscopy, bias-dependent imaging, and numerical simulations are attributed to nanoscale In-rich clusters in the quantum well. No such features are observed in samples exhibiting lower luminescence efficiencies. Together, these studies demonstrate the ability to image, and distinguish, nanoscale variations in subsurface electronic properties arising from either monolayer thickness fluctuations or compositional inhomogeneities in InxGa1-xN/GaN quantum-well structures. (c) 2005 American Vacuum Society.