Strong internal electric fields (of the order of MV/cm) influence the electronic properties and light emission process of InGaN quantum wells. We have used electron holography to directly profile the potential and charge distribution across a GaN/In 0.18Ga 0.82N/GaN quantum well structure, and present here an analysis of the fine-scale potential variations at the quantum well. The potential profiles show a drop of 0.6±0.2V across, and an internal electric field of -2.2±0.6MV/cm in the quantum well. An analysis based on Poisson's equation suggests that the field is caused by electronic charges with a peak density of 8×10 20cm -3, corresponding to a sheet charge density of 0.027C/m 2. Free electron and hole densities of the order of 10 20cm -3 are confined separately in the quantum well. These free carriers screen only part of the electric field due to the polarization effect. © 2002 American Institute of Physics.