Locally resolved imaging of internal electric fields in GaN/GaInN quantum wells by differential phase contrast microscopy

InGaN/GaN-based laser diodes emitting in the green spectral range are still difficult to achieve. The efficiency ''droop'' in the green spectral range is strongly believed to be a consequence of the quantum confined Stark effect (QCSE), due to inner piezoelectric fields in the material, caused by strain at the interfaces. Attempts are made to reduce the piezoelectric (PE) fields by choosing semi- or non-polar crystal facets for the growth of quantum wells. It is necessary to measure the existing PE fields in order to determine whether the various approaches actually can reduce the QCSE and to foster more efficient light emission. We present first results using differential phase contrast (DPC) in a (S)TEM, where we measure directly the beam deflection due to the inner PE fields. The specimens contain quantum wells grown on different facets. The DPC images display the PE fields in strong contrast and laterally highly resolved over a large field of view. We observe effects not only in the quantum wells but also adjacent to them in the substrate layer and around stacking faults.