Measuring local lattice polarity in AlN and GaN by high resolution Z-contrast imaging: The case of (0001) and (1(1)over-bar00) GaN quantum dots

By using a probe Cs corrected transmission electron microscope, the local lattice polarity, i.e., the local stacking of atoms, in AlN and GaN is determined. The N atomic columns are not directly resolved, but determined from the tunnel positions of the structure. The method is first tested on GaN quantum dots grown on polar (0001) AlN surfaces. Then it is applied to GaN quantum dots grown on nonpolar (1 (1) over bar 00) AlN surfaces. In the (11 (2) over bar0) plane, (1 (1) over bar 00) GaN dots have an asymmetrical shape and grow faster along the +c=[0001] direction than along -c=[000 (1) over bar].