It was recalled that epitaxial layers frequently contained a high density of threading dislocations, which could exceed values of 109/cm2 for ZnO and GaN. In order to study the electrical activity of single dislocations, off-axis electron holography in a transmission electron microscope was used. The electrostatic potential in the vicinity of charged dislocations was deduced from the reconstructed phase of the electron wave, which also provided access to the charge density at the dislocation. On the basis of the Read electrostatic potential for a screened line charge, a refined model was proposed in which a charge density with a finite spatial distribution at the dislocation core was assumed. Upon comparing the measured and theoretically expected potentials, charge densities of between 5 x 1019 and 5 x 1020/cm3 were found, in cylinders around the dislocation lines, which had radii of up to 5nm. An important aspect of this work was an analysis of dislocations seen in cross-sectional transmission electron microscopy samples, where the dislocation lines were oriented perpendicularly to the electron beam. In this regard, the cross-sectional geometry had both advantages and drawbacks. The electrostatic potentials due to piezoelectric charges in the dislocation strain field were considered and were found to be insignificant with respect to the dislocation charges.

Probing the Electrostatic Potential of Charged Dislocations in n-GaN and n-ZnO Epilayers by Transmission Electron Holography. E.Müller, D.Gerthsen, P.Brückner, F.Scholz, T.Gruber, A.Waag: Physical Review B, 2006, 73[24], 245316 (9pp)