Andreas Thust

Aberration measurement in HRTEM: Implementation and diagnostic use of numerical procedures for the highly precise recognition of diffractogram patterns

The precise characterisation of the instrumental imaging properties in the form of aberration parameters constitutes an almost universal necessity in quantitative HRTEM, and is underlying most hardware and software techniques established in this field. We focus in this paper on the numerical analysis of individual diffractograms as a first preparatory step for further publications on HRTEM aberration measurement. The extraction of the defocus and the 2-fold astigmatism from a diffractogram is a classical pattern recognition problem, which we believe to have solved in a near-optimum way concerning precision, speed, and robustness. The newly gained measurement precision allows us to resolve fluctuations of the defocus and the 2-fold astigmatism and to assess thereby the optical stability of electron microscopes. Quantitative stability criteria are elaborated, which may serve as helpful guidelines for daily work as well as for microscope acceptance tests.

Atomic-resolution imaging of lattice imperfections in semiconductors by combined aberration-corrected HRTEM and exit-plane wavefunction retrieval

With improvements in the instrumental information limit and the simultaneous minimization of image delocalization, high-resolution transmission electron microscopy is presently enjoying increased popularity for the atomic-scale imaging of lattice imperfections in solid-state materials. In this study, the benefits of a combination of spherical aberration-corrected imaging and numerical retrieval of the exit-plane wavefunction from a focal series of micrographs are illustrated by highlighting their combined use for atomic-scale characterization of lattice defects frequently observed in common semiconductor materials. Thus, experimental analyses will review the core structure of Lomer dislocations at In0.3Ga0.7As/GaAs heterointerfaces and focus on atomic lattice displacements associated with extrinsic stacking faults in GaAs, as well as on the core structure of chromium implantation-induced Frank partial dislocations in GaN at directly interpretable contrast features. Supplementary, practical advantages of the retrieval of the exit-plane wavefunction for the subsequent numerical elimination of residual lens aberrations are demonstrated.

The Error of Aberration Measurements in HRTEM Using Zemlin Tableaus

In modern high-resolution transmission electron microscopy lens aberrations, in particular the strong third-order spherical aberration of the objective lens, severely limit the conditions for a directly interpretable imaging of object structures. In recent years hardware aberration correction [1] and software aberration correction of reconstructed exit wave functions [2, 3] have become feasible, thus fully exploiting the information limit of an instrument. Both methods require an accurate aberration measurement which is used to align the lens corrector [1, 4] or to correct an experimental exit wave function numerically [2, 3].