Influence of free surfaces on dislocation elastic fields: advancing dislocation characterisation in GaN for non-destructive surface observation techniques

Dislocations are linear crystalline defects that induce long-range elastic fields inside materials. Dislocations can significantly alter the positions of atoms in their neighbourhood, enabling their detection through near-surface electron diffraction techniques, such as conventional electron backscatter diffraction (EBSD), High-Resolution Electron Backscatter Diffraction (HR-EBSD), or electron channelling contrast imaging (ECCI). In this contribution, we explore the effect of free surfaces on dislocation elastic fields, by comparison with bulk signatures, in order to assess the possible influence of free surfaces on dislocation detection and characterisation using near surface electron diffraction techniques. To this end, we employ a field dislocation mechanics (FDM) model numerically approximated by a fast Fourier transform (FFT) algorithm, which allows to model any dislocation configuration with the effect of free surfaces. In addition to strain, rotations and stress fields, the model is further used to generate virtual curvature and associated geometrically necessary dislocation (GND) density maps that can be typically measured in EBSD to characterise dislocations. We apply the workflow to threading dislocations (TDs) in GaN semiconductor [0001] layers deposited on a Si substrate. As a model material system featuring mostly straight dislocations, GaN allows for the assessment of the possible applicability of EBSD, HR-EBSD and ECCI to characterise dislocations near the surface. Our findings reveal that the elastic fields can differ significantly between the surface and the bulk, which can lead to potential misinterpretations in dislocation characterisation. Additionally, the necessary measures required for the unambiguous characterisation and classification of dislocations are established.