Abstract
Diffusion is one of the fundamental processes that govern the structure, processing, and properties of
materials and it plays a crucial role in determining device lifetimes. However, direct observations of
diffusion processes have been elusive and limited only to the surfaces of materials. Here we use an
aberration-corrected electron microscope to locally excite and directly image the diffusion of single Ce and
Mn dopants inside bulk wurtzite-type AlN single crystals, identifying correlated vacancy-dopant and
interstitial-dopant kick-out mechanisms. Using a 200 kV electron beam to supply energy, we observe a
higher frequency of dopant jumps for the larger and heavier Ce atoms than the smaller Mn atoms. These
observations confirm density-functional-theory-based predictions of a decrease in diffusion barrier for large
substitutional atoms. The results show that combining depth sensitive microscopy with theoretical
calculations represents a new methodology to investigate diffusion mechanisms, not restricted to surface
phenomena, but within bulk materials.
