Imaging Impurities in Semiconductor Nanostructures

Atomic impurities are critical for many technologies. They are used to engineer the optical and electronic properties of semiconductors for applications such as transistors, solar cells, lightemitting diodes (LEDs), and lasers, as well as to store energy for applications such as batteries and electrochemical cells. While the characterization and understanding of impurities in bulk semiconductors is well developed, new challenges arise at the nanoscale. In particular, methods are needed to characterize structures that may only contain a few impurity atoms. With such techniques, a fundamental understanding of how atomic impurities affect the properties of semiconductor nanostructures could be more fully developed. In this review, we give a brief introduction to the benefits and challenges associated with the incorporation of impurities in nanoscale structures, a process known as doping. We then focus on techniques used to characterize and image atomic impurities in semiconductor nanostructures. Advances in electron microscopy allow researchers to probe the dynamics of impurity incorporation with in situ transmission electron microscopy (TEM), and techniques such as electron energy loss spectroscopy (EELS) coupled with annular dark-field scanning transmission electron microscopy (ADF-STEM) allow individual atomic impurities in semiconductor nanostructures to be detected and imaged. Likewise, techniques such as atom probe tomography (APT) enable the full atomic reconstruction of nanoscale materials.