Peering under the surface of semiconductors using scanning tunneling microscopy

Scanning tunneling microscopy (STM) is a workhorse tool for understanding the structural and electronic properties of surfaces at the atomic scale. This is a consequence of the tunneling current coming from the evanescent decay of wavefunctions in vacuum, and the sample’s integrated density of electronic states. Defects in semiconductors offer a tantalizing opportunity for STM to look underneath the surface layer of atoms, which could provide new insight into non-equilibrium processes during thin film growth. Biasing the tunnel junction inside the semiconductor gap should reduce the sensitivity to the semiconductor, and enhance the sensitivity to defects which have electronic states inside the gap. This creates a situation where a subsurface defect can produce a larger tunneling current than the surface layer of semiconducting atoms. Here, we examine how accurately STM can be used to locate the depth of phosphorus, boron, carbon and tin defects in silicon. Spanning a range of defects, from those that are electrically active to those which strain the lattice, and comparison to electron density calculations using density functional theory enables a deeper understanding of the limitations to sub-surface imaging. This potentially opens up a new tool to understand non-equilibrium growth processes which are used to exceed conventional limits to, e.g., composition in group IV epitaxy.