Imaging individual dopant atoms on cleavage surfaces of wurtzite-structure compound semiconductors

We report the identification of bulk-dopant atoms in (1120) and (1010) cleavage surfaces of wurtzite CdSe in atomically resolved scanning tunneling microscopy images. The In dopant atoms give rise to elevations (hillocks) of up to 5 nm in diameter in the empty- and occupied-state images. This contrast is simulated and it shows that the dopant atoms are positively charged. Hillocks with different symmetries with respect to the underlying lattice are correlated with different subsurface locations of the In-dopant atoms. Dopant atoms can be observed up to a depth below the surface of 3 to 5 layers. A quantitative analysis of the concentration of dopant atoms in the bulk yield the same values for both surfaces and agrees well with the In content of the crystal. Similar features in CdS(1010) surfaces are also attributed to In-dopant atoms.