The scaling of electronic devices such as field effect transistors to nanometer dimensions requires more precise control of individual dopants in semiconductor nanostructures, as statistical fluctuations can impact device performance and functionality. Toward this end, the scanning tunneling microscope (STM) is emerging as a useful tool for its capabilities of atomic manipulation, imaging and tunneling spectroscopy. I will discuss our STM studies of Mn acceptors within the surface layer of a p-doped GaAs crystal . We start by sublimating Mn adatoms onto the GaAs (110) surface, prepared by cleavage in ultrahigh vacuum. A voltage pulse applied with the STM tip allows us to replace a Ga atom in the surface with the Mn atom, thus forming a single Mn acceptor. We find that the properties of Mn acceptors can be tuned by control of the local electrostatic environment. For example, the STM tip can be used to position As vacancies (VAs), which are positively charged. Direct Coulomb repulsion causes a reduction in the hole-Mn binding energy as VAs is moved nearby. Tunneling spectroscopy allows us to quantify this effect, through the shift of an in-gap acceptor resonance toward lower energy. In addition, vacancy-induced band bending provides a method for tuning the ionization state of the Mn acceptors, as evidenced by a ring-like feature in STM images. Tunable control over single dopants in semiconductors is promising for next-generation classical- and quantum-based information technologies.
Lee, D. H. and Gupta, J. A. Tunable Field Control Over the Binding Energy of Single Dopants by a Charged Vacancy in GaAs. Science 330, 1807-1810 (2010).