To realize the potential of quantum point defects for next-generation computing paradigms, deterministic placement of the defects is necessary, along with improved control over the nanoscale local environment. Toward this end, the scanning tunneling microscope is emerging as a useful tool for its capabilities of atomic manipulation, imaging and tunneling spectroscopy. I will discuss our recent STM studies of adatoms and acceptors at (110) surfaces of III-V semiconductors. On undoped InSb(110), we demonstrate how individual In adatoms can affect tunneling rates into the surface by more than 100x over micron-scale areas [1]. The spatial extent and magnitude of the tunneling effect are widely tunable by imaging conditions such as bias voltage, set current and photoillumination. We attribute the effect to occupation of a (+/0) charge transition level by a tunneling electron from the tip, and switching of the associated adatom-induced band bending. The effect in STM topographic images is well reproduced by transport modeling of filling and emptying rates as a function of the tip position. We attribute the remarkable spatial extent of the adatom ionization effect to the low concentration and low binding energy of the residual donors in the undoped InSb crystal. To more directly probe and control the spin properties of magnetic dopants, we have applied methods for spin-polarized STM to Fe acceptors in GaAs. Fe adatoms are found to substitute for surface-layer Ga upon adsorption at ~10 K, and the STM tip is then used to sweep away resultant Ga adatoms through repeated imaging at positive voltage. Tellingly, STM imaging at positive voltage with an antiferromagnetic bulk Cr tip indicates Fe acceptors with a range of apparent heights, as well as reversible switching of contrast. Both effects are absent in repeated imaging with nonmagnetic PtIr tips, suggesting a magnetic origin. While the time scales for static alignment of the Fe spins are much shorter than STM imaging times, our observations are in qualitative agreement with recent advances in dynamical spin transport theory of exchange-coupled spins [2]. These studies show that tunable control over single dopants and their environment in semiconductors is becoming a realistic route for next-generation classical- and quantum-based information technologies, while at the same time informing the design of conventional nanoscale devices. 

[1] Mueller, Sara M., Dongjoon Kim, Stephen R. McMillan, Steven J. Tjung, Jacob J. Repicky, Stephen Gant, Evan Lang, et al. “Tunable Tunnel Barriers in a Semiconductor via Ionization of Individual Atoms.” Journal of Physics: Condensed Matter 33, no. 27 (May 2021): 275002. https://doi.org/10.1088/1361-648X/abf9bd. 

[2] McMillan, Stephen R., Nicholas J. Harmon, and Michael E. Flatte. “Image of Dynamic Local Exchange Interactions in the Dc Magnetoresistance of Spin-Polarized Current through a Dopant.” Physical Review Letters 125, no. 25 (December 17, 2020): 257203. https://doi.org/10.1103/PhysRevLett.125.257203.