Tuning the electronic states of individual Co acceptors in GaAs

Scanning tunneling microscope studies of individual impurities in semiconductors explore challenges associated with future nanoscale electronics and can provide insight into how new materials properties such as ferromagnetic ordering arise from impurity interactions. Atomic manipulation and tunneling spectroscopy were employed to characterize and control the acceptor states of Co atoms substituted for Ga in the GaAs(110) surface. Three states were observed whose appearance in tunneling spectra was sensitive to the tip position within the acceptor complex. The energy of these states did not follow bending of the host bands due to the tip-induced electric field, but did respond to the Coulomb potential of nearby charged defects, such as As vacancies. By applying voltage pulses with the scanning tunneling microscope tip, the vacancies could be positioned on the surface, thus enabling tunable control over the Co acceptor states.