Charge Switching of Donor Ensembles in a Semiconductor*

We report charge switching of well-defined groups of individual donors in GaAs controlled by a scanning tunneling tip with atomic resolution [1,2]. Lateral positioning and voltage tuning of the band-bended region under the sharp tip allows to measure at low temperature, T = 5 K, the ionization threshold of each given donor site with high precision. The changing charge states of surrounding donors give rise to discrete ``Coulomb ladder'' steps in the screened electrostatic potential V(r) which is extracted for the nearest donor under the tip. In certain geometrical configurations we observe bias- and time-dependent flicker spectra. \\[4pt] [1] K.Teichmann et al., PRL 101, 076103 (2008)\\[0pt] [2] A.P.Wijnheijmer et al., PRL 102, 166101 (2009)