Imaging the high-frequency charging dynamics of a single impurity in a semiconductor on the atomic scale
As electronic devices approach the atomic limit, the charge dynamics of individual dopant atoms increasingly constrain performance, stability, and coherence. In scanning tunnelling microscopy (STM), donor ionization is typically interpreted as a static threshold process arising from tip-induced band bending. Here we show that the ionization of individual sulfur donors in InAs is intrinsically dynamic and governed by the local electric field. Using MHz-frequency STM noise spectroscopy with atomic-scale spatial mapping, we resolve pronounced random telegraph noise that is invisible in time-averaged tunnelling spectra. A bias-dependent model quantitatively links the noise spectra to microscopic ionization and neutralization processes of the donor states, enabling direct extraction of nanosecond charge-state lifetimes. The switching rate is strongly bias dependent, demonstrating that the electric field continuously drives charge-state transitions. Unexpectedly, we show that the degenerately doped bulk leads to a sharp bias-dependent onset of donor ionization as the donor level crosses the Fermi level, giving rise to a characteristic shoulder in the noise power spectrum that is captured by our model. These results establish donor ionization as a non-equilibrium dynamical process with nontrivial contribution by the bulk electrons, and identify impurity switching as a universal nanoscale charge-noise mechanism relevant to quantum devices.