Reversibly switching between the hydrogenic substitutional donor configuration and a previously unknown negatively charged interstitial configuration of silicon (Si) impurities in the surface layer of gallium arsenide (GaAs) was observed. The unexpected negatively charged state of Si${}_{\mathrm{Ga}}$ stresses that the surface dominates the properties of dopant atoms close to it. We find that the negatively charged state is favorable in the case of the bare surface, whereas the donor configuration is only favorable with the tip of the scanning tunneling microscope (STM) nearby. The Si atom randomly switches between both bistable configurations. The bistable behavior was characterized with STM as a function of the applied voltage, the tunneling current, the temperature, and the local environment. The voltage dependence suggests a similar potential landscape as derived for DX${}^{-}$ centers in bulk GaAs. Increased switching rates at higher currents point on an inelastic process, although with a rather low efficiency. The switching rate is constant below 20 K, whereas it increases above 20 K. This indicates a nonthermal process below 20 K, probably elesatic excitations in combination with quantum tunneling, whereas the switching is thermally activated at higher temperatures.

• Received 17 February 2011

DOI:https://doi.org/10.1103/PhysRevB.84.075459