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We study sub-surface arsenic dopants in a hydrogen terminated Si(001) sampleat 77 K, using scanning tunnelling microscopy and spectroscopy. We observe anumber of different dopant related features that fall into two classes, whichwe call As1 and As2. When imaged in occupied states the As1 features appear asanisotropic protrusions superimposed on the silicon surface topography, andhave maximum intensities lying along particular crystallographic orientations.In empty-state images the features all exhibit long-range circular protrusions.The images are consistent with buried dopants that are in the electricallyneutral (D0) charge state when imaged in filled states, but become positivelycharged (D+) through electrostatic ionisation when imaged under empty stateconditions, similar to previous observations of acceptors in GaAs. Densityfunctional theory (DFT) calculations predict that As dopants in the third layerof the sample induce two states lying just below the conduction band edge,which hybridize with the surface structure creating features with the surfacesymmetry consistent with our STM images. The As2 features have the surprisingcharacteristic of appearing as a protrusion in filled state images and anisotropic depression in empty state images, suggesting they are negativelycharged at all biases. We discuss the possible origins of this feature.

Investigating individual arsenic dopant atoms in silicon using low-temperature scanning tunnelling microscopy