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

TL;DR

This study uses low-temperature scanning tunnelling microscopy and spectroscopy to analyze sub-surface arsenic dopants in silicon, revealing different charge states and surface features consistent with DFT predictions.

Contribution

It provides detailed imaging and characterization of arsenic dopants in silicon at the atomic level, including charge states and surface interactions, supported by DFT calculations.

Findings

As1 features are neutral in filled states and positively charged in empty states.

As2 features are negatively charged at all biases.

DFT predicts dopant-induced states near the conduction band edge.

Abstract

We study sub-surface arsenic dopants in a hydrogen terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant related features that fall into two classes, which we call As1 and As2. When imaged in occupied states the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography, and have 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 electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D+) through electrostatic ionisation when imaged under empty state conditions, similar to previous observations of acceptors in GaAs. Density functional theory (DFT) calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled state images and an isotropic depression in empty state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.