We investigated Silicon donors in highly n-doped GaAs (6.5x1018cm-3) by Cross Sectional Scanning Tunneling Microscopy in UHV at 8K. Subsurface donors were studied by spatially resolved I(V)-spectroscopy and simultaneous determination of the local barrier height with a dI/dz modulation technique. At different bias voltage different scenarios for the relevant tunnelling processes are discussed. Previous spectroscopic studies concentrated on features which appear for negative sample voltages in the band-gap [1]. To extend the established model we set our focus on the positive voltage range.
A spectrum taken on a donor compared with one recorded on the free surface shows additional features. At large negative voltages the differential conductance dI/dV shows peaks within the band gap. This feature is well known and can be explained with the tip induced band bending (TIBB) in combination with the potential of the donor [2]. Confined states build up in this quasi zero dimensional potential. The bias voltage position of this state is related to the magnitude of TIBB connected with the barrier height. At small negative voltages tunnelling out of the donor state is observed.
In addition to the known features at negative voltages, our measurements on single donors show an additional transport channel for larger positive bias voltages. The minimal bias voltage of the current onset is obtained if the tip is localized above the donor. The current distribution has a circular symmetric structure. The diameter is bias dependent, and can extend up to several nanometers around the donor. Our analysis gives strong indication that this ring-like feature is caused by the potential of the tip, and is dependent on the magnitude of the TIBB. We conclude that tunnelling through or into the donor is supressed at small TIBB. At higher voltages and higher TIBB, tunneling out of the filled donor states is possible.
This work was supported by the DFG, SFB 602.
[1]R. M. Feenstra, G. Meyer, F. Moresco and K. H. Rieder Phys Rev B 66 165204 (2002)
[2]M. Wenderoth, M. A. Rosentreter, K. J. Engel, A. J. Heinrich, M. A. Schneider and R. G. Ulbrich Europhys. Lett. 45 579 (1999) |