Constant Current mode

In STM bias voltage is applied between a sharp conductive tip and a conductive sample, so when the sample is approached to a few angstroms from the tip, tunneling current occurs, that indicates proximity of the tip to the sample with very high accuracy. In Constant Current mode (CCM) of operation when scanning sample surface the scanner keeps the current constant by feedback circuit. So vertical displacement of the scanner (feedback signal) reflects surface topography.

STM gives true atomic resolution on some samples even at ambient conditions. Scanning tunneling microscopy can be applied to study conductive surfaces or thin nonconductive films and small objects deposited on conductive substrates. The speed of scanning in CCM is restricted by usage of feedback system. Larger scanning speeds can be obtained by usage of Constant Height mode (CHM), but CCM allows to investigate the samples with developed relief.

The tunnel currents registered in the course of the measurement are sufficiently small - up to 0.03 nA (with a special STM head - up to 0.01 nA), so it is possible to investigate also low conductivity surfaces, in particular biological objects.

Among the STM disadvantages one can mention the complexity of the results interpretation for some surfaces since the surface image received in the STM investigation mode is determined not only by the surface relief but also by the density of states, bias voltage sign and value, current value etc. For example on the highly oriented pyrolitic graphite surface one can see only each second atom. It is concerned with special arrangement of wave functions density of states.

References

1. Rep. Prog. Phys. 55, 1165-1240 (1992).