Constant Height 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 Height mode (CHM) of operation the scanner of STM moves the tip only in plane, so that current between the tip and the sample surface visualizes the sample relief. Because in this mode the adjusting of the surface height is not needed a higher scan speed can be obtained. CHM can only be applied if the sample surface is very flat, because surface corrugations higher than 5-10 A will cause the tip to crash. The weak feedback is still present to maintain a constant average tip-sample distance. As the information on the surface structure is obtained via the current, a direct gauging of height differences is no longer possible.

STM gives true atomic resolution on the 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 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 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).