As in Non-contact EFM in Contact EFM the cantilever is biased directly by
Vtip=Vdc + Vac sin(wt), where Vac
is referred to as the driving voltage. Scanning is executed as in usual Constant Force mode and simultaneously the electric forces are measured. The capacitive force Fcap(z) between the tip and a surface at potential Vs is
Fcap(z) =(1/2) (Vtip - Vs)2(dC/dz)
where C(z) is the tip-surface capacitance dependent on tip geometry, surface topography and tip-surface separation z. In contact EFM the influence of the first harmonic of Fcap(z) on the cantilever oscillation is registered.
Contact EFM can be applied to semiconductor structures. The current flow through the contact area may decrease those forces, but it is extremely small, since there are usually thick (a few nanometers) oxide layers on the tip and sample surface.
As an example data of GaAlAs geterostructures measuring are given.
- Rev. Sci. Instr. 70, 1735 (1999).