"A major improvement
of this new method
is that the surfaces
do not need to touch
each other.
It only requires very
weak electric fields,
which are not likely
to influence the
electrical or
chemical
structure
of the material."
The Kelvin Probe Principles
When two conducting materials with different work functions
are brought together, for example via an external wire contact, electrons in the
material with the lower work function flow to the one with the higher work
function. If these materials are made into a parallel plate capacitor, equal and
opposite surface charges form. Measuring the contact potential is then
exquisitely simple: an external potential is applied to the capacitor until the
surface charges disappear, and at this point the external potential equals the
contact potential. Kelvin realized this experimentally measured using two large
flat polished disc's of Zinc and Copper and a gold-leaf electroscope to measure
the charge transfer upon electrical contact. In 1932 William Zisman of Harvard University introduced a new
method to measure the contact potential. He mounted a vibrating reference
surface, or tip, just above a sample electrode. The output voltage varies
periodically as the tip vibrates, and the peak-to-peak voltage depends upon the
difference between the contact potential and the external voltage. Changes in
contact potential can then be detected by determining the external potential
that yields a minimum or "null" output voltage. This technique lead to
development of systems that automatically track shifts in the contact potential
due to changes in the work function of the sample. A major improvement of this new method is that the surfaces
do not need to touch each other. It only requires very weak electric fields,
which are not likely to influence the electrical or chemical structure of the
material. Biasing of the sample rather than the tip will reduce noise, and the
use of a low impedance current sensitive amplifier rather than a high impedance
voltage sensitive amplifier minimizes the interference from parasitic
capacities. However this isn't the whole story: real-world commercial systems
inherently produce talkover from the driver (the device used to vibrate the
tip).
This can substantially influence work function measurements and
consequently an "off-null" technique has been developed by Baikie, see for
instance, Baikie et al, 'Noise and the Kelvin Method, Review of Scientific
Instruments, Vol. 62, page 1326, 1991. This system has the added advantage of
allowing accurate measurements of mean capacity which can be used to perform
scanning measurements at a constant height allowing the user to automatically
perform measurements under identical experimental parameters. Without this
feature the apparent work function difference will be different each time the
user starts an experiment. It is important to recognize that the Kelvin Probe is a
relative technique capable of approximately 1 mV relative resolution. The work
function of the tip must therefore be known in order to obtain the absolute work
function of the sample. This problem has been addressed by illuminating a low
work function reference sample with monochromatic ultraviolet light, and then
measuring the energy at which current starts to flow. This technique can detect
changes in absolute work function of 30-50 meV, see Baikie et al, 'Work Function
study of rhenium oxidation using an ultra-high vacuum scanning Kelvin probe,
Journal of Applied Physics, Vol. 88, page 4371, 2000.
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Single Electron Transistor Device results scan
Kelvin Probe 50micron tip measuring a Single Electron Transistor Device
Scanning Kelvin Probe
UHV Kelvin Probe
Solar Panels
Scanning Kelvin Probe
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