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Scanning Kelvin Probe to investigate electrical surface states of the sample

Dedicated to measure electron work function of the semiconductor sample surface.

Description

Scanning Kelvin Probe allows measurements of work function of different semiconducting and conducting materials. It can also determine charge distribution on a sample surface.

Measurements of work function is particularly useful in semiconductor physics as it provides the position of Fermi level of examined material.

Kelvin Probe equipped with light source makes it possible to examin  surface states of electrons of the sample. Surface states of electrons play significant role in electric charge transfer, which is especially important for photovoltaic materials and in photoelectrochemistry.

Possibility of scanning the sample surface and to examine its electrical properties enables precise assessment of the quality of the material, its homogeneity and enables gathering the series of data that may be used to evaluate work function more precisely.

System components

The instrument consists of probe on the stand, electronic control system and Faraday cage.

  • The sample is located on XY table on one of two provided stands. Motorized table enables to move the sample in range of 5 cm in both, X and Y, directions.
  • Each time the probe approaches the sample being examined, an accurate distance between the sample surface and probe is measured with 50 µm precision. In our method, the vertical axis precision is not important for the determination of work function.
  • The probe is equipped with a laser pointer that illuminates a spot the probe will hoover above during measuremnt.
  • The light source helps see the sample under the Faraday cage.
  • Above the probe the optical fiber adapter is mounted. The optical fiber allows to illuminate the sample with light from a monochromator or from a LED revolver.
  • During sample handling, the probe is hidden under metal cover which prevents the probe from being accidentally damaged.
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The probe tip

The probe tip is the component fully designed and manufactured by Instytut Fotonowy. It provides:

  • large signal even from a distance of 0.5 mm above the sample, thus it matters not if the sample surface is rough or polished.
  • transparency that lets light through. This way the sample can be illuminated by a perpendicular light beam.

The tip oscillations are generated with an electromagnet.

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Sample holders

The Kelvin Probe set includes two types of sample stands:

  • Sample with bottom contact

 

  • Freely shaped solid state sample with top contact

Faraday Cage

The whole measurement instrument is covered with Faraday cage which shields the system from the surroundings. The grounding socket is situated on the cage.

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Specifications

 

The following specification may be changed in terms of further process of appliance development.

 

Voltage range-5 V .. 5 V
Voltage measurement nominal resolution0.15 mV
Measurement technology2-channel lock-in amplifier
Sample holdermotorized translation stage X-Y,
Sample standstand for sample layer/ stand for solid state sample
Range of stage movement5 cm x 5 cm
Min sample move step1 µm
Tip typeAu, mesh, dia. 2.5 mm
Tip Z positioningmotorized Z stage
Positioning with respect to a samplelight barrier
Tip Z positioning resolution10 µm
Spectral range for wave guides200 nm .. 2000 nm (depending on a waveguide)
Liquid waveguides selection (Lumatec)Wybór światłowodów Lumatec
Auxiliary sensorshumidity and temperature
Internal Faraday cage illuminationwhite LED
X-Y positioning aidlaser sample placement indicator
PC connectivityUSB 2.0
Size40 x 40 x 45 cm
Weight10 kg

Exemplary results

Aluminium surface

Distribution of the electric potential on the sample surface:

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Ambient temperature during measurement:

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Air humidity during measurement:

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Metal surface sample with periodic holes

Evaluated CPD values on surface:

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Preview during ongoing measurement

Distribution of the electric potential on the sample surface:

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Ambient temperature during ongoing measurement:

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Air humidity during ongoing measurement:

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