Purpose of sheet resistance in thin film devices

[chpolyz]

In optoelectronics and other nanotechnology applications (like organic photovoltaics, organic LEDs etc) most layers need to be electrically characterized among other types of measurements. Thus, sheet resistance is measured.

An example are the 4-point-probe resistivity measurements. Yet, what happens is to impose current that travels almost horizontally to the surface of the film and measure a voltage between 2 other points of the same surface.

Consider that a movement of charge carriers is realized vertically to the film surface due to voltage, in order to reach the top or bottom electrodes of the device (an organic photovoltaic in this case, which consists of many consecutive sandwiched layers)..

My question is, why do we need to measure the resistance of charge carriers that travel alongside the horizontal surface of a film? In such a way we find the "horizontal" resistance of the film and not the vertical one...

thank you

 

[Baluncore]

As part of the manufacturing process it is necessary to know the quality of each step in the process.
By measuring the horizontal characteristics of individual layers, data relevant to each step in the process can be gathered without the confounding effect of the interfaces between layers.

 

[chpolyz]

Suppose we realize a single thin film, then perfectly sandwich it between two sheet electrodes and measure the resistance. Would we expect the same value as with the sheet resistance ("horizontal" one)?

 

[Baluncore]

In the simple case, yes.

 

[chpolyz]

By "simple" you mean homogeneous film, I guess...However, what if the film were not? Can you propose any supporting literature?

Thanks a lot for your replies