Piezoelectronic Transistor - Microindentor Results

We can measure piezoresistance by applying pressure to a film, and monitoring the change in electrical resistivity. To do this, we use a microindentor that allows us to exert a controlled force on the sample.

The sample is constructed for vertical current flow through a small aperture – that way we can make a quantitative analysis of the sample resistivity. The piezoresistor is deposited on a tungsten coated substrate that is masked by silicon nitride with small opening. SmSe is deposited, followed by a titanium nitride layer to protect from oxidation. Next, the sample is capped with a thick aluminum layer. The aluminum distributes the load from the indenter ball, so the experiment is not dominated by any sharp projections or asperities of the microindentor. Finally, the aluminum, SmSe and titanium nitride are lithogrophically patterned into large pads to isolate the top contact.

The indentation experiment is done by carefully positioning the tip of a tungsten carbide ball over the silicon nitride aperture and applying force. The aluminum is deformed by the ball, creating a uniform force field that extends down through the piezoresistive layer. During compression, current is driven vertically through the sample, and the resistance voltage is monitored. With increasing pressure, the resistance drops.

Extensive modeling is needed to convert the applied force into a pressure. A strength of this experiment is that the configuration allows highly quantitative measurement of piezoresistance. Because the apparatus uses extensive mechanical equipment to deliver calibrated forces, it is only suitable for slow, quasistatic measurements. Analysis of high-frequency characteristics will require integrated devices. Nonetheless, indenter results have enabled us to verify and perfect piezoresistive thin films down to nanoscale thicknesses.

More details of the experiment can be found in this link: Giant Piezoresistive On/Off Ratios in Rare-Earth Chalcogenide Thin Films Enabling Nanomechanical Switching,