The Effect of Electrostatic Screening on a Nanometer Scale Electrometer

Abstract

We investigate the effect of electrostatic screening on a nanoscale silicon MOSFET electrometer. We find that screening by the lightly doped p-type substrate, on which the MOSFET is fabricated, significantly affects the sensitivity of the device. We are able to tune the rate and magnitude of the screening effect by varying the temperature and the voltages applied to the device, respectively. We show that despite this screening effect, the electrometer is still very sensitive to its electrostatic environment, even at room temperature.

Contact-Independent Measurement of Electrical Conductance of a Thin Film with a Nanoscale Sensor

“…We fabricate an n-channel MOSFET according to standard CMOS methods, as previously described. 9,10 We begin with a p-type silicon substrate that is doped with boron at a level of 4 × 10 15 cm −3 . We implant phosphorous ions in two regions of the silicon substrate with dimensions 100 × 150 µm 2 , and separated by 100 µm, to serve as the source and drain electrodes.…”

“…The charge transient is analyzed as follows: − We treat the amorphous germanium as a distributed RC network, as shown in Figure a. When the amorphous germanium is subject to a step voltage at one edge, charge diffuses through the film with a diffusion constant of D = 1/ R sq C , where R sq is the resistance per square, and C is the capacitance per unit area of the film.…”

“…For example, we find G aGe = 1.3 × 10 –17 S at 44 K. As we increase the temperature to 54 K, the rate Γ increases, and the conductance increases to G aGe = 9.4 × 10 –17 S. We note that the measurement of conductance of the film comes from the time decay of the transient in the MOSFET, which is independent of the characteristics of the MOSFET as long as the transient signal is small enough that the conductance of the MOSFET versus gate voltage is linear and large enough to be measured above the noise of the MOSFET. For this reason, our technique is insensitive to discrete changes in the conductance of the MOSFET when we change the temperature or when we apply a voltage to the gold electrodes, which act as gates of the MOSFET . More detail on the derivation of the conductance from the charge transient can be found in the Supporting Information.…”