Neil M. Zimmerman

Accuracy of electron counting using a 7‐junction electron pump

We have operated a 7‐junction electron pump as an electron counter with an error per pumped electron of 15 parts in 109 and an average hold time of 600 s. The accuracy and hold time are sufficient to enable a new fundamental standard of capacitance. We compare the measured accuracy of the pump as a function of pumping speed and temperature with theoretical predictions based on a model which includes stray capacitance.

Single electron tunneling transistor with tunable barriers using silicon nanowire metal-oxide-semiconductor field-effect transistor

We have achieved the operation of single-electron tunneling (SET) transistors with gate-induced electrostatic barriers using silicon nanowire metal-oxide-semiconductor field-effect transistor (MOSFET) structures. The conductance of tunnel barriers is tunable by more than three orders of magnitude. By using the flexible control of the tunable barriers, the systematic evolution from a single charge island to double islands was clearly observed. We obtained excellent reproducibility in the gate capacitances: values on the order of 10 aF, with the variation smaller than 1 aF. This flexibility and controllability both demonstrate that the device is highly designable to build a variety of SET devices based on complementary metal-oxide-semiconductor technology.

Submicron gap capacitor for measurement of breakdown voltage in air

We have developed a new method for measuring the value of breakdown voltage in air for electrode separations from 400nmto45μm. The electrodes used were thin film Au lines evaporated on sapphire. The resulting capacitors had an area of 80×80μm2. We demonstrate the ability to deduce the value of the separation of the plates by the value of the capacitance. The data acquired with this method do not agree with Paschen’s law for electrode separations below 10μm, as expected from previous experiments. Amongst the improvements of our method are the measurement of plate separation and the very small surface roughness (average of 6nm).

Correlation between microstructure, electronic properties and flicker noise in organic thin film transistors

We report on observations of a correlation between the microstructure of organic thin films and their electronic properties when incorporated in field-effect transistors. We present a simple method to induce enhanced grain growth in solution-processed thin film transistors by chemical modification of the source-drain contacts. This leads to improved device performance and gives a unique thin film microstructure for fundamental studies concerning the effect of structural order on the charge transport. We demonstrate that the 1∕f flicker noise is sensitive to organic semiconductor thin film microstructure changes in the transistor channel.

Excellent charge offset stability in a Si-based single-electron tunneling transistor

We have measured the long-term drift and the short-term 1/f noise in the charge offset Q0(t) in two Si-based single-electron tunneling transistors (SETTs). In contrast to metal-based SETTs, these devices show excellent charge stability, drifting by less than 0.01e over weeks. The short-term 1/f noise magnitude is similar to the metal-based devices, demonstrating that different mechanisms are responsible for the short-term noise versus the long-term drift. Finally, we show that, in addition to the excellent stability over time, it may be possible to make the devices more robust with respect to voltage-induced instability as well.

Current quantization due to single-electron transfer in Si-wire charge-coupled devices

We observe a quantized current due to single-electron transfer in a small charge-coupled device, which consists of a narrow Si-wire channel with fine gates; the gate is used to form a tunable barrier potential. By modulating two barrier potentials under the fine gates with phase-shifted pulse voltages, quantized numbers of electrons are injected into and extracted from the charge island sandwiched by the two barriers. Current plateaus due to single-electron transfer are clearly observed at 20 K with frequencies up to 100 MHz and a current level of 16 pA.

Why the long-term charge offset drift in Si single-electron tunneling transistors is much smaller (better) than in metal-based ones: Two-level fluctuator stability

A common observation in metal-based (specifically, those with AlOx tunnel junctions) single-electron tunneling (SET) devices is a time-dependent instability known as the long-term charge offset drift. This drift is not seen in Si-based devices. Our aim is to understand the difference between these, and ultimately to overcome the drift in the metal-based devices. A comprehensive set of measurements shows that (1) brief measurements over short periods of time can mask the underlying drift, (2) we have not found any reproducible technique to eliminate the drift, and (3) two-level fluctuators (TLFs) in the metal-based devices are not stable. In contrast, in the Si-based devices the charge offset drifts by less than 0.01e over many days, and the TLFs are stable. We also show charge noise measurements in a SET device over four decades of temperature. We present a model for the charge offset drift based on the observation of nonequilibrium heat evolution in glassy materials, and obtain a numerical estimate in go...

Electrical metrology with single electrons

This paper is mostly a review of the progress made at NIST in pursuing a capacitance standard based on the charge of the electron. We briefly introduce the Coulomb blockade, which is the basic physical phenomenon allowing control of single electrons, describe two types of single-electron tunnelling (SET) device and describe the metrology goals and payoffs achievable from SET devices. We then discuss the electron-counting capacitance standard (ECCS): the motivation, previous experimental work on various critical elements, present status and future prospects. This last part includes using the ECCS for a practical representation of capacitance, as well as pointing out that we can close the quantum metrology triangle without needing a large-value current standard. Finally, we briefly review other SET-based metrological applications.

A seven-junction electron pump: design, fabrication, and operation

We have developed a seven-junction electron pump for use in a new standard of capacitance based on measuring the voltage produced when a known charge is placed on a capacitor. This new pump, with an error per pumped electron of 15/spl times/10/sup -9/, is about 30 times more accurate than a five-junction pump made previously at NIST. By careful design of the pump geometry, we have reduced the effect of cross capacitance and simplified device operation. Our fabrication recipe produces small, stable tunnel junctions relatively quickly and reliably. We have developed a method of tuning the pump for highly accurate electron counting. This tuning can be quickly repeated whenever fluctuations in background charges degrade accuracy.

Uncertainty budget for the NIST electron counting capacitance standard, ECCS-1

We measure a cryogenic, vacuum-gap capacitor by two methods: (1) charging it with a known number of electrons and measuring the resulting voltage, and (2) using a capacitance bridge traceable to the SI farad. We report a detailed uncertainty budget for the comparison of the two methods and find that they agree within a relative standard uncertainty of 9.2 × 10−7. This comparison closes the quantum metrology triangle with the same uncertainty.