Peter Hadley

Single-crystal organic field-effect transistors based on dibenzo-tetrathiafulvalene

We report on the fabrication and characterization of field-effect transistors based on single crystals of the organic semiconductor dibenzo-tetrathiafulvalene (DB-TTF). We demonstrate that it is possible to prepare very-good-quality DB-TTF crystals from solution. These devices show high field-effect mobilities typically in the range 0.1–1?cm2/V?s. The temperature dependence was also studied revealing an initial increase of the mobility when lowering the temperature until it reached a maximum, after which the mobility decreased following a thermally activated behavior with activation energies between 50 and 60?meV. Calculations of the molecular reorganization energy and intermolecular transfer integrals for this material were also performed and are in agreement with the high mobility observed in this material.

Negative differential resistance due to single-electron switching

We present the multilevel fabrication and measurement of a Coulomb-blockade device displaying tunable negative differential resistance (NDR). Applications for devices displaying NDR include amplification, logic, and memory circuits. Our device consists of two Al/AlxOy islands that are strongly coupled by an overlap capacitor. Our measurements agree excellently with a model based on the orthodox theory of single-electron transport.

Single-electron inverter

A single-electron inverter was fabricated that switches from a high output to a low output when a fraction of an electron is added to the input. For the proper operation of the inverter, the two single-electron transistors that make up the inverter must exhibit voltage gain. Voltage gain was achieved by fabricating a combination of parallel-plate gate capacitors and small tunnel junctions in a two-layer circuit. Voltage gain of 2.6 was attained at 25 mK and remained larger than one for temperatures up to 140 mK. The temperature dependence of the gain agrees with the orthodox theory of single-electron tunneling.

Simulating Hybrid Circuits of Single-Electron Transistors and Field-Effect Transistors

An exact model for a single-electron transistor was developed within the circuit simulation package SPICE. This model uses the orthodox theory of single-electron tunneling and determines the average current through the transistor as a function of the bias voltage, the gate voltage, and the temperature. Circuits including single-electron transistors, field-effect transistors (FETs), and operational amplifiers were then simulated. In these circuits, the single-electron transistors provide the charge sensitivity while the FETs tune the background charges, provide gain, and provide low output impedance.

Logic circuits based on carbon nanotubes

Abstract We demonstrate logic circuits with field-effect transistors based on single carbon nanotubes. A new technique is used for achieving local gates in nanotube field-effect transistors that provide excellent capacitive coupling between the gate and nanotube, enabling the transistor to be ambipolar. The transistors show favorable device characteristics such as a high gain, a large on-off ratio, and room-temperature operation. Importantly, it also allows for the integration of multiple devices on a single chip. Indeed, we demonstrate 1-, 2-, and 3-transistor circuits that exhibit a wide range of digital logic operations such as an inverter, a logic NOR, and an AC ring oscillator.

Dynamical states and stability of linear arrays of Josephson junctions

We consider a one‐dimensional array of dc‐biased Josephson junctions shunted by a load of passive circuit elements. The load serves to couple the ac Josephson effect oscillations in the various junctions, giving rise to dynamical states of the system that do not appear for a single junction. Our results demonstrate two distinct phase‐locked states of the array, hysteresis, and chaotic behavior depending on the load and the value of the bias current. Implications of these results for local oscillator applications of such arrays are also discussed.

Field effect transistors based on poly(3-hexylthiophene) at different length scales

In this paper we report on thin film transistors based on drop casting solutions of regioregular poly(3-hexylthiophene) (P3HT) over prefabricated gold electrodes. This polymer is known to self-organize into a lamellar structure in chloroform resulting in high field-effect mobilities. We studied the dependency of the charge carrier mobility of devices prepared from solution in chloroform with electrode spacings ranging from 5 µm to 20 nm. It was found that the overall trend was that the mobility decreased as the electrode spacing was made smaller, indicating that the transport properties on closely spaced electrodes were dominated by the contacts. Applying an ac voltage during the preparation of such films resulted in lower mobilities. However, P3HT in p-xylene forms fibres, which were aligned between the electrodes by applying an ac field. Films of aligned fibres with mobilities as high as 0.04 cm2 V−1 s−1 were prepared.

Fabrication of multilayer single‐electron tunneling devices

A reliable process has been developed for the fabrication of multilevel single‐electron tunneling (SET) devices. Using this process, we have fabricated SET devices with Au‐SiO‐Al and Al‐AlOx‐SiO‐Al overlap capacitors. The SET transistors exhibit voltage gain and, despite the complex device structure, have a low charge noise (7×10−5e/√Hz). Moreover, the use of overlap capacitors in SET devices results in a reduction of cross capacitances down to 8%.

Phase locking of Josephson junction arrays

We report the results of a stability analysis of coherent oscillations in series arrays of Josephson junctions with a matched resistive load. We find that arbitrarily large, dc biased arrays of Josephson junctions will phase lock most strongly when the capacitance parameter βc ≊1, and the bias current is about twice the critical current of the individual junctions.

Electron Beam-Induced Current (EBIC) in solution-processed solar cells

Electron Beam-Induced Current (EBIC) measurements were used to produce 2D maps for investigating the homogeneity of solar cells. These maps are acquired by scanning the electron beam of a scanning electron microscope over a small area and using a programmable sample stage to move the solar cell under the scan area. The electron beam generates electron-hole pairs in the solar cell much like light does in normal solar cell operation. Solution-processed solar cells where the active layer consisted of purely inorganic or purely organic materials were measured. Since the electron beam irreversibly damages organic material, it was important to ensure that the measurements were made before the materials were altered.