Tom C. T. Geuns

High-performance all-polymer integrated circuits

In this letter, we demonstrate the integration of all-polymer field-effect transistors in fully functional integrated circuits with operating frequencies of several kHz. One of the key items is an approach to incorporate low-Ohmic vertical interconnects compatible with an all-polymer approach. Inverters, NAND gates, and ring oscillators with transistor channel lengths down to 1 μm have been constructed. Inverters show voltage amplification at moderate biases and pentacene seven-stage ring oscillators show switching frequencies of a few kHz. The potential to realize large integrated circuits is demonstrated by a 15 bit code generator circuit using several hundreds of devices. The proposed concept was evaluated for three solution-processable organic semiconductors.

Organic field-effect transistors and all-polymer integrated circuits

Electrical properties of field-effect transistors made of different solution processable organic semiconductors are described. The temperature and gate-voltage dependence of the mobility is shown and theoretically described using a model based on the variable-range hopping of charge carriers in an exponential density of states. Furthermore, a technology has been developed to make all-polymer integrated circuits. It involves reproducible fabrication of field-effect transistors on flexible substrates, where the semiconducting, conducting and insulating parts are all made of polymers. Integrated circuits consisting of more than 300 field-effect transistors are demonstrated.

Upscaling, integration and electrical characterization of molecular junctions

The ultimate target of molecular electronics is to combine different types of functional molecules into integrated circuits, preferably through an autonomous self-assembly process. Charge transport through self-assembled monolayers has been investigated previously, but problems remain with reliability, stability and yield, preventing further progress in the integration of discrete molecular junctions. Here we present a technology to simultaneously fabricate over 20,000 molecular junctions-each consisting of a gold bottom electrode, a self-assembled alkanethiol monolayer, a conducting polymer layer and a gold top electrode-on a single 150-mm wafer. Their integration is demonstrated in strings where up to 200 junctions are connected in series with a yield of unity. The statistical analysis on these molecular junctions, for which the processing parameters were varied and the influence on the junction resistance was measured, allows for the tentative interpretation that the perpendicular electrical transport through these monolayer junctions is factorized.

Electrical characteristics of conjugated self-assembled monolayers in large-area molecular junctions

We have studied the electrical characteristics of close-packed monolayers of conjugated para-phenylene oligomers as a function of molecular length in large-area molecular junctions. An exponential increase in resistance with molecular length is observed, Rexp (βL), with β=0.26±0.04 A-1 and β=0.20±0.06 A-1 for dithiol and monothiol derivatives, respectively. The decay coefficients are lower than previously determined experimentally using scanning probe or breakjunction techniques. We tentatively explain the low values by the forced planer geometry of the self-assembled molecules.

Doped polyaniline polymer fuses: Electrically programmable read-only-memory elements

We demonstrate polymeric electrically programmable read-only-memory elements based on camphorsulfonic-acid–doped polyaniline lines. Their working mechanism relies on irreversible reduction of the electrical conductivity by Joule heating like electrical safety fuses. The heating power is supplied electrically. The critical power required to “blow up” the fuse is strongly reduced by notches. The influenc eo f the notch design can be predicted reasonably well using a simple thermal model. The critical power becomes less than 1 mW for fuses with notches narrower than 2 m. This power can be delivered by organic transistors already at modest voltages, opening the way of integration of these memory elements in all-polymer circuits.

Universal Scaling of the Charge Transport in Large-Area Molecular Junctions

Charge transport through alkanes and para-phenylene oligomers is investigated in large-area molecular junctions. The molecules are self-assembled in a monolayer and contacted with a top electrode consisting of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonic acid) (PEDOT:PSS). The complete set of J(V,T) characteristics of both saturated and π-conjugated molecules can be described quantitatively by a single equation with only two fit parameters. The derived parameters, in combination with a variation of the bulk conductivity of PEDOT:PSS, demonstrate that the absolute junction resistance is factorized with that of PEDOT:PSS.

Up-Scaling Graphene Electronics by Reproducible Metal-Graphene Contacts

Chemical vapor deposition (CVD) of graphene on top of metallic foils is a technologically viable method of graphene production. Fabrication of microelectronic devices with CVD grown graphene is commonly done by using photolithography and deposition of metal contacts on top of the transferred graphene layer. This processing is potentially invasive for graphene, yields large spread in device parameters, and can inhibit up-scaling. Here we demonstrate an alternative process technology in which both lithography and contact deposition on top of graphene are prevented. First a prepatterned substrate is fabricated that contains all the device layouts, electrodes and interconnects. Then CVD graphene is transferred on top. Processing parameters are adjusted to yield a graphene layer that adopts the topography of the prepatterned substrate. The metal-graphene contact shows low contact resistances below 1 kΩ μm for CVD graphene devices. The conformal transfer technique is scaled-up to 150 mm wafers with statistically similar devices and with a device yield close to unity.

Circuit yield of organic integrated electronics

Research on organic electronics is focussed on materials and on the performance of discrete devices. Reliability and circuit yield is largely unexplored. Yield, based on measurements on digital organic circuits up to 1000 transistors, is described. The causes of yield loss are analyzed and design solutions to improve the yield are discussed.

Polymeric integrated circuits: fabrication and first characterisation

A technology to fabricate polymeric integrated circuits on 150-mm foils is presented. The technology is demonstrated with functional code generators. The integration level is about 700 transistors. The yield of the circuits has been measured as function of the complexity and has been correlated with intrinsic noise margin of the logic gates.

Extending the voltage window in the characterization of electrical transport of large-area molecular junctions

A large bias window is required to discriminate between different transport models in large-area molecular junctions. Under continuous DC bias, the junctions irreversibly break down at fields over 9 MV/cm. We show that, by using pulse measurements, we can reach electrical fields of 35 MV/cm before degradation. The breakdown voltage is shown to depend logarithmically on both duty cycle and pulse width. A tentative interpretation is presented based on electrolysis in the polymeric top electrode. Expanding the bias window using pulse measurements unambiguously shows that the electrical transport exhibits not an exponential but a power-law dependence on bias.