Harry J. Wondergem

Bottom-up organic integrated circuits

Self-assembly—the autonomous organization of components into patterns and structures—is a promising technology for the mass production of organic electronics. Making integrated circuits using a bottom-up approach involving self-assembling molecules was proposed in the 1970s. The basic building block of such an integrated circuit is the self-assembled-monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. In the SAMFETs fabricated so far, current modulation has only been observed in submicrometre channels, the lack of efficient charge transport in longer channels being due to defects and the limited intermolecular π–π coupling between the molecules in the self-assembled monolayers. Low field-effect carrier mobility, low yield and poor reproducibility have prohibited the realization of bottom-up integrated circuits. Here we demonstrate SAMFETs with long-range intermolecular π–π coupling in the monolayer. We achieve dense packing by using liquid-crystalline molecules consisting of a π-conjugated mesogenic core separated by a long aliphatic chain from a monofunctionalized anchor group. The resulting SAMFETs exhibit a bulk-like carrier mobility, large current modulation and high reproducibility. As a first step towards functional circuits, we combine the SAMFETs into logic gates as inverters; the small parameter spread then allows us to combine the inverters into ring oscillators. We demonstrate real logic functionality by constructing a 15-bit code generator in which hundreds of SAMFETs are addressed simultaneously. Bridging the gap between discrete monolayer transistors and functional self-assembled integrated circuits puts bottom-up electronics in a new perspective.

Controlled Deposition of Highly Ordered Soluble Acene Thin Films: Effect of Morphology and Crystal Orientation on Transistor Performance

Controlling the morphology of soluble small molecule organic semiconductors is crucial for the application of such materials in electronic devices. Using a simple dip-coating process we systematically vary the film drying speed to produce a range of morphologies, including oriented needle-like crystals. Structural characterization as well as electrical transistor measurements show that intermediate drying velocities produce the most uniformly aligned films.

Monolayer coverage and channel length set the mobility in self-assembled monolayer field-effect transistors

The mobility of self-assembled monolayer field-effect transistors (SAMFETs) traditionally decreases dramatically with increasing channel length. Recently, however, SAMFETs using liquid-crystalline molecules have been shown to have bulk-like mobilities that are virtually independent of channel length. Here, we reconcile these scaling relations by showing that the mobility in liquid crystalline SAMFETs depends exponentially on the channel length only when the monolayer is incomplete. We explain this dependence both numerically and analytically, and show that charge transport is not affected by carrier injection, grain boundaries or conducting island size. At partial coverage, that is when the monolayer is incomplete, liquid-crystalline SAMFETs thus form a unique model system to study size-dependent conductance originating from charge percolation in two dimensions.

Position-controlled epitaxial III-V nanowires on silicon

We show the epitaxial integration of III–V semiconductor nanowires with silicon technology. The wires are grown by the VLS mechanism with laser ablation as well as metal–organic vapour phase epitaxy. The hetero-epitaxial growth of the III– Vn anowires on silicon was confirmed with x-ray diffraction pole figures and cross-sectional transmission electron microscopy. We show preliminary results of two-terminal electrical measurements of III–V nanowires grown on silicon. E-beam lithography was used to predefine the position of the nanowires. (Some figures in this article are in colour only in the electronic version)

Thermal expansion of cubic Si3N4 with the spinel structure

The thermal expansion coefficient of cubic Si3N4 with the spinel structure was determined with high temperature X-ray diffraction. The experimental value agrees well with the lattice parameter predicted by first principles methods, and is significantly larger than the value for β-Si3N4. This difference is discussed in terms of the chemical bonding in these two modifications of Si3N4.

Wide-range three-dimensional reciprocal-space mapping: a novel approach applied to organic monodomain thin films

An X-ray method is presented to characterize thin films with unknown crystal structure with specific crystal orientations. The method maps large volumes of the reciprocal space by a series of pole-figure measurements using a standard texture goniometer. The data can be used for lattice indexing and texture evaluation and in subsequent steps for a complete structural thin-film characterization. The application of the method is demonstrated on an organic monodomain thin film consisting of uniaxially aligned crystallites.

Cross-sectional studies of epitaxial growth of InP and GaP nanowires on Si and Ge

Heteroepitaxial growth of III-V wires on Si and Ge was performed using either MOVPE or laser ablation. The epitaxial relation between wire and substrate was studied using cross-sectional TEM and X-ray diffraction. For both GaP wires on Si and InP wires on Ge perfect epitaxy was observed. In the initial stage of the growth process, the gold particles partly sink into the Si substrate. During subsequent wire growth, the Si that had been dissolved in the gold particle is excreted above the Si surface. Due to the interaction of the gold particle with the substrate, the final substrate/wire interface displays a roughness on the nanometer scale.