Constantin Augustin Dutu

Towards all-organic field-effect transistors by additive soft lithography.

Unconventional nanofabrication is attractive for organic electronics because of its potential impact in manufacturing low-cost electronics starting from soluble precursors that can be processed and patterned via a sustainable technology. So far, the major endeavor aimed at the development of organicbased devices has been through the design of new materials, novel synthetic procedures and purification methods, optimized conditions for thin film growth, and original methods for nanofabrication. In particular, a strong effort was devoted to the technological control of organic semiconductors in transistors. Yet, only a limited number of studies have focused on new approaches for low cost fabrication of electrodes and their integration with the organic materials. Successful examples of unconventional electrode manufacturing include stencil printing of Au nanoparticles, inkjet printing, Ag electroless plating followed by microcontact patterning, lamination, microtransfer printing of Ag nanoparticles, metal transfer printing, and soft lithography. Although inkjet printing is probably the most straightforward example of an additive process where both the electrodes and the active layers can be realized on the same platform, the fabrication of the electrodes and the active layers often relies on different processes, specifications, and platforms. Precisely, standard microfabrication approaches consisting of photolithography and/or electron-beam lithography followed by vacuum metallization are generally used for the source and drain definition, while wet methods (spin-coating, layer-by-

Highly Ordered Conjugated Polymer Nanoarchitectures with Three-Dimensional Structural Control

Conductive polymers are a class of materials with vast potential for tomorrows ultra-large-scale technologies as they combine structural and functional diversity with flexible synthesis and processing approaches. A missing component, with their subtle chemical structure, is reliable building at nanoscale. Here we report on the patterning of polyaniline, a prototypical conjugated polymer, with an unprecedented areal patterning order and density exceeding 0.25 teradot/inch(2). With template-confined growth, through platinum-surface-catalyzed polymerization of aniline, highly ordered arrays of distinct polyaniline nanowires are produced with a typical diameter <or=15 nm and aspect ratio higher than 20. Up-scaling is straightforward. Complex three-dimensional structural control is achieved through a direct pattern transfer via resist- and dose-modulated electron beam lithography. The morphology-modulated nanowires self-assemble in key-lock type architectures induced by the structure asymmetry and nonuniformity of the capillary forces associated with the re-entrant features.

Very low effective Schottky barrier height for erbium disilicide contacts on n-Si through arsenic segregation

The segregation of As+ ions implanted into thin Er films deposited on n-Si substrates is studied after ErSi2−x formation. The same lowering of the effective Schottky barrier height (SBH) below 0.12 eV is obtained at moderate annealing temperatures, regardless of the redistribution of As dopants at the ErSi2–x/Si interface. On the other hand, if the implanted dose is slightly enhanced, the annealing temperature required to reach sub-0.12-eV effective SBH can be further reduced. This process enables the formation of very low effective SBH ErSi2–x/n-Si contacts with a low thermal budget.

Nanowire-templated microelectrodes for high-sensitivity pH detection

A highly sensitive pH capacitive sensor has been designed by confined growth of vertically aligned nanowire arrays on interdigited microelectrodes. The active surface of the device has been functionalized with an electrochemical pH transducer (polyaniline). We easily tune the device features by combining lithographic techniques with electrochemical synthesis. The reported electrical LC resonance measurements show considerable sensitivity enhancement compared to conventional capacitive pH sensors realized with microfabricated interdigited electrodes. The sensitivity can be easily improved by changing only the thickness of the functional layer.

Vertical single nanowire devices based on conducting polymers

A simple scheme for single conducting polymer nanowire fabrication and device integration is presented. We discuss a combined top-down and bottom-up approach for the sequential, precise manufacture of vertical polyaniline nanowires. The method is scalable and can be applied on rigid as well as on flexible substrates. The kinetics of the template-confined growth is presented and discussed. We further study the electrical behavior of single vertical polyaniline nanowires and address the fabrication of crossbar latches using a criss-cross arrangement of electrodes. The as-synthesized polyaniline nanowires display electric conductivities reaching values as high as 0.4 S cm⁻¹.

Erbium silicide growth in the presence of residual oxygen

The chemical changes of Ti/Er/n-Si(100) stacks evaporated in high vacuum and grown ex situ by rapid thermal annealing were scrutinized. The emphasis was laid on the evolution with the annealing temperature of (i) the Er-Si solid-state reaction and (ii) the penetration of oxygen into Ti and its subsequent interaction with Er. For that sake, three categories of specimens were analyzed: as-deposited, annealed at 300°C, and annealed at 600°C. It was found that the presence of residual oxygen into the annealing atmosphere resulted in a substantial oxidation of the Er film surface, irrespective of the annealing temperature. However, the part of the Er film in intimate contact with the Si bulk formed a silicide (amorphous at 300°C and crystalline at 600°C) invariably free of oxygen, as testified by x-ray photoelectron spectroscopy depth profiling and Schottky barrier height extraction of 0.3 eV at 600°C. This proves that, even if Er is highly sensitive to oxygen contamination, the formation of low Schottky barrier Er silicide contacts on n-Si is quite robust. Finally, the production of stripped oxygen-free Er silicide was demonstrated after process optimization.

Tuning the surface conditioning of trapezoidally shaped silicon nanowires by (3-aminopropyl)triethoxysilane.

We report on the electrical performance of silane-treated silicon nanowires configured as n + – p – n + field effect transistors. The functionalization of the silicon oxide shell with (3-aminopropyl)triethoxysilane controls the formation of the conduction channel in the trapezoidal cross-section nanowires. By carefully adjusting the surface conditioning protocol, robust electrical characteristics were achieved in terms of device-to-device reproducibility for the studied silicon nanowire transistors: the standard deviation displays a fourfold decrease for the threshold voltage together with a sevenfold improvement for the subthreshold slope.

Sub-10-nm nanogap fabrication by silicidation

We developed a simple and reliable method for the fabrication of sub-10-nm wide nanogaps. The self-formed nanogap is based on the stoichiometric solid-state reaction between metal and Si atoms during silicidation. The nanogap width is determined by the metal layer thickness. Our proposed method provides nanogaps with either symmetric or asymmetric electrodes, as well as multiple nanogaps within one unique process step. Therefore, this method allows for high throughput and large-scale production.

Hybrid synthesis and processing schemes for highly-ordered polyaniline nanoarchitectures

We report on a newly developed technology for the nanoscale processing of the conducting polyaniline (PANI) with an unprecedented areal patterning order and density control exceeding 0.25 teradot/inch2. A simple two-step process is put forward to hierarchically build a large variety of functional PANI nanostructures on virtually any type of flexible or rigid substrates. Using template confinement, through Pt catalyzed electroless growth, highly-ordered arrays of distinct PANI nanowires are produced. Complex three-dimensional (3D) structural control is achieved through a direct pattern transfer using a novel type of resist- and dose-modulated 3D electron beam lithography. The method is scalable and provides a generic approach for nanopatterning surfaces with functional polymers. Aspects of the nanoscale PANI growth mechanism are discussed and the highly controllable, sub-picogram scale fabrication is emphasized. Simple schemes for single PANI nanowire fabrication, processing and device integration are presented.