Etienne Menard

Pattern Transfer Printing by Kinetic Control of Adhesion to an Elastomeric Stamp

An increasing number of technologies require large-scale integration of disparate classes of separately fabricated objects into spatially organized, functional systems1,2,3,4,5,6,7,8,9. Here we introduce an approach for heterogeneous integration based on kinetically controlled switching between adhesion and release of solid objects to and from an elastomeric stamp. We describe the physics of soft adhesion that govern this process and demonstrate the method by printing objects with a wide range of sizes and shapes, made of single-crystal silicon and GaN, mica, highly ordered pyrolytic graphite, silica and pollen, onto a variety of substrates without specially designed surface chemistries or separate adhesive layers. Printed p–n junctions and photodiodes fixed directly on highly curved surfaces illustrate some unique device-level capabilities of this approach.

Intrinsic charge transport on the surface of organic semiconductors.

The air-gap field-effect technique enabled realization of the intrinsic (not limited by static disorder) polaronic transport on the surface of rubrene (C42H28) crystals over a wide temperature range. The signatures of this intrinsic transport are the anisotropy of the carrier mobility, mu, and the growth of mu with cooling. Anisotropy of mu vanishes in the activation regime at low temperatures, where the transport is dominated by shallow traps. The deep traps, introduced by x-ray radiation, increase the field-effect threshold without affecting mu, an indication that the filled traps do not scatter polarons.

GaAs photovoltaics and optoelectronics using releasable multilayer epitaxial assemblies

Compound semiconductors like gallium arsenide (GaAs) provide advantages over silicon for many applications, owing to their direct bandgaps and high electron mobilities. Examples range from efficient photovoltaic devices to radio-frequency electronics and most forms of optoelectronics. However, growing large, high quality wafers of these materials, and intimately integrating them on silicon or amorphous substrates (such as glass or plastic) is expensive, which restricts their use. Here we describe materials and fabrication concepts that address many of these challenges, through the use of films of GaAs or AlGaAs grown in thick, multilayer epitaxial assemblies, then separated from each other and distributed on foreign substrates by printing. This method yields large quantities of high quality semiconductor material capable of device integration in large area formats, in a manner that also allows the wafer to be reused for additional growths. We demonstrate some capabilities of this approach with three different applications: GaAs-based metal semiconductor field effect transistors and logic gates on plates of glass, near-infrared imaging devices on wafers of silicon, and photovoltaic modules on sheets of plastic. These results illustrate the implementation of compound semiconductors such as GaAs in applications whose cost structures, formats, area coverages or modes of use are incompatible with conventional growth or integration strategies.

A printable form of silicon for high performance thin film transistors on plastic substrates

Free-standing micro- and nanoscale objects of single crystal silicon can be fabricated from silicon-on-insulator wafers by lithographic patterning of resist, etching of the exposed top silicon, and removing the underlying SiO2 to lift-off the remaining silicon. A large collection of such objects constitutes a type of material that can be deposited and patterned, by dry transfer printing or solution casting, onto plastic substrates to yield mechanically flexible thin film transistors that have excellent electrical properties. Effective mobilities of devices built with this material, which we refer to as microstructured silicon (μs-Si), are demonstrated to be as high as 180cm2∕Vs on plastic substrates. This form of “top down” microtechnology might represent an attractive route to high performance flexible electronic systems.

Semiconductor Wires and Ribbons for High- Performance Flexible Electronics

This article reviews the properties, fabrication and assembly of inorganic semiconductor materials that can be used as active building blocks to form high-performance transistors and circuits for flexible and bendable large-area electronics. Obtaining high performance on low temperature polymeric substrates represents a technical challenge for macroelectronics. Therefore, the fabrication of high quality inorganic materials in the form of wires, ribbons, membranes, sheets, and bars formed by bottom-up and top-down approaches, and the assembly strategies used to deposit these thin films onto plastic substrates will be emphasized. Substantial progress has been made in creating inorganic semiconducting materials that are stretchable and bendable, and the description of the mechanics of these form factors will be presented, including circuits in three-dimensional layouts. Finally, future directions and promising areas of research will be described.

Hall Effect in the Accumulation Layers on the Surface of Organic Semiconductors

We have observed the Hall effect in the field-induced accumulation layer on the surface of single-crystal samples of a small-molecule organic semiconductor rubrene. The Hall mobility muH increases with decreasing temperature in both the intrinsic (high-temperature) and trap-dominated (low-temperature) conduction regimes. In the intrinsic regime, the density of mobile field-induced charge carriers extracted from the Hall measurements, nH, coincides with the density n calculated using the gate-channel capacitance and becomes smaller than n in the trap-dominated regime. The Hall data are consistent with the diffusive bandlike motion of field-induced charge carriers between trapping events.

Bendable single crystal silicon thin film transistors formed by printing on plastic substrates

Bendable, high performance single crystal silicon transistors have been formed on plastic substrates using an efficient dry transfer printing technique. In these devices, free standing single silicon objects, which we refer to as microstructured silicon (μs‐Si), are picked up, using a conformable rubber stamp, from the top surface of a wafer from which they are generated. The μs‐Si is then transferred, to a specific location and with a controlled orientation, onto a thin plastic sheet. The efficiency of this method is demonstrated by the fabrication of an array of thin film transistors that exhibit excellent electrical properties: average device effective mobilities, evaluated in the linear regime, of ∼240cm2∕Vs, and threshold voltages near 0V. Frontward and backward bending tests demonstrate the mechanical robustness and flexibility of the devices.

High-speed mechanically flexible single-crystal silicon thin-film transistors on plastic substrates

This letter describes the fabrication and properties of bendable single-crystal-silicon thin film transistors formed on plastic substrates. These devices use ultrathin single-crystal silicon ribbons for the semiconductor, with optimized device layouts and low-temperature gate dielectrics. The level of performance that can be achieved approaches that of traditional silicon transistors on rigid bulk wafers: effective mobilities>500cm/sup 2//V/spl middot/s, ON/OFF ratios >10/sup 5/, and response frequencies > 500 MHz at channel lengths of 2 /spl mu/m. This type of device might provide a promising route to flexible digital circuits for classes of applications whose performance requirements cannot be satisfied with organic semiconductors, amorphous silicon, or other related approaches.

Spin on Dopants for High-Performance Single-Crystal Silicon Transistors on Flexible Plastic Substrates

Free-standing micro/nanoelements of single-crystal silicon with integrated doped regions for contacts provide a type of material that can be printed onto low-temperature device substrates, such as plastic, for high-performance mechanically flexible thin-film transistors (TFTs). We present simple approaches for fabricating collections of these elements, which we refer to as microstructured silicon (μs-Si), and for using spin-on dopants to introduce doped regions in them. Electrical and mechanical measurements of TFTs formed on plastic substrates with this doped μs-Si indicate excellent performance. These and other characteristics make the material potentially useful for emerging large area, flexible ‘macroelectronic’ devices.Free-standing micro/nanoelements of single-crystal silicon with integrated doped regions for contacts provide a type of material that can be printed onto low-temperature device substrates, such as plastic, for high-performance mechanically flexible thin-film transistors (TFTs). We present simple approaches for fabricating collections of these elements, which we refer to as microstructured silicon (μs-Si), and for using spin-on dopants to introduce doped regions in them. Electrical and mechanical measurements of TFTs formed on plastic substrates with this doped μs-Si indicate excellent performance. These and other characteristics make the material potentially useful for emerging large area, flexible ‘macroelectronic’ devices.

Collapse of stamps for soft lithography due to interfacial adhesion

Collapse of elastomeric elements used for pattern transfer in soft lithography is studied through experimental measurements and theoretical modeling. The objective is to identify the driving force for such collapse. Two potential driving forces, the self-weight of the stamp and the interfacial adhesion, are investigated. An idealized configuration of periodic rectangular grooves and flat punches is considered. Experimental observations demonstrate that groove collapse occurs regardless of whether the gravitational force promotes or suppresses such collapse, indicating that self-weight is not the driving force. On the other hand, model predictions based on the postulation that interfacial adhesion is the driving force exhibit excellent agreement with the experimentally measured collapse behavior. The interfacial adhesion energy is also evaluated by matching an adhesion parameter in the model with the experimental data.