K. W. Baldwin

Paper-like electronic displays: Large-area rubber-stamped plastic sheets of electronics and microencapsulated electrophoretic inks

Electronic systems that use rugged lightweight plastics potentially offer attractive characteristics (low-cost processing, mechanical flexibility, large area coverage, etc.) that are not easily achieved with established silicon technologies. This paper summarizes work that demonstrates many of these characteristics in a realistic system: organic active matrix backplane circuits (256 transistors) for large (≈5 × 5-inch) mechanically flexible sheets of electronic paper, an emerging type of display. The success of this effort relies on new or improved processing techniques and materials for plastic electronics, including methods for (i) rubber stamping (microcontact printing) high-resolution (≈1 μm) circuits with low levels of defects and good registration over large areas, (ii) achieving low leakage with thin dielectrics deposited onto surfaces with relief, (iii) constructing high-performance organic transistors with bottom contact geometries, (iv) encapsulating these transistors, (v) depositing, in a repeatable way, organic semiconductors with uniform electrical characteristics over large areas, and (vi) low-temperature (≈100°C) annealing to increase the on/off ratios of the transistors and to improve the uniformity of their characteristics. The sophistication and flexibility of the patterning procedures, high level of integration on plastic substrates, large area coverage, and good performance of the transistors are all important features of this work. We successfully integrate these circuits with microencapsulated electrophoretic “inks” to form sheets of electronic paper.

Additive, nanoscale patterning of metal films with a stamp and a surface chemistry mediated transfer process: Applications in plastic electronics

We describe a method for contact printing metal patterns with nanometer features over large areas. This nanotransfer printing (nTP) technique relies on tailored surface chemistries to transfer metal films from the raised regions of a stamp to a substrate when these two elements are brought into intimate physical contact. The printing is purely additive, fast (<15 s contact time), and it occurs in a single processing step at ambient conditions. Features of varying dimensions, including sizes down to ∼100 nm, can be printed with edge resolution better than 15 nm. Electrical contacts and interconnects for high-performance organic transistors and complementary inverter circuits have been successfully fabricated using nTP.

Contact resistance in organic transistors that use source and drain electrodes formed by soft contact lamination

Soft contact lamination of source/drain electrodes supported by gold-coated high-resolution rubber stamps against organic semiconductor films can yield high-performance organic transistors. This article presents a detailed study of the electrical properties of these devices, with an emphasis on the nature of the laminated contacts with the p- and n-type semiconductors pentacene and copper hexadecafluorophthalocyanine, respectively. The analysis uses models developed for characterizing amorphous silicon transistors. The results demonstrate that the parasitic resistances related to the laminated contacts and their coupling to the transistor channel are considerably lower than those associated with conventional contacts formed by evaporation of gold electrodes directly on top of the organic semiconductors. These and other attractive features of transistors built by soft contact lamination suggest that they may be important for basic and applied studies in plastic electronics and nanoelectronic systems based ...

Formation of a high quality two‐dimensional electron gas on cleaved GaAs

We have succeeded in fabricating a two-dimensional electron gas (2DEG) on the cleaved (110) edge of a GaAs wafer by molecular beam epitaxy (MBE). A (100) wafer previously prepared by MBE growth is reinstalled in the MBE chamber so that an in situ cleave exposes a fresh (110) GaAs edge for further MBE overgrowth. A sequence of Si-doped AlGaAs layers completes the modulation-doped structure at the cleaved edge. Mobilities as high as 6.1×10^5 cm^2/V s are measured in the 2DEG at the cleaved interface.

Electron mobilities exceeding 107 cm2/V s in modulation‐doped GaAs

A modulation‐doped Al0.35Ga0.65As/GaAs single interface structure with a 700 A undoped setback grown by solid‐source molecular beam epitaxy (MBE) shows a Hall mobility of 11.7×106 cm2/V s at a carrier density of 2.4×1011 electrons/cm2 measured in van der Pauw geometry after exposure to light at 0.35 K. This is the highest carrier mobility ever measured in a semiconductor. Similar Al0.32Ga0.68As/GaAs structures with 1000–2000 A setbacks show Hall mobilities in the dark at 0.35 K as high as 4.9×106 cm2 /V s for carrier densities of 5.4×1010 electrons/cm2 and lower.

Modulation doping in GexSi1−x/Si strained layer heterostructures

We report the first observation of the modulation doping effect in Si/Ge0.2Si0.8 heterojunctions grown by molecular beam epitaxy. Peak hole mobilities of ∼3300 cm2 V−1 s−1 have been observed at 4.2 K. These values, although nonoptimum, are comparable to the best reported values for holes in Si/SiO2 inversion layers. Low temperature, angular dependent, Shubnikov–de Haas measurements have demonstrated the two‐dimensional nature of the hole gas and yield a surface carrier density of 3.5×1011 cm−2. From the temperature dependence of the Shubnikov–de Haas amplitudes a hole effective mass of 0.30±0.02mo has been derived. Identical measurements on n‐type heterojunctions having the same Ge content (x=0.2) have failed to show a sustained enhancement of mobility at low temperatures, indicating that ΔEv≫ΔEc.

Soft, conformable electrical contacts for organic semiconductors: High-resolution plastic circuits by lamination

Soft, conformable electrical contacts provide efficient, noninvasive probes for the transport properties of chemically and mechanically fragile, ultrathin organic semiconducting films. When combined with high-resolution printing and lamination techniques, these soft contacts also form the basis of a powerful technique for fabricating flexible plastic circuits. In this approach, a thin elastomeric film on a plastic substrate supports the electrodes and interconnections; laminating this substrate against another plastic substrate that supports the gate, dielectric and semiconductor levels establishes effective electrical contacts and completes the circuits. In addition to eliminating many of the problems associated with traditional layer-by-layer fabrication strategies, this lamination scheme possesses other attractive features: the transistors and circuit elements are naturally and efficiently encapsulated, and the active organic semiconductor layer is placed near the neutral mechanical plane. We demonstrate the features of soft, laminated contacts by fabricating large arrays of high-performance thin film transistors on plastic substrates by using a wide variety of organic semiconductors.

High-power laser light source for near-field optics and its application to high-density optical data storage

A laser light source for high-resolution near-field optics applications with an output power exceeding 1 mW (104 times the power from previous sources) and small (300 nm square to less than 50 nm square) output beam size is demonstrated. The very-small-aperture laser (VSAL) tremendously expands the range of applications possible with near-field optics and increases the signal-to-noise ratios and data rates obtained in existing applications. As an example, 250-nm-diam marks corresponding to 7.5 Gb/in.2 storage density have been recorded and read back in reflection and transmission on a rewritable phase-change disk at 24 Mb/s with a 250-nm-square aperture VSAL. VSALs potentially enable data storage densities of over 500 Gb/in.2 (up to 100 times today’s magnetic or optical storage densities).

Differential adhesion of amino acids to inorganic surfaces

A fundamental, yet underexplored, materials system is the interface between biological molecules and inorganic surfaces. In an elemental approach to this problem, we have systematically examined the adhesion of amino acids to a series of inorganic surfaces including metals, insulators, and semiconductors. Significant differential adhesion is observed over the full complement of amino acids, determined largely by amino acid side-chain charge. Extensive mapping of the amino acid adhesion versus materials in multiple solutions is presented, with preliminary mechanisms derived from concentration and pH dependence. These results provide an empirical basis for building peptide to inorganic surface structures, and, using this adhesion data, we design inorganic nanostructures that are shown to selectively bind to prescribed primary peptide sequences.

12.3: Flexible, Active-Matrix Display Constructed Using a Microencapsulated Electrophoretic Material and an Organic-Semiconductor-Based Backplane

We report on the fabrication of a prototypical flexible, active-matrix display incorporating a microencapsulated electrophoretic material and an active matrix backplane constructed with an organic semiconductor using all low temperature processes. This prototype has attributes desirable for an electronic paper display: flexibility, an ink-on-paper appearance, and low-power consumption.