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The continuous scaling of electronic devices has brought Si based Complementary-Metal –Oxide-Semiconductor (CMOS) technology close to its limits, requiring novel materials and approaches to increase performance. Group III-V semiconductor nanowires have evolved as promising candidates due to their superior carrier mobility, saturation velocity and capability for bandgap engineering and integration in heterostructures, as well as their potential for use in active photonic devices. However, CMOS compatible, epitaxial integration of III-Vs on Si(100) remains challenging, mainly due to the formation of crystal defects, significantly reducing device performance. Selective Area Growth (SAG)1,2 and epitaxial lateral overgrowth (ELO)3–5 have both been demonstrated to reduce defects due to lattice mismatch. In a more recent method referred to as Template-Assisted Selective Epitaxy (TASE)6–10 nanostructures are grown entirely within pre-defined hollow template cavities containing a small Si seed, thus minimizing the...

GaSb nanostructures integrated on Si substrates are of high interest for p-type transistors and mid-IR photodetectors. Here, we investigate the metalorganic chemical vapor deposition and properties of GaSb nanostructures monolithically integrated onto silicon-on-insulator wafers using template-assisted selective epitaxy. A high degree of morphological control allows for GaSb nanostructures with critical dimensions down to 20 nm. Detailed investigation of growth parameters reveals that the GaSb growth rate is governed by the desorption processes of an Sb surface layer and, in turn, is insensitive to changes in material transport efficiency. The GaSb crystal structure is typically zinc-blende with a low density of rotational twin defects, and even occasional twin-free structures are observed. Hall/van der Pauw measurements are conducted on 20 nm-thick GaSb nanostructures, revealing high hole mobility of 760 cm2/(V s), which matches literature values for high-quality bulk GaSb crystals. Finally, we demonstrate a process that enables cointegration of GaSb and InAs nanostructures in close vicinity on Si, a preferred material combination ideally suited for high-performance complementary III-V metal-oxide-semiconductor technology.

We report on the fabrication and characterization of silicon nanowire tunnel diodes. The silicon nanowires were grown on p-type Si substrates using Au-catalyzed vapor-liquid-solid growth and in situ n-type doping. Electrical measurements reveal Esaki diode characteristics with peak current densities of 3.6 kA/cm(2), peak-to-valley current ratios of up to 4.3, and reverse current densities of up to 300 kA/cm(2) at 0.5 V reverse bias. Strain-dependent current-voltage (I-V) measurements exhibit a decrease of the peak tunnel current with uniaxial tensile stress and an increase of 48% for 1.3 GPa compressive stress along the111growth direction, revealing the strain dependence of the Si band structure and thus the tunnel barrier. The contributions of phonons to the indirect tunneling process were probed by conductance measurements at 4.2 K. These measurements show phonon peaks at energies corresponding to the transverse acoustical and transverse optical phonons. In addition, the low-temperature conductance measurements were extended to higher biases to identify potential impurity states in the band gap. The results demonstrate that the most likely impurity, namely, Au from the catalyst particle, is not detectable, a finding that is also supported by the excellent device properties of the Esaki diodes reported here.

A wide variety of methods are now available for the synthesis of colloidal particle having controlled shapes, structures, and dimensions. One of the main challenges in the development of devices that utilize micro- and nanoparticles is still particle placement and integration on surfaces. Required are engineering approaches to control the assembly of these building blocks at accurate positions and at high yield. Here, we investigate two complementary methods to create particle assemblies ranging from full layers to sparse arrays of single particles starting from colloidal suspensions of gold and polystyrene particles. Convective assembly was performed on hydrophilic substrates to create crystalline mono- or multilayers using the convective flow of nanoparticles induced by the evaporation of solvent at the three-phase contact line of a solution. On hydrophobic surfaces, capillary assembly was investigated to create sparse arrays and complex three-dimensional structures using capillary forces to trap and organize particles in the recessed regions of a template. In both methods, the hydrodynamic drag exerted on the particle in the suspension plays a key role in the assembly process. We demonstrate for the first time that the velocity and direction of particles in the suspension can be controlled to perform assembly or disassembly of particles. This is achieved by setting the temperature of the colloidal suspension above or below the dew point. The influence of other parameters, such as substrate velocity, wetting properties, and pattern geometry, is also investigated. For the particular case of capillary assembly, we propose a mechanism that takes into account the relative influences of these parameters on the motion of particles and that describes the influence of temperature on the assembly efficiency.

Microfluidic devices are well suited for the miniaturization of biological assays, in particular when only small volumes of samples and reagents are available, short time to results is desirable, and multiple analytes are to be detected. Microfluidic networks (MFNs), which fill by means of capillary forces, have already been used to detect important biological analytes with high sensitivity and in a combinatorial fashion. These MFNs were coated with Au, onto which a hydrophilic, protein-repellent monolayer of thiolated poly(ethyleneglycol) (HS-PEG) was self-assembled, and the binding sites for analytes were present on a poly(dimethylsiloxane) (PDMS) sealing cover. We report here a set of simple methods to extend previous work on MFNs by integrating binding sites for analytes inside the microstructures of MFNs using microcontact printing (muCP). First, fluorescently labeled antibodies (Abs) were microcontact-printed from stamps onto planar model surfaces such as glass, Si, Si/SiO2, Au, and Au derivatized with HS-PEG to investigate how much candidate materials for MFNs would quench the fluorescence of printed, labeled Abs. Au coated with HS-PEG led to a fluorescence signal that was approximately 65% weaker than that of glass but provided a convenient surface for printing Abs and for rendering the microstructures of the MFNs wettable. Then, proteins were inked from solution onto the surface of PDMS (Sylgard 184) stamps having continuous or discontinuous micropatterns or locally inked onto planar stamps to investigate how the aspect ratio (depth:width) of microstructures and the printing conditions affected the transfer of protein and the accuracy of the resulting patterns. By applying a controlled pressure to the back of the stamp, Abs were accurately microcontact-printed into the recessed regions of MFNs if the aspect ratio of the MFN microstructures was lower than approximately 1:6. Finally, the realization of a simple assay between Abs (used as antigens) microcontact-printed in microchannels and Abs from solution suggests that this method could become useful to pattern proteins in microstructures for advanced bioanalytical purposes.

The thin-film transistor (TFT) array of liquid-crystal displays (LCDs) comprises a number of metallic, semiconducting, and insulating layers, which need to be deposited and patterned accurately with very high yields on a (large) glass substrate. We are exploring how to fabricate the gate metal lines of the TFT array in an entirely new and potentially cost-effective wayby depositing the metal layer of the TFT array using electroless deposition (ELD) and by patterning the gates using microcontact printing (μCP). To achieve this goal, we separately explore first the plating conditions to deposit a gate metal on 15 in. glass substrates, and second the printing process to finally combine them later in the work. Here, we review in depth the metallization of the glass by ELD of NiB as gate material, and we demonstrate the patterning of the gate layer using a conventional photoengraving process (PEP, i.e., photolithography and wet etching). We selected NiB because this material can fulfill the conductivity requir...

We present a method to pattern an electroless-deposited metal layer based on electroplating a mask and patterning this mask using microcontact printing (μCP) and wet etching. This method starts by derivatizing a glass substrate with an amino-functionalized silane, 3-(2-aminoethylamino)propyltrimethoxysilane (EDA-Si) from solution and using the amine functions of the grafted silane to immobilize Pd/Sn colloids from an acidic suspension onto the glass. The Pd/Sn colloids initiate the electroless deposition (ELD) of a 150 nm thick NiB layer onto the glass. The as-deposited NiB has a specific resistivity of 22 μΩ cm and can be covered with 50 nm of an electroplated Cu layer. Microcontact printing a protective monolayer of eicosanethiol (ECT) onto the Cu and etching it selectively with a cyanide-based etch bath defines the pattern of the Cu mask. This pattern is transferred into the NiB underlayer using a sulfuric acid-based wet etch. The ECT monolayer, the Cu mask, and the Pd/Sn residue remaining on the glass...

We developed strategies based on self-assembly principles to etch substrates patterned with monolayer resists with high selectivity and etch directionality. Our strategies exploit the defined composition and order of these ultrathin resists and overcome their imperfections. Defects in a monolayer can be healed by additives present in an etch bath. Alternatively, large molecules that cannot diffuse through defects can be employed as etchants. It is also possible to taper structures using the competition between etching and the side-growth of a self-assembling etch barrier, nucleating from the originally patterned monolayer. The application of these concepts lets defect-sensitive monolayers become robust and versatile resists, which should promote their acceptance and use in microtechnology.

We investigate self-assembled monolayers on polycrystalline Pd composed of eicosanethiol (ECT). X-ray photoelectron spectroscopy reveals a composition and structure of the ECT monolayer on Pd similar to those on Au: the sulfurs are close to the Pd and underneath the alkyl chains. Monolayers formed from a 0.4 mM solution of ECT in ethanol are 2.8 nm thick and have advancing and receding contact angles with water of ∼115° and ∼90°, respectively. They are 0.3 nm thicker than those on Au and have a surprisingly low receding contact angle. Monolayers of ECT microcontact printed onto 100-nm-thick evaporated Pd films confer good protection of the Pd in a ferrichloride wet etch. It is therefore possible to create Pd patterns with high contrast and with lateral dimensions smaller than 200 nm. The Pd patterns can initiate electroless deposition of NiB, NiP, or Cu but are difficult to use as a mask for etching a less noble metal such as Cu due to the formation of a galvanic cell during etching. Instead, this galvan...

The capability of microcontact printing (μCP) to transfer chemical reagents from an elastomeric stamp to a substrate is used here to direct electroless deposition of Cu. The stamp is inked with a Pd(II) catalytic precursor, which it prints onto a Ti-coated substrate. This application of μCP is challenged by several interdependent parameters such as the appropriate surface chemistry of the stamp, the choice of the ink, the control over the transfer of the catalyst during printing, the activation of the catalyst, and the electroless plating of Cu in general. We address these issues and suggest a reliable, but not general, method to combine microcontact printing and electroless deposition of Cu. This method uses hydrophilization of poly(dimethyl)siloxane stamps and inking with the ethanol-soluble [(CH3−(CH2)16−CN)2PdCl2] catalytic precursor. Printing this complex onto a thin Ti layer evaporated onto Si/SiO2 provides a high-level transfer of the Pd(II) complex from the stamp to the surface with simultaneous a...

We report the formulation of siloxane polymers for high-resolution, high-accuracy stamps for soft lithography. With this technique, a molecular, polymeric, or liquid ink is applied to the surface of a stamp and then transferred by conformal contact to a substrate. Stamps for this technique are usually made of a commercial siloxane elastomer with appropriate mechanical properties to achieve conformal contact but are incapable of printing accurate, submicrometer patterns. To formulate better stamp polymers, we used models of rubber-like elasticity as guidelines. Poly(dimethylsiloxane) networks were prepared from vinyl and hydrosilane end-linked polymers and vinyl and hydrosilane copolymers, with varying mass between cross-links and junction functionality. The polymer formulations were characterized by strain at break as well as compression modulus and surface hardness measurements. This resulted in the identification of bimodal polymer networks having mechanical properties that allow the replication of high...

Microcontact printing of proteins proves to be an excellent means of directly patterning biomolecules on solid substrates. Monolayer quantities of protein equilibrated on the surface of a hydrophobic, elastomeric stamp are immobilized there to rinses with buffer. These biomolecules can nevertheless transfer with >99% efficiency from the stamp to a substrate after just 1 s of contact. This capability allows the simple creation of functional patterns of proteins at scales that involve the placement of

This article describes the design, function, and application of simple microfluidic networks as conduits for the patterned delivery of chemical reactants onto a substrate. It demonstrates how such networks, made in an elastomer, allow simultaneous delivery of functionally distinct molecules onto targeted regions of a surface (Delamarche, E. et al. Science 1997, 276, 779−781). Microfluidic networks generally consume less than microliter quantities of solution and are thus well suited for use when the required reagents are scarce or precious, as often occurs in experiments and technologies that place biochemicals on solid planar substrates. We illustrate some of the particular challenges of doing chemistry inside the narrow confines of capillaries defined by fluidic networks, in addition to the advantages attendant to this approach, in the context of forming patterned arrays of different, and functional, immunoglobulins useful in highly localized biological assays.

We have demonstrated microcontact printing (muCP) of self-assembled monolayers in the millisecond regime. The contact formation and separation of the stamp and substrate was studied with high-speed video recordings. Using high ink concentrations and contact times as short as 1 ms, we printed monolayers of hexadecanethiol on Au, which served as a selective etch resist. High-speed muCP yields defect-free monolayers that are independent of the dimensions of the printed patterns, have high contrast between printed and unprinted areas, and enable perfect reproducibility of prints.