DOI: 10.1021/cg100538z
2010, Vol. 10 2842–2846
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Epitactically Interpenetrated High Quality ZnO Nanostructured Junctions on Microchips Grown by the Vapor-Liquid-Solid Method Seid Jebril,† Hanna Kuhlmann,† Sven M€ uller,‡ Carsten Ronning,§ Lorenz Kienle, Viola Duppel,^ Yogendra Kumar Mishra,† and Rainer Adelung*,† †
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Functional Nanomaterials, Technische Fakult€ at der Universit€ at Kiel, Kaiserstrasse 2, 24143 Kiel, ottingen, Germany, Germany, ‡II. Physikalisches Institut, Friedrich-Hund-Platz 1, 37077 G€ § Institute for Solid State Physics, University of Jena, Max-Wien-Platz 1, 07743 Jena, Germany, Synthesis and Real Structure, Technische Fakult€ at der Universit€ at Kiel, Kaiserstrasse 2, ur Festk€ orperforschung, 24143 Germany, and ^Max-Planck-Institut f€ Heisenbergstrasse 2, 70569 Stuttgart, Germany Received April 23, 2010; Revised Manuscript Received May 30, 2010
ABSTRACT: The usability of nanostructures in electrical devices such as gas sensors critically depends on the ability to form high quality contacts and junctions, at least in a two terminal setup. For the fabrication of various nanostructures, vapor-liquid-solid (VLS) growth is meanwhile a widely spread and very efficient technique for many semiconductors. However, as already demonstrated in the literature, forming contacts with the VLS grown structures to utilize them in a device is typically tedious. Either the substrate material has to be the same, such as the VLS material, or a laborious procedure has to be used to connect one side with the other. As a strikingly simple alternative approach, we report that the ability of VLS grown nanostructures to interpenetrate each other in a homoepitactical manner can be used to form a connecting bridge between two gold contact lines on a microchip. Other methods of direct growth are already established, but they lead only to a touching of the nanowires and not to interpenetration. The ZnO interpenetration junctions can be directly used as electrical devices, and just as a proof of principle, they were employed as UV-light dependent resistor, field effect transistor (FET), or gas sensor. Si, III-V and II-VI, compound semiconducting nanowires can be formed by the efficient approach of vapor-liquid-solid (VLS) growth.1,2 Different catalysts, such as tin,3 copper,4 and gold,1,5 have been used for the growth of 1D ZnO structures. ZnO, a wide direct band gap semiconductor (3.37 eV), is one of the most studied semiconductors in the past decade. Beside its semiconducting, piezoelectric, and pyroelectric nature, it has probably the richest variety of different nanostructures.3,6,7 Grown morphologies include nanowires, nanospirals, nanobelts, nanocombs, etc.6 Their high surface to volume ratios and superior mechanical stability promote them as the ideal candidates for sensors,8 FET devices, or switches.6 The catalyst seed layer has an immense influence on selecting certain nanostructures, and control over growth can be gained by simply varying the Au layer thickness.9 Also, an alignment of these esthetically pleasing structures is possible by combining the VLS growth with conventional or unconventional microstructuring techniques10,11 on crystalline substrates. Microstructuring gives a well determined nucleation point for the subsequent VLS growth; crystalline substrates, e.g., GaN,10 induce a vertical growth. However, forming crystalline contacts with the VLS grown nanostructures to utilize them in a device is still tedious, because either a crystalline substrate material has to be compatible with the VLS material or a laborious procedure has to be used. Such bridges between two contacts were fabricated from several semiconductors of, e.g., GaAs, Si, GaN, or ZnO. Typically, this requires a careful process control and a large amount of process steps; see, for example, refs 12-15. Often the nano-
structures are grown in an epitaxial relation with a substrate material in order to direct the growth of the nanowires, which limits the flexibility for device application. Nevertheless, the great potential of these structures has already been demonstrated, e.g., as biosensors, but still requires manual and individual contacting by e-beam lithography and focused ion beam techniques.16 Here we report the growth of epitactically interpenetrated ZnO nanosail junctions with varying densities *Corresponding author. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 06/15/2010
on a microstructured chip by the VLS approach, and we also report their electrical properties as well as sensoric responses. This is different from earlier reports on nonepitaxial junctions between VLS grown wires by a touching of wires,17 which were also directly grown on Pt-covered microcontact lines. The sample preparation for the growing of ZnO nanosails was done in the following steps. First, the deposition of gold, used as a predefined site for the ZnO growth and as the circuit path, was done on the microstructured-photoresist coated silicon substrate (76 mm diameter, p-doped, 1-10 Ω cm resistivity, 380 μm thick, Æ100æ oriented and coated with 100 nm thick thermally grown SiO2). The microstructure patterning was done by using conventional photolithography. About 3 nm of titanium (Ti) as adhesion promoter was used prior to the gold film deposition to ensure that the deposited gold will not be removed during the resist removing process. This resist lift-off process, used to obtain the gold circuit, was done by dipping into acetone and using an ultrasonic bath. Both the gold film and the adhesion promoter (Ti) were deposited by using magnetron sputtering at an Ar pressure of about 3.0 10-3 mbar (with a base pressure of 8.6 10-7 mbar) and rate of 1.2 and 0.5 A˚/s, respectively. For the sake of film uniformity, sample rotation was used during metal deposition. For the synthesis of the nanostructures by the VLS process,6,18 pure ZnO powder (99.99% from Goodfellows) as source material was put in an alumina boat and placed at the center of a tube furnace, which was heated to 1350 C. Silicon substrates covered with Au contacts were placed at the position in the furnace with a sample temperature of about 1000 C. The furnace was heated up to 1350 C with a ramp of 190 C per hour and was kept at 1350 C for 5 min. During the entire process, an argon gas flow of 50 SCCM with a system pressure of about 100 mbar was applied. However, the flow direction was switched during the growth process. Only during the growth at 1350 C was the argon gas flow direction adjusted to transport the ZnO vapor from source to substrate; an opposite gas flow direction was applied during heating-up and cooling-down. For detailed investigation, the ZnO nanosails were scratched from an Au support and analyzed by high resolution transmission r 2010 American Chemical Society
Communication electron microscopy (HRTEM) with a Philips CM 30ST microscope (LaB6 cathode, 300 kV, CS = 1.15 mm). SAED (selected area electron diffraction) and PED (precession electron diffraction19) were carried out using a diaphragm, which limited the diffraction to a circular area of 2500 A˚ in diameter. The PED technique allows recording diffraction patterns close to the kinematic approximation. HRTEM micrographs (multislice formalism) were simulated with the EMS program package20 (spread of defocus, 70 A˚; illumination semiangle, 1.2 mrad). All images were evaluated (including Fourier filtering) with the programs Digital Micrograph 3.6.1 (Gatan) or Crisp (Calidris). Chemical analyses by Energy-dispersive X-ray spectroscopy (EDX) were performed with a Si/Li detector (Noran, Vantage System). Here, we demonstrate the contact formation in VLS growth by simply using the ability of individual crystals to interpenetrate and self-organize each other during a straightforward VLS growth. In the conventional VLS process, aligned nanostructures are grown from a liquid catalyst droplet which initiate and guide the growth of the nanostructures. The precursor decomposes on the molten catalyst droplet that supersaturates with the nanostructure forming species and precipitates at the bottom, accompanied by the formation of a nanostructure at the interface. In our case, the junctions were made by growing VLS structures directly on two neighboring gold circuit paths of a microchip; bridges over predefined gaps will be formed by an interpenetration of the grown VLS structures. No special substrate has to be used in order to create these bridges. By controlling the width and distance of the gap on the conventionally microstructured contact lines or the growth time, the density of intersecting nanostructures bridging the gap can be controlled in order to achieve a massively parallel set of intersections. Here, a triangular sheetlike geometry is selected and denoted as “nanosails” in the following,7 which are well suited for the here described penetration approach. Advantageously, a gold substrate of 50 nm thickness will effect the triangular growth, so the nanosails can be directly obtained on 50 nm thick gold circuit paths. Smaller gold layer thicknesses lead to wire growth,18 because of the break up into clear separated Au nanoclusters during heating, enabling pure VLS growth of ZnO nanowires. In contrast, for thick layers and thus excess of the catalysts, no separated Au clusters are formed, and this lead to a combination of VLS and VS growth driven also by polarity effects. Several details of such a growth are described in the literature;7,21-24 however, the exact details of the sail growth are not yet understood, because of the phase separation of the involved components during cooling down, always leading to an ambiguous situation at room temperature examination of the products. These 50 nm thick circuit paths have a reasonably good conductivity (resistance
