Vertical Germanium Nanowire Arrays in Microfluidic Channels for Charged Molecule Detection

Abstract

We demonstrate the fabrication of vertical germanium nanowire ͑Ge NW͒ molecular sensor arrays that are suitable for biological field-effect-transistor sensing, using conventional semiconductor fabrication processes. The proposed fabrication process enabled production of freestanding vertical NW arrays with a Schottky junction top contact. The arrays' dc current-voltage characteristics could be modulated by altering the solution pH in contact with the HfO 2 gate insulator-coated nanowire surface. A sensitivity of 50 nS/pH with 1 m 2 sensor area is demonstrated for undoped vertical Ge NW arrays.One-dimensional devices such as semiconductor nanowires ͑NWs͒, have attracted much interest because of their unique characteristics. Previous studies have shown the potential of NWs for interconnects, field-effect transistors ͑FETs͒, 1,2 and sensors. 3 For sensor device applications, several authors have reported on NW-based devices in which a very high sensitivity is obtained by the selective binding of molecules to the wire surface, thus inducing charge transfer and a conductance change in the wire. 1 However, in previous research, NWs were first removed from their host substrate, collected in solution, and finally deposited onto another substrate. Therefore, most NW-based devices reported to date sit horizontally on the substrate. From the fluid dynamics viewpoint of target molecule capture and detection in solution, freestanding FET structures are preferable, as the amount of NW surface exposed to the fluid is increased and the functionalized NW surface region at which molecules bind can be far away from the solution/fluidic channel boundary layer across which molecules must, in the absence of convection or stirring, diffuse to reach the wire surface. 4 In addition, an enhanced electrolyte-insulator-semiconductor capacitance is expected for freestanding, "surround-gate" channel structures. 5 In the present research, we have demonstrated reproducible pH sensing using vertical, freestanding Ge NW devices fabricated within a microfluidic channel on a Si͑111͒ wafer. Furthermore, the Au catalyst particles at the tips of the NWs were removed during the fabrication process, which may improve its adaptation to a conventional semiconductor process. Because of its conventional processes to fabricate the sensor array, the devices can be easily reproduced with a reliable sensing performance. Figure 1 shows an overview of the fabrication process. A lowresistivity silicon substrate served as the common bottom electrode for the NW array sensors. The substrate used in this study was As-doped silicon ͑111͒ with 0.05-0.09 ⍀ cm resistivity. To fabricate microfluidic channels, 140 nm silicon oxide and 860 nm silicon nitride were formed by furnace oxidation and chemical vapor deposition ͑CVD͒, respectively. The silicon dioxide film functioned as an etch stop layer during silicon nitride dry etching and as a buffer layer between the silicon substrate and the relatively higher-stress silicon nitride. The silicon nitride film formed by low-pres...

Germanium Nanowires as Sensing Devices: Modelization of Electrical Properties

“…Germanium nanowires (NWs) are one-dimensional semiconducting nanostructures that are currently being investigated for the development of innovative molecular sensors, among other nanodevices. Convincing and encouraging results have been already obtained by using these nanostructures to electrically detect various molecules [ 1 , 2 , 3 , 4 ].…”

Size-dependent chemical transformation, structural phase change, and optical properties of nanowires

“…Benefits gained from high-surface-to-volume ratios, as well as electrical or optical confinement in the radial direction, are accessible due to the long lengths of the nanowires, which allow electrical connections and interconnection to be made via conventional lithographic techniques. In the cases where benefits gained from radial confinement are unaffected by nanowire length, nascent techniques have potential for scaling single-nanowire devices to multi-nanowire systems: microfluidic assembly [18, 19], electric field [20, 21], dielectrophoresis [22-24], mechanical transfer [25-27], optical tweezers [28-31], for instance, have potential for aiding in precise placement within smaller-scale systems, while techniques such as Langmuir-Blodgett [32-34], branched nanowire growth [35-37], or 3-D assembly [38, 39] could eventually mature sufficiently for large-scale deterministic system assembly.…”

One-Dimensional Nanostructures: Microfluidic-Based Synthesis, Alignment and Integration towards Functional Sensing Devices

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