5. ZnO Single Nanowire Based Printed-Schottky Diodes
Fig. 5.2: Schematic diagrams showing the printing steps SDP-SP. LIR: Local ink reservoir,µC: Micro-cantilever
of DI water to avoid the agglomeration of NWs into random clusters due to high surface tension in DI water. The optimized ZnO SNWs solution helps to place the NWs at a reasonable distance on the substrate, which helps in targeting an SNW for printing metal contact pads to complete the device fabrication. AgNP contact pads are printed using MCP technology, which uses a molecular printing system (Make: BioForce Nanosciences) containing a surface patterning tool (SPT) consisting of a silicon micro-cantilever print-head [36, 55, 114]. The micro-cantilever has a 5-10 µm wide micro- channels through which the ink to be printed flows or is temporarily stored. The mechanism of AgNP contact printing is described in the following subsection.
5.2.2 AgNP Contact Pad fabrication
The micro-cantilever tip can be used to perform SPT drag printing (SDP) and dip-ink printing with spot overwrite printing (DIPSOP) [36]. The micro-cantilever is kept under UV/O3 exposure for at least 30 minutes before using it for printing experiments. This turns the surface of micro-cantilever channels solvophilic to the AgNP ink to ensure that a large density of ink particles remain attached to it during the drag and dipping process [22, 112].
After selecting a ZnO SNW,∼10 µL of AgNP ink is drop-casted as a local ink reservoir (LIR)∼ 1000µm away from one of the ends of the selected ZnO SNW. The micro-cantilever is positioned over the AgNP drop and slowly brought down using the coarse and fine z-axis control settings in NanoWare software installed with the printing system [55]. When the gap between the micro-cantilever and the
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5.2 Fabrication of ZnO SNW Schottky diode
drop is less than 100µm, then both get focussed together in an optical camera attached to the printing software. Further, the focus of the camera is re-adjusted to the tip of the micro-cantilever to avoid the spillover (proximity of micro-cantilever and drop may lead to the direct hit of whole surface pattering tool to the ink source).
SDP technique is used only for metal contact pads deposition where the micro-cantilever tip is dragged to a shorter length (∼ 100 - 300 µm) in such a way that the dragged portion of the ink is not separated from the LIR. When the micro-cantilever tip is lifted up, the ink diffuses from LIR, which is a bigger ink-spot of average diameter near 1 to 2 mm containing a high volume of the ink.
However, in this work, SDP is used to assist the targeted deposition of the metal ink to overlap the ZnO SNW properly. To achieve this, the micro-cantilever tip is made to deliberately touch the surface of the substrate being dipped in the LIR and is dragged to a longer length (∼ 1000 µm) such that a small volume of ink is separated from the source LIR. When the micro-cantilever tip is lifted up gradually, the separated volume of the ink is uniformly distributed within a portion of the dragged area without touching the LIR due to the longer length and small volume of ink deposition. The above-mentioned process is repeated until the ink is uniformly deposited in the entire area dragged by the micro-cantilever tip. The complete printing mechanism is shown in Figure 5.2.
We have referred this phenomenon of uniform distribution of the separated ink (from the source LIR) throughout the SPT dragged region once the micro-cantilever tip is lifted up, as SDP assisted self-printing (SDP-SP) because the dynamics of the printing is totally governed by the volume of the separated ink, gradient of the ink density and the ink-substrate mutual interaction.
Utilizing SDP-SP, a rectangular contact strip of AgNP is printed, overlapping one end of the ZnO SNW. The overlap is achieved by stopping the drag of the ink closest possible to the ZnO SNW end (preferably ∼ 5 to 2 µm before NW) and allowing the ink to automatically diffuse and self- print the remaining distance with the help of high ink-density gradient between the micro-cantilever dragged region and the gap-region between the AgNP ink and ZnO SNW. The ink dragged by the micro-cantilever can also be placed directly over the SNW by gently touching its ends. The direct overlapping of ink is more feasible for SNW channel having length>10 µm. Then the entire substrate is annealed from 50◦C to 200◦C with a rate of 8◦C/minute. To ensure proper connection between the contact pad and the SNW, two to three DIPSOP based AgNP micro-spots (average diameter ∼ 5-10 µm) are printed by ultra-fine positioning. The targeted hit region ensures that 1-2 µm of the TH-2495_146102016
5. ZnO Single Nanowire Based Printed-Schottky Diodes
Fig. 5.3: (a) Optical image of dispersed ZnO SNWs on SiO2/Si surface (b) FESEM image of ZnO SNW after annealing of drop-casted dispersion with a scale of 1 µm (c) EDS spectra of ZnO SNW (d) SAED pattern showing ZnO SNW as single crystalline material
peripheral region of the micro-spot touches the end of nanowire without completely drowning it with AgNP ink. The DIPSOP spots are annealed properly to complete the fabrication of one-side of metal contact. The above-mentioned steps are repeated to form the AgNP metal contacts to the other side of ZnO SNW. DIPSOP is used to thicken and strengthen the printed AgNP layer at the AgNP-ZnO SNW interface to avoid any chance of discontinuity in the AgNP printed layer and reduce the effect of contact resistance and surface defects at interface, on the electrical characteristics of the Schottky diodes.
There are modifications between our previous chapters and the present chapter, and their differences are briefly summarized in Table 5.1. The details of process development to accomplish the current device are shown in Figure 5.2.
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