Nanowire Placement with Ink Jet Heads

We have shown that thermal ink jet print heads can be used to place GaN nanowires on patterned substrates. The semiconductor nanowires had diameters ranging from 70 to 300 nm and lengths from 5 μm to 20 μm. They were dispersed in alcohol-water solutions for loading into ink reservoirs. To avoid clogging, the thermal ink jet heads were chosen with drop weights from 72 to 165 ng. The thermal ink jet method was successfully used to place nanowires across narrow gaps in metal patterns. When using a low-power optical microscope to align the nozzle with substrate pattern features, the placement accuracy is much higher than with micropipette placement. For unknown reasons, nanowires would not pass through piezoelectric ink jet heads. These experiments demonstrate that ink jet technology holds promise for low-cost, rapid, massively parallel placement and processing of nanowires for optoelectronic, electronic, and sensor applications. Introduction Semiconductor nanostructures provide opportunities for novel device architectures and improved performance and yield, but handling of these materials efficiently will require new manufacturing technologies. In this paper we describe placement of GaN nanowires on patterned Si substrates to within 35 μm by use of thermal ink jet technology. We have previously shown that nanowires can be dispersed in solvent solutions onto patterned planar surfaces and partially directed with applied electric fields through a dielectrophoretic mechanism [1]. The adhesion between a dispersed nanowire and its substrate is sufficient to survive subsequent conventional photolithographic processing, thus enabling the deposition of electrical contact pads. For example, bridge structures such as the one illustrated in Fig. 1 have been made in this way. The yield offered by this method is limited, however, by the ~5 μL drop size produced by a metered pipette. This volume is equivalent to a spherical drop 2 mm in diameter that disperses over a substrate area on the order of 1 cm. Although pipettes with metered volumes down to 200 nL are commercially available, this still reduces the drop volume by only a factor of 40. In contrast, ink jet drop volumes around 150 pL can readily be achieved, allowing confinement of the nanowires in a solvent sphere of 70 μm diameter. Although placement yield varies significantly with geometrical parameters such as the number and area of acceptable sites and the density of nanowires in the solvent, we expect to improve device yields from below ~10 % afforded by the dielectrophoresis process to greater than 90 % with the ink-jet techniques. By addressing the efficient coupling of the nanoscale semiconductor device to the macro world, ink jet dispersal could demonstrate a path for full utilization of semiconductor nanostructures in a manufacturing process. GaN and related compounds are particularly attractive for nanostructure growth because conventional bulk and epitaxial growth methods produce material with high defect density and therefore low yields. Native substrates for the nitride semiconductors are only now becoming available in small diameters (1 to 8 cm) and at high cost. GaN nanowires with diameters on the order of 200 nm and lengths of 10 μm contain sufficient material to sustain operation as diode lasers, light emitting diodes, detectors, or field effect transistors [2,3,4]. The superior optical and crystalline quality [5,6] that we have demonstrated for nanowires relative to epitaxial material makes them valuable from a performance standpoint alone, and avoiding expensive native substrates also lowers manufacturing costs. Alloys of GaN with InN and AlN span a large range of the optical spectrum, from 0.7 eV to 6.2 eV, thus making them suitable for applications from ultraviolet medical and sterilization systems to solar cells and telecommunications. Figure 1. GaN nanowire placed on metal pads with dielectrophoresis guiding followed by processing with conventional photolithography to add electrical