Nanoimprint lithography (NIL) is an attractive method for its ability to quickly and cheaply pattern nano-scaled
dimensions, and is an enabling technology for patterning large area substrates. The benefits of NIL are demonstrated
through its application towards large area nanowire image arrays. In this work, we have fabricated and characterized top
down silicon nanowire detector arrays by using UV curing NIL and deep Reactive Ion Etching techniques. Fabricated
devices show over 106 gain value at low incident light power, which is comparable to high sensitivity of an e-beam
written lithography device. This technology is suitable for fabrication of high density, addressable imager arrays.
Nanowire photodetectors of a variety of materials have been attracting increased attention due to their potential for very
high sensitivity detection. Silicon photodetectors are of particular interest for detection in the visible spectrum, having
many benefits including cost of substrate, ease of processing, and ability for integration with conventional fabrication
techniques. Using top-down fabrication techniques results in additional benefits of precise control of number, geometry,
and placement of these wires. To demonstrate the potential of these devices, top-down, vertical silicon nanowire
phototransistor arrays have been fabricated using ebeam lithography and deep reactive ion and inductively coupled
plasma etching. These devices show a much higher phototransistive gain over nanowire photodiodes with similar
geometry under illumination from a 635nm laser. Low temperature measurements also show the dependence of dark
current and sensitivity on temperature. The mechanism responsible for this gain is shown to be dominated by the large
surface-to-volume ratio of nanowires where charge capture and recombination at the surface creates a radial gate bias
which is modulated with light intensity. 3D numerical simulations validate this mechanism and further show the
dependence of device behavior on nanowire doping, geometry, and surface state density. This will allow for the precise
engineering of these devices to achieve the maximum sensitivity obtainable as we strive for the ultimate goal of single
photon resolution.
Vertical silicon nanowire detectors with high phototransistive gain have been demonstrated and the principles
responsible for the high gain have been reported in recent publications. The emphasis of this paper is (a) the fabrication
technology of silicon nanowire array detectors that can be integrated with Si VLSI and (b) the ability of sub-bandgap
detection to achieve ultrawide band (from UV to IR) responsivity. We have demonstrated responsivity of greater than
100 A/W at 1550 nm for single crystal silicon nanowires to detect picowatts of IR light, the highest record ever reported
for single crystal silicon detectors.
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