Ordered arrays of silicon nanowires were fabricated by etching, and their bunching characteristics were parametrically studied by varying the diameter, the length, and the pitch. The diameter to length ratio was found to be critical for the nanowires to stand vertically without bunching. For a length of 650 nm, 40 nm and larger diameter nanowires were vertical, whereas for a length of 400 nm, 34 nm and larger diameter nanowires were vertical. Further, the phase change between the bunching and vertical nature of nanowires happens abruptly and was verified by finite element modeling of the deflections of the nanowire tips for different diameters. The detailed experimental study provides guidelines for silicon nanowire arrays being considered for different applications including solar cells, optical waveguides, and sensors.

A semi-analytical model for the optical properties of silicon nanowires (SiNWs) is presented. Results from the model offer a good physical understanding of the optical behavior of SiNWs. It is shown that light trapping within ordered nanowires only happens over a small wavelength band (20-75 nm) that is dependent on the diameter of the nanowires, not length. Furthermore, wavelength tunable absorption peaks can be achieved in ordered SiNWs by adjusting the geometrical parameters. A good match between the model and experimental results confirms the validity of the proposed effective index model.

We propose a new nanowire based, junction-less phototransistor, that consists of a channel with both wide and narrow regions to ensure efficient light absorption and low dark current, respectively. While the light is absorbed in the wide region, the narrow region allows for ease of band engineering. We also find that a nanowire in the source can further boost the optical gain. The proposed device, which can potentially detect very low light intensities, does not rely on complicated doping profiles, but instead uses suitably designed gates. Our calculations show the detection of a photon flux as low as 35 per second.

A system concept is described for a pushbroom airborne optical fire detection instrument operating in the visible and near-infrared spectral regions. In the design concept, several detection modules are used simultaneously, each having a camera lens, beam splitter, spectral filters, silicon linear array, InGaAs linear array, and signal processing. Calculations indicate that dual-wavelength signal processing should allow cool (600 K) incipient fires as small as 0.1 m in extent to be identified against the expected background of diffuse and specular sunlight.

A design of a photodetector with multiple gates and a modified channel, incorporating silicon nanowires, is proposed, and its performance is modeled. Working under lateral bipolar action, the gate/channel geometry increases the device photocurrent in the accumulation mode and leads to photoresponsivities of greater than 10 4 A/W.