Person:

Park, Hyunsung

Conventional image sensors achieve color imaging using absorptive organic dye filters. These face considerable challenges however in the trend toward ever higher pixel densities and advanced imaging methods such as multispectral imaging and polarization-resolved imaging. In this dissertation, we investigate the optical properties of vertical silicon nanowires with the goal of image sensor applications. First, we demonstrate a multispectral imaging system that uses a novel filter that consists of vertical silicon nanowires embedded in a transparent medium. Second, we demonstrate pixels consisting of vertical silicon nanowires with integrated photodetectors. We show that their spectral sensitivities are governed by nanowire radius, and perform color imaging. In addition, we demonstrate polarization-resolving photodetectors consisting of silicon nanowires with elliptical cross sections. Finally, we discuss a dual detector device. Each pixel consists of vertical silicon nanowires (incorporating photodetectors) formed above a silicon substrate (that also incorporates a photodetector). Our method is very practical from a manufacturing standpoint because all filter functions are defined at the same time through a single lithography step. In addition, our approach is conceptually different from current filter-based methods, as absorbed light in our device is converted to photocurrent, rather than discarded. This ultimately presents the opportunity for very high photon efficiency.

Multispectral imaging is a powerful tool that extends the capabilities of the human eye. However, multispectral imaging systems generally are expensive and bulky, and multiple exposures are needed. Here, we report the demonstration of a compact multispectral imaging system that uses vertical silicon nanowires to realize a filter array. Multiple filter functions covering visible to near-infrared (NIR) wavelengths are simultaneously defined in a single lithography step using a single material (silicon). Nanowires are then etched and embedded into polydimethylsiloxane (PDMS), thereby realizing a device with eight filter functions. By attaching it to a monochrome silicon image sensor, we successfully realize an all-silicon multispectral imaging system. We demonstrate visible and NIR imaging. We show that the latter is highly sensitive to vegetation and furthermore enables imaging through objects opaque to the eye.