Nanophotonics in absorbing III-V nanowire arrays
Sammanfattning: We have studied the interaction of light with an array of vertically oriented III-V semiconductor nanowires both theoretically and experimentally. For the theoretical studies, electromagnetic modeling has been employed. This modeling shows that with proper tuning of the nanowire diameter, the absorption per volume semiconductor material can be 20 times higher in the nanowires than in a corresponding bulk semiconductor sample. This enhancement occurs when nanophotonic resonances show up in the nanowires. We have shown that the optical response of a nanowire array can be described by electrostatics for small-diameter nanowires and by geometrical optics for large-diameter nanowires. None of these two limit cases showed resonances, motivating the interest for the intermediate nanophotonic regime where the diameter of the nanowires is comparable to the wavelength of the incident light. Supported by theoretical analysis, we have shown experimentally a resonant photodetection response in an InAsSb nanowire array in the infrared region, which is of potential interest for thermal imaging and chemical analysis. Furthermore, we have demonstrated a solar cell based on InP nanowires. The nanowire-array solar cell showed an efficiency of 13.8 % and converted more than 70 % of the above-bandgap photons into electron-hole pairs that contributed to the short-circuit current, even though the nanowires covered only 12 % of the surface. By combining the electromagnetic modeling with reflectance measurements, we have developed an optical method for simultaneously measuring both the diameter and the length of nanowires in large-area arrays. The accuracy of the method is comparable to that of scanning electron microscopy. Furthermore, we have developed tools for studying the crystal-phase dependent optical response of III-V materials. The studies showed that a tuning of the crystal phase, which is possible in the nanowire geometry, can be used for enabling and disabling strongly absorbing nanophotonic resonances in nanowire arrays.
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