Engineering the carrier lifetime and switching speed in Si-based mm-wave photomodulators

For a diverse range of semiconductor devices, the charge carrier lifetime is an essential characteristic. However, the carrier lifetime is difficult to control, as it is usually determined by a variety of recombination processes. For indirect bandgap materials, it is well known that effective carrier lifetimes can be improved by passivating the surface, effectively extinguishing surface-related recombination processes. However, for some applications, such as photomodulators for sub-infrared radiation, it is beneficial to tailor lifetimes to specific values, in this particular case trading off between photo-efficiency and switching speed. In this paper, we design a new type of silicon-based metamaterial with a tunable electron-hole lifetime. By periodically patterning a dielectric surface passivation layer, we create a metamaterial whereby the filling fraction of passivated relative to unpassivated areas dictates the effective charge carrier lifetime. We demonstrate tunable lifetimes between 200 mu s and 8 ms in a 670 mu m thick Si wafer, though in principle our approach allows one to generate any lifetime between the fully passivated and unpassivated limits of a bulk semiconductor. Finally, we investigate the application of these metamaterials as photomodulators, finding switching times that depend upon both the photoexcitation intensity, wafer thickness, and the carrier lifetime.