Nanometre-Scale Germanium Photodetector Enhanced by Optical Antennas

The use of optics to make connections within and between electronic chips has been the subject of research for over 20 years because it could solve many of the problems experienced in electrical systems. A critical challenge for the convergence of optics and electronics is that the micrometre scale of optics is significantly larger than the nanometre scale of modern electronic devices. In the conversion from photons to electrons by photodetectors, this size incompatibility often leads to substantial penalties in power dissipation, area, latency and noise. A photodetector can be made smaller by using a subwavelength active region which, however, could result in very low responsivity because of the diffraction limit of the light. In our first approach to tackle this problem, we use a C-shaped nano-aperture antenna in a thin metal layer to enhance the photocurrent response of a subwavelength photodetector. The single C-shaped aperture, without any other supporting surface structures, can collect light from a large area and concentrate it into a tiny volume of semiconductor. We demonstrated the first antenna-enhanced photodetector at near-infrared wavelengths. In our second approach, we exploit the idea of a dipole antenna from radio waves, but at near infrared wavelengths (∼ 1.3 mum), to concentrate radiation into a nanometre-scale Ge photodetector. Despite the small antenna size (∼ 380 nm long) and the different properties of metals at such high frequencies (∼ 230 THz), the antenna has qualitatively similar behavior to the common radio-frequency half-wave (i.e., half wavelength long) Hertz dipole. It gives a relative enhancement of 20 times in the resulting photocurrent in the subwavelength Ge detector element, which has an active volume of 0.00072 mum 3, two orders of magnitude smaller than previously demonstrated detectors at such wavelengths. Photodetectors are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures and ultra-low electrical and optical power operations. Finally, we show that such an antenna-enhanced photodetector can be fabricated in a CMOS process. For the first time, plasmonic effects have been demonstrated in Si CMOS, paving the way for plasmonics to become the next wave of chip-scale optoelectronic technology. It is noteworthy that although strong optical fields associated with metallic nanostructures have been studied extensively in recent years, we are among the very first in studying the interaction of these strong fields with semiconductors and the further transformation of the optical energy into electricity.