September 16, 2008
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 micrometer scale of optics is significantly larger than the nanometer 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 work is the first demonstration of a plasmonic-enhanced semiconductor 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 µm), to concentrate radiation into a nanometer-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 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 _m3, two orders of magnitude smaller than previously demonstrated detectors at such wavelengths. Finally, we integrate an antenna-enhanced photodetector on a commercial CMOS chip, which is the first demonstration of any plasmonic effect in Si CMOS. 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 operation.
Salman Latif received the B.Eng (Honors) degree in Electrical Engineering from McGill University, Montreal, Canada, in 2002, and the M.S degree in Electrical Engineering from Stanford University, Stanford, CA in 2004. He is currently a Ph.D student in Electrical Engineering at Stanford University. He is interested in the integration of optoelectronic devices with Silicon electronics for interconnect and clocking applications, and is currently involved in the design and characterization of low capacitance CMOS compatible photodetectors.