Resonant Germanium Nanoantenna Photodetectors Linyou Cao, † Joon-Shik Park, †,‡ Pengyu Fan, † Bruce Clemens, † and Mark L. Brongersma* ,† † Geballe Laboratory for Advanced Materials, Stanford University, California 94305 and ‡ Nanomechatronics Research Center, Korea Electronics Technology Institute, Gyeonggi, 463-816, Republic of Korea ABSTRACT On-chip optical interconnection is considered as a substitute for conventional electrical interconnects as microelectronic circuitry continues to shrink in size. Central to this effort is the development of ultracompact, silicon-compatible, and functional optoelectronic devices. Photodetectors play a key role as interfaces between photonics and electronics but are plagued by a fundamental efficiency-speed trade-off. Moreover, engineering of desired wavelength and polarization sensitivities typically requires construction of space-consuming components. Here, we demonstrate how to overcome these limitations in a nanoscale metal-semiconductor-metal germanium photodetector for the optical communications band. The detector capitalizes on antenna effects to dramatically enhance the photoresponse (>25-fold) and to enable wavelength and polarization selectivity. The electrical design featuring asymmetric metallic contacts also enables ultralow dark currents (∼20 pA), low power consumption, and high-speed operation (>100 GHz). The presented high-performance photodetection scheme represents a significant step toward realizing integrated on-chip communication and manifests a new paradigm for developing miniaturized optoelectronics components. KEYWORDS Photodetector, antenna, Mie resonance, nanophotonics, nanowire G e is considered to be one of the most promising materials for near-infrared (1.3-1.6 μm) photode- tectors in integrated optical circuits because of its large absorption coefficient (related to its direct gap at 0.8 eV) and compatibility with standard silicon-processing technology. 1-4 Substantial resources have been devoted to develop compact on-chip Ge photodetectors featuring high speed and responsivity. 5-12 For example, detector elements have been embedded in optical resonators to enhance responsivity 7 and waveguide-based detectors have been developed to get around the infamous efficiency-speed trade-off. 5,6,10 Unfortunately, these structures offer limited opportunities for scaling, and seamless integration with nanoscale electronic components is precluded. Nanometallic (i.e., plasmonic) light concentration structures have enabled further shrinking of device dimensions below the diffraction limit and absorption depth of a semiconductor without significant loss in responsivity. 9,11 However, one would ideally avoid the intrinsic heating losses in metals. Another desirable feature attracting significant attention is the integration of wavelength and polarization selectivity into photodetectors that usually only sense the signal intensity. 13-16 This would facilitate communication encoded in polarization states of light 17 and valuable demultiplexing of wavelength division multiplexed (WDM) signals, greatly extending the available bandwidth of optical interconnec- tion. 18 Efforts to increase functionality again require the use of bulky external structures, 9,13-16 akin to the attempts to enhance responsivity. Polarization and wavelength selective detection has been realized by adding filters capable of screening specific polarization or wavelength of light, includ- ing grating couplers, 13 wire grid polarizers, 16 interdigitated metallic electrodes, 14 or planar cavities. 15 Here, we for the first time propose a photodetector that combines the strong intrinsic optical resonance effects in semiconductor nanow- ires with the excellent high-speed, low-noise performance of metal-semiconductor-metal (MSM) photodetectors. We demonstrate that the resonances can be engineered to boost the absorption of light in a CMOS-compatible Ge nanowire- based MSM detector by over ∼25-fold compared to non- resonant designs. The nature of the resonance also naturally offers polarization and wavelength selectivity at the impor- tant 1.55 μm communication wavelength. Owing to its small dimension and the use of asymmetric metal contacts (one side Ohmic and the other Schottky), the photodetector also features a very low dark current, can work at zero bias, and is expected to have very high operation speed (>100 GHz). It was recently shown that semiconductor nanowires (NWs) can exhibit a strongly enhanced and tunable photo- response. 19-22 Physically, the enhanced light-matter inter- action arises from the coupling of incident light to leaky mode resonances (LMRs) supported by the nanowires; 19 sufficiently large NWs can be thought of as a cylindrical cavity antenna 23 that can trap light in circulating orbits by multiple total internal reflections from the periphery, as illustrated in Figure 1a upper inset. It is worth noting that planar detectors exploiting Fabry-Pe ´ rot resonances in thin films are expected to experience smaller field enhancements than wire detectors as they rely on relatively weak reflections at normal incidence. To exploit the LMRs achieving optimal photodetection performance at near-infrared wavelengths, * To whom correspondence should be addressed, brongersma@stanford.edu. Received for review: 11/6/2009 Published on Web: 03/15/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 1229 DOI: 10.1021/nl9037278 | Nano Lett. 2010, 10, 1229–1233