Surface plasmon enhanced responsivity in a waveguided germaniummetal-semiconductor-metal photodetectorFang-Fang Ren,a?Kah-Wee Ang, Junfeng Song, Qing Fang, Mingbin Yu, Guo-Qiang Lo,and Dim-Lee KwongInstitute of Microelectronics, Agency of Science and Technology Research (A*STAR), 11 Science Park Road,Singapore Science Park II, Singapore 117685?Received 11 June 2010; accepted 12 August 2010; published online 30 August 2010?The authors report on high transverse magnetic ?TM?-mode responsivity in a waveguidedgermanium Schottky-barrier metal-semiconductor-metal photodetector on silicon-on-insulatorsubstrate for operating wavelength at 1550 nm. The employed aluminum interdigitated electrodesact as a one-dimensional rectangular grating above the depletion layer. By means of properlydesigned finger dimensions, surface plasmon polariton resonances can be excited at the interface ofmetal and silicon interfacial layer due to grating coupling. The resulting strong field intensities reachinto active region, enabling high absorption under TM injection. At a voltage of 1 V, the TM-modephotocurrent is measured over three times than that of transverse electric mode, in spite of therelatively larger TM insertion loss in the silicon waveguide. © 2010 American Institute of Physics.?doi:10.1063/1.3485064?Surface plasmon polaritons ?SPPs? are electromagneticwaves that are strongly coupled to free electron oscillationon the metal surface and propagate along the dielectric-metalinterface.1The presence of strong and localized optical in-tensities proved extremely efficient for semiconductor pho-todetectors in absorption enhancement and footprint shrink-age but most of design schemes focused on discretecomponents for operation with free-space illumination.2–4Infact, an electrophotonic integrated circuit on a single chipplatform requires waveguide-integrated photodetectors to re-ceive optical signals through the input waveguides. So far,the research into this area is lagging behind.Inourpreviouswork,insulator ?Ge-on-SOI? photodetectors have been actively pur-sued owing to the large absorption coefficient and integrationcompatibility with silicon ?Si? complementary metal-oxide-semiconductor ?CMOS? process technology.5–8For efficientcoupling and propagating of transverse electric ?TE?-modelight wave, our SOI substrate usually feature a Si core layerwith very tight vertical confinement. A drawback found is therelatively lower transverse magnetic ?TM? mode response,which mainly resulted from the larger TM insertion loss ?in-cluding fiber-to-waveguide coupling loss and propagationloss?.In this work, we explore the use of SPPs in awaveguided Ge-on-SOI metal-semiconductor-metal ?MSM?photodetector with interdigitated electrodes to achieve largeTM enhancement at a standard communication wavelengthof 1550 nm. The plasmon-enhancement regime is realized byproperly choosing the finger width and spacing in terms ofthe Bloch theorem and momentum conservation.Figure 1?a? illustrates the cross-section structural geom-etry of our designed Ge-on-SOI MSM photodetector with aneffective device width W of 25 ?m and length L of 50 ?m.Figure 1?b? shows the top-view structure schematic of theinterdigitated electrodes, which are defined by the figurewidth w and spacing s ?or periodicity a=w+s?. These elec-germanium-on-silicon-on-trodes effectively form a one-dimensional ?1D? metallic rect-angular grating with finite width, length, and height on thetop surface of the massive multilayer. Taking into accountthe compatibility to Si-CMOS processes, the nontraditionalplasmonic metal aluminum ?Al? is employed in this work. Acrystalline Si interfacial layer of 20 nm is designed to beinserted between the metal layer and the Ge region so as toincrease the Schottky barrier and thus suppress the dark cur-rent of device.6Figure 1?c? depicts a perspective view of ourwaveguide Si core layer with a thickness of 220 nm and awidth of 500 nm. The TE/TM-polarized ?electric/magneticfield parallel to the x axis? light wave propagates along thez-direction. When reaching the active regime, the Si wave-guide width is adjusted with a taper to ensure efficient cou-pling of incidence photon from the routing Si waveguide upinto the Ge region for absorption.In our designed scheme, the SPP excitation is permittedby the counter-directional coupling between the guided modepropagating in the Si core layer and the copropagating SPPwave in the vicinity of Al–Si interface with the assistance ofBragg grating.9,10The first parameter to be chosen is theperiodicity of the 1D Al grating, which can be roughly givenby the momentum conservation law,11a?Author to whom correspondence should be addressed. Electronic mail:fangfang.ren@gmail.com.(a)(b)waszxBuried SiO2Ge500 nm20 nmSi220 nmSiSiO2Alh=400 nm400 nmzyws(c)500nmx220nmSiyxFIG. 1. Scheme of the Ge-on-SOI MSM photodetector with interdigitatedelectrodes. ?a? Cross-sectional view of the device. ?b? Top view of the in-terdigitated fingers. ?c? Perspective view of the Si core layer with a thicknessof 220 nm and a width of 500 nm where light propagates in the z-direction.APPLIED PHYSICS LETTERS 97, 091102 ?2010?0003-6951/2010/97?9?/091102/3/$30.00© 2010 American Institute of Physics97, 091102-1Downloaded 12 Oct 2010 to 222.200.137.171. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
