IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 17, SEPTEMBER 1, 2011 1183 Mode Density Reduction and Coupling in Microdisk LASERs Processed on a 200-mm CMOS Pilot Line F. Mandorlo, P. Rojo Romeo, J.-M. Fedeli, H. MD Sohrab, and R. Orobtchouk Abstract—Efficient compact sources are necessary to consider on chip optical interconnections in integrated circuits. Properties of III–V microdisk light amplification by stimulated emission of radiation (LASERs) coupled to Si waveguides and fabricated on a 200-mm complementary metal–oxide–semiconductor (CMOS) pilot line are investigated. Index Terms—Complementary metal–oxide–semiconductor (CMOS), coupling, design, heterogeneous integration, light ampli- fication by stimulated emission of radiation (LASER), microdisk, silicon photonics. I. INTRODUCTION O PTICAL links have been considered to reduce the in- terconnection impact on consumption and performances for the longest interconnections in integrated chips [1], [2], for instance to provide a very high bandwidth between different cores. To compete with electrical solutions, low power and CMOS (Complementary Metal–Oxide–Semiconductor) com- patible LASER (Light Amplification by Stimulated Emission of Radiation) sources must be fabricated. In this article, we discuss the experimental electrooptical properties of the first microdisk LASERs fabricated on a CMOS pilot line. The fabrication flow is based on molecular bonding of subwavelength thick III–V dies [3], [4] onto a 200 mm SOI (Silicon On Insulator) wafer containing Si 220 nm height waveguides and grating couplers for light collection. Reduction of mode competition in these LASERs taking elec- trodes into account is experimentally confirmed and compati- bility with CMOS voltage operation is achieved. II. DESIGN OF MICRODISK LASERS In a microdisk, Modes with the highest Quality factor ( ) are located at the edge of the cavity and are called Whispering Gallery Modes (WGMs). Each of their polarization, quasi Transverse Electric (TE) or Transverse Magnetic (TM), is Manuscript received November 16, 2010; revised March 28, 2011; accepted May 14, 2011. Date of publication May 23, 2011; date of current version August 03, 2011. This work was supported by the European project FP7-ICT STREP WADIMOS. F. Mandorlo, P. Rojo Romeo, and H. MD Sohrab are with the University of Lyon, Lyon Institute of Nanotechnology (INL) UMR CNRS 5270, Ecully, F-69134, France (e-mail: Fabien.Mandorlo@EC-Lyon.Fr). J.-M. Fedeli is with the CEA LETI, Minatec, 38054 Grenoble, Cedex 9, France. R. Orobtchouk is with the University of Lyon, Lyon Institute of Nanotech- nology (INL) UMR CNRS 5270, INSA de Lyon, Villeurbanne, F-69621, France. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2157335 Fig. 1. Partial slice view of an integrated microdisk with a low index central part, its contacts, and a vertically coupled waveguide. Optical modes are ex- tracted from 3-D finite-difference time-domain (FDTD) simulations. numbered with 3 integers: the radial , azimuthal and vertical orders [5]. In the frequency domain, the Free Spectral Range (FSR) is the constant spacing between two modes where only the azimuthal order differ. This parameter does not depend very much on and for the most confined modes with low radial and vertical orders [6]. Consequently, the spectrum of such resonators is constituted of different combs (quoted ) where and mainly describe the vertical and radial optical confinement. An integrated microlaser (Fig. 1), with a III–V gain mate- rial including InAsP Quantum Wells (QWs), requires at least two electrodes: the bottom one is a thin doped layer (typ. 80 to 100 nm thick) obtained by partial etching of the gain material while the second one is positioned on top of the cavity, on a III–V n-doped layer. For CMOS compliance, gold free contacts are used and the device as a whole is embedded in silica. Last, light is collected by a Si waveguide underneath the resonator. Due to contact closeness and fabrication imperfections [7], [8], -factors are limited to low values (typ. few tens of thou- sands) compared to the intrinsic ones [9], [10], even for the most confined WGMs. To ensure that one mode in will reach the lasing regime, we propose to optimize the design so that modes of other combs have significantly higher losses. A. Injection Scheme and Mode Density Reduction From a semianalytical model [5], only modes with exist in 550 nm thick microdisks. To restrict light emission to TE modes at 1.55 m wavelength, compressive-strained QWs are centered in the III–V membrane [11]. As the extension of WGMs inside the resonator increases with the radial order , absorption in the top contact can be advantageously used to select the comb [5]. Then, lasing competition mainly occurs between the few WGMs in lo- cated in the emission range ( nm) of the QWs [12]. To re- duce useless carrier recombinations, the inner part of the device is replaced by silica (Fig. 1): modes with a high radial order are eliminated while frequencies and field distribution are kept un- changed for the most confined ones (typ. ). For this reason, this resonator is closer to a microdisk than to a microring. Last, 1041-1135/$26.00 © 2011 IEEE