Engineering the heterogeneously integrated III-V/SOI tunable laser

S. Keyvaninia, G. Roelkens, D. Van Thourhout Photonics Research Group, Department of Information Technology - Ghent University

Sint-Pietersnieuwstraat 41, B-9000 Ghent

Recently reported state-of-the-art adhesively bonded III-V/Silicon laser diodes are fabricated using the adhesive DVS-BCB. The efficiency of the laser is mostly related to the coupling efficiency between the III-V and SOI waveguide layer. In this paper, an adiabatically tapered heterogeneously integrated III-V laser is engineered for high coupling efficiency, while at the same time resulting in a short length device and the optimum structure with respect to the fabrication tolerances. Moreover, a tunable laser design in order to demonstrate suitability of this technique in the realization of an array of single wavelength lasers is presented and discussed. Introduction Photonic integrated circuits offer the potential of realizing low-cost, compact optical functions. Silicon-on-insulator (SOI) is a promising material platform for this photonic integration, as one can rely on the massive electronics processing infrastructure to process the optical components. However, the integration of a Si laser is hampered by its indirect bandgap. Silicon Raman lasers have been demonstrated but external optical pumps are required [1]. Recently, some research groups proposed to integrate a direct bandgap III-V material on top of a SOI waveguide substrate to achieve stimulated light emission and to couple this stimulated emission to the underlying SOI waveguide circuit. This technique, semiconductor wafer bonding, allows the dense integration of high-quality III-V epitaxial layers on top of a Si platform by transferring the III-V layer stack from its original growth substrate to the SOI wafer. Recently, several devices based on evanescent coupling were demonstrated. The first, hybrid III-V/SOI Fabry-Perot laser (emitting at 1.55μm), was reported in 2006 [2], followed by a DBR [3] and DFB lasers [4], in 2008. These devices were based on molecular wafer bonding procedure, which is based on van der Waals forces and as this is a short-range force, sub-nm rms roughness of the surfaces is required. Therefore, this technique may not be robust enough for fabrication where such strict requirements are difficult to meet. In the second approach, heterogeneous integration based on DVS-BCB adhesive bonding as an alternative method is actively pursued in the photonics research group at Ghent University [4].The major challenge in this method is the light coupling from the top active III-V layer into the bottom SOI waveguides because of the additional polymer layer between them. For realizing highly efficient coupling and compact devices, we introduce an adiabatically tapered coupler for III-V layers and SOI waveguides. Finally we also introduce a tunable single mode laser based on this technique, useful for telecom applications. Adiabatic III-V/silicon taper Laser Design In the adhesive BCB bonding method, the epitaxial III-V structure is bonded on top of the SOI waveguide, using a spincoated DVS-BCB adhesive layer. The adiabatic taper III-V/silicon laser and the general layout of the structure are given in Figure 1. It comprises the n-type InP spacer layer and the mesa structure on top of it. The mesa is

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