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technology  PICs


Figure 2 a) Bonded wafer,in this case on a transparent Pyrex wafer,allowing us to inspect the interface,showing no defects in this case b) Multiple bonded InP dies on a patterned SOI substrate c) Cross-section of bonded InP wafer on silicon waveguide


the advantage of avoiding a polymer layer in the stack, but they are more sensitive to surface roughness and contamination. Note that in all cases the overall integration philosophy remains the same.


Once the materials have adhered to one another, the substrate of the bonded material is removed to leave a high-quality III-V epitaxial layer on the silicon platform. The thickness of this III-V structure can range from less than 100 nm to 2 µm – its value should reflect the target application. From that point onwards, fabrication can exploit standard waferscale processes. This means that the accuracy of alignment between III-V devices and silicon waveguides is determined by lithographic processes. These deliver far tighter tolerances than the flip-chip process used for integrating prefabricated devices on top of a wafer.


Making lasers ...


Our device development began with the fabrication of telecom-wavelength lasers based on InP and related material. We constructed a portfolio of emitters, ranging from simple Fabry-Pérot lasers to


Another, more common approach for us is to concentrate light into the III-V layers and couple it back to the silicon layer using evanescent tapers. By creating suitable structures in the silicon layer, it


single-longitudinal-mode, DFB-type lasers – with the grating defined in the silicon layer – and more complex devices, such as tuneable lasers. The latter, which featured advanced filters in the silicon layer to provide wavelength-selective feedback, have typical threshold currents of 25-50 mA and deliver output powers up to 10 mW.


One of the biggest challenges associated with the fabrication of PICs incorporating lasers is realising efficient coupling of light out of this device and into silicon. This can be achieved with the ‘hybrid laser’, a design originally proposed by UCSB, which maintains the optical mode within the silicon waveguide – its evanescent tail extends into the III-V gain layers on top. We have adopted this approach to build a DFB laser using our BCB-bonding process (see Figure 3). This device contains eight InGaAlAs based quantum wells, optimized for emission at 1.3 µm.


Figure 3.Ultra-dense wavelength demultiplexer chip developed in the EU-funded project BOOM.a) Top view of chip.b) Eye diagram for detector under 10 Gbit/s operation.c) Full optical to electrical response of the device


58 www.compoundsemiconductor.net March 2013


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