Si-based light emitter in an integrated photonic circuit for smart biosensor applications

Integrated optics concerns mainly the generation, guiding, and detection of light. Especially bio sensing needs systems that incorporate electronic and photonic devices for the detection of harmful substances, like synthetic estrogens or plasticizers. We present recent developments in the integration of Si-based light emitters into a photonic circuit for a planar optical waveguide-based bio detection system. The growing demand for sensitive biochemical sensors in the environmental control, medicine or process technology results in the development of integrated sensors, which should show a high resolution over a wide concentration regime. In our first approach we deal with the integration of a Si-based light emitting device (LED) into a photonic circuit for the detection of harmful biological substances. Light injection into a waveguide is commonly obtained by using an external source coupled to the waveguide, e.g. an optical fiber via total internal reflection. For simplifying this injection process, we built Si-based LEDs consisting of a metal-oxide-semiconductor (MOS) structure, in which the oxide film contains group-IV and/or rare earth elements, incorporated by ion-beam synthesis [1, 2]. The Si-based LED exhibits strong electroluminescence, tunable from the visible up to the UV region depending on the rare-earth element (e.g. Gd, Tb, Eu, Nd, Er). Currently, the Si-based LEDs are already available and best efficiencies were achieved by Tb or Er implantation with an external quantum and power efficiency of 16% and 0.3%. LOCOS (local oxidation of silicon) processing and an additional layer of SiON were applied to the device to improve the electrical stability and operation time. Our concept bases upon a Si-based photonic circuit which consists of the integrated LED, working as the light source, a newly fabricated dielectric strip-waveguide below a bioactive layer and a receiver. The dielectric strip-waveguide has a Si3N4 or SiON core, in which the light should be guided, and a cladding of SiO2. The receiver should be a photodiode (e.g. Ge, Si). In this work, we focus on the development and characterization of the dielectric waveguides. For theoretical pre-analysis we are using the finite element method by the FlexPDE software for calculating the mode profiles and resonance frequencies according to the cross sections of the structures. The fabrication of the waveguides was done by plasma enhanced chemical vapor deposition (PECVD), photolithography and electron beam lithography. Obtained SEM results enabled an improvement of the fabrication recipe of the waveguides by using an additional Al-masking during the reactive ion etching (RIE). Furthermore, a new measurement setup is built up, which enables transmission measurements and the inspection of the beam profiles as well as the damping factors of the structures in dependence on their cross sections. In future, the theoretical calculations are going to be compared with the experimental results of transmission and beam profiling measurements. Moreover, the Si-based LED should be coupled with the waveguide e.g. by Bragg grating. Finally, this lab-on-a-chip system is showing a high potential to become an all-round applicable integrated sensor system, without using any external light sources or relay lenses, which is why it should be easily portable and customizale.