Abstract
The performance of strained silicon devices based on the deposition of a top silicon nitride layer with high stress have been thoroughly analyzed by means of simulations and experimental results. Results clearly indicate that the electro-optic static response is basically governed by carrier effects. A first evidence is the appearance of a variable optical absorption with the applied voltage that should not occur in case of having a purely electro-optic Pockels effect. However, hysteresis and saturation effects are also observed. We demonstrate that such effects are mainly due to the carrier trapping dynamics at the interface between the silicon and the silicon nitride and their influence on the silicon nitride charge. This theory is further confirmed by analyzing identical devices but with the silicon nitride cladding layer optimized to have intrinsic stresses of opposite sign and magnitude. The latter is achieved by a post annealing process which produces a defect healing and consequently a reduction of the silicon nitride charge. Raman measurements are also carried out to confirm the obtained results.
Highlights
The silicon platform has the unique capability for enabling a monolithic integration of photonic and electronic circuits with a low cost standardized fabrication process
Variations of the carrier distribution can affect the electric field inside the waveguide and the modulation induced by the Pockels effect
The obtained results confirm that carrier effects can play a prominent role in the performance of strained silicon devices
Summary
The silicon platform has the unique capability for enabling a monolithic integration of photonic and electronic circuits with a low cost standardized fabrication process. The demonstration of Pockels effect in strained silicon by Jacobsen et al.[6] opened the door to a new route for a CMOS compatible integration of fast and low loss electro-optic modulators at minimum complexity and cost. An electro-optic response induced by the free carrier distribution has been demonstrated by using cladding materials with different and opposite fixed charge concentrations and interface traps densities[23]. The injection of free carriers in the silicon waveguide in response to an applied electric field has been proposed as a possible mechanism responsible of the electro-optic response[24]. The influence of interface charging dynamics and stressing conditions on the electro-optic response of strained silicon devices is discussed. Results provide an additional confirmation of the strong contribution of carrier effects taking place in the electro-optic response
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