Silicon photonic modulators are a key component for electro-optic transmitter within data centers. Electro-refractive modulators relying on free carrier plasma dispersion in Mach-Zehnder interferometer have become the most popular solution. Accumulation–based capacitive modulators are an efficient approach, which can reduce the modulation power consumption. In this work we study the behavior of capacitive modulators with polycrystalline silicon to form the capacitance. The modulators are made within the standard fabrication flow with only few add-ons. In this work we show that furnace annealing conditions and excimer laser annealing conditions during the polycrystalline silicon formation enhance the modulator bandwidths.
Silicon photonic modulators are a key component for electro-optic transmitters within data centers. Electro-refractive modulators relying on free carrier plasma dispersion in a Mach-Zehnder interferometer (MZI) have become the most popular solution. Among the different electrical configurations, PN silicon modulators show a high bandwidth but at the price of a low efficiency. Accumulation–based capacitive modulators are an alternative, allowing to reduce the modulator power consumption. Additionally, strained SiGe exhibits a stronger plasma dispersion effect than silicon for holes. In this work we study the behavior of capacitive modulators with a thin layer of strained SiGe. The modulator fabrication process is based on the standard process flow with only few add-ons. In the first demonstration we show that the thin SiGe layer improved the modulator efficiency by 25%. In addition, further improvement is possible by optimization of the SiGe deposition condition to maximize the SiGe layer stress.
Optical signal modulation is presently done using Si pn junctions which cause phase shifting due to Soref effect and, put in a Mach-Zehnder configuration, produce interference and generate amplitude modulation. The drawback of pn junctions is the relatively low phase shifting efficiency which consequently inflicts high power consumptions on the electrical driver. An alternative device to pn junctions was developed and consists of introducing capacitive structures within the optical waveguide. The proposed device has the same cross-section foot-print but is much shorter due to improved efficiencies. Typical pn-junctions can generate phase shifts of < 20°/mm for given implantation conditions and the capacitive structure developed produces shifts of > 60°/mm for the same implantation conditions. The device is made up of crystal Si, a thin SiO2 capacitor dielectric and poly-Si. Benchmarking the two phase shifters with respect to insertion losses, we observe that the proposed device is promising.
Another material exhaustively used in CMOS technologies is Si3N4. In the data-communication bandwidths, the index contrast between Si3N4 (n = 1.95) and SiO2 (n=1.45) is smaller than that with Si (n = 3.5). Thus, nitride waveguides have lower optical mode confinements and are thus less sensitive to insertion losses caused by line edge roughness and wavelength shifting incurred by process variations. Moreover, the temperature induced index variations are 5 times les in Si3N4 than Si. Therefore, the use of nitride to fabricate devices in silicon photonics looks advantageous. However, high speed electro-optic devices are challenging in Si3N4. Consequently, a co-integration of both materials is essential. We developed a fabrication method and associated devices which allow to transfer the signal to and fro Si and Si3N4. We present some devices in each layer to illustrate the benefits.
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