Radiation Damage in Silicon Photonic Mach–Zehnder Modulators and Photodiodes

Radiation-hard optical links are the backbone of read-out systems in high-energy physics (HEP) experiments at CERN. The optical components must withstand large doses of radiation and strong magnetic fields and provide high data rates. Radiation hardness is one of the requirements that become more demanding with every new generation of HEP experiment. Previous studies have shown that vertical cavity surface emitting lasers, on which the current optical links are based, will not be able to withstand the expected radiation levels in the innermost regions of future HEP experiments. Silicon photonics (SiPh) is currently being investigated as a promising alternative technology to address this challenge. We irradiated SiPh Mach-Zehnder modulators (MZMs) with different design parameters to evaluate their resistance against ionizing radiation. We confirm that SiPh MZMs with a conventional design do not show a phase shift degradation when exposed to a 20-MeV neutron fluence of $3\cdot 10^{16}~{\rm n/cm^{2}}$ . We further demonstrate that custom-designed MZMs with shallow etch optical waveguides and high doping concentrations in their p-n junctions exhibit a strongly improved radiation hardness over devices with a conventional design when irradiated with X-rays. We also found that MZMs withstood higher radiation levels when they were irradiated at a low temperature. In contrast, larger reverse biases during irradiation led to a faster device degradation. Simulations indicate that a pinch-off of holes is responsible for the device degradation. Photodiodes (PDs) were also tested for their radiation hardness as they are needed in silicon photonic transceivers. X-ray irradiation of building-block germanium-silicon PDs showed that they were not significantly affected.