Radiation Effects
Over the past few decades, silicon photonics has revolutionized information transfer towards optical signal routing on-chip. By seamlessly integrating photonic integrated circuits (PICs) with traditional electronics, there have been significant improvements in data transmission efficiency and bandwidth. Beyond terrestrial applications, the interest in silicon photonics for aerospace applications has grown considerably in recent years driven by its ability to minimize size, weight and power consumption of communication systems. However, for silicon photonic components to be viable for space applications, their performance under harsh conditions (radiation, temperature, shock/vibration) must be thoroughly evaluated. Our group, in collaboration with the Institute of Space and Defense Electronics at Vanderbilt University, investigates the reliability of various photonic integrated circuit (PIC) components, including photodiodes, modulators, passive photonic elements etc. under harsh environmental conditions. By combining experimental testing, detailed device characterization, and advanced simulations, we aim to understand and mitigate radiation-induced degradation mechanisms that impact the performance of these components in aerospace applications.
Highly focused pulsed laser systems are commonly used to inject charge into circuits to emulate high energy particles passing through a device. In most cases, sub-bandgap wavelength photons are used to deposit charge through nonlinear, multiphoton absorptive processes that spatially confine charge generation to only the focus of the laser. While pulsed laser testing is extensively used for qualitative measurements, development of first-principle simulation tools for nonlinear optical energy deposition is critical for correlating pulsed laser measurements with traditional radiation testing techniques. Adaption of a commercial FDTD solvers to incorporate nonlinear absorption as well as free carrier effects has allowed for the simulation of 3D distribution of deposited energy from a single laser pulse. Incorporation of these optical simulations into charge transport code allows for complete simulation of the pulsed laser testing technique: from laser pulse to electrical signal output.
Ge-Si photodiodes with different configurations are studied in harsh radiation environments, with modest sensitivities to 1.8-MeV proton and 10-keV X-ray irradiation. Defects within the bulk and at the Ge/SiO2 interface of the depletion region contribute to an increase in dark current, which increases the noise floor ~4.25 dB for signal detection on the PIC. No measurable change to the S21 bandwidth of the devices was found after irradiation. While the performance changes in dark current are relatively small in the scheme of overall device operation, as PICs become increasing complex, their performance in harsh radiation environments must be considered.
